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The ratio φ 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 φ 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 φ than with the critical parameter proposed by Gore and Crowe. The critical point at which no modification occurs is close to φ=1, irrespective of the type of two-phase dispersed flow.

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 27.2mm. 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 Eotvos number.

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 λ of the bubble diameter to the subchannel hydraulic diameter, (2) when λ>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 VT of a small bubble (λ 0.9) is presented, and (5) the rising velocity VB in laminar and turbulent flow conditions are well evaluated by substituting the proposed VT models and the ratio of the maximum liquid velocity to the mean liquid velocity into the Nicklin correlation.

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 , 1.39⩽Eo⩽5.74 and , where M is the Morton number, Eo the Eotvos number and dVL/dy the velocity gradient of the shear flow. A net transverse lift coefficient CT was evaluated by making use of all the measured trajectories and an equation of bubble motion. It was confirmed that CT for small bubbles is a function of the bubble Reynolds number Re, whereas CT for larger bubbles is well correlated with a modified Eotvos number Eod which employs the maximum horizontal dimension of a deformed bubble as a characteristic length. An empirical correlation of CT was therefore summarized as a function of Re and Eod. 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.

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, λ, 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 λ 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 λ.Copyright © 2014 by ASME

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...

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 liluid-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.

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.

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 accu...

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.

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.

The flow patterns of the air-water two-phase flow in horizontal and horizontal-downward declined pipes were studied experimentally in order to obtain basic information on the flow characteristics of gas-liquid two-phase flows in hilly-terrain pipelines. It is one of the special characteristics of the flow pattern in these hilly-terrain pipelines that the developing and/or unsteady flow patterns occupy almost whole pipes. Flow pattern maps were presented through a new classification and definition. The concept of “basic and auxiliary flow patterns” was introduced. The flow state was expressed by the combination of a basic flow pattern and some auxiliary flow patterns. The flow behavior greatly changes along hilly-terrain pipelines. It is impossible to predict the flow patterns with existing flow pattern maps for single pipes.

For next-generation nuclear reactors, hybrid safety systems which consist of active and passive safety systems have been planned. Steam generators using U bend pipelines with upper and lower horizontal pipes will be used as one of the passive safety systems. Liquid is generated due to condensation of steam in pipelines if these pipelines are used as condensers. It is supposed that flow patterns in the horizontal parts of these pipelines are different from those in horizontal pipes without U bend. An experimental study was carried out using a U bend pipeline with upper and lower horizontal pipe which has a lower tank and a water supply section at midway of lower horizontal pipe. Flow patterns were observed at four parts, and their maps were compared with those in the previous studies.

The flow patterns of the air-water two-phase flow in horizontal-upward inclined pipes were studied experimentally in order to obtain basic information on the flow characteristics of gas-liquid two-phase flows in hilly-terrain pipelines. Flow regime maps in horizontal-upward inclined pipes were expressed by the combination of a basic flow pattern and some auxiliary flow patterns. In addition to the effect of inclination angle on the flow pattern, their mutual relation between one in horizontal and others in inclined part was described. Comparisons of the flow regime maps with those obtained in single horizontal or upward inclined pipes without bending part were carried out.

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 λ = 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 δt, 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 50×50 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.

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 JSME

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.Copyright © 2011 by JSME

Flow structures and temperature fluctuations in a closed branch pipe connected to a high velocity and high temperature flow in a main pipe were investigated experimentally. A straight pipe and a pipe with a bend were tested for uniform and non-uniform temperature conditions. Visualization of the flow patterns, measurement of the flow velocity and measurement of the liquid and wall temperature distributions were conducted. The flow pattern near the stagnant point in the branch pipe was a spiral flow. The time averaged velocity distributions were linear and the structure of the spiral flow was a simple forced vortex with a vertical circulation. The spiral flow velocity was fluctuated with a long period of several tens seconds. It was shown that large liquid temperature fluctuations were initiated when the spiral flow penetrated into a thermal stratified layer in the bend. The liquid temperature fluctuations caused the wall temperature fluctuations and two types of the wall temperature fluctuations were observed.Copyright © 2010 by ASME

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 indicated 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.

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 1/5th 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 QG 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.

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 κ and dimensionless shear rate Sr on drag forces under the conditions of −6.3 ≤ logM ≤ −4.8, 0.94 ≤ κ7.04, 1 < Re ≤ 23 and 3.2 ≤ ω ≤ 6.0 s−1, where M is the Morton number and ω the magnitude of the velocity gradient of the continuous phase. As a result, we confirmed that (1) the drag coefficient CD of a drop in a linear shear flow takes a higher value than the drag coefficient CD0 of the same-size drop in a stagnant liquid, (2) CD/CD0 increases with Sr, and (3) the augmentation of CD becomes large as κ increases.

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, and , in the present experiments were 0.1 < < 2.0 m/s and 0.04 < < 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 – 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 & Ishii’s flow pattern transition model, and (3) the boundary between churn and annular flows is well predicted by the Mishima & Ishii’s model.

In this paper, we present simulation results of two- and three-dimensional motions of drops in a shear flow based on the lattice Boltzmann method (LBM), where a macroscopic fluid flow results from averaging collisions and propagations of mesoscopic particles. The binary fluid model in LBM used here can reproduce two-phase interface in a self-organizing way by repulsive interaction between particles consistent with the van der Waals-Cahn-Hilliard free energy theory. A finite difference scheme is applied to the lattice-Boltzmann equations governing time evolution of velocity distributions of particle number density. When a drop is suspended in an immiscible second liquid with the same mass and viscosity between moving parallel plates, the numerical results of deformation of drop agree with theoretical solutions and previous numerical results obtained by the volume-of-fluid (VOF) method. Breakup motions of drops in LBM are also reasonable in comparison with the critical Reynolds and capillary numbers predicted by the VOF method. In the simulations of two-drop interaction, it is shown that the breakup motion depends on not only number density of drops but also initial positioning of their volumetric center away from a halfway cross section between the plates.

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 shown 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. (author)

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 Eotvos 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.Copyright © 2002 by ASME

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.Copyright © 2002 by ASME