Koji Akagawa

Flow instabilities in parallel-channel flow systems of gas-liquid two-phase mixtures

Abstract Parallel-channel two-phase flow systems with compressible capacities and the negative-slope characteristics of the pressure drop vs flow rate curve suffer from a non-uniform flow distribution and/or pressure drop oscillation, depending on the pressure drop characteristics of each channel, under certain operating conditions. The modes of flow distribution are quite similar to those observed in boiling channels. Pressure drop oscillations are subdivided into two types: relaxation oscillation and quasi-static oscillation. The modes of oscillation observed in the parallel-channel system are a single-channel mode, a U-tube mode and a multi-channel mode. These modes of oscillation are closely related to the type of oscillation. Upstream compressibility in the gas feed pipe has a destabilizing effect on the oscillation. On the other hand, upstream compressibility in the liquid feed pipe has a stabilizing effect on the oscillations of the gas flow rate and the pressure drop between the headers of parallel channels, but induces a small oscillation of the liquid flow rate. These non-uniform flow distribution and oscillation patterns are analyzed; the results of the analysis are found to be in good agreement with the experimental results. Thus, the characteristics of the non-uniform flow distribution and the oscillation can be estimated by the method of analysis presented in this paper.

Flow pattern transition and heat transfer of inverted annular flow

Abstract The characteristics of flow pattern transitions and heat transfer of a steady inverted annular flow (IAF) in a vertical tube are studied experimentally and analytically using freon R-113 as a working fluid. The heat transfer characteristics are divided into three regions and a heat transfer characteristic map is proposed. Flow patterns along the tube change from IAF to dispersed flow (DF) through inverted slug flow (ISF) for lower mass fluxes and through agitated inverted annular flow (AIAF) for higher mass fluxes. A flow pattern map is proposed and the boundaries of the flow patterns agree well with those of the heat transfer characteristic map. A turbulent boundary layer analysis is applied to IAF using modified Reichardt equations. Radial velocity and temperature profiles of both the gas and liquid phases are obtained. The predicted heat transfer coefficients agree well with the experimental results. The flow pattern transition criterion from IAF to AIAF is predicted by the analytical results and the Kelvin-Helmholtz instability and that from AIAF to DF is predicted by a previous drift flux model and a void fraction of 0.7. The predicted criteria agree well with the experimental results.

Experimental study on flow pattern and heat transfer of inverted annular flow

Abstract Experimental results are presented on flow pattern and heat transfer in the regions from inverted annular flow to dispersed flow in a vertical tube using freon R-113 as a working fluid at atmospheric pressure to discuss the correspondence between them. Axial distributions of heat transfer coefficient are measured and flow patterns are observed. The heat transfer characteristics are divided into three regions and a heat transfer characteristics map is proposed. The flow pattern changes from inverted annular flow (IAF) to dispersed flow (DF) through inverted slug flow (ISF) for lower inlet velocities and through agitated inverted annular flow (AIAF) for higher inlet velocities. A flow pattern map is obtained which corresponds well with the heat transfer characteristic map.

Characteristics of natural circulation flow in a bend loop with a U-shape-type cooler

Abstract An experiment on transient single phase natural circulation in a closed loop was conducted to study the core cooling behaviour by natural circulation in the loop-type PWR for marine application. The system employed a loop that had two U-shaped, and three inverse U-shaped flow paths, and a small elevation difference between the heater and the cooler. The experiment was conducted at atmospheric pressure with the heater input held constant ( Q = 0.5 kW to 3.0 kW ), and the cooling water flow rate of the cooler constant during the transient following tripping of the pump. From the experiment, a flow stability map was obtained which correlated heater input versus elevation differences. Comparison between experimental and numerical results obtained by RETRAN-02 analyses yielded general agreement.

Visualization of dynamic behavior of thermal stratification by liquid crystal.

An experimental study on dynamic behavior of a thermal stratification in a tank was carried out. The temperature and flow fields were visualized by using the liquid crystal tracer, and the dynamic phenomena of the thermal stratification and the thermal plumes were recorded by VTR. When the upper wall was cooled rapidly, the cold plumes formed in the vicinity of the upper wall, moved downward and finally emerged into the thermal stratification. The liquid was well mixed by the cold plumes, and the thermal stratification disappeared.

Prediction of pressure surge in a one-component two-phase bubbly flow

Abstract Shock phenomena caused by a rapid valve closure in a one-component, two-phase bubbly flow were investigated experimentally using Refrigerant R-113 as the working fluid, and a theoretical analysis was conducted. The pressure transient in a one-component, two-phase bubbly flow is characterized by the existence of an exponential pressure decay just after the initial sharp pressure rise. This phenomenon is due to the mass transfer between the vapor and the liquid, that is, condensation. The profile of the pressure transient at each location along the channel was calculated numerically by solving the fundamental equations. By this analysis, the effects of heat transfer and mass transfer between the two phases were clarified. Simplified estimation methods for the potential surge and the equilibrium pressure rise are presented.

A Fundamental Study on Flow Characteristics in Boiler Furnaces : 2nd Report, A Quantitative Evaluation on Mixing Characteristics in Furnaces

In the first report the experimental results of the flow patterns and the concentration distribution of tracer fluid in a two-dimensional model in which perforated plates were installed instead of a tube bundle were reported. In the present report, in order to evaluate these characteristics quantitatively, numerical calculations of the flow pattern and the transient concentration distribution based on the turbulence energy model (one equation model) have been conducted. The effects of the magnitude of flow resistance of the tube bundle on the flow pattern and the mixing performance were clarified quantitatively referring the comparisons with the experimental results.

Studies of Swirling Annular-mist Two-phase Flow : 2nd Report, Analysis of Liquid Film Flowrate Along a Tube

The liquid film flowrate along a tube (40 mm ID, 5 m length) in swirling downward annular-mist air-water flows, induced by different swirlers at the inlet of the test tube in each run, was measured. The effects of the angle of torsion and length of swirlers, the gas velocity, and the liquid flowrate on the liquid film flowrate along the tube were clarified by experiments. A theoretical analysis of the liquid film flowrate at an outlet of swirler and along a tube was conducted. The calculated results agreed well with the experimental results.

Analyses of Shock Phenomena in a Bubbly Flow by Two-Velocity Model and Homogeneous Model

Shock phenomena in a two-phase flow caused by a quick valve closure in the cooling system of a nuclear reactor have attracted special interest for the safety assessment of LOCA. In this report, analyses of the shock phenomena in a two-component two-phase bubbly flow by a two-velocity model, a homogeneous model, and a pressure wave reflection model are presented. Based on the comparisons of these analytical results with experimental results, the modelling and calculation techniques for various cases of void fraction distribution along the tube are discussed.

Horizontal Liquid Film-mist Two-phase Flow : 1st Report, Concentration Distribution and Diffusivity of Entrained Liquid Droplets

The entrainment flow rate distribution, the gas velocity profile, and the concentration profile of droplets across the channel cross section in fully developed region of a horizontal rectangular channel of 150mm width and 50mm height were measured. The concentration profile of droplets was expressed by a simple equation based on a constant diffusion coefficient model. From this equation the effects of gravity and turbulent diffusion of droplets on the concentration profile were evaluated. The characteristic mean setting velocity of a group of droplets with various diameters was derived, and using this value the mean diffusion coefficient of the group of droplets was obtained.