Yoshifusa Sato

Two-phase flow in vertical noncircular channels

Abstract Experiments have been performed for vertical two-phase flow of air-water mixtures through several noncircular channels. Frictional pressure drops are discussed in terms of the correlation method for single-phase turbulent flow developed first in this study. Results for rising velocity of large gas bubble and mean void fraction are also discussed. Flow pattern boundaries are presented and compared with each other as to channel geometry.

Performance of an air-lift pump for conveying coarse particles

Experiments were made with either uniform spherical particles or non-uniform spherical particles using air and water as the working fluids. In the experiments, the diameter of the upriser, the diameters of the particles and the submergence ratio were changed systematically. The obtained data showed that the operation performance of an air-lift pump can be clearly characterized by the triangular relationship between the flow rates of the water and particles discharged and the flow rate of the air supplied. A model which describes the flows in the upriser is proposed based on a momentum equation. The experimental and predicted characteristics of the flow are shown to be in good agreement.

Prediction of the single-phase turbulent mixing rate between two parallel subchannels using a subchannel geometry factor

This paper presents a simple method for predicting the single-phase turbulent mixing rate between adjacent subchannels in nuclear fuel bundles. In this method, the mixing rate is computed as the sum of the two components of turbulent diffusion and convective transfer. Of these, the turbulent diffusion component is calculated using a newly defined subchannel geometry factor F* and the mean turbulent diffusivity for each subchannel which is computed from Elders equation. The convective transfer component is evaluated from a mixing Stanton number correlation obtained empirically in this study. In order to confirm the validity of the proposed method, experimental data on turbulent mixing rate were obtained using a tracer technique under adiabatic conditions with three test channels, each consisting of two subchannels. The range of Reynolds number covered was 5000–66 000. From comparisons of the predicted turbulent mixing rates with the experimental data of other investigators as well as the authors, it has been confirmed that the proposed method can predict the data in a range of gap clearance to rod diameter ratio of 0.02–0.4 within about ±25% for square array bundles and about ±35% for triangular array bundles.

Flow redistribution due to void drift in two-phase flow in a multiple channel consisting of two subchannels

Abstract Void drift in two-phase flow is studied experimentally using a geometrically simple, vertical channel consisting of two interconnected subchannels. Data on the flow redistributions of both air and water along the channel axis are obtained and presented for the following two multiple channels: one with two circular subchannels of different cross-sectional area and the other with two identical circular subchannels. The data are analysed by a simple one-dimensional subchannel code taking account of the effects of void drift and turbulent mixing between subchannels, i.e. incorporating both the void-settling model of Lahey et al. and a term similar to that in the COBRA code in the momentum equation. The flow redistribution process can be explained by the analysis.

Prediction of turbulent mixing rates of both gas and liquid phases between adjacent subchannels in a two-phase slug-churn flow

Abstract This paper presents a slug-churn flow model for predicting turbulent mixing rates of both gas and liquid phases between adjacent subchannels in a BWR fuel rod bundle. In the model, the mixing rate of the liquid phase is calculated as the sum of the three components, i.e. turbulent diffusion, convective transfer and pressure difference fluctuations between the subchannels. The components of turbulent diffusion and convective transfer are calculated from Sadatomi et al.s [Nucl. Eng. Des. 162 (1996) 245–256] method, applicable to single-phase turbulent mixing, by considering the effect of the increment of liquid velocity due to the presence of gas phase. The component of the pressure difference fluctuations is evaluated from a newly developed correlation. The mixing rate of the gas phase, on the other side, is calculated from a simple relation of mixing rate between gas and liquid phases. The validity of the proposed model has been confirmed with the turbulent mixing rates data of Rudzinski et al. [Can. J. Chem. Eng. 50 (1972) 297–299] as well as the present authors.

The turbulent mixing rate and the fluctuations of static pressure difference between adjacent subchannels in a two-phase subchannel flow

Turbulent mixing rate between adjacent subchannels in a two-phase flow has been known to be strongly dependent on the flow pattern. In this study, flow visualization was made to investigate the mechanism of the turbulent mixing between subchannels in a two-phase flow under hydrodynamic equilibrium conditions. The test channel was a vertical multiple channel consisting of two identical rectangular subchannels, and the working fluids were air and water. It was observed in slug-churn flows that a large scale inter-subchannel liquid flow occurs in front of the nose of a large gas bubble and behind the tail when the bubble axially passes through the subchannel, and thus a high turbulent mixing rate of the liquid phase results. In order to know driving force of such a large scale inter-subchannel flow, measurement of instantaneous static pressure difference between the subchannels was also made. The result showed that there is a close relationship between the liquid phase turbulent mixing rate and the magnitude of the pressure difference fluctuations.

A PROPOSAL FOR TREATMENT OF TURBULENT MIXING IN A TWO-PHASE SUBCHANNEL FLOW

Abstract We propose a practical method for the treatment of turbulent mixing rate in a two-phase subchannel flow in a hydrodynamic non-equilibrium state. Based on the assumption that the fundamental modes of the inter-subchannel fluid transfer in such a state are turbulent mixing, void drift, and diversion cross flow, the turbulent mixing rate is considered to be equal to that in the hydrodynamic equilibrium state that the flow will attain. The applicability of the method is examined by experiments concerning the axial variation in tracer concentration in a non-equilibrium flow without diversion cross flow. A good agreement is seen between the calculations and the measurements.

An experimental method for measurement of void fraction distribution in a quasi-2D two-phase flow using image processing

Abstract To understand the multidimensional behavior of a two-phase gas-liquid flow, an experimental method for a two-dimensional (2D) flow is proposed. A test channel was constructed in which a quasi-2D, uniform two-phase flow occurs. The channel is a vertical, narrow, concentric annular space made up of two large-diameter pipes. It is found that using an image processing technique with this channel enables us to visualize a 2D two-phase air-water flow and to measure the void fraction distribution. The proposed experimental method is described, and several examples of void fraction distribution around a test body are presented.

Measurement of turbulent diffusivity of both gas and liquid phases in quasi-2D two-phase flow

Abstract The turbulent diffusion process has been studied experimentally by observing a tracer plume emitted continuously from a line source in a uniform, quasi-2D two-phase flow. The test section was a vertical, relatively narrow, concentric annular channel consisting of two large pipes. Air and water were used as the working fluids, and methane and acid orange II were used as tracers for the respective phases. Measurements of local, time-averaged tracer concentrations were made by means of a sampling method and image processing for bubbly flows and churn flows, and the turbulent diffusivity, the coefficient of turbulent diffusion, was determined from the concentration distributions measured. The diffusivities for the gas and liquid phases, ϵ DG and ϵ DL respectively, are presented and compared with each other in this paper. When a flow is bubbly, ϵ DG is close to or slightly smaller than ϵ DL . In a churn flow, on the contrary, ϵ DG is much greater than ϵ DL . Regarding bubbly flow, a plausible model on turbulent diffusivity of the liquid phase is presented and examined by the present data.

Treatment of Turbulent Mixing between Subchannels in a Hydrodynamic Non-Equilibrium Two-Phase Flow.

Based on the assumption that the fundamental modes of the fluid transfer between subchannels are turbulent mixing, void drift and diversion cross flow, we have proposed and examined a treatment for the turbulent mixing in a hydrodynamic non-equilibrium two-phase flow. In this treatment, the turbulent mixing rate in such a flow is assumed to be equal to the rate in a hydrodynamic equilibrium state that the flow will attain. In order to obtain data needed for the examination, experiments on the axial variations in tracer concentrations for both phases were made in several non-equilibrium flows. An analysis of the data showed that the assumption and treatment are valid.