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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. 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.

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.

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.

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≥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) Whenσ≤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.

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.

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.