Shigeo Hosokawa

Transverse migration of single bubbles in simple shear flows

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.

Terminal velocity of single bubbles in surface tension force dominant regime

Abstract 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.

Simulation of Bubble Motion under Gravity by Lattice Boltzmann Method

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 Eötvös 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 Laplaces 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.

Lateral Migration of Single Bubbles Due to the Presence of Wall

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

Shapes and Rising Velocities of Single Bubbles rising through an Inner Subchannel

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 accurately predicted by a theoretical model proposed by Tomiyama et al., (4) an empirical correlation of VT for cell-Taylor bubbles (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.

A compact fibre LDV with a perforated beam expander

The authors describe an optical fibre laser Doppler velocimeter (FLDV) system which consists of a manipulator that is easy to handle and a small probe. The manipulator, with a tilting mechanism of 0.3 mu m resolution, has been developed to achieve a high transmitting efficiency of laser power into a single-mode fibre. A beam expander with a perforated lens is used in the probe in order to improve the quality of the measurement volume and to reject light scattered at the front lens. The performance of the FLDV was examined by measuring the tangential velocity of a rotating disc and the longitudinal velocity of turbulent water in a pipe at three flow rates. The results show an excellent performance of the FLDV.

Two-phase Flow Patterns in a Four by Four Rod Bundle

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} < 2:0 m/s and 0:06 < {JG} < 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 Ishiis flow pattern transition model, and (3) the boundary between churn and annular flows is close to the Mishima and Ishiis model.

Countercurrent Flow Limitation at the Junction between the Surge Line and the Pressurizer of a PWR

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.

Swirling Annular Flow in a Steam Separator

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.

Effects of fluid properties on CCFL characteristics at a vertical pipe lower end

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–40 wt% or air–60 wt% 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 (JG* and JL*) for the pipe diameters of D = 14, 20, and 27 mm. A CCFL correlation was derived using the ratio μG/μL of the viscosities of the gas and liquid phases, μG and μL, as a correction term representing effects of fluid properties, where JG*1/2(μG/μL)−0.07 was expressed by a cubic function of JL*1/2(μG/μL)0.1. In the correlation, the constant C indicating the value of JG*1/2(μG/μL)−0.07 at JL* = 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.