Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

We measured the radial and axial temperature distributions of a saturated steam-air mixture in a vertical circular pipe (diameter, 49.5 mm
cooling height, 610 mm) in our previous study. Those of a superheated steam-air mixture in the same experimental setup were measured in this study. The condensation heat flux qc and heat transfer coefficient hc for the superheated mixture were evaluated, and they were compared with those for the saturated mixture. There was no large difference in qc between the saturated and superheated mixtures. The heat transfer coefficient hc (which was evaluated for the saturation temperature) for the superheated mixture agreed with that for the saturated mixture. hc computed with the diffusion layer model for saturated temperature agreed well with the hc measured for the superheated mixture.

DOI： 10.1016/j.nucengdes.2020.110948

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Publishing type：Research paper (scientific journal)   Publisher：Begell House  

DOI： 10.1615/multscientechn.2021040158

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Publishing type：Research paper (international conference proceedings)   Publisher：American Society of Mechanical Engineers  

Abstract

This paper presents an analysis of the performance of the Spatial Filter Velocimetry (SFV) technique applied to external flows across tube bundles to evaluate the benefits of using low particle concentration in a complex geometry. The experimental vector velocity fields were obtained in a tube bundle test section made of acrylic with 20 rows of 4 tubes of 20mm O.D. mounted in a normal triangular configuration and transversal pitch per diameter ratio of 1.25. The SFV results reliability was evaluated by comparing the volumetric flow rates estimated by the experimental vector velocity fields with the volumetric flow rates measured by the flowmeter, showing good agreement. Additionally, the experimental results are used to check the validity of numerical results obtained through CFD simulation. The experiments were conducted for Reynolds numbers of 909, 1842 and 3902. The velocity profiles obtained through the SFV technique agree with the CFD simulations results, which indicate consistent results for both analyzes. Therefore, the numerical and experimental results indicate that the SFV technique provides accurate velocity data and is suitable to be applied to complex geometries, especially in regions away of the test section wall due to the good visibility provided by low particle concentration.

DOI： 10.1115/fedsm2020-20251

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s10494-020-00143-z

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Other Link： http://link.springer.com/article/10.1007/s10494-020-00143-z/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s40430-020-02293-7

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Other Link： http://link.springer.com/article/10.1007/s40430-020-02293-7/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.ijmultiphaseflow.2020.103207

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japanese Society for Multiphase Flow  

DOI： 10.3811/jjmf.2019.012

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DOI： 10.1016/j.nucengdes.2019.110223

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DOI： 10.1615/MultScienTechn.2019031210

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DOI： 10.1615/MultScienTechn.2019029476

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Inc.  

Aspect ratios of ellipsoidal bubbles in clean and contaminated systems were measured to investigate the effects of surfactant on the bubble aspect ratio. The bubble diameter ranged from 0.80 to 5.8 mm. Air was used for the gas phase and the glycerol-water solutions were used for the liquid phase. The experimental ranges of the Morton number, M, the bubble Reynolds number, Re, the Eötvös number, Eo, the Weber number, We, and the Tadaki number, Ta, were as follows: 10−7 &lt
M &lt
10−3, 3.6 × 10−1 &lt
Re &lt
1.7 × 102, 1.1 × 10−1 &lt
Eo &lt
5.9, 5.0 × 10−3 &lt
We &lt
3.7 and 5.3 × 10−2 &lt
Ta &lt
3.6. Triton X-100 and 1-octanol were used for surfactant. The terminal velocities of bubbles were confirmed to be independent of the surfactant concentration, and therefore, the bubbles were fully-contaminated from the point of view of the terminal velocity. By making use of the aspect ratio data, the applicability of available aspect ratio correlations was examined and an empirical correlation was proposed. The applicability of the proposed correlation was also examined for sodium dodecyl sulphate (SDS) and 1-decanol. As a result, the following conclusions were obtained: (1) the aspect ratio database, which is of use for validating and developing shape correlations, was obtained, (2) the available aspect ratio correlations are not applicable to the present data for fully-contaminated ellipsoidal bubbles, (3) the aspect ratios are well correlated in terms of a combination of the Eötvös and bubble Reynolds numbers, which proves that all the relevant forces, i.e. the viscous, inertial, buoyant and surface tension forces, should be taken into account to correlate bubble shapes, and (4) the empirical correlation of the aspect ratio developed by using the Triton X-100 and 1-octanol data also gives good evaluations for bubbles contaminated with SDS and 1-decanol in spite of largely different adsorption-desorption characteristics.

DOI： 10.1016/j.expthermflusci.2018.03.015

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

Experiments on air–water two-phase swirling annular flows in a vertical pipe of 40 mm diameter were carried out at atmospheric pressure and room temperature to investigate interfacial and wall friction factors, fi and fw. The friction factors were evaluated using measured pressure drops and void fractions. Measurements of liquid film thickness and flow observation were also conducted. The ranges of the gas and liquid volume fluxes, JG and JL, were 12.5 ≤ JG ≤ 20.0 m/s and 0.03 ≤ JL ≤ 0.11 m/s, respectively. The main conclusions obtained are as follows: (1) fi and fw in swirling annular flows are several times larger than those in non-swirling flows, (2) fi is well correlated in terms of the liquid volume fraction and the gas Reynolds number, ReG, (3) ReG and the liquid Reynolds number, ReL, are required for correlating fw, and (4) the liquid film thicknesses in two-phase swirling flows in a one-fifth model of a BWR separator are well predicted using the two-fluid model and the correlations of fi and fw developed based on the experimental data.

DOI： 10.1016/j.nucengdes.2018.01.043

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

Spatiotemporal filter velocimetry (SFV) was extended to Lagrangian measurements with boundary-fitted measurement areas, and was applied to flows about single spherical drops of glycerol-water solution falling in stagnant silicon oil under clean and contaminated conditions to examine its applicability to the estimation of the Marangoni stress and surfactant concentration at a moving interface. Effects of bulk concentration of surfactant on the velocity field, the Marangoni stress and the surface concentration of surfactant were discussed from the measured data. As a result, we confirmed that accurate velocity distribution in the vicinity of the interface measured by SFV enables us to evaluate interfacial velocity and interfacial shear stresses and to estimate the Marangoni stress, interfacial tension and surfactant concentration at the interface with the assumption of negligible surface viscosity. The flow inside the drop and the interfacial velocity become weak due to the Marangoni stress caused by the gradient of surfactant concentration at the interface as the bulk concentration of surfactant increases. These results demonstrate that SFV is of great use in experimental analysis of adsorption and desorption kinetics at a moving interface.

DOI： 10.1016/j.ijmultiphaseflow.2017.08.007

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Mass transfer rates, k(L), of single carbon-dioxide (CO2) bubbles rising through vertical pipes filled with electrolyte aqueous solutions were measured to investigate the effects of the presence of electrolyte on k(L) Sodium chloride (NaCI) was used for electrolyte and its concentration was varied from 3.5 to 14 wt.%. The pipe diameters, D, were 12.5, 18.2 and 25.0 mm and the bubble diameter, d, ranged from 5 to 25 mm. The diameter ratio, lambda(= d/D), was varied from 0.2 to 1.7, to cover various bubble shapes, i.e. ellipsoidal, cap, semi-Taylor bubbles and Taylor bubbles. The conclusions obtained are as follows: (1) k(L) in the NaCI aqueous solutions decreases with increasing the NaCI concentration mainly due to the reduction of the diffusion coefficient of CO2 in the liquid phase, (2) the Sherwood numbers, Sh, of Taylor bubbles of L/D > 1 in the NaCI aqueous solutions can be well evaluated using the available Sherwood number correlation for clean Taylor bubbles, where L is the length of a Taylor bubble, (3) Sh of ellipsoidal, cap, semi-Taylor bubbles and Taylor bubbles of LID <= 1 in the clean water and NaCI aqueous solutions can be well correlated in terms of the Peclet number and the dimensionless bubble diameter d* = d/d(T), where d(T) is the bubble diameter at the transition from the ellipsoidal-cap bubble regime to the semi-Taylor bubble regime, and (4) the Sh correlations give good predictions for long-term bubble dissolution processes in NaCI aqueous solutions. (c) 2017 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijheatmasstransfer.2017.07.087

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Lift coefficients, C-L, of single bubbles in linear shear flows are measured to investigate effects of the bubble shape, the liquid velocity gradient and the fluid property on C-L. The range of the Morton number, M, tested is from logM = -6.6 to -3.2. The shapes of bubbles are spherical and ellipsoidal. A correlation of bubble aspect ratio for single bubbles in infinite stagnant liquids proposed in our previous study can give good evaluations for bubbles in the linear shear flows. The C-L of spherical bubbles at low bubble Reynolds numbers, Re, depend on the dimensionless shear rate Sr and Re and decrease with increasing Re. These characteristics agree with the Legendre-Magnaudet correlation. The use of a single dimensionless group such as Re, the Eotvos number, the Weber number and the Capillary number cannot correlate C-L of non-spherical bubbles. The trend of the critical Re for the reversal of the sign of C-L is the same as that for the onset of oscillation of bubble motion, which supports the mechanism proposed by Adoua et al., at least within the range of -6.6 <= logM <= -3.2. An experimental database of C-L is provided for validation of available C-L models and CFD. (C) 2017 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijmultiphaseflow.2017.07.003

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

The swirler and the 1st pick-off ring (POR) of a gas-liquid separator are improved to reduce the pressure drop, DPT, in the separator while keeping the flow rate of separated liquid, WST, high. Two swirler designs are presented, i.e. V8 (swirler with eight vanes) and V6 (swirler with six vanes), whose diameters of the hubs supporting the vanes are smaller than that of the normal swirler, N8, and the nose shape of vanes are coned. The swirlers are installed into a one-fifth scale separator with three PORs. The gap width of the 1st POR is reduced to the value of the liquid film thickness in the barrel of the separator and the rear shape of the 1st POR is smoothed. The performance of the downscaled separator with the proposed swirlers and the 1st POR are evaluated under the nominal operating condition of the Hyper BWR and a higher quality condition. As a result, the following conclusions are obtained in the present experimental range: (1) DPT in the separator is reduced by 23% using V8, and V8 gives the same WST as N8, confirming that the decrease in the hub diameter and the coned-nose shape of vanes are effective, (2) V6 can realize further reduction of DPT (38%) without deteriorating the liquid-separation rate, (3) the reduced gap width of the 1st POR and a tapered shape of the downstream side of the 1st POR are also effective to decrease the pressure drop, i.e. the improved POR decreases DPT by 42% and 54% by combining with V8 and V6, respectively, and (4) the improved separators also show good performances in DPT and WST at the higher quality condition. (C) 2017 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nucengdes.2017.07.002

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：THE JAPANESE SOCIETY FOR MULTIPHASE FLOW  

<p>One-region computations with the annular flow model were done for counter-current flow limitation (CCFL) at the sharp-edged lower end of vertical pipes to evaluate effects of diameters and fluid properties on CCFL characteristics. CCFL characteristics computed with several correlations for interfacial friction factors were compared with CCFL data and the correlation proposed by Bharathan et al. (which is a function of void fraction) was selected. The adjustment factors were obtained to give good agreement between CCFL characteristics computed with the correlation by Bharathan et al. and CCFL data and were correlated with the viscosity ratio of gas and liquid phases and the dimensionless diameter. By using the correlation for interfacial friction factors by Bharathan et al. modified with the viscosity ratio of gas and liquid phases and the dimensionless diameter, effects of diameters and fluid properties on CCFL characteristics were computed.</p>

DOI： 10.3811/jjmf.31.37

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Mass transfer rates, k(L), of single carbon dioxide bubbles contaminated with alcohols in vertical pipes of 12.5, 18.2 and 25.0 mm diameters were measured to investigate effects of the carbon chain length of the alcohols on kL. Four straight-chain alcohols, i.e. 1-pentanol, 1-heptanol, 1-octanol and 1-decanol, were used. A wide range of bubble diameter, d, covered ellipsoidal and Taylor bubbles. Effects of the alcohol concentration, C-sol, on k(L) were also investigated to make clear the values of C-sol corresponding to fully contaminated conditions. As a result, the following conclusions were obtained. (1) The kL of bubbles contaminated with alcohols decrease with increasing C-sol, and they become constant at high C-sol, i.e. bubbles in the latter C-sol ranges are fully-contaminated from the point of view of mass transfer. On the other hand, the rising velocities decrease with increasing C-sol due to the reduction of surface tension even when k(L) are independent of Cob (2) The k(L). of ellipsoidal bubbles fully-contaminated with alcohols decrease with increasing the carbon chain length, whereas the reduction rate of kL becomes smaller and smaller as the carbon chain length increases. The k(L). of fully-contaminated ellipsoidal bubbles can be expressed by accounting for the adsorption constant of alcohol in an available kL correlation. (3) The increase in the carbon chain length also decreases k(L) of fully-contaminated Taylor bubbles, whereas the effect of carbon chain length on k(L), becomes smaller with increasing d. (C) 2017 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijheatmasstransfer.2017.01.058

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本混相流学会  

<p>One-region computations with the annular flow model were done for counter-current flow limitation (CCFL) at the sharp-edged lower end of vertical pipes to evaluate effects of diameters and fluid properties on CCFL characteristics. CCFL characteristics computed with several correlations for interfacial friction factors were compared with CCFL data and the correlation proposed by Bharathan et al. (which is a function of void fraction) was selected. The adjustment factors were obtained to give good agreement between CCFL characteristics computed with the correlation by Bharathan et al. and CCFL data and were correlated with the viscosity ratio of gas and liquid phases and the dimensionless diameter. By using the correlation for interfacial friction factors by Bharathan et al. modified with the viscosity ratio of gas and liquid phases and the dimensionless diameter, effects of diameters and fluid properties on CCFL characteristics were computed.</p>

DOI： 10.3811/jjmf.31.37

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

Lift coefficients, CL, of single bubbles in linear shear flows are measured to investigate effects of the bubble shape, the liquid velocity gradient and the fluid property on CL. The range of the Morton number, M, tested is from logM = − 6.6 to − 3.2. The shapes of bubbles are spherical and ellipsoidal. A correlation of bubble aspect ratio for single bubbles in infinite stagnant liquids proposed in our previous study can give good evaluations for bubbles in the linear shear flows. The CL of spherical bubbles at low bubble Reynolds numbers, Re, depend on the dimensionless shear rate Sr and Re and decrease with increasing Re. These characteristics agree with the Legendre-Magnaudet correlation. The use of a single dimensionless group such as Re, the Eötvös number, the Weber number and the Capillary number cannot correlate CL of non-spherical bubbles. The trend of the critical Re for the reversal of the sign of CL is the same as that for the onset of oscillation of bubble motion, which supports the mechanism proposed by Adoua et al., at least within the range of −6.6 ≤ logM ≤ −3.2. An experimental database of CL is provided for validation of available CL models and CFD.

DOI： 10.1016/j.ijmultiphaseflow.2017.07.003

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：International Congress on Advances in Nuclear Power Plants, ICAPP  

The purpose of this study was to evaluate the applicability and issues for numerical simulations to counter-current flow limitation (CCFL) at the lower end of a vertical pipe. Numerical simulations were done for CCFL at the lower end of a vertical pipe simulating steam generator U-tubes with the k-m SST turbulence model implemented in the CFD software FLUENT63.26, and CCFL characteristics were evaluated at pressure P = 0.1-7 MPa. After CCFL had been reproduced at the lower end of a vertical pipe, the effects of pressure on CCFL characteristics were evaluated. The numerical simulation results obtained with the κ-σ SST turbulence model showed that Jl∗1/2 increased in the range of P = 0.1-1 MP a and decreased in the range of P = 1-7 MPa as P increased. The trend of Jl1/2 in the range of P &gt
1 MPa was opposite to that of ROSA-IV/LSTF data at high pressure and temperature. CCFL data at high pressure and temperature are limited in the ROSA-IV/LSTF data
therefore, further studies including the accumulation of experimental data are needed in order to evaluate the applicability of numerical simulations to CCFL at the lower end of a vertical pipe at P &gt
1 MPa.

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：THE JAPANESE SOCIETY FOR MULTIPHASE FLOW  

<p>Characteristics of counter-current flow limitation (CCFL) inside vertical pipes with sharp-edged upper end (CCFL-P/SE) and with round-edged upper and lower ends (CCFL-P) were evaluated to clarify the dependence of the limiting locations. The CCFL is defined by the relationship between the gas and liquid volumetric fluxes (<i>J_G</i> and <i>J_L</i>) under a quasi-steady state after onset of flooding. CCFL correlations were constructed for CCFL-P/SE and CCFL-P based on the previous experimental data. The correlations were compared to the characteristics of CCFL at the vertical pipes of the sharp-edged upper end (CCFL-U) and the sharp-edged lower end (CCFL-L). CCFL-P has the highest liquid volumetric flux when CCFL occurs, and the lowest is CCFL-L. For the Wallis correlation, selection of the characteristic length of <i>w</i> = <i>D</i>^{(1-<i>β</i>)}<i>L</i>^<i>β</i> (where <i>D</i> and <i>L</i> respectively denote the diameter and the Laplace capillary length) is quite important. Our results have indicated that CCFL-P/SE and CCFL-P are also expressed by <i>β</i> ≈ 0.5, CCFL-U is expressed by <i>β</i> = 1, and CCFL-L is expressed by <i>β</i> = 0.</p>

DOI： 10.3811/jjmf.30.392

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

Experiments on air-water two-phase swirling flows in a one-fifth scale model of a steam separator with three pick-off rings (PORs) are carried out to investigate characteristics of two-phase swirling flows and separator performance. In addition to flow observation, liquid film thickness, flow rates of separated liquid and pressure drops are measured. The ranges of the gas and liquid volume fluxes, J(G) and J(L), tested are 12.0 &lt;= J(G) &lt;= 17.8 m/s and 0.05 &lt;= J(L) &lt;= 0.11 m/s, respectively. The main conclusions obtained are as follows: (1) the liquid film thicknesses at the PORs are smaller than the gap widths of the PORs under the nominal operating condition, which results in a high liquid-separation rate, (2) the increase in J(G) decreases the flow rate of unseparated liquid, whereas the flow rate of unseparated liquid is independent of J(L) except for low J(G) conditions, under which the liquid film thicknesses at the 2nd and 3rd PORs tend to be larger than the gap widths of PORs, (3) the presence of the 2nd and 3rd PORs realizes high liquid-separation rates by capturing the liquid unseparated at the 1st POR and the pressure drops at the 2nd and 3rd PORs are much smaller than that at the 1st POR, and (4) the pressure drops at the swirler and the 1st POR, which are the main source of the pressure drop of the separator under the nominal operating condition, increase with J(G), whereas J(L) increases only the latter. (C) 2016 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nucengdes.2016.08.030

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：化学工学会  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

DOI： 10.1299/transjsme.15-00685

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Improvement of efficiency of a gaslift pump is required for lifting submarine resources economically. We propose a gaslift pump utilizing phase change of working fluid to improve the efficiency by avoiding air compression. A working fluid, which has a boiling point lower than surrounding temperature, is supplied as liquid to an evaporator near the bottom of a riser pipe through an adiabatic pipe. The working fluid boils at the evaporator due to heating from the surrounding liquid. The generated gas is injected into the riser pipe instead of compressed air and forms gas-liquid two-phase flow, by which the water is drawn from the bottom opening of the riser pipe. The working fluid is separated from the drawn water at the top of the pipe, and it flows into a cooler to supply the condensed liquid into the adiabatic pipe again. This pump needs the energy only to cool the working fluid, i.e. no energy is required for compressing gas. The feasibility of the pump is confirmed through the experiments using a prototype gaslift pump using a hydro-fluorocarbon as the working fluid. Prediction of the flow rate of drawn water is also carried out. (C) 2016 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.applthermaleng.2016.05.010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

In this study, we measured counter-current flow limitation (CCFL) characteristics in an inverted U-tube (18.4 mm diameter and 1.0 m straight-part length) simulating steam generator (SG) U-tubes under conditions of steam condensation at pressures of 0.1-0.14 MPa. Differential pressure P between the top of the inverted U-tube and the lower tank was measured, and the flow patterns wave estimated by comparing the waveforms of P with those in air-water experiments. As a result, we classified the flow patterns under CCFL conditions into CCFL-P, CCFL-L and CCFL-T. The falling water flow rate under CCFL conditions slightly increased as the pressure increased and the cooling water temperature decreased (subcooling of cooling water increased). In the case of CCFL-L, CCFL characteristics in the inverted U-tube were between those in air-water and saturated steam-water experiments at 0.1 MPa. Furthermore, we derived a Wallis type CCFL correlation and its uncertainty from CCFL data, including previously measured data, i.e., J*(1/2)(G) + 0.88J(L)*(1/2) = 0.76 +/- 0.05.

DOI： 10.1080/00223131.2015.1057253

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：THE JAPANESE SOCIETY FOR MULTIPHASE FLOW  

A gas-liquid separation cycle is one of the technologies that can improve the cooling capacity. In this study, the availability of a conventional gas-liquid separator for the room air conditioner of 7.1 kW class was evaluated and an improved gas-liquid separator that has an inflow pipe with a bottom hole and some side holes was proposed. The proposed gas-liquid separator was further improved for easy manufacturing and the gas separation efficiency and the cooling capacity when the gas-liquid separation cycle was applied to the room air conditioner of 7.1 kW class were investigated. As a result, approximately complete gas-liquid separation was realized and the cooling capacity was improved by about 3%.

DOI： 10.3811/jjmf.30.49

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Other Link： https://jlc.jst.go.jp/DN/JLC/20022423281?from=CiNii

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Aspect ratios E of ellipsoidal bubbles in infinite stagnant clean liquids are measured for log M = -6.6, -5.5, -4.9 and -3.9, where M is the Morton number. An empirical correlation of E applicable to a wide range of the Morton number is proposed by making use of the present data and Sugihara's data at log M = -11 (2007). The aspect ratio in this correlation is expressed in terms of the combination of the Eotvos number and the bubble Reynolds number to account for the effects of the inertial, viscous, buoyant and surface tension forces on E. Terminal velocities of ellipsoidal bubbles are accurately predicted by using the proposed correlation and a drag correlation proposed by Rastello et al. (2011). (C) 2015 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijmultiphaseflow.2015.10.003

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Mass transfer from single fully contaminated carbon dioxide bubbles rising through vertical pipes was measured to investigate the effects of surfactants. The diameter ratio of the bubble diameter to the pipe diameter was varied to cover various bubble shapes from ellipsoidal to Taylor bubbles. Triton X-100 and 1-octanol served as surfactants. The mass transfer rates, k(L), of contaminated bubbles in the ellipsoidal regime were smaller than those of clean bubbles due to the interface immobilization. The k(L) of Taylor bubbles contaminated with Triton X-100 and 1-octanol showed varying trends because of the difference in the surfactant distributions at the bubble interfaces which strongly depend on the Hatta number. The Sherwood numbers of contaminated bubbles were well correlated by introducing the bubble diameter, at which the bubble shape transits from ellipsoidal to Taylor bubbles.

DOI： 10.1002/ceat.201500063

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

The purpose of this study is to derive a counter-current flow limitation (CCFL) correlation and evaluate its uncertainty for steam generator (SG) U-tubes in a pressurized water reactor (PWR). Experiments were conducted to evaluate effects of the liquid viscosity on CCFL characteristics using air-40wt% or air-60wt% glycerol water solution and saturated steam-water at atmospheric pressure with vertical pipes simulating the lower part of the SG U-tubes. The steam-water experiments confirmed that CCFL characteristics could be expressed in terms of the Wallis parameters (J(G)* and J(L)*) for the pipe diameters of D = 14, 20, and 27mm. A CCFL correlation was derived using the ratio mu(G)/mu(L) of the viscosities of the gas and liquid phases, mu(G) and mu(L), as a correction term representing effects of fluid properties, where J(G)(*1/2)(mu(G)/mu(L))(-0.07) was expressed by a cubic function of J(L)(*1/2)(mu(G)/mu(L))(0.1). In the correlation, the constant C indicating the value of J(G)(*1/2)(mu(G)/mu(L))(-0.07) at J(L)* = 0 was (1.04 +/- 0.05), and this uncertainty of +/- 0.05 would cover most of the previous experimental data including the ROSA-IV/LSTF data at 1, 3, and 7 MPa.

DOI： 10.1080/00223131.2014.990533

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SOC CHEMICAL ENG JAPAN  

Dissolution of a carbon dioxide (CO2) bubble in the downward flow of water or glycerol-water solutions in a vertical pipe is measured by using an image processing method. Numerical predictions of bubble dissolution using a Sherwood number Sh correlation for a bubble in stagnant water, which was proposed in our previous study, are also carried out to examine its applicability to single bubbles in several fluid property systems. The correlation can predict the bubble dissolution well, during which large bubbles change from Taylor to small bubbles, not only in water but also in glycerol-water solutions. A small bubble in a 50 wt% solution predicted by using the correlation, however, dissolves slightly faster than the experiments in the diameter range of 5-8 mm. CO2 concentration fields visualized by using laser-induced fluorescence show that this is due to the difference in wake structures in the stagnant liquid and in the downward flow, that is, vortex shedding with shape oscillation in the stagnant liquid enhances mass transfer from small bubbles; whereas the closed wake in the downward flow reduces the mass transfer rate. This, in turn, implies that the Sh correlation can give good predictions for long-term bubble dissolution in various stagnant liquids.

DOI： 10.1252/jcej.14we241

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本機械学会  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

Micro-bubbles are in use in many industrial fields such as water treatment, purification of lake water, chemical engineering, washing processes and housing equipment by virtues of their large interfacial area concentration and long residence time in liquid. A pressurized dissolution method based on decompression of liquid with dissolved gas is one of promising methods for generating fine micro-bubbles at high number density. Since the mechanism of micro-bubble generation is not clarified yet, design and improvement of micro-bubble generators are based on trial and error. In this study, effects of liquid volume flux at a decompression nozzle and dissolved gas concentration in the upstream region of the nozzle on diameter and number density of generated micro-bubbles are examined to understand generation mechanism of micro-bubbles in a pressurized dissolution method. The diameter and the number density of micro-bubbles are measured by using phase Doppler anemometry (PDA) in the downstream region of the nozzle, and the flow patterns in the nozzle are visualized by using a high-speed camera. The experimental results show that diameter and number density of generated micro-bubbles depend on cavitation pattern at the nozzle, and that cavitation bubbles containing not only vapor but also non-condensable gas become micro-bubbles due to their shrink caused by condensation of the vapor in the downstream region of the nozzle. (C) 2014 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.expthermflusci.2014.09.010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Begell House Inc.  

Sound generated by vapor bubbles condensing in subcooled water was measured to investigate effects of the degree, ΔT, of subcooling. Saturated steam from a boiler was injected into a horizontal water pipe flow through an orifice of 1 mm in diameter. The duct diameter was 16 mm and the Reynolds number of water was about 5 × 104. The subcooling was varied from 10 to 40 K. The sound level increased with increasing ΔT for ΔT ≤ 20 K, whereas it was independent of ΔT at higher ΔT. Video images of condensing bubbles revealed that the interfacial velocity of a condensing bubble had the same trend as that of the sound level, and therefore, the interfacial velocity in condensation is the key in the sound level. The rapid condensation at high ΔT induced emission of microbubbles and secondary pressure pulse generation. The latter also contributes to the increase in the sound level.

DOI： 10.1615/MultScienTechn.v27.i1.40

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Begell House Inc.  

Sound generated by vapor bubbles condensing in subcooled water was measured to investigate effects of the degree, ΔT, of subcooling. Saturated steam from a boiler was injected into a horizontal water pipe flow through an orifice of 1 mm in diameter. The duct diameter was 16 mm and the Reynolds number of water was about 5 × 104. The subcooling was varied from 10 to 40 K. The sound level increased with increasing ΔT for ΔT ≤ 20 K, whereas it was independent of ΔT at higher ΔT. Video images of condensing bubbles revealed that the interfacial velocity of a condensing bubble had the same trend as that of the sound level, and therefore, the interfacial velocity in condensation is the key in the sound level. The rapid condensation at high ΔT induced emission of microbubbles and secondary pressure pulse generation. The latter also contributes to the increase in the sound level.

DOI： 10.1615/MultScienTechn.v27.i1.40

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Multi-fluid simulations of heterogeneous bubbly flows in an air-water bubble column were carried out to verify the speculation that a heterogeneous bubbly flow is predictable without turbulence models such as k-epsilon and LES models, provided that the velocity fluctuation caused by large-scale vortical flow structures prevails over the bubble-induced and shear-induced turbulences. Experiments on the heterogeneous air-water bubbly flows in a rectangular bubble column were also carried out to obtain experimental data of the mean velocity, fluctuation velocity and void fraction for validation of the numerical method. A small LDV probe developed in our previous study was utilized to measure the liquid velocity in the column at high spatial and high temporal resolutions. The distribution of void fraction was measured using an electrical conductivity probe. The conclusions obtained under the present experimental conditions are as follows: (1) the small LDV probe is of great use in measuring the distributions of mean and fluctuation velocities of the liquid phase at high spatial and high temporal resolutions, (2) the velocity fluctuation in the heterogeneous regime in the bubble column is mainly due to large-scale vortical structures, and (3) the multi-fluid model can give good predictions of a heterogeneous bubbly flow without using the turbulence models, provided that large-scale vortical structures in the flow prevail over the bubble-induced and shear-induced turbulences. (C) 2014 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijmultiphaseflow.2014.05.008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：化学工学会  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

Numerous pipes branch off from the main pipes in power plant facilities. Main pipe flow initiates a cavity flow in a downward branch pipe with a closed end and a thermally stratified layer may form in the branch pipe if there is significant temperature difference in the main and branch pipe fluids. Fluctuation of a thermally stratified layer may initiate thermal fatigue crack in the branch pipe. In the present study, flow structures and temperature fluctuations in a branch pipe are investigated by experiments and numerical simulations to understand detailed behavior of the layer in a straight pipe and in a bent pipe. The penetration length of the main flow is measured for various main pipe flow velocities. The flow patterns in a straight pipe are divided into three regions by visualization with a tracer method, i.e., cavity flow in region 1, disturbed flow in the transition region, and spiral flow in region 2. The fluid temperature fluctuation in a straight pipe after the increase of main pipe flow velocity is attenuated in several hundred seconds since the thermal stratified layer goes down under the area into which the spiral vortex reaches. The fluid temperature in a bent pipe fluctuates when the spiral vortex reaches its lowest point after a long time period. Periodical velocity fluctuations during several tens second period are observed in the spiral flow. The mechanism of temperature fluctuation near the thermal stratified layer is considered with respect to the interference by the spiral flow and the fixed thermal stratified layer at the bent section by the cold water provided from the horizontal section. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nucengdes.2013.07.006

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Mass transfer from single carbon dioxide bubbles in vertical pipes is measured using a stereoscopic image processing method to develop a mass transfer correlation applicable to a wide range of bubble diameters in standard pipe sizes. The diameters of pipes used are 12.5, 18.2, and 25.0 mm and the bubble diameter ranges from 5 to 26 mm. The ratio, lambda, of the bubble diameter to the pipe diameter is varied from 0.2 to 1.8, which covers various bubble shapes such as spheroidal, wobbling, cap, and Taylor bubbles. Measured Sherwood numbers, Sh, strongly depend on bubble shape, i.e., Sh of Taylor bubbles differs from that of spheroidal and wobbling bubbles. The Sherwood numbers are correlated in terms of the Peclet number Pe and lambda. The applicability of the proposed correlation to long-term bubble dissolution process is examined through comparisons between measured and predicted dissolution processes. The predictions are carried out by solving mass conservation equations not only for carbon dioxide but also for nitrogen and oxygen. Good agreements are obtained in the dissolution processes for various bubble sizes and the three pipe diameters. (C) 2013 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijheatmasstransfer.2013.10.031

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Lack of local void fraction data in a rod bundle makes it difficult to validate a numerical method for predicting gas-liquid two-phase flow in the bundle. Distributions of local void fraction and bubble velocity in each subchannel in a 4x4 rod bundle were, therefore, measured using a double-sensor conductivity probe. Liquid velocity in the subchannel was also measured using laser Doppler velocimetry (LDV) to obtain relative velocity between bubbles and the liquid phase. The size and pitch of rods were 10 and 12.5mm, respectively. Air and water at atmospheric pressure and room temperature were used for the gas and liquid phases, respectively. The volume fluxes of gas and liquid phases ranged from 0.06 to 0.15m/s and from 0.9 to 1.5m/s, respectively. Experimental results showed that the distributions of void fraction in inner and side subchannels depend not only on lift force acting on bubbles but also on geometrical constraints on bubble dynamics, i.e. the effects of rod walls on bubble shape and rise velocity. The relative velocity between bubbles and the liquid phase in the subchannel forms a non-uniform distribution over the cross-section, and the relative velocity becomes smaller as bubbles approach the wall due to the wall effects.

DOI： 10.1080/00223131.2013.862189

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Size distributions of micro-bubbles and concentrations of dissolved oxygen in water in a square duct downstream of the decompression nozzle were measured to investigate the time evolution of bubble diameter and the mass transfer of dissolved gas between bubbles and water after bubble generation in a pressurized dissolution method. A numerical simulation based on the Rayleigh-Plesset equation was also carried out to predict time evolutions of bubble diameter and concentration of dissolves gas. The validity of the prediction was discussed through the comparison between the predictions and the experiments. As a result, the following conclusions were obtained: (1) When cavitation does not occur in the decompression nozzle, few micro-bubbles are generated at the nozzle and the mass transfer rate between bubbles and water in the downstream region of the nozzle is low due to a low interface area concentration. The mass transfer due to bubble nucleation is negligibly small in the downstream region of the decompression nozzle in spite of the supersaturated concentration of the dissolved gas. (2) When cavitation occurs in the nozzle, a lot of micro-bubbles are generated at the nozzle and therefore, the mass transfer rate between the phases becomes high. Hence, the bubble diameter and the void fraction increase and the concentration of dissolved gas in water decreases with the time elapsed after the bubble generation. (3) The proposed numerical method can reasonably predict time evolution of bubble size distributions, void fractions and concentrations of dissolved gas, provided that a reliable initial condition is available. Since the numerical simulation assumes that no bubble nucleation occurs in the downstream region of the nozzle, the agreement between the prediction and the experiments proves low influence of the bubble nucleation on the mass transfer between the phases.

DOI： 10.1063/1.4872124

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：SOC CHEMICAL ENG JAPAN  

Liquid slugs in oil and gas two-phase flow in hilly-terrain pipelines are often generated at V-shaped elbows and induce strong pressure fluctuation. The knowledge on characteristic of slugs such as slug frequency, velocity and length is required when designing the pipelines. In this study, numerical simulation based on a three-dimensional two-fluid model is carried out to predict liquid slugs generated at the elbows. The free surface model for the interfacial area density is employed to describe the interaction between the phases. Comparisons with experimental data show that the two-fluid model is able to well predict the liquid slug behavior in the ascending pipe at the downstream of the V-shaped elbow and the slug characteristics.

DOI： 10.1252/kakoronbunshu.40.275

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER SOC MECHANICAL ENGINEERS  

Mass transfer from single carbon dioxide bubbles rising through contaminated water in a vertical pipe of 12.5 mm diameter was measured to investigate effects of surfactant. The bubble diameter was widely varied to cover various bubble shapes such as spheroidal, wobbling, cap and Taylor bubbles. The gas and liquid phases were 99.9 % purity carbon dioxide and a surfactant solution made of purified water and Triton X-100. Comparison of mass transfer rates between contaminated and clean bubbles made clear that the surfactant decreases the mass transfer rates of small bubbles. The Sherwood number of small bubbles in the extreme cases, i.e. zero and the highest surfactant concentrations, is well correlated in terms of the bubble Reynolds number, Schmidt number and the ratio, lambda, of the bubble diameter to pipe diameter. The Sherwood numbers at intermediate surfactant concentration, however, are not well correlated using available correlations. The mass transfer rates of Taylor bubbles also decrease with increasing the surfactant concentration. They however increase with the diameter ratio and approaches that of clean Taylor bubbles as 2, increases. The main cause of this tendency was revealed by interface tracking simulations, i.e. the surfactant adsorbs only in the bubble tail region and the nose-to-side region is almost clean at high lambda.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Size distributions of micro-bubbles and concentrations of dissolved oxygen in water in a square duct downstream of the decompression nozzle were measured to investigate the time evolution of bubble diameter and the mass transfer of dissolved gas between bubbles and water after bubble generation in a pressurized dissolution method. A numerical simulation based on the Rayleigh-Plesset equation was also carried out to predict time evolutions of bubble diameter and concentration of dissolves gas. The validity of the prediction was discussed through the comparison between the predictions and the experiments. As a result, the following conclusions were obtained: (1) When cavitation does not occur in the decompression nozzle, few micro-bubbles are generated at the nozzle and the mass transfer rate between bubbles and water in the downstream region of the nozzle is low due to a low interface area concentration. The mass transfer due to bubble nucleation is negligibly small in the downstream region of the decompression nozzle in spite of the supersaturated concentration of the dissolved gas. (2) When cavitation occurs in the nozzle, a lot of micro-bubbles are generated at the nozzle and therefore, the mass transfer rate between the phases becomes high. Hence, the bubble diameter and the void fraction increase and the concentration of dissolved gas in water decreases with the time elapsed after the bubble generation. (3) The proposed numerical method can reasonably predict time evolution of bubble size distributions, void fractions and concentrations of dissolved gas, provided that a reliable initial condition is available. Since the numerical simulation assumes that no bubble nucleation occurs in the downstream region of the nozzle, the agreement between the prediction and the experiments proves low influence of the bubble nucleation on the mass transfer between the phases.

DOI： 10.1063/1.4872124

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Spectrum analysis of fluctuating velocity is one way for understanding turbulence characteristics. It is however not easy to acquire accurate velocity data at high data acquisition rates in LDV measurements due to uncontrollable tracer particles. A molecular tagging method is one of the methods to form controllable tags in flow fields. We also developed spatiotemporal filter velocimetry (SFV) which can measure velocity with spatial resolution and accuracy as high as LDV. In this study, the spatiotemporal filter velocimetry was therefore coupled with a molecular tagging method to obtain time-series velocity data with high spatial resolutions ant high data acquisition rates. Photobleaching reaction was used to form molecular tags in a flow field. The developed system was applied to single-phase and bubbly flows in a vertical square duct to examine its applicability to spectrum analysis of single phase and bubbly flows. As a result, we confirmed that the SFV coupled with molecular tagging was of use for accurate velocity measurements at high data acquisition rates, and therefore, for spectrum analysis of fluctuating velocity of turbulent single phase and bubbly flows. The turbulence spectra in the vicinity of the wall showed that the bubbles traveling in the vicinity of the wall enhance turbulence intensity in the vicinity of the wall and the turbulence generated by the bubbles dissipates through a pseudo cascade process.

DOI： 10.1063/1.4872108

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

Optical measurement is an effective and accurate method to understand flow structures of multiphase flows. However, we frequently encounter difficulty in optical access due to the presence of interface and the complex geometry of a flow field. In the present study, we have developed small LDV optics to avoid the difficulty in optical access by installing the LDV probe into a channel component or by submerging the LDV probe into a flow field. The developed LDV optics consists of a manipulator and the probe. The manipulator splits a beam emitted from an Ar-ion laser into four beams. Each beam is introduced into an optical fiber through beam adjustment optics, and is transmitted to the probe through the fiber. The diameter of the probe is 7 mm. The laser beams emitted from the fibers in the probe are focused by a front lens of the probe and form a measurement volume. Scattered light from tracer particles are received by the probe and collected into a receiving fiber installed in the probe. The developed system is applied to laminar and turbulent flows in a rectangular duct for validation. Comparisons with a conventional LDV system show that the accuracy and reliability of the developed system are the same level as the conventional system. The application of the developed system to single-phase and bubbly flows in a 4 x 4 rod bundle demonstrates that the system has a large potential of measuring single-phase and multiphase flows in a channel with a complex geometry. (c) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nucengdes.2013.06.009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Spatial filter velocimetry (SFV) is extended to three-dimensional three-component velocimetry by coupling the SFV processing with a tomographic technique, which reconstructs three-dimensional particle distributions from stereoscopic particle images recorded by two cameras. Since time duration of a ghost particle traveling through an interrogation area of SFV is shorter than the transit time of a real tracer particle, a validation method of velocity data based on a particle transit time is used to eliminate ghost particles generated in the particle reconstruction process. Effect of a ghost particle on velocity evaluation is not significant due to the short transit time of a ghost particle. The developed system is applied to a free jet, and an impinging jet discharged from a circular pipe. Three-dimensional distributions of time-averaged velocity vectors of the jets are successfully measured by the developed tomographic spatial filter velocimetry (Tomo-SFV) in spite of using only two cameras. The peak-locking errors, which are apt to occur in particle image velocimetry measurements, do not appear in Tomo-SFV measurements.

DOI： 10.1007/s00348-013-1597-0

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：American Nuclear Society  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Spatial filter velocimetry (SFV) based on spatial filtering of time-series particle images proposed by Hosokawa and Tomiyama is applied to a bubble column to examine its applicability to bubble velocity measurement. The edge detection process is added in the SFV processing to reduce the size of image pattern and to obtain accurate velocity data. It is demonstrated that SFV is able to measure the bubble velocity in various spatial scales with the same accuracy as PTV. The measured mean bubble velocity does not depend on the interrogation size in the measurements. To the contrary, the standard deviation of measured bubble velocity becomes higher as the interrogation size decreases due to the superposition of interfacial wave velocity on the measured velocity. Simultaneous measurement of bubble and liquid velocities is also carried out for a planar bubble plume. The results clearly show that SFV can measure bubble and liquid velocities simultaneously.

DOI： 10.1007/s00348-013-1538-y

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

Turbulence intensities and Reynolds stresses of bubble-induced pseudo turbulence in air-water laminar bubbly flows in a vertical pipe of 20 mm diameter are measured at a low liquid Reynolds number, Re = 900. A pipe made of fluorinated ethylene propylene resin, which has the same refractive index as water, is utilized to enable the LDV measurement of Reynolds stresses. Experimental data show that (1) turbulence kinetic energy of bubble-induced turbulence is proportional to local void fraction, (2) the ratio among axial, radial and azimuthal components of turbulence intensity is 2:1:1, (3) Sato's eddy viscosity model underestimates the Reynolds shear stress of bubble-induced pseudo turbulence, whereas it qualitatively well represents the Reynolds stress distribution, (4) the Reynolds shear stress of bubble-induced turbulence depend not only on velocity gradient but also on gradient of void fraction and (5) the dissipation process of bubble-induced turbulence is analogous to that of shear-induced turbulence. (C) 2013 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.ijheatfluidflow.2013.01.004

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Optical measurement is an effective and accurate method to understand flow structures of multiphase flows. However, we frequently encounter difficulty in optical access due to the presence of interface and the complex geometry of a flow field. In the present study, we have developed small LDV optics to avoid the difficulty in optical access by installing the LDV probe into a channel component or by submerging the LDV probe into a flow field. The developed LDV optics consists of a manipulator and the probe. The manipulator splits a beam emitted from an Ar-ion laser into four beams. Each beam is introduced into an optical fiber through beam adjustment optics, and is transmitted to the probe through the fiber. The diameter of the probe is 7 mm. The laser beams emitted from the fibers in the probe are focused by a front lens of the probe and form a measurement volume. Scattered light from tracer particles are received by the probe and collected into a receiving fiber installed in the probe. The developed system is applied to laminar and turbulent flows in a rectangular duct for validation. Comparisons with a conventional LDV system show that the accuracy and reliability of the developed system are the same level as the conventional system. The application of the developed system to single-phase and bubbly flows in a 4 × 4 rod bundle demonstrates that the system has a large potential of measuring single-phase and multiphase flows in a channel with a complex geometry. © 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nucengdes.2013.06.009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

Turbulence models such as the k-epsilon model have been used in numerical predictions of turbulent flows in rod bundles. However, the validation of the turbulence models is not an easy task mainly due to the lack of experimental data on turbulent flows in rod bundles. In this study, we therefore measured three-dimensional velocity and turbulence intensities of water flows in a 2 x 2 rod bundle by using a particle image velocimetry (PIV) system. The diameter and pitch of the rods were 20 and 25 mm, respectively. The mean velocity ranged from 1 to 2 m/s. To measure the velocity distributions in the subchannels, the rod in the measurement region was made of fluorinated ethylene propylene (FEP) resin, the refractive index of which is almost the same as that of water. The time-averaged velocity and turbulent intensities at each measurement point were obtained from 10,000 instantaneous velocities. Time-series velocity distributions were also acquired to understand the motion of turbulence eddies in the subchannel. The experimental results indicate that the axial mean velocity along the line normal to the rod in the inner subchannel agrees well with the one-seventh power law, and that the velocity fluctuation in the direction normal to the rod wall becomes weak in the rod gap. Numerical simulations using the k-epsilon models were carried out to examine their applicability to the turbulent flow in the rod bundle. They gave reasonable predictions for the distribution of the axial velocity in the rod bundle and the flow rate in each subchannel. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nucengdes.2011.11.035

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Slugging of a stagnant liquid at a V-shaped elbow in a hilly-terrain pipeline is experimentally investigated to obtain information on flow regime and critical conditions of the slugging. According to flow observation using a high-speed camera, the flow is classified into seven flow regimes, i.e. static puddle, wavy puddle, periodic slug flow, periodic semi-slug flow, aperiodic semi-slug flow, film flow and pebble flow. The onset of the slugging is well predicted not by the Mishima-Ishii's model, but by the Wallis parameter used for flooding in a pipe. This means that the slugging at the V-shaped elbow is similar to the flooding caused by interfacial shear stress. An empirical correlation based on a modified Wallis parameter is proposed to represent the effects of inclination angle, pipe diameter and fluid properties on the onset of slugging.

DOI： 10.1299/kikaib.78.1292

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

High accuracy and high spatial resolution are required in measurements of fluid velocity for detailed flow diagnostics. In this study, we proposed spatial filter velocimetry (SFV) based on a frequency analysis of time-series spatially filtered particle images. Since this method can measure velocity from one particle in a measurement region, it enables us to measure the velocity with high accuracy and high spatial resolution. We developed a SFV system and applied it to laminar and turbulent flows in a duct to examine its performance. Comparisons between the velocities measured by SFV and LDV confirmed that SFV accurately measures the mean velocity and turbulent intensity with spatial and temporal resolutions as high as LDV.

DOI： 10.1007/s00348-011-1259-z

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Understanding turbulence kinetic energy (TKE) budget in gas-liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. In this study, a molecular tagging velocimetry based on photobleaching reaction was applied to turbulent bubbly flows with sub-millimeter bubbles in a vertical square duct to examine the applicability of the k-epsilon models to the bubbly flows. Effects of bubbles on TKE budget are discussed and a priori tests of the standard and low Reynolds number k-epsilon models are carried out to examine the applicability of these models to the bubbly flows. The conclusions obtained are as follows: (1) The photobleaching molecular tagging velocimetry is of use for validating turbulence models. (2) The bubbles increase the liquid velocity gradient in the near wall region, and therefore, enhance the production and dissipation rates of TKE. (3) The k-epsilon models can reasonably evaluate the production rate of TKE in the bubbly flows. (4) The modulations of diffusion due to the bubbles have different characteristics from the diffusion enhancement due to shear-induced turbulence. Hence, the k-epsilon models fail in evaluating the diffusion rate in the near wall region in the bubbly flows. (5) The k-epsilon models represent the trends of the production, dissipation, and diffusion rates of epsilon in the bubbly flow, although more accurate experimental data are required for quantitative validation of the epsilon equation.

DOI： 10.1007/s00348-011-1109-z

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Bubble tracking simulations of bubbly flows in a rectangular column are carried out to examine the applicability of a bubble tracking method to flows involving the bubble-induced pseudoturbulence. Experimental data of void fraction, mean liquid and bubble velocities, and liquid fluctuation velocity are obtained for validation. As a result, the following conclusions are obtained: (1) The order of liquid fluctuation velocity is similar to that of the bubble relative velocity, and the fluctuation is well scaled in terms of the bubble terminal velocity and the void fraction, as Risso &amp
Ellingsen suggested [J. Fluid Mech., vol. 440, pp. 235-268, 2001]. (2) The Reynolds shear stress is not produced by the bubble motion when both void fraction and liquid velocity are uniform. (3) The Reynolds stress model proposed by Lopez de Bertodano et al. [Int. J. Multiphase Flow, vol. 20, pp. 805-818, 1994] underestimates the normal stress, whereas the bubble tracking simulation with a spatial resolution comparable to the bubble size gives better predictions than the model. (4) The fluctuation in liquid velocity induced by bubbles is partly resolved with a spatial resolution comparable to the bubble size, and subgrid turbulence models do not have much influence on predictions. (5) Eddy viscosity models would be a reasonable choice to capture the shear-induced turbulence in the near-wall region in bubbletracking simulations with an inevitably insufficient spatial resolution for large eddy simulation. © 2012 by Begell House, Inc.

DOI： 10.1615/MultScienTechn.v24.i3.20

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER SOC MECHANICAL ENGINEERS  

High spatial and temporal resolutions are required in velocity measurements for turbulent flows to examine turbulence characteristics accurately. In this study, we proposed a spatial filter velocimetry (SFV) based on a frequency analysis of time-series spatially-filtered particle images. Since this method can measure velocity from one particle in a measurement region, it enables us to measure the velocity with high spatial and temporal resolutions. We developed a SFV system and applied it to laminar and turbulent flows in a duct to examine its performance. Through comparisons between the velocities measured by SFV and LDV, we confirmed that SFV accurately measures the mean velocity and turbulent intensity with spatial and temporal resolutions as high as LDV.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Size of micro-bubbles is widely distributed in the range of one to several hundreds micrometers and depends on generation methods, flow conditions and elapsed times after the bubble generation. Although a size distribution of micro-bubbles should be taken into account to improve accuracy in numerical simulations of flows with micro-bubbles, a variety of the size distribution makes it difficult to introduce the size distribution in the simulations. On the other hand, several models such as the Rosin-Rammler equation and the Nukiyama-Tanazawa equation have been proposed to represent the size distribution of particles or droplets. Applicability of these models to the size distribution of micro-bubbles has not been examined yet. In this study, we therefore measure size distribution of micro-bubbles generated by a pressurized dissolution method by using a phase Doppler anemometry (PDA), and investigate the applicability of the available models to the size distributions of micro-bubbles. Experimental apparatus consists of a pressurized tank in which air is dissolved in liquid under high pressure condition, a decompression nozzle in which micro-bubbles are generated due to pressure reduction, a rectangular duct and an upper tank. Experiments are conducted for several liquid volumetric fluxes in the decompression nozzle. Measurements are carried out at the downstream region of the decompression nozzle and in the upper tank. The experimental results indicate that (1) the Nukiyama-Tanasawa equation well represents the size distribution of micro-bubbles generated by the pressurized dissolution method, whereas the Rosin-Rammler equation fails in the representation, (2) the bubble size distribution of micro-bubbles can be evaluated by using the Nukiyama-Tanasawa equation without individual bubble diameters, when mean bubble diameter and skewness of the bubble distribution are given, and (3) an evaluation method of visibility based on the bubble size distribution and bubble number density is proposed, and the evaluated visibility agrees well with the visibility measured in the upper tank.

DOI： 10.1063/1.3694707

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Size of micro-bubbles is widely distributed in the range of one to several hundreds micrometers and depends on generation methods, flow conditions and elapsed times after the bubble generation. Although a size distribution of micro-bubbles should be taken into account to improve accuracy in numerical simulations of flows with micro-bubbles, a variety of the size distribution makes it difficult to introduce the size distribution in the simulations. On the other hand, several models such as the Rosin-Rammler equation and the Nukiyama-Tanazawa equation have been proposed to represent the size distribution of particles or droplets. Applicability of these models to the size distribution of micro-bubbles has not been examined yet. In this study, we therefore measure size distribution of micro-bubbles generated by a pressurized dissolution method by using a phase Doppler anemometry (PDA), and investigate the applicability of the available models to the size distributions of micro-bubbles. Experimental apparatus consists of a pressurized tank in which air is dissolved in liquid under high pressure condition, a decompression nozzle in which micro-bubbles are generated due to pressure reduction, a rectangular duct and an upper tank. Experiments are conducted for several liquid volumetric fluxes in the decompression nozzle. Measurements are carried out at the downstream region of the decompression nozzle and in the upper tank. The experimental results indicate that (1) the Nukiyama-Tanasawa equation well represents the size distribution of micro-bubbles generated by the pressurized dissolution method, whereas the Rosin-Rammler equation fails in the representation, (2) the bubble size distribution of micro-bubbles can be evaluated by using the Nukiyama-Tanasawa equation without individual bubble diameters, when mean bubble diameter and skewness of the bubble distribution are given, and (3) an evaluation method of visibility based on the bubble size distribution and bubble number density is proposed, and the evaluated visibility agrees well with the visibility measured in the upper tank.

DOI： 10.1063/1.3694707

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Understanding turbulence kinetic energy (TKE) budget in gas-liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. Simultaneous measurement of velocity and velocity gradients with spatial resolution smaller than the Kolmogorov scale is required to evaluate the TKE budget experimentally. We therefore proposed a molecular tagging velocimetry based on photobleaching reaction (PB-MTV) and applied it to turbulent flows in a square duct to demonstrate the possibility of evaluation of TKE budget. Although the accuracy of measured velocity and velocity gradients were examined in the previous study, the accuracy of the TKE budget has not been examined yet due to lack of information on the TKE budget. In this study, we measure TKE budget in a turbulent water flow in a square duct by using the PB-MTV at the same turbulent Reynolds number condition as DNS data provided by Iwamoto, and compare the measured data with the DNS data to validate PB-MTV for evaluation of TKE budget. The photobleaching is an irreversible photodegradation of a fluorescent dye induced by intense illumination. Uranin (fluoresceine sodium salt) is used for the fluorescent dye which is photobleached by an intense laser beam (wave length lambda = 488.0 nm) to form a molecular tag. Velocity and velocity gradients are evaluated from translation and deformation of the molecular tag between two consecutive images with a time interval delta t, respectively. The size of the tag is about 40 to 100 micrometers, and smaller than the Kolmogorov scale in the turbulent flow. The cross-section and length of the duct are 50x50 mm and 1.6 m, respectively. The measured result indicates that the TKE budget evaluated by PB-MTV agrees well with the DNS data. The PB-MTV is also applied to turbulent bubbly flows in the square duct.

DOI： 10.1063/1.3694718

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Understanding turbulence kinetic energy (TKE) budget in gas-liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. Simultaneous measurement of velocity and velocity gradients with spatial resolution smaller than the Kolmogorov scale is required to evaluate the TKE budget experimentally. We therefore proposed a molecular tagging velocimetry based on photobleaching reaction (PB-MTV) and applied it to turbulent flows in a square duct to demonstrate the possibility of evaluation of TKE budget. Although the accuracy of measured velocity and velocity gradients were examined in the previous study, the accuracy of the TKE budget has not been examined yet due to lack of information on the TKE budget. In this study, we measure TKE budget in a turbulent water flow in a square duct by using the PB-MTV at the same turbulent Reynolds number condition as DNS data provided by Iwamoto, and compare the measured data with the DNS data to validate PB-MTV for evaluation of TKE budget. The photobleaching is an irreversible photodegradation of a fluorescent dye induced by intense illumination. Uranin (fluoresceine sodium salt) is used for the fluorescent dye which is photobleached by an intense laser beam (wave length lambda = 488.0 nm) to form a molecular tag. Velocity and velocity gradients are evaluated from translation and deformation of the molecular tag between two consecutive images with a time interval delta t, respectively. The size of the tag is about 40 to 100 micrometers, and smaller than the Kolmogorov scale in the turbulent flow. The cross-section and length of the duct are 50x50 mm and 1.6 m, respectively. The measured result indicates that the TKE budget evaluated by PB-MTV agrees well with the DNS data. The PB-MTV is also applied to turbulent bubbly flows in the square duct.

DOI： 10.1063/1.3694718

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：HINDAWI PUBLISHING CORPORATION  

An experimental study on countercurrent flow limitation (CCFL) in vertical pipes is carried out. Effects of upper tank geometry and water levels in the upper and lower tanks on CCFL characteristics are investigated for air-water two-phase flows at room temperature and atmospheric pressure. The following conclusions are obtained: (1) CCFL characteristics for different pipe diameters are well correlated using the Kutateladze number if the tank geometry and the water levels are the same; (2) CCFL occurs at the junction between the pipe and the upper tank both for the rectangular and cylindrical tanks, and CCFL with the cylindrical tank occurs not only at the junction but also inside the pipe at high gas flow rates and small pipe diameters; (3) the flow rate of water entering into the vertical pipe at the junction to the rectangular upper tank is lower than that to the cylindrical tank because of the presence of low frequency first-mode sloshing in the rectangular tank; (4) increases in the water level in the upper tank and in the air volume in the lower tank increase water penetration into the pipe, and therefore, they mitigate the flow limitation.

DOI： 10.1155/2012/754724

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：HINDAWI PUBLISHING CORP  

Steam generated in a reactor core and water condensed in a pressurizer form a countercurrent flow in a surge line between a hot leg and the pressurizer during reflux cooling. Characteristics of countercurrent flow limitation (CCFL) in a 1/10-scale model of the surge line were measured using air and water at atmospheric pressure and room temperature. The experimental results show that CCFL takes place at three different locations, that is, at the upper junction, in the surge line, and at the lower junction, and its characteristics are governed by the most dominating flow limitation among the three. Effects of inclination angle and elbows of the surge line on CCFL characteristics were also investigated experimentally. The effects of inclination angle on CCFL depend on the flow direction, that is, the effect is large for the nearly horizontal flow and small for the vertical flow at the upper junction. The presence of elbows increases the flow limitation in the surge line, whereas the flow limitations at the upper and lower junctions do not depend on the presence of elbows.

DOI： 10.1155/2012/174838

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Begell house  

Bubble tracking simulations of bubbly flows in a rectangular column are carried out to examine the applicability of a bubble tracking method to flows involving the bubble-induced pseudoturbulence. Experimental data of void fraction, mean liquid and bubble velocities, and liquid fluctuation velocity are obtained for validation. As a result, the following conclusions are obtained: (1) The order of liquid fluctuation velocity is similar to that of the bubble relative velocity, and the fluctuation is well scaled in terms of the bubble terminal velocity and the void fraction, as Risso &amp
Ellingsen suggested [J. Fluid Mech., vol. 440, pp. 235-268, 2001]. (2) The Reynolds shear stress is not produced by the bubble motion when both void fraction and liquid velocity are uniform. (3) The Reynolds stress model proposed by Lopez de Bertodano et al. [Int. J. Multiphase Flow, vol. 20, pp. 805-818, 1994] underestimates the normal stress, whereas the bubble tracking simulation with a spatial resolution comparable to the bubble size gives better predictions than the model. (4) The fluctuation in liquid velocity induced by bubbles is partly resolved with a spatial resolution comparable to the bubble size, and subgrid turbulence models do not have much influence on predictions. (5) Eddy viscosity models would be a reasonable choice to capture the shear-induced turbulence in the near-wall region in bubbletracking simulations with an inevitably insufficient spatial resolution for large eddy simulation. © 2012 by Begell House, Inc.

DOI： 10.1615/MultScienTechn.v24.i3.20

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A micro-bubble generator based on a pressurized dissolution method can generate fine micro-bubbles with a high bubble number density. The mean diameter and the number density of micro-bubbles depend on the concentration of dissolved gas in the upstream region of a decompression nozzle. In this study, the mean diameter and the number density of micro-bubbles in the downstream region of the decompression nozzle are measured by using a phase Doppler anemometry (PDA) to examine effects of the dissolved gas concentration on the mean diameter and the number density of micro-bubbles generated by the pressurized dissolution method. Experiments are conducted for several dissolved gas concentrations in the upstream region of the decompression nozzle under bubble cavitation and sheet cavitation conditions in the decompression nozzle. The experimental results indicate that, (1) influence of the dissolved gas concentration on flow patterns in the decompression nozzle (bubble cavitation and sheet cavitation) is weak, (2) the Sauter mean diameter of micro-bubbles increases as dissolved gas concentration increases, (3) the gradient of Sauter mean diameter with respect to the dissolved gas concentration in a sheet cavitation case is higher than that in a bubble cavitation case, (4) micro-bubbles are generated even when the dissolved gas concentration is lower than the saturated concentration at the atmospheric pressure provided that a sheet cavitation takes place at the nozzle, and (5) the bubble number density increases with the dissolved gas concentration in bubble cavitation cases whereas it does not depend on the concentration in sheet cavitation cases. Copyright © 2011 by ASME.

DOI： 10.1115/AJK2011-10035

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Understanding turbulence kinetic energy (TKE) budget in gas-liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. Simultaneous measurement of velocity and velocity gradients with a spatial resolution smaller than the Kolmogorov scale is required to evaluate the TKE budget experimentally. We therefore proposed a molecular tagging velocimetry based on photobleaching reaction (PB-MTV) and applied it to turbulent flows in a square duct to demonstrate the possibility of evaluation of TKE budget. In this study, we improved PB-MTV in its processing speed by utilizing GPGPU(General Purpose Graphic Processing Unit) to increase sample number in measurements. We measured TKE budget in a turbulent water flow in a square duct by using the PB-MTV at the same turbulent Reynolds number as DNS data provided by Horiuchi, and compared the measured data with the DNS data to validate PB-MTV for evaluation of TKE budget. We also measured TKE budget in a bubbly flow in the square duct to examine effects of bubbles on TKE budget. As a result, we found that (1) PB-MTV can accurately evaluate TKE budget in turbulent flows, (2) bubbles affect the production and diffusion rates of TKE and do not affect the dissipation rate so much, and (3) the model proposed by Troshko and Hassan can reasonably estimate the production rate of the bubble-induced pseudo turbulence. © 2011 The Japan Society of Mechanical Engineers.

DOI： 10.1299/kikaib.77.2263

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Understanding turbulence kinetic energy (TKE) budget in gas-liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. Simultaneous measurement of velocity and velocity gradients with a spatial resolution smaller than the Kolmogorov scale is required to evaluate the TKE budget experimentally. We therefore proposed a molecular tagging velocimetry based on photobleaching reaction (PB-MTV) and applied it to turbulent flows in a square duct to demonstrate the possibility of evaluation of TKE budget. In this study, we improved PB-MTV in its processing speed by utilizing GPGPU(General Purpose Graphic Processing Unit) to increase sample number in measurements. We measured TKE budget in a turbulent water flow in a square duct by using the PB-MTV at the same turbulent Reynolds number as DNS data provided by Horiuchi, and compared the measured data with the DNS data to validate PB-MTV for evaluation of TKE budget. We also measured TKE budget in a bubbly flow in the square duct to examine effects of bubbles on TKE budget. As a result, we found that (1) PB-MTV can accurately evaluate TKE budget in turbulent flows, (2) bubbles affect the production and diffusion rates of TKE and do not affect the dissipation rate so much, and (3) the model proposed by Troshko and Hassan can reasonably estimate the production rate of the bubble-induced pseudo turbulence. © 2011 The Japan Society of Mechanical Engineers.

DOI： 10.1299/kikaib.77.2263

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：THE JAPANESE SOCIETY FOR MULTIPHASE FLOW  

Effects of liquid volumetric flux <i>J_L</i> and flow pattern in a decompression nozzle on the mean diameter and number density of micro-bubbles generated by using a pressurized dissolution method are experimentally investigated. The bubble diameter and number density are measured by using a phase Doppler anemometry at the exit of the decompression nozzle. The flow pattern in the decompression nozzle was observed by using a high speed camera. The experimental results indicate that the diameter and number density strongly depends on the flow pattern in the decompression nozzle, and their characteristics are classified into three regimes: (1) few bubbles are generated when no cavitation occurs, (2) when bubble cavitation takes place, the mean bubble diameter keeps low values which do not depend on <i>J_L</i> so much and the bubble number density increases with <i>J_L</i>, and (3) the mean bubble diameter increases with <i>J_L</i> whereas the number density does not depend on <i>J_L</i> when flow pattern in the nozzle is sheet cavitation.

DOI： 10.3811/jjmf.24.462

CiNii Books

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Other Link： https://jlc.jst.go.jp/DN/JALC/00362111260?from=CiNii

Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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DOI： 10.3811/jjmf.23.547

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Since the conservation equation of turbulence kinetic energy (TKE) involves correlations among local instantaneous gradients of fluctuating velocities, it is indispensable to measure the velocity gradients as well as the velocity components. The authors developed a molecular tagging method using photobleaching (PB-MTV), which can measure not only velocity but also its gradient by making use of the translation and deformation of a tag. We applied the PB-MTV to turbulent bubbly flows in a square duct. The experimental results indicate that the presence of bubbles increases not only the production rate but also the dissipation rate of TKE. Although the low Reynolds number k-z model well evaluates the production rate of TKE in the bubbly flow, modification of the diffusion term is required for the bubbly flows.

DOI： 10.1299/jsmemecjsm.2010.9.0_153

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BEGELL HOUSE INC  

Cavitation in two-dimensional and cylindrical nozzles with various length-to-diameter (L/D) ratios and its effects on liquid jet deformation are investigated by carrying out simultaneous high-speed visualization of the cavitation and jet. The applicability of a modified cavitation number to quantitative prediction of supercavitation formation for nozzles with various LID ratios is also examined. As a result, the following conclusions are obtained. (1) The modified cavitation number a c can predict supercavitation in nozzles of various shapes and L/D ratios. (2) When cavitation length is less than twice the nozzle diameter D, the cavitation length normalized by the diameter (L(cav)/D), rather than by the nozzle length (L(cav)/L), can be expressed as a unique function of sigma(c). (3) Once a cavitation film is formed in long nozzles (L/D &lt;= 4), the cavitation zone suddenly extends to near the exit even for very long nozzles (L/D = 8). (4) In short nozzles (L/D = 2), incipient cavitation consisting of tiny bubbles can reach the vicinity of the exit. However, a hydraulic flip instead of a cavitation film is formed even at a low flow rate. (5) In long nozzles (L/D = 8), hydraulic flip does not occur even at very high flow rates, which is preferable for a plain orifice atomizer. (6) Cavitation clouds are shed and collapse near the exit to induce a large deformation of the discharged jet, not only in moderately long (L/D = 4) but also in shorter and longer nozzles.

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

Diameters of micro-bubbles are to range from about one to several hundreds urn, and therefore it is difficult to measure the correct diameter distribution using a single measurement method. In this study, diameters of bubbles generated by a pressurized dissolution method are measured by using phase Doppler anemometry (PDA) and an image processing method, which is based on the Sobel filter and Hough transform. The diameter distribution and the Sauter mean diameter of micro bubbles are evaluated based on the diameters measured by both methods. Experiments are conducted for several mass flow rates of dissolved gas and of air bubbles entrained in the upstream of the decompression nozzle to examine effects of the entrained bubbles on bubble diameter. The experimental result indicates that diameter distribution of micro bubbles can be accurately measured for a wide range of diameter by using PDA and the image processing method. The mean diameter of micro-bubbles generated by gasification of dissolved gas is smaller than that generated by breakup of air bubbles entrained in the upstream of the decompression nozzle. As the mass flow rate of the entrainment of air bubbles in the upstream of the decompression nozzle increases, the mean bubble diameter increases and the bubble number density decreases.

DOI： 10.1299/kikaib.76.765_763

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

High spatial and temporal resolutions are required in velocity measurements for turbulent flows to examine turbulence characteristics accurately. In this study, we proposed a spatial filter velocimetry (SFV) based on a frequency analysis of time-series spatially-filtered particle images. Since this method can measure velocity from one particle in a measurement region, it enables us to measure the velocity with high spatial and temporal resolutions. We developed a SFV system and applied it to laminar and turbulent flows in a duct to examine its feasibility. Through comparisons between the velocities measured by SFV and LDV, we confirmed that SFV accurately measures the mean velocity and turbulent intensity with spatial and temporal resolutions as high as LDV.

DOI： 10.1299/jsmemecjo.2010.7.0_57

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Language：English   Publishing type：Research paper (international conference proceedings)  

Influence of bubbles on turbulence properties depends on the ratio of bubble size dB to turbulence length scale lt and the ratio of relative velocity VR between bubbles and liquid to turbulence velocity v′. Since most studies on turbulent bubbly flow have been carried out for flows with large dB/lt, little is known regarding the turbulence modulation in bubbly flows with small dB/l t and large VR/v′. Hence, turbulence properties are measured for bubbly flows with small dB/lt and large VR/v′ using a molecular tagging method based on a photobleaching reaction (PB-MTV). This method can measure not only velocity components but also the gradients by making use of the translation and deformation of a tag and enables us to evaluate the turbulence kinetic energy (TKE) budget. The experimental result indicates that the boundary between the linear sublayer and the log region shifts toward the wall by addition of bubbles in the buffer layer. The bubbles migrating toward the vicinity of the wall increase the liquid velocity gradient at the wall, that is, the shear-induced turbulence is enhanced in the near wall region. When the bubble size is comparable to the Kolmogorov scale, the influence of the bubble-induced pseudo turbulence on the TKE budget is not prominent, in spite of the large relative velocity. The expression of eddy viscosity used in the low-Reynolds-number k-ε model is applicable to the bubbly flows in which the bubble-induced pseudo turbulence is not prominent. The expression of eddy viscosity used in the standard k-ε model is also applicable, provided that accurate boundary conditions for frictional velocity, TKE, and dissipation rate of TKE are given. © 2010 by Begell House, Inc.

DOI： 10.1615/MultScienTechn.v22.i3.30

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Language：English   Publishing type：Research paper (international conference proceedings)  

Influence of bubbles on turbulence properties depends on the ratio of bubble size dB to turbulence length scale lt and the ratio of relative velocity VR between bubbles and liquid to turbulence velocity v′. Since most studies on turbulent bubbly flow have been carried out for flows with large dB/lt, little is known regarding the turbulence modulation in bubbly flows with small dB/l t and large VR/v′. Hence, turbulence properties are measured for bubbly flows with small dB/lt and large VR/v′ using a molecular tagging method based on a photobleaching reaction (PB-MTV). This method can measure not only velocity components but also the gradients by making use of the translation and deformation of a tag and enables us to evaluate the turbulence kinetic energy (TKE) budget. The experimental result indicates that the boundary between the linear sublayer and the log region shifts toward the wall by addition of bubbles in the buffer layer. The bubbles migrating toward the vicinity of the wall increase the liquid velocity gradient at the wall, that is, the shear-induced turbulence is enhanced in the near wall region. When the bubble size is comparable to the Kolmogorov scale, the influence of the bubble-induced pseudo turbulence on the TKE budget is not prominent, in spite of the large relative velocity. The expression of eddy viscosity used in the low-Reynolds-number k-ε model is applicable to the bubbly flows in which the bubble-induced pseudo turbulence is not prominent. The expression of eddy viscosity used in the standard k-ε model is also applicable, provided that accurate boundary conditions for frictional velocity, TKE, and dissipation rate of TKE are given. © 2010 by Begell House, Inc.

DOI： 10.1615/MultScienTechn.v22.i3.30

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

To evaluate turbulence energy budget in bubbly flows, an image processing method in a photobleaching molecular tagging velocimetry is improved for accurate evaluation of velocity gradients. Turbulence properties in single-phase and two-phase dilute-bubbly flows in a square duct are measured using the improved method. As a result, the following conclusions are obtained: (1) The axial velocity and axial turbulent intensity measured by the present method agree well with those measured by laser Doppler velocimetry not only for the single-phase flow but also for the dilute-bubbly flow. (2) The present method can measure velocity components and velocity gradients in the vicinity of the wall, and therefore the present method is of great use in understanding the mechanism of turbulence generation and dissipation near the wall. (3) The present method can provide detailed information on turbulence structure such as turbulence kinetic energy budget. (4) Bubbles tend to increase not only the turbulence production but also the turbulence dissipation.

DOI： 10.1007/s00348-009-0690-x

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Numerical simulations based on a three-dimensional two-way bubble tracking method are carried out to predict bubble motions in a square duct with an obstacle and in a two-by-three rod bundle with a grid spacer. Comparisons between measured and predicted bubble motions demonstrate that the two-way bubble tracking method gives good predictions for trajectories of small bubbles in the upstream side of the grid spacer in the rod bundle geometry. The predicted bubble trajectories clearly show that bubbles are apt to migrate toward the rod surface in the vicinity of the bottom of the grid spacer. Analysis of forces acting on the bubbles confirms that pressure gradient force induced by the presence of the spacer is the main cause of the bubble lateral migration toward the rod surface. Motions of steam bubbles at a nominal operating condition of a pressurized water reactor (PWR) are also predicted by using the bubble tracking method, which indicates that steam bubbles also migrate toward the rod surface at the upstream side of the spacer due to the spacer-induced pressure gradient force.

DOI： 10.1299/jpes.3.393

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Effects of pick-off ring configuration on the separator performance of a downscaled model of a steam separator for a boiling water nuclear reactor are examined using various types of pick-off rings. The experiments are conducted using air and water. Pressure drops in a barrel and a diffuser and diameters and velocities of droplets at the exit of the barrel are measured using differential pressure transducers and particle Doppler anemometry, respectively. The separator performance does not depend on the shape of the pick-off ring but strongly depends on the width of the gap between the pick-off ring and the barrel wall. The pressure drop in the barrel is well evaluated using the interfacial friction factor for unstable film flows. Carry-under can be estimated using a droplet velocity distribution at the exit of the separator. © 2009 by ASME.

DOI： 10.1115/1.3078701

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：THE JAPANESE SOCIETY FOR MULTIPHASE FLOW  

Partial length rods (PLR) are used in fuel bundles of BWR to reduce pressure drops in two-phase regions and to optimize the power distribution. Since little is known about effects of PLR on two-phase flows, air-water two-phase flow around PLRs in a four by four rod bundle is visualized by using a high-speed video camera. The experimental apparatus consists of acrylic channel box and transparent rods. Air and water at atmospheric pressure and room temperature are used for the gas and liquid phases, respectively. The ranges of the gas and liquid volume fluxes, <I>J<SUB>G</SUB></I> and <I>J<SUB>L</SUB></I>, are 0.4 < <I>J<SUB>L</SUB></I> < 2.0 m/s and 0.06 < <I>J<SUB>G</SUB></I> < 8.85 m/s, which cover typical two-phase flow patterns appearing in the fuel bundle. As a result, the following conclusions are obtained. (1) In churn flows at high <I>J<SUB>L</SUB></I>, the flow pattern in the downstream of PLR transits to slug flow, and the flow patterns in the surrounding subchannels transit to bubbly flow due to the redistribution of gas flow. (2) In annular flow, the liquid film on the PLR forms a liquid column above the end cap of PLR. Drops are generated by column breakup and deposit on liquid films on the neighboring rods. (3) The liquid film thickness on the surface of neighbor rods facing the PLR increases and it reduces that on their opposite surface in the downstream of PLR.

DOI： 10.3811/pmfr.4.61

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Other Link： https://jlc.jst.go.jp/DN/JALC/00329084262?from=CiNii

Language：English   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Physics Publishing  

Diameters of micro-bubbles are apt to range from about one mm to several-hundred mm, and therefore, it is difficult to measure a correct diameter distribution using a single measurement method. In this study, diameters of bubbles generated by a pressurized dissolution method are measured by using phase Doppler anemometry (PDA) and an image processing method, which is based on the Sobel filter and Hough transform. The diameter distribution and the Sauter mean diameter of micro bubbles are evaluated based on the diameters measured by both methods. Experiments are conducted for several mass flow rates of dissolved gas and of air bubbles entrained in the upstream of the decompression nozzle to examine effects of the entrained bubbles on bubble diameter. As a result, the following conclusions are obtained: (1) Diameter distribution of micro bubbles can be accurately measured for a wide range of diameter by using PDA and the image processing method. (2) The mean diameter of micro-bubbles generated by gasification of dissolved gas is smaller than that generated by breakup of air bubbles entrained in the upstream of the decompression nozzle. (3) The mean bubble diameter increases with the entrainment of air bubbles in the upstream of the decompression nozzle at a constant mass flow rate of dissolved gas. © 2009 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/147/1/012016

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Physics Publishing  

Diameters of micro-bubbles are apt to range from about one mm to several-hundred mm, and therefore, it is difficult to measure a correct diameter distribution using a single measurement method. In this study, diameters of bubbles generated by a pressurized dissolution method are measured by using phase Doppler anemometry (PDA) and an image processing method, which is based on the Sobel filter and Hough transform. The diameter distribution and the Sauter mean diameter of micro bubbles are evaluated based on the diameters measured by both methods. Experiments are conducted for several mass flow rates of dissolved gas and of air bubbles entrained in the upstream of the decompression nozzle to examine effects of the entrained bubbles on bubble diameter. As a result, the following conclusions are obtained: (1) Diameter distribution of micro bubbles can be accurately measured for a wide range of diameter by using PDA and the image processing method. (2) The mean diameter of micro-bubbles generated by gasification of dissolved gas is smaller than that generated by breakup of air bubbles entrained in the upstream of the decompression nozzle. (3) The mean bubble diameter increases with the entrainment of air bubbles in the upstream of the decompression nozzle at a constant mass flow rate of dissolved gas. © 2009 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/147/1/012016

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

DOI： 10.1299/kikaib.75.749_68

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Physics Publishing  

Diameters of micro-bubbles are apt to range from about one mm to several-hundred mm, and therefore, it is difficult to measure a correct diameter distribution using a single measurement method. In this study, diameters of bubbles generated by a pressurized dissolution method are measured by using phase Doppler anemometry (PDA) and an image processing method, which is based on the Sobel filter and Hough transform. The diameter distribution and the Sauter mean diameter of micro bubbles are evaluated based on the diameters measured by both methods. Experiments are conducted for several mass flow rates of dissolved gas and of air bubbles entrained in the upstream of the decompression nozzle to examine effects of the entrained bubbles on bubble diameter. As a result, the following conclusions are obtained: (1) Diameter distribution of micro bubbles can be accurately measured for a wide range of diameter by using PDA and the image processing method. (2) The mean diameter of micro-bubbles generated by gasification of dissolved gas is smaller than that generated by breakup of air bubbles entrained in the upstream of the decompression nozzle. (3) The mean bubble diameter increases with the entrainment of air bubbles in the upstream of the decompression nozzle at a constant mass flow rate of dissolved gas. © 2009 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/147/1/012016

Scopus

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Language：English   Publishing type：Research paper (scientific journal)  

Radial distributions of void fraction αG, bubble aspect ratio E, phasic velocities VG and VL and turbulent kinetic energy k in bubbly pipe flows are measured using an image processing method and a laser Doppler velocimetry. Multi-fluid simulations are conducted to examine applicability of state-of-the-art closure relations to the turbulent bubbly pipe flows. The experimental results indicate that aspect ratio of bubbles in the near wall region takes a higher value than that of free rising bubbles due to the presence of wall, and that the change in the aspect ratio induces decrease in relative velocity between bubbles and liquid in the near wall region. Drag coefficient CD of a bubble in a bubbly pipe flow tends to increase with magnitude of shear flow, and the effect of shear flow on CD is estimated by the correlation proposed by Legendre and Magnaudet (1998). Comparison between the simulated and the measured results indicate that the effects of bubble shape and shear flow on drag force acting on bubbles should be taken into account for accurate predictions of bubbly pipe flows. The turbulence models proposed by Lopez de Bertodano et al. (1994) and by Hosokawa and Tomiyama (2004a) give good predictions for turbulence modification caused by bubbles. © 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ces.2009.09.017

Scopus

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.3811/jjmf.22.403

CiNii Books

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Cavitation in a cylindrical nozzle and liquid jets discharged from the nozzle are simultaneously visualized using a high-speed camera to investigate the mechanism of liquid jet atomization induced by the fully-developed cavitation. Three mirrors are used to capture the front view of liquid jet interfaces and the side view of cavitation clouds within a frame of the camera. The high-speed visualization confirms that the collapse of a cavitation cloud near the nozzle exit induces a ligament formation not only for 2D nozzles but also for cylindrical nozzles. The visualization also finds that the collapse of a large cavitation cloud tends to cause the formation of a large ligament, while that of a small cloud results in a small ligament.

DOI： 10.1299/jfst.3.633

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

The effects of nozzle geometry on cavitation in the nozzle of pressure atomizers and the liquid jet are examined using various two-dimensional (2D) nozzles with different geometries. Then, whether or not the conventional cavitation numbers can be used to predict the formation of supercavitation, in which liquid jet atomization is enhanced, is examined. As a result, we confirm that (1) the thickness of the cavitation zone increases with the ratio <i>C_u</i> of the cross-sectional area upstream of the nozzle to that of the nozzle, (2) the spray angle increases with <i>C_u</i>, (3) the formation of supercavitation can be predicted using the cavitation number σ_c' in which the effects of the flow contraction and the frictional pressure drop are taken into account, and (4) the conventional cavitation numbers σ, σ₂ and σ₃ cannot predict the formation of supercavitation in nozzles with different geometries.

DOI： 10.1299/jfst.3.622

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Effects of inlet bubble diameter on void and bubble size distributions in poly-dispersed bubbly flow in a rectangular bubble column under high gas volumetric flux conditions are experimentally investigated. Three types of gas distributors are used to form bubbly flows with different inlet bubble diameters at a constant gas flow rate. The conclusions obtained are as follows : (1) the bubble size distribution reaches an equilibrium state at which the birth rate of bubbles belonging to a bubble group due to bubble breakup and coalescence is equal to the death rate at high gas volumetric flux, (2) distributions of void fraction and bubble diameter are not affected by inlet bubble diameters except for the region close to the air diffuser, and (3) the elevation at which the bubble size distribution reaches the equilibrium state becomes lower as the gas volumetric flux increases.

CiNii Books

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：THE JAPANESE SOCIETY FOR MULTIPHASE FLOW  

Air-water two-phase swirling flows in a downscaled model of a steam separator for a boiling water nuclear reactor have been measured while paying attention to the effects of pick-off-ring (POR) shape on separation performance. As a result, the following conclusions are obtained. (1) In an annular swirling flow, the ratio <I>W<SUB>s</SUB><SUP>*</SUP></I> of the separated liquid flow rate to the total liquid flow rate increases with <I>J<SUB>G</SUB></I> and does not depend on <I>J<SUB>L</SUB></I> and POR shape so much. (2) Separation performance becomes lower and does not depend on POR shape so much when the liquid film thickness becomes larger than the size of the gap between the barrel wall and the POR wall. The gap size should be determined so as to be smaller than the maximum film thickness. (3) The POR shape strongly affects the separation performance when the liquid film thickness is smaller than the gap size.

DOI： 10.3811/pmfr.3.67

CiNii Books

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Other Link： https://jlc.jst.go.jp/DN/JALC/00322311785?from=CiNii

Language：English   Publishing type：Research paper (international conference proceedings)  

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DOI： 10.1299/jfst.3.172

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Air-water swirling flows in a one-fifth model of a steam separator in a boiling water nuclear reactor are measured to obtain a database for modeling and verification of numerical methods for predicting swirling flows in the separator. Flow patterns, liquid film thicknesses, separated flow rates and the ratio <i>W_s</i>^* of the separated flow to the total liquid flow are measured using a high-speed camera, a laser focus displacement meter and flowmeters. Main conclusions obtained are as follows: (1) liquid transfer from droplets to liquid film is caused not only by droplet deposition but also by the collection of droplets on the vanes of the swirler, (2) <i>W_s</i>^* increases with the gas volume flux <i>J_G</i> and does not depend on the liquid volume flux <i>J_L</i> so much because a large centrifugal force caused by the swirler makes most of droplets in the gas core deposit on the liquid film before the separation and (3) a local peak appears in the axial distribution of film thickness, the position of which corresponds to the location where the droplet deposition caused by the centrifugal force has completed.

DOI： 10.1299/jpes.2.1120

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER SOC MECHANICAL ENGINEERS  

In the case of loss of the residual heat removal system under mid-loop operation during shutdown of the pressurized water reactor (PWR) plant, steam generated in a reactor core and condensed water in a steam generator (SG) form a countercurrent flow in a hot leg. In this study, in order to improve a counter-current flow model of a transient analysis code, experiments were conducted using a scale-down model of the PWR hot leg, and flow patterns and counter-current flow limitation (CCFL) characteristics were measured. A rectangular duct, whose height is about 1/5(th) of the hot leg diameter, was used to simulate the hot leg, and air and water at atmospheric pressure and room temperature were used for gas and liquid phases.
In the horizontal section, as air flow rate Q(G) increases, the flow pattern transits from a stratified flow to wavy flow, and then wavy to wavy-mist flow. When the latter transition takes place, water flow from the horizontal duct to the lower tank is to be restricted. Flow patterns in the elbow section are the same as those in the horizontal section. Wavy flow is not formed in the inclined section, where the transition to wavy-mist flow occurs due to the inflow of wavy-mist flow generated in the horizontal section. Flow patterns in the elbow and inclined section are strongly affected by those in the horizontal section.
CCFL characteristics are well correlated with the Wallis-type correlation, and the onset of CCFL well corresponds to the transition from wavy flow to wavy-mist flow.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER SOC MECHANICAL ENGINEERS  

Several experiments have reported that departure from nucleate boiling (DNB) in a pressurized water reactor (PWR) fuel assembly often takes place just upstream of a grid spacer. Since DNB inception is to be tightly related with an extremely high void fraction in the vicinity of a heated wall, it is speculated that one of the causes of DNB inception is lateral migration of bubbles toward a fuel rod due to a flow induced by the grid spacer. Therefore, experiments are conducted to investigate the motion of small bubbles and liquid velocity profiles just upstream of a grid spacer in a two by three rod bundle. Tap water and air are used for the liquid and gas phases, respectively. The bulk liquid velocity ranges from 1.0 to 4.0 m/s and experiments are conducted at atmospheric pressure and room temperature. Trajectories of single small bubbles are recorded using a high-speed video camera. Mean liquid velocities and turbulence distributions upstream of the spacer are calculated based on 10,000 instantaneous velocity distributions measured by Particle Image Velocimetry (PIV). Discussion on the effects of the gird spacer on bubble motion is made based on the difference between the measured bubble trajectories and liquid streamlines. Numerical simulations using a three-dimensional two-way bubble tracking method are also carried out. The calculated trajectories of single bubbles and liquid velocity distributions are compared with the measured data. The main conclusions obtained are as follows: (1) just upstream of the grid spacer, bubbles are apt to migrate toward the rods and accumulate in the vicinity of the rod surface, (2) a centrifugal force caused by a curved streamline just upstream of the grid spacer is the main cause of lateral migration of bubbles toward the rods, and (3) the two-way bubble tracking method gives good predictions for trajectories of small bubbles upstream of the grid spacer and for liquid velocity profiles.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER SOC MECHANICAL ENGINEERS  

Effects of pick-off ring configuration on the separation performance of a downscaled model of a steam separator for a boiling water nuclear reactor are examined using various types of pick-off rings. The experiments are conducted using air and water. Pressure drops in a barrel and a diffuser, and diameters and velocities of droplets at the exit of the barrel are measured using differential pressure transducers and particle doppler anemometry, respectively. As a result, the following conclusions are obtained: (1) the separation performance does not depend on the shape of pick-off ring, but strongly depends on the width of the gap between the pick-off ring and the barrel wall, (2) the pressure drop in the barrel is well evaluated using the interfacial friction factor for unstable film flows, and (3) the radial distribution of droplet velocity at the exit of the separator is of use for the evaluation of carry-under.

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Cavitation in two-dimensional (2D) nozzles and liquid jet in the vicinity of the nozzle exit were visualized using high-speed cameras to investigate the effects of cavitation on liquid jet under various conditions of cavitation and Reynolds numbers sigma and Re. Liquid velocity in the nozzle was measured using a laser Doppler velocimetry to examine the effects of cavitation on the flow in the nozzle and liquid jet. As a result, the following conclusions were obtained: (1) cavitation in the nozzles and liquid jet can be classified into the four regimes: (no cavitation, wavy jet), (developing cavitation, wavy jet), (super cavitation, spray) and (hydraulic flip, flipping jet), (2) liquid jet near the nozzle exit depends on cavitation regime, (3) cavitation and liquid jet are not strongly affected by Re but by a, and (4) strong turbulence induced by the collapse of cavitation clouds near the exit plays an important role in ligament formation. (c) 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijheatmasstransfer.2006.12.033

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ATOMIC ENERGY SOC JAPAN  

Air-water two-phase flow patterns in a four by four square lattice rod bundle consisting of an acrylic channel box of 68 mm in width and transparent rods of 12 mm in diameter were observed by utilizing a high speed video camera, FEP (fluorinated ethylene propylene) tubes for rods, and a fiberscope inserted in a rod. The FEP possesses the same refractive index as water, and thereby, whole flow patterns in the bundle and local flow patterns in subchannels were successfully visualized with little optical distortion. The ranges of gas and liquid volume fluxes, (J(G)) and (J(L)), in the present experiments were 0.1 &lt; (J(L)) &lt; 2.0 m/s and 0.06 &lt; (J(G)) &lt; 8.85 m/s, which covered typical two-phase flow patterns appearing in a fuel bundle of a boiling water nuclear reactor. As a result, the following conclusions were obtained: (1) the region of slug flow in the (J(G))-(J(L)) flow pattern diagram is so narrow that it can be regarded as a boundary between bubbly and chum flows, (2) the boundary between bubbly and chum flows is close to the boundary between bubbly and slug flows of the Mishima and Ishii's flow pattern transition model, and (3) the boundary between churn and annular flows is close to the Mishima and Ishii's model.

DOI： 10.3327/jnst.44.894

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.3811/pmfr.2.9

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.3811/pmfr.2.55

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Shapes of single drops rising through infinite stagnant liquids under terminal conditions were measured using a high-speed camera for a wide range of fluid properties. Aspect ratios <i>E</i> and distortion factors γ of drops were evaluated from drop images. Effects of initial shape deformation, surfactants and viscosity ratio κ on <i>E</i> and γ were investigated. As a result, the following conclusions were obtained. (1) Initial shape deformation does not affect <i>E</i> and γ of single drops. (2) Addition of surfactants decreases <i>E</i> and breaks the fore-and-aft symmetry of a drop shape especially in systems of low Morton numbers <i>M</i>. (3) The Tadaki number <i>Ta</i> is the best dimensionless number among the Eötvös number <i>Eo</i>, the Weber number <i>We</i> and <i>Ta</i> for correlating <i>E</i> of drops, and an empirical correlation of <i>E</i> in terms of <i>Ta</i> was proposed for clean drops under the conditions of -11.6 ≤ log <i>M</i> ≤ -0.9, 1.5 × 10^-2 ≤ <i>Re</i> ≤ 8.5 × 10², 1.7 × 10^-2 ≤ <i>Eo</i> ≤ 9.3, 7.4 × 10^-3 ≤ <i>Ta</i> ≤ 3.6 and 0.1 ≤ κ ≤ 100. (4) The viscosity ratio κ does not affect <i>E</i> so much but influences γ. For κ > 1, γ decreases with increasing the drop diameter, that is, the front part of a drop is flatter than the rear part. On the contrary, the rear part is flatter than the front part when κ < 1. The fore-and-aft symmetry always holds for drops with κ ∼ 1.0. (5) Various drop shapes were presented as a database utilized for verification of numerical methods such as an interface tracking method and a boundary element method.

DOI： 10.1299/jfst.2.184

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Several experiments have reported that Departure from Nucleate Boiling (DNB) in a Pressurized Water Reactor (PWR) fuel assembly often takes place in upstream part of a grid spacer, and it was supposed that bubble accumulation induced by flow around an obstacle is one of reasons of the DNB inception. Hence, experiments were conducted to investigate the motion of small bubbles in high-speed flows in a vertical duct containing an obstacle. The bubble trajectories and averaged bubble distributions were measured by an image processing method. The experimental results indicated that small bubbles migrate toward the opposite wall of the obstacle near the upstream edge of the obstacle. The lateral translation of bubbles was larger than that of the liquid phase, which could be induced by the difference in inertia between bubbles and the liquid phase.

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Language：English   Publishing type：Research paper (scientific journal)  

Multiphase flows and turbulent flows are inevitably three-dimensional and, therefore, a three-dimensional three-component PIV (3D3C-PIV), which can measure a three-dimensional distribution of three velocity components in a flow, is required for the understanding of a three-dimensional flow structure. In this study, we developed a 3D3C-PIV by making use of the intensity gradients of a two-color laser beam. The significant features of the present method are (i) a three-dimensional distribution of three velocity components is measurable, (ii) simultaneity in measured vectors is guaranteed perfectly, (iii) the optical system is simple and economical because it does not require scanning optics, high-repetition light sources, and cameras, (iv) the calibration of optical distortion required in a stereoscopic PIV/PTV is not necessary , and (v) the algorithm of image processing is simpler compared with 3D-PTV. Feasibility tests were carried out by applying the developed method to a jet flow and a flow around drops. As a result, the feasibility and potential of the proposed method was confirmed.

DOI： 10.1615/MultScienTechn.v19.i3.40

Scopus

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Language：English   Publishing type：Research paper (scientific journal)  

Air-water two-phase flow patterns in a four by four square lattice rod bundle consisting of an acrylic channel box of 68mm in width and transparent rods of 12mm in diameter were observed by utilizing a high speed video camera, FEP (fluorinated ethylene propylene) tubes for rods, and a fiberscope inserted in a rod. The FEP possesses the same refractive index as water, and thereby, whole flow patterns in the bundle and local flow patterns in subchannels were successfully visualized with little optical distortion. The ranges of gas and liquid volume fluxes, (JG) and (JL), in the present experiments were 0:1 ≪ (JL) &lt
2:0 m/s and 0:06 &lt
(JG) &lt
8:85 m/s, which covered typical two-phase flow patterns appearing in a fuel bundle of a boiling water nuclear reactor. As a result, the following conclusions were obtained: (1) the region of slug flow in the (JG)-(JL) flow pattern diagram is so narrow that it can be regarded as a boundary between bubbly and churn flows, (2) the boundary between bubbly and churn flows is close to the boundary between bubbly and slug flows of the Mishima and Ishii’s flow pattern transition model, and (3) the boundary between churn and annular flows is close to the Mishima and Ishii’s model. © 2007 Taylor and Francis Group, LLC.

DOI： 10.1080/18811248.2007.9711327

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER SOC MECHANICAL ENGINEERS  

Dissolution of single carbon dioxide (CO2) bubbles in a vertical pipe of 25 mm in diameter are measured to examine the effects of the ratio lambda of sphere-volume equivalent bubble diameter d to pipe diameter D, liquid Reynolds number Re-L and surfactants on mass transfer. The bubble diameter d and Reynolds number Re-L are varied from 5.0 to 26 mm (lambda = 0.20-1.0) and from 0 to 3100, respectively. Millipore water, tap water and water contaminated with Triton X-100 are used for the liquid phase. Mass transfer coefficients k(L) are evaluated from changes in d. The k(L) decreases with increasing lambda for bubbles in stagnant millipore water because of the decrease in bubble rising velocity due to the wall effect. Measured Sherwood numbers Sh do not depend on Re-L because a turbulent fluctuation velocity in bulk liquid flow is Much smaller than a relative velocity between a bubble and liquid. The mass transfer correlation for a bubble in a stagnant liquid proposed by Johnson et at. is applicable to a bubble in pipe flow, provided that a correct relative velocity between a bubble and liquid is substituted in the correlation. Due to the retardation of capillary wave, mass transfer coefficients for bubbles in contaminated water becomes smaller than those in millipore and tap waters.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER SOC MECHANICAL ENGINEERS  

Motions of single silicon oil drops rising in linear shear flows of glycerol-water solutions are measured to investigate the effects of the viscosity ratio kappa and dimensionless shear rate Sr on drag forces under the conditions of -6.3 &lt;= logM &lt;= -4.8, 0.94 &lt;= kappa &lt;= 7.04, 1 &lt; Re &lt;= 23 and 3.2 &lt;= omega &lt;= 6.0 s(-1), where M is the Morton number and omega the magnitude of the velocity gradient of the continuous phase. As a result, we confirmed that (1) the drag coefficient C-D of a drop in a linear shear flow takes a higher value than the drag coefficient CD of the same-size drop in a stagnant liquid, (2) C-D/C-D0 increases with Sr, and (3) the augmentation of C-D becomes large as kappa increases.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER SOC MECHANICAL ENGINEERS  

Dissolution of single carbon dioxide (CO2) bubbles in a vertical pipe of 25 mm in diameter are measured to examine the effects of the ratio lambda of sphere-volume equivalent bubble diameter d to pipe diameter D, liquid Reynolds number Re-L and surfactants on mass transfer. The bubble diameter d and Reynolds number Re-L are varied from 5.0 to 26 mm (lambda = 0.20-1.0) and from 0 to 3100, respectively. Millipore water, tap water and water contaminated with Triton X-100 are used for the liquid phase. Mass transfer coefficients k(L) are evaluated from changes in d. The k(L) decreases with increasing lambda for bubbles in stagnant millipore water because of the decrease in bubble rising velocity due to the wall effect. Measured Sherwood numbers Sh do not depend on Re-L because a turbulent fluctuation velocity in bulk liquid flow is Much smaller than a relative velocity between a bubble and liquid. The mass transfer correlation for a bubble in a stagnant liquid proposed by Johnson et at. is applicable to a bubble in pipe flow, provided that a correct relative velocity between a bubble and liquid is substituted in the correlation. Due to the retardation of capillary wave, mass transfer coefficients for bubbles in contaminated water becomes smaller than those in millipore and tap waters.

DOI： 10.1299/jfst.3.667

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Language：Japanese   Publisher：Atomic Energy Society of Japan  

DOI： 10.11561/aesj.2007s.0.236.0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC MECHANICAL ENGINEERS  

Cavitation in nozzles of liquid injectors is known to affect the atomization of a discharged liquid jet. To understand how cavitating How in a nozzle enhances the liquid jet atomization, liquid velocity distribution of cavitating flow in a two-dimensional transparent nozzle was measured using a Laser Doppler Velocimetry (LDV) system. As a result, the following conclusions were obtained: (1) The inception of cavitation occurs near the outer edge of separated boundary layer (SBL), where the time-averaged local velocity takes the highest value and the time-averaged pressure is almost equal to the vapor saturation pressure. (2) When the cavitation number sigma is greater than 0.78 (in no cavitation and developing cavitation regimes), the reattachment of SBL occurs in the middle of the nozzle. A large velocity fluctuation, which appears just downstream of SBL, decreases near the nozzle exit. Hence the wavy jet is formed in these regimes. (3) For sigma &lt;= 0.65 (in super cavitation regime), the lateral flow directing from the core region toward the side walls just upstream of the nozzle exit is a major cause of the increase in the spray angle and drastic enhancement of liquid jet atomization. The strong turbulence just upstream of the exit must play an important role in the formation of ligaments on liquid jet interface.

DOI： 10.1299/jsmeb.49.1253

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Coalescence of two planar air bubbles in stagnant water filled in two parallel plates was experimentally investigated. To examine the effects of bubble wake on coalescence, the bubble diameter, initial vertical and horizontal distances between two bubbles were chosen as experimental parameters. The vertical distance <i>x</i> from the bubble release point, at which coalescence took place, was measured. The number of coalesced bubble pairs was counted for about 100 bubble pairs, and the coalescence probability was calculated as the ratio of the number of the coalesced bubble pairs to that of the total bubble pairs. With decreasing the bubble diameter or with increasing the initial distance between two bubbles, the mean vertical distance <i>x</i> increased and the coalescence probability decreased. There was a strong correlation between the distribution of coalescence probability and that of bubble wake velocity, i.e. the wake structure does play a significant role in the bubble coalescence process and affects the coalescence probability.

DOI： 10.1299/jfst.1.94

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Terminal velocities of single drops rising through infinite stagnant liquids under wide ranges of fluid properties were measured to examine the effects of initial shape deformation, surfactants and the viscosity ratio. As a result, the following conclusions were obtained: (1) the terminal velocity <i>V_T</i> of a drop is not affected by initial shape deformation due to strong viscous damping of shape oscillation, (2) the drop drag coefficient <i>C_D</i> is a function of the drop Reynolds number <i>Re</i>, the viscosity ratio κ and surfactant concentration, (3) surfactants increase <i>C_D</i> and the influence of surfactants on <i>C_D</i> becomes weaker as κ increases, which is consistent with the Levich's drag model, (4) the effect of κ on <i>C_D</i> can be evaluated by the factor in the Levich's model, [(2+3κ)/(1+κ)], even at intermediate drop <i>Re</i> numbers, and (5) the combination of the Levich's model and Schiller & Nauman's correlation gives good estimation of <i>C_D</i> of single drops in clean and fully contaminated systems under the conditions of -11.6 <log₁₀<i>M</i> < -0.9, 1.7×10^-1 < <i>Re</i> < 2×10², 1.7×10^-2 < <i>Eo</i> < 12.1 and 0.1 < κ < 100.

DOI： 10.1299/jfst.1.72

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DOI： 10.3811/pmfr.1.65

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Cavitation and internal flow in the nozzle of a liquid injector are known to affect the atomization of a discharged liquid jet. In the present study cavitation in a two-dimensional (2D) nozzle and the injected water jet were visualized using an ultra high-speed camera. As a result, the following conclusions were obtained: (1) The behavior of cavitation in the 2D nozzle and liquid jet agreed qualitatively with the previous observations of cylindrical nozzles. (2) When the cavitation number σ was 1.18, the inception of small cavitation bubbles was observed near the nozzle inlet apart from the side wall. (3) The number of cavitation bubble clouds increased with decreasing of σ (0.75&ge;σ<1.18), and the streamwise length of the cavitation region increased toward the middle of the nozzle (developing cavitation). The bubble clouds were clearly observed to recirculate in the separated boundary layer. In the case a wavy jet was observed with no significant effects of cavitation on liquid jet atomization. (4) With a slight decrease of σ from 0.75, the cavitation region suddenly extended to the nozzle exit (super cavitation). The shape of the cavitation region near the inlet became smooth, which imply the formation of a long cavitation bubble near the inlet. At the downstream half of the nozzle, clouds of cavitation appeared and collapsed intermittently within the nozzle. In the case atomization was promoted to form spray with ligaments and droplets.

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Cavitation and internal flow in the nozzle of a liquid injector are known to affect the atomization of a discharged liquid jet. In order to obtain the knowledge on the mechanism how cavitating flow in a nozzle enhances the liquid jet atomization, liquid velocity distributions of cavitating flows in a two dimensional transparent nozzle were measured using a Laser Doppler Velocimetry (LDV) system. As a result, the following conclusions were obtained: (1) The inception of cavitation occurs near the outer edge of the separated boundary layer (SBL), where the mean local velocity takes the highest value and the mean pressure is lower than the vapor saturation pressure. (2) When the cavitation number σ&ge;0.78 (No Cavitation or Developing Cavitation), the reattachment of SBL occurs in the middle of the nozzle. A large velocity fluctuation, which appears in and just downstream of SBL, decreases near the nozzle exit. This may be the reason of the wavy jet. (3) When σ&ge;0.65 (Super Cavitation), the lateral flow from the core region toward the side walls just upstream of the nozzle exit may be the major cause of the increase in spray angle and a drastic promotion of liquid jet atomization. The strong turbulence just upstream of the exit may also play a role on the formation of ligaments at liquid jet interface.

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Language：Japanese   Publisher：The Visualization Society of Japan  

To verify the validity of a numerical method to predict multi-phase dispersed flows of which the density varies with fluid temperature and solute concentration, solute concentration and velocity distributions of a salt water flow were measured using an imaging technique. The aspect ratio and rising velocity of a single drop in a density stratified salt water were also measured. Calculated results of these flows agreed well with the measured results.

DOI： 10.3154/jvs.26.Supplement1_301

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Air-water two-phase flow patterns in a four by four square lattice rod bundle consisting of an acrylic channel box of 68 mm in width and transparent rods of 12 mm in diameter were observed by utilizing a high speed video camera, FEP (fluorinated ethylene propylene) tubes for rods, and a fiberscope inserted in a rod. The FEP possesses the same refractive index as water, and thereby, whole flow patterns in the bundle and local flow patterns in subchannels were successfully visualized with little optical distortion. The ranges of liquid and gas volume fluxes, 〈JG〉 and 〈JL〉, in the present experiments were 0.1 &lt
〈JL〉 &lt
2.0 mis and 0.04 &lt
〈JG〉 &lt
8.85 m/s, which covered typical two-phase flow patterns appearing in a fuel bundle of a boiling water nuclear reactor. As a result, the following conclusions were obtained: (1) the region of slug flow in the 〈JG〉 - 〈JL〉 flow pattern diagram is so narrow that it can be regarded as a boundary between bubbly and churn flows, (2) the boundary between bubbly and churn flows is close to the boundary between bubbly and slug flows of the Mishima &amp
Ishii's flow pattern transition model, and (3) the boundary between churn and annular flows is well predicted by the Mishima &amp
Ishii's model. Copyright © 2006 by ASME.

DOI： 10.1115/ICONE14-89332

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Computational Multi-Fluid Dynamics, CMFD, has come to play an important role in practical engineering and fundamental researches on multiphase flows. Since a multiphase flow is often comprised of interdependent elementary phenomena with length scales ranging from mesoscale to macroscale, a multipurpose CMFD should cover a wide range of scales by the combination of different numerical methods such as interface tracking, bubble tracking and averaging methods. This report reviews recent activities on CMFD for bubbly flows, in particular, an improvement of an interface tracking method based on volume tracking, two types of hybrid CMFD methods based on a combination of interface tracking and bubble tracking methods and that of interface tracking and averaging methods, and work in progress heading toward the hybrid combination of the three methods.

DOI： 10.1615/MultScienTechn.v18.i1.40

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Publisher：Atomic Energy Society of Japan  

DOI： 10.11561/aesj.2006f.0.258.0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

The ratio phi of the eddy viscosity induced by the dispersed phase to the shear-induced eddy viscosity was confirmed to be an appropriate parameter for correlating turbulence modification in gas-solid two-phase pipe flows in our previous study. The purpose of this study is to examine the applicability of phi to gas-liquid and solid-liquid two-phase dispersed upflows in vertical pipes. Experiments were, therefore, conducted under various conditions, in which liquid, gas and solid volumetric fluxes, particle diameter and pipe diameter were chosen as parameters. The velocities of continuous and dispersed phases and the phase fractions were measured by LDV and an image processing method. It was confirmed that measured turbulence modification is better correlated with phi than with the critical parameter proposed by Gore and Crowe. The critical point at which no modification occurs is close to phi = 1, irrespective of the type of two-phase dispersed flow. (C) 2004 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.ijheatfluidflow.2004.02.001

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

Shapes and rising velocities of single bubbles rising through vertical square ducts were measured under stagnant, laminar and turbulent flow conditions to examine the applicability of available empirical correlations for singe bubbles in vertical pipes to those in square ducts. Experiments were carried out by using three different ducts, the widths W of which were 13.0, 16.2 and 27.2 mm. The ratio λ of sphere-volume equivalent bubble diameter d to W was varied from 0.2 to 2.0. As a result, it was confirmed that (1) empirical correlations for evaluating terminal velocities of a bubble in a pipe are applicable to a bubble in a square duct, provided that the characteristics length of duct is evaluated as (4A/π)0.5 where A is the flow area, and (2) bubble shapes in square ducts are well expressed as a function of the liquid Reynolds number and the Eötvös number.

DOI： 10.1299/kikaib.70.356

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Most of unintrusive velocimetry techniques utilize small tracer particles to visualize a flow field. The addition of tracer particles in flows induces errors and flow disturbance due to their poor followability and accumulation on gas-liquid interface. A molecular tagging velocimetry based on photobleaching reaction is developed in this study. The most significant features of the developed method are as follows: (a) local instantaneous velocity, strain rate and rotation are measurable, (b) the measurement can be performed at arbitrary points in a flow field, and (c) tracer particles disturbing a flow are not required. The method was applied to a laminar pipe flow and flows around a bubble and a drop. It is confirmed that (1) the method can accurately measure radial distributions of liquid velocity and shear stress in the pipe flow, (2) local instantaneous velocity distributions around single bubbles and a drop are successfully measured, (3) the time evolution of velocity field is measurable by forming multiple tags in a matrix arrangement, and (4) the liquid velocity in the vicinity of gas-liquid interface can be measured, provided that tags are formed there.

DOI： 10.1615/MultScienTechn.v16.i4.40

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Measurements of velocity and pressure distributions around a bubble are of fundamental importance to model the forces acting on the bubbles and to verify detailed numerical methods for the prediction of flow in nuclear reactors. The measurements of velocity distributions around a bubble have been conducted to understand the interaction between liquid flow and bubbles. However there are few studies on pressure distributions around a bubble for the lack of measurement method. In this study, we developed a method for evaluating a pressure distribution by making use of velocity data obtained by a particle image velocimetry (PIV) or a particle tracking velocimetry (PTV), and applied it to laminar pipe flows, laminar flows around single particles and single bubbles in a pipe to examine its accuracy and applicability to the flow around single bubbles. As a result, we could confirm that the method can evaluate the pressure distribution in various laminar flows, provided that the velocity data possess a good quality and a flow of concern is two-dimensional. The proposed method therefore has a potential to provide the important information for modeling of the bubble motion and verification of CFD methods such as interface tracking and lattice Boltzmann methods.

DOI： 10.1080/18811248.2003.9715416

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The effect of electric and gravitational field on detachment and motion of gas bubbles was studied by injecting nitrogen in a fluoroinert liquid (FC-72) at ambient temperature and pressure through an orifice (about 0.1 mm diameter) drilled in a horizontal tube. In such a way, it was possible to investigate the mechanical effects in bubble dynamics separately from the thermal and mass exchange ones involved in boiling. An electric field of nearly cylindrical geometry was generated around the tube by imposing a D.C. potential drop V (0-18 kV) to a eight-rod cylindrical "squirrel cage" surrounding it. The apparatus was operated in microgravity conditions in the dropshaft of JAMIC in Hokkaido, Japan. Bubble size, detachment frequency and velocity were measured by digital processing of high-speed video images. The results showed that in the absence of electric field bubble detachment did not take place at low gas flow rate; conversely at higher gas flow, the dynamical effects were sufficient to induce bubble detachment even in the absence of the buoyant force. The application of electric field was confirmed to be effective in promoting bubble detachment at values of diameter greater but of the same order of magnitude as in normal gravity, and in providing a force to remove the bubbles away from the orifice. (C) 2003 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0301-9322(03)00030-2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ATOMIC ENERGY SOC JAPAN  

Shapes and velocities of single air bubbles rising through stagnant and flowing waters in an inner subchannel are measured by making use of fluorocarbon tubes. It is confirmed that, (1) bubble shapes and motions in the subchannel are by far different from those in simple geometry, and they depend on the ratio. of the bubble diameter to the subchannel hydraulic diameter, (2) when lambda&gt;0.9, a part of a bubble intrudes into neighboring subchannels, and thereby a kind of void drift takes place even with a single bubble, (3) the terminal velocity V-T of a small bubble (lambda&lt;0.9) is accurately predicted by a theoretical model proposed by Tomiyama et al., (4) an empirical correlation of V-T for cell-Taylor bubbles (X &gt; 0.9) is presented, and (5) the rising velocity V-B in laminar and turbulent flow conditions are well evaluated by substituting the proposed V-T models and the ratio of the maximum liquid velocity to the mean liquid velocity into the Nicklin correlation.

DOI： 10.3327/jnst.40.136

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ATOMIC ENERGY SOC JAPAN  

Shapes and velocities of single air bubbles rising through stagnant and flowing waters in an inner subchannel are measured by making use of fluorocarbon tubes. It is confirmed that, (1) bubble shapes and motions in the subchannel are by far different from those in simple geometry, and they depend on the ratio. of the bubble diameter to the subchannel hydraulic diameter, (2) when lambda&gt;0.9, a part of a bubble intrudes into neighboring subchannels, and thereby a kind of void drift takes place even with a single bubble, (3) the terminal velocity V-T of a small bubble (lambda&lt;0.9) is accurately predicted by a theoretical model proposed by Tomiyama et al., (4) an empirical correlation of V-T for cell-Taylor bubbles (X &gt; 0.9) is presented, and (5) the rising velocity V-B in laminar and turbulent flow conditions are well evaluated by substituting the proposed V-T models and the ratio of the maximum liquid velocity to the mean liquid velocity into the Nicklin correlation.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WORLD SCIENTIFIC PUBL CO PTE LTD  

This study describes the numerical simulations of two-phase interfacial deformations using the binary fluid (BF) model in the lattice Boltzmann method (LBM), where a macroscopic fluid involves mesoscopic particles repeating collisions and propagations and an interface is reproduced in a self-organizing way by repulsive interaction between different kinds of particles. Schemes for the BF model axe proposed to simulate motions of immiscible two phases with different mass densities. For higher Reynolds number, the finite difference-based lattice Boltzmann scheme is applied to the kinetic equations of particles, which include convection terms to reduce the diffusivity of each phase volume. In addition, two parameters are introduced into the BF model to adjust surface tension and interfacial thickness independently. The numerical results of three-dimensional bubble motion under gravity and two-dimensional droplet deformation under shear stress indicate that the lattice-Boltzmann BF model with the proposed schemes would be applicable to simulating interfacial dynamics of immiscible two-phase fluids.

DOI： 10.1142/S0217979203017308

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

Lateral migration of a deformed single bubble rising in the vicinity of a vertical flat plate was measured to evaluate a wall force acting on the bubble. Experimental results indicated that a wall force coefficient Cw is a function of the bubble Reynolds number Re and Morton number M under the condition of low bubble Reynolds number, whereas it is a function of the Eötvös number Eo for high bubble Reynolds number condition. Empirical correlations of Cw were deduced from the experimental results. It was confirmed that the wall force model proposed by one of the authors and the proposed correlations of Cw are applicable for the bubbles under the condition of -6.0. ≤ log10M ≤ -2.5, 2.0 ≤ E0 ≤10.0 and 0.4 ≤ Re ≤ 65, and have a potential for the application to a bubble in an air-water system, provided that the bubble does not collide with the wall.

DOI： 10.1299/kikaib.69.2214

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

Liquid slugs in petroleum pipelines in hilly terrain are mainly generated at V-shaped elbows and induce high pressure fluctuation. The slug characteristics generated at elbows such as slug frequency, slug velocity and length are therefore required for the design of pipelines. In this study, we measured the velocity, length and frequency of liquid slug generated at V-shaped elbows. The effects of pipe diameter and inclination angle on the slug characteristics were experimentally examined by using three different pipe diameters and three inclination angles. As a result, it was confirmed that (1) the slug velocity is proportional to the total volume flux, and thereby it is well estimated by a drift flux correlation, (2) the length of a slug unit is well correlated with the ratio of the liquid volume flux to the total volume flux and (3) the slug length can be expressed as a function of the gas volume flux, the slug velocity and the void fraction in a gas bubble region.

DOI： 10.1299/kikaib.69.2208

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

The effect of electric and gravitational field on detachment and motion of gas bubbles was studied by injecting nitrogen in a fluoroinert liquid (FC-72) at ambient temperature and pressure through an orifice (about 0.1 mm diameter) drilled in a horizontal tube. In such a way, it was possible to investigate the mechanical effects in bubble dynamics separately from the thermal and mass exchange ones involved in boiling. An electric field of nearly cylindrical geometry was generated around the tube by imposing a D.C. potential drop V (0-18 kV) to a eight-rod cylindrical "squirrel cage" surrounding it. The apparatus was operated in microgravity conditions in the dropshaft of JAMIC in Hokkaido, Japan. Bubble size, detachment frequency and velocity were measured by digital processing of high-speed video images. The results showed that in the absence of electric field bubble detachment did not take place at low gas flow rate
conversely at higher gas flow, the dynamical effects were sufficient to induce bubble detachment even in the absence of the buoyant force. The application of electric field was confirmed to be effective in promoting bubble detachment at values of diameter greater but of the same order of magnitude as in normal gravity, and in providing a force to remove the bubbles away from the orifice. © 2003 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0301-9322(03)00030-2

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Sufficient knowledge of turbulence modification is indispensable to understand the turbulence structure and to develop a turbulence model in gas-liquid two-phase bubbly flow in vertical pipes. There is, however, little information on the interaction between shear-induced turbulence and bubbles in the near wall region, whereas many researches have been carried out on the turbulence properties in gas-liquid bubbly flow in a vertical pipe. In this study, we measured liquid velocity in the near wall region where the shear-induced turbulence is generated and interacts with bubbles. The measured result indicated that (1) bubbles increase the gradient of mean liquid velocity and therefore the shear-induced turbulence is enhanced in the near wall region, and (2) bubbles break up the developed large-scale shear-induced eddy and obstruct the transfer of turbulence energy from the near wall region to the core region.

DOI： 10.1299/jsmemecjo.2003.3.0_267

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Terminal velocity V-T of a single bubble rising through an infinite stagnant liquid in surface tension force dominant regime was investigated theoretically and experimentally. A theoretical V-T model, which is applicable to a distorted spheroidal bubble with a high bubble Reynolds number, was deduced from a jump condition and a potential flow theory for a flow about an oblate spheroid. Experiments were conducted using air and water to measure bubble trajectories, shapes and velocities. As a result, it was confirmed that (1) the primal cause of widely scattered V-T in this regime is not surfactant concentration but initial shape deformation, (2) small initial shape deformation results in a low V-T and a high aspect ratio, whereas large initial shape deformation results in a high V-T and a low aspect ratio, (3) the primal role of surfactants in this regime is to cause the damping of shape oscillation, by which a contaminated bubble behaves as if it were a clean bubble with low initial shape deformation, and (4) the proposed model gives good predictions of V-T for single distorted bubbles. (C) 2002 Elsevier Science Ltd. All rights reserved.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Trajectories of single air bubbles in simple shear flows of glycerol-water solution were measured to evaluate transverse lift force acting on single bubbles. Experiments were conducted under the conditions of -5.5 less than or equal to log(10) M less than or equal to -2.8, 1.39 less than or equal to Eo less than or equal to 5.74 and 0 less than or equal to \dV(L)/dy\ less than or equal to 8.3 s(-1), where M is the Morton number, Eo the Eotvos number and dV(L)/dy the velocity gradient of the shear flow. A net transverse lift coefficient C-T was evaluated by making use of all the measured trajectories and an equation of bubble motion. It was confirmed that C-T for small bubbles is a function of the bubble Reynolds number Re, whereas C-T for larger bubbles is well correlated with a modified Eotvos number Eo(d) which employs the maximum horizontal dimension of a deformed bubble as a characteristic length. An empirical correlation Of C-T was therefore summarized as a function of Re and Eo(d). The critical bubble diameter causing the radial void profile transition from wall peaking to core peaking in an air-water bubbly flow evaluated by the proposed CT correlation coincided with available experimental data. (C) 2002 Elsevier Science Ltd. All rights reserved.

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Lateral migration of a single bubble rising in the vicinity of a vertical flat plate was measured to evaluate a wall force acting on the bubble. Experimental results indicated that a wall force coefficient CW3 is a function of the bubble Reynolds number Re and the Eötvös number Eo. Empirical correlations of CW3 were deduced for bubbles and particles. It was confirmed that the wall force model proposed by one of the authors and the proposed correlations of CW3 are applicable not only to high viscousity systems but also for low viscousity systems, provided that a bubble does not collide with the wall.

DOI： 10.1115/FEDSM2002-31148

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

A three-dimensional one-way Bubble Tracking Method has been proved to give good predictions for the spatial evolution of flow pattern, mean void fraction, void fluctuation, mean pressure drop and pressure drop fluctuation in developing bubble flows in vertical pipes. In this study, the spatial evolution of radial developing bubble flow. As a result, it is confirmed that the method gives good predictions of local void fractions, provided that bubble size distributions at an inlet were available.

DOI： 10.1299/jsmemecjo.2002.3.0_325

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

Most of interface tracking simulations conducted so far have been based on the assumption that the gas velocity is equal to the liquid velocity on gas-liquid interfaces. To the contrary, the theory of jump conditions presented by Ishii and Delhaye implies that an interfacial slip would exist when interfacial entropy generation takes place. In the present study, we therefore measured velocity distributions in the vicinity of a flat gas-liquid interface using an LDV system with a high spatial resolution of about 10 μm to examine whether or not the interface slip exist. To accurately measure the interface position, we utilized a secondary measurement volume formed by beams reflected by the interface. As a result, we could confirm that the interface slip does exist and its magnitude is large where the interfacial shear stress is large, i.e. where a large interfacial momentum transfer takes place.

DOI： 10.1299/kikaib.68.2050

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In this paper, we describes the simulations of two- and three-dimensional interfacial motions in shear flow based on the lattice Boltzmann method (LBM), in which a macroscopic fluid flow results from averaging collision and translation of mesoscopic particles and an interface can be reproduced in a self-organizing way by repulsive interaction between particles. A new scheme in the binary fluid model is proposed to simulate motions of immiscible two phases with different mass densities, and examined in numerical analysis of bubble motions under gravity in a circular tube and deformation of bubble under shear stress. For higher Reynolds numbers, a finite difference-based lattice Boltzmann scheme is applied to the kinetic equations of particle to improve numerical stability, which can capture break-up motions of bubble. Parallel computing in LBM is also discussed briefly for efficient speeding up.

DOI： 10.1115/FEDSM2002-31150

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

We describe the numerical simulation results of bubble motion under gravity by the lattice Boltzmann method(LBM), which assumes that a fluid consists of mesoscopic fluid particles repeating collision and translation and a multiphase interface is reproduced in a self-organizing way by repulsive interaction between different kinds of particles. The purposes in this study are to examine the applicability of LBM to the numerical analysis of bubble motions, and to develop a three-dimensional version of the binary fluid model that introduces a free energy function. We included the buoyancy terms due to the density difference in the lattice Boltzmann equations, and simulated single- and two-bubble motions, setting flow conditions according to the Eotvos and Morton numbers. The two-dimensional results by LBM agree with those by the Volume of Fluid method based on the Navier-Stokes equations. The three-dimensional model possesses the surface tension satisfying the Laplace's law, and reproduces the motion of single bubble and the two-bubble interaction of their approach and coalescence in circular tube. These results prove that the buoyancy terms and the 3D model proposed here are suitable, and that LBM is useful for the numerical :analysis of, bubble motion under gravity.

DOI： 10.1080/18811248.2001.9715037

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Sufficient knowledge of Reynolds stress is indispensable to understand the turbulence structure and to develop a turbulence model in gas-liquid two-phase bubbly flow. There is, however, little information on the Reynolds shear stress as well as radial and azimuthal turbulence intensities, whereas many researches have concerned on the axial turbulence intensities in a gas-liquid bubbly flow in a vertical pipe. In this study, we utilized a pipe made of fluoro carbon which has the same refractive index with water, and which enabled us to measure the two velocity components simultaneously using LDV in air-water two-phase bubbly flow in vertical pipes. The measured result indicated that all the components of Reynolds stress are affected by the presence of bubbles. Bubble-induced agitation of a liquid velocity field was particularly prominent under a laminar flow condition. Turbulence enhancement as well as suppression due to the presence of bubbles was observed.

DOI： 10.1299/jsmeptec.2001.0_393

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Language：Japanese   Publisher：THE JAPANESE SOCIETY FOR MULTIPHASE FLOW  

Three-phase flows are encountered in air-lift pumps, preheaters and reactors, and other industrial plants. It is necessary to clarify not only macroscopic characteristics such as pressure drops and volume fractions of each phase but also local characteristics such as spatial distnbutions of local volume fractions and local velocities of each phase. In the present review, measurement techniques of volumetric fluxes and volumetric fractions of each phase, radial distributions of gas and solid volume fractions and liquid velocities were described for the three-phase bubbly flow. Those of length and velocities of large bubble liquid slug and film thickness of large bubble were also described for the three-phase slug flow Furthermore, application of measurement techniques for two-phase flows to three-phase flows were examined.

DOI： 10.3811/jjmf.14.270

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Other Link： https://jlc.jst.go.jp/DN/JALC/00152027313?from=CiNii

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

Terminal velocities of single solid particles or bubbles are one of the most fundamental knowledges required for modeling and simulation of dispersed two-phase flows in vertical pipes. However, there are few available correlations which can cover a wide range of parameters, i. e., the ratio of a particle diameter to a pipe diameter, particle density as well as the falling motions of particles such as rectilinear and rocking. In this study, we proposed a semi-empirical model of terminal settling velocity which can cover a wide range of parameters, based on a singular pressure loss about a particle. Experiments were also carried out to develop an empirical correlation based on the proposed semi-empirical model of terminal settling velocity of a particle falling through a Iiluid-filled pipe. It was confirmed through comparisons of terminal velocities between the measurements and the developed correlation that the proposed model can give good estimations of terminal velocities under various terminal settling conditions.

DOI： 10.1299/kikaib.66.652_3071

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Very high data rate Phase Doppler Measurements were carried out in order to demonstrate the spray characteristics at each cycle and how each injection differed from each other. Conventional time-averaged data analysis can hardly provide information to analysis cyclic variation of spray formation and droplet dynamics so that a cycle-resolved PDA system was developed in the study. A direct gasoline injector for two-stroke marine engine was used for the experiment. For data analysis, droplet dynamics and characteristics of different droplet diameter were examined. The results show mat cycle variation of injector was remarkable, the maximum spray tip velocity differed from 63 m/s to 93 m/s even for the consecutive injection. The data rate obtained was over 40 kHz (Max: 85 kHz) and bin width was carefully examined to show the spray collision to air and entrained air motion. The high speed cycle formed large velocity fluctuation and small vortex time scale, while the low speed cycle was not so clear for the vortex entrainment. © Society of Automotive Engineers, Inc.

DOI： 10.4271/970631

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE PUBL B V  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：XIAN JIAOTUNG UNIV PRESS  

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Seeding particles for laser Doppler velocimeter (LDV) measurement have been developed which are of uniform diameter, spherical shape, low loose weight and high melting temperature. Evaluation test results show that the use of the particles can increase the signal quality substantially and extend the measurable field of the LDV, which can hardly be accessed by conventional measurement methods. Copyright © 1994 Verlag GmbH &amp
Co. KGaA, Weinheim

DOI： 10.1002/ppsc.19940110204

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

The flowfields in gas turbine combustors have not been well understood in either experiments or numerical simulations. To increase the accuracy of flow prediction, numerical errors, which come from turbulence model, mesh size, iteration number, and inlet conditions, should be reduced with the assistance of measurements. However, so far very few experimental data have been obtained to evaluate the numerical models and the predicted results.
The purpose of the present study is to obtain the inlet conditions for a combustor in an isothermal model of the gas turbine combustor that was made to demonstrate the fundamental flow structure of swirling flow and jet flow interactions, in order to compare predicted results with measured results and demonstrate the influence of inlet conditions on the flowfield. A very small fiber laser-Doppler velocimeter was used to measure radial velocity in order to understand the recirculating flow and flow interactions. The momentum ratio of the swirl mass flow rate to the jet mass flow rate was taken to be 1.78-7.01, which would cover all loading conditions. The results indicate that the radial velocity at the swirler exit should be given; otherwise the radial velocity should be half the axial velocity, and the swirl velocity at the jet exit also should be considered, so we propose a way to determine swirl velocity at the jet exits. The predicted flow patterns and turbulent kinetic energy profiles show the major flow structure that can be seen in the experiments. The errors due to the influence of inlet conditions on the flowfield could be reduced and the accuracy of prediction increased if the proposed modification is used.

Web of Science

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Web of Science

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Language：Japanese   Publishing type：Rapid communication, short report, research note, etc. (scientific journal)   Publisher：日本流体力学会  

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Language：Japanese   Publishing type：Lecture material (seminar, tutorial, course, lecture, etc.)  

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

A measurement method of local instantaneous velocity gradients in turbulent flows are required to evaluate turbulence kinetic energy (TKE) budget. Spatiotemporal Filter Velocimetry (SFV) is therefore applied to evaluation of TKE budget by coupling with a molecular tagging method using photobleaching reaction. Measurements are carried out not only for single-phase turbulent flow but also for bubbly flows with two different bubble diameters to examine effects of bubble diameter on the velocity distribution and TKE budget. As a result, the following conclusions are obtained: (1) Effect of bubble diameter on the distribution of axial mean velocity is weak under the fixed void distribution and the fixed interaction forces between the phases. (2) Effect of bubble diameter on TKE budget is not so large in the bubbly flows whereas the presence of bubbles tend to reduce the magnitudes of the production and diffusion rates.

CiNii Books

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

It is important in design and development of reactor cores to increase knowledge of turbulent flows in rod bundles. In this study, we therefore measured velocity in single-phase and bubbly flows by installing a small LDV probe into a rod of a 2x2 rod bundle. The diameter and the pitch of the rods were 10 and 13 mm, respectively. The liquid volume flux was 1 m/s, and the gas volume flux in the bubbly flow was 0.08 m/s. The local time-averaged velocity and turbulent intensity were obtained from 10,000 instantaneous velocities at each measurement point. As a result, the following conclusions were obtained: (1) The distribution of liquid velocity in the bubbly flow is flatter than that in the single-phase flow, and the velocity gradient near the wall increases due to the presence of bubbles. (2) Local wall shear stress on the rod wall depends on the azimuthal localtion, and the distributions of dimensionless liquid velocities near the wall in inner subchannel agree with the universal dimensionless mean velocity profile of wall turbulence.

CiNii Books

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

CiNii Books

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

CiNii Books

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

CiNii Books

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Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (other)   Publisher：日本機械学会流体工学部門  

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

CiNii Books

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Presence of interface in gas-liquid two-phase flow makes it difficult to measure liquid velocity using LDV at a high void fraction. In the present study, we have developed small LDV optics to avoid the difficulty in optical access by submerging the LDV probe into a flow field. The diameter of the probe is 7 mm. The developed system is applied to bubbly flow in a bubble column to examine its applicability to bubbly flow at a high void fraction. Comparisons with a conventional LDV system show that the accuracy and reliability of the developed system are the same level as the conventional system, and that the developed system can measure liquid velocity in bubbly flow at high void fractions, in which the conventional LDV cannot measure the liquid velocity. The measured data show that the axial mean velocity and turbulence intensity increase with the gas volumetric flux J_G not only at low JG but also at high J_G.

CiNii Books

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Spatial filter velocimetry (SFV) based on spatial filtering of time-series particle images proposed by Hosokawa and Tomiyama is applied to a planar bubble plume to examine its applicability to simultaneous measurement of bubble and liquid velocities. The edge detection process is added in the SFV processing to reduce the size of image pattern and to obtain accurate bubble velocity. Bubble regions in the recorded images of the planer bubble plume are detected by using a labeling process, and the particle and bubble images are generated by masking the bubble region and by inversing the bubble region, respectively. Then, SFV processing is applied to particle and bubble images to obtain liquid and bubble velocities, respectively. The measured results clearly demonstrate the potential of SFV for simultaneous measurements of liquid and gas velocities in two-phase flows.

CiNii Books

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Validation of turbulence models is not an easy task mainly due to lack of experimental data on turbulent flows in rod bundles. In this study, we therefore measured distributions of velocity and turbulence intensities of water flow in a 2x2 rod bundle by using a stereoscopic PIV system. The diameter and pitch of the rods were 20 and 25 mm, respectively. Area-averaged velocity was 2 m/s. Local time-averaged velocity and turbulent intensities were obtained from 10,000 instantaneous velocities at each measurement point. Experimental results showed that horizontal components of turbulence intensity in the cross-section are anisotropic near the rod gap. Since eddies larger than the gap width cannot exist in the rod gap, the turbulence spectrum takes low values in the rod gap compared with center region of subchannel. To the contrary, difference in the turbulence spectrum of eddies smaller than the gap width is small between the rod gap and the center of subchannel.

CiNii Books

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Although area-averaged void fractions in subchannels have been measured frequently in literatures, distribution of local void fraction in a rod bundle have been rarely reported. Little is also known on bubble motion in a rod bundle. Distributions of local void fraction and bubble velocity in a 4x4 rod bundle were, therefore, measured using a conductance probe in this study. Liquid velocity was also measured using LDV. Experimental results showed that the distributions of the void fractions in the inner and side subchannels depend not only on lift force acting on bubbles but also on geometrical interaction between the bubble and the rod wall. The relative velocity between bubbles and liquid phase in the subchannel possesses non-uniform distribution over the cross-section. This is caused by increase in drag force acting on bubbles due to bubble deformation induced by interaction between the bubbles and the rod wall.

CiNii Books

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Improvement of steam separators in ABWR is required to decrease pressure drops at high steam qualities without degrading the separation performance. In this study, shapes of the swirler and 1^<st> pick-off ring (POR) were modified to reduce the pressure drop, and experiments using a one-fifth scale model of the separator were carried out to evaluate its performance. Experimental data showed that the modifications successfully reduce the pressure drop and carry-under (CU) in the separator without degrading the separation performance.

CiNii Books

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

CiNii Books

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

CiNii Books

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Language：Japanese  

CiNii Books

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