The progress of the two-phase flow research can be divided into four stages. These periods are 1948-1958, 1960-1970, 1971-1979, and 1980-1990. In this 7th Report, the development of the theoretical analyses on flow oscillations and flow instabilities in natural circulation loops in boilers and nuclear reactors at the initial stage of the second period has been described. The various analytical methods, that is, the lumped-parameter model, the distributed-parameter model, the phase plane analysis, the density wave model, etc. have been introduced.

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The present study is concerned with particle motion in the linear shear flows with constant velocity gradient in a duct. Effects of the velocity gradients at a high Reynolds number on lift acted on a spherical particle are investigated by photographing trajectories of particles suspending in the shear flows using a high speed video and numerical simulation of particle distribution in the duct using the method of Direct Simulation Monte Carlo. The Reynolds number (relative air velocity×particle diameter/kinematic viscosity of air) is ranging from 950 to 2700. The characteristics of the duct flows are discussed after measuring the velocity, and turbulence intensity, or auto-correlation and cross-correlation using hot-wire anemometer and FFT digital spectrum analyzer in order to make clear that the test duct is suitable to the experiments. In the numerical simulation by DSMC the particle-particle and particle-duct wall collisions are taken into account. From the results of numerical simulation, it is shown that particles move from the higher velocity side to the lower velocity side of the shear flows, and this result agrees with the high speed video picture of particle trajectories in the near part downstream of the particle supply part. But, in the far part, particles are scattered randomly due to the particle collisions. In conclusion, It is made clear that lift is applied on a sphere from the higher velocity side to the lower velocity side on the linear shear flows at high Reynolds number.The present study is concerned with particle motion in the linear shear flows with constant velocity gradient in a duct. Effects of the velocity gradients at a high Reynolds number on lift acted on a spherical particle are investigated by photographing trajectories of particles suspending in the shear flows using a high speed video and numerical simulation of particle distribution in the duct using the method of Direct Simulation Monte Carlo. The Reynolds number (relative air velocity×particle diameter/kinematic viscosity of air) is ranging from 950 to 2700. The characteristics of the duct flows are discussed after measuring the velocity, and turbulence intensity, or auto-correlation and cross-correlation using hot-wire anemometer and FFT digital spectrum analyzer in order to make clear that the test duct is suitable to the experiments. In the numerical simulation by DSMC the particle-particle and particle-duct wall collisions are taken into account. From the results of numerical simulation, it is shown that particles move from the higher velocity side to the lower velocity side of the shear flows, and this result agrees with the high speed video picture of particle trajectories in the near part downstream of the particle supply part. But, in the far part, particles are scattered randomly due to the particle collisions. In conclusion, It is made clear that lift is applied on a sphere from the higher velocity side to the lower velocity side on the linear shear flows at high Reynolds number.

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To provide data necessary for modeling turbulent mixing between subchannels in a nuclear fuel rod bundle, three experiments were made in series for equilibrium two-phase flows, in which net mass exchange does not occur between subchannels for each phase. The first one was the measurement of turbulent mixing rates of both gas and liquid phases by a tracer technique, using air and water as the working fluids. Three kinds of vertical test channels consisting of two subchannels were used. The data have shown that the turbulent mixing rate of each phase in a two-phase flow is strongly dependent on flow regime. So, to see the relation between turbulent mixing and two-phase flow configuration in the subchannels, the second experiment, flow visualization, was made. It was observed in slug and churn flows that a lateral inter-subchannel liquid flow of a large scale is caused by the successive axial transit of large gas bubbles in each subchannel, and the turbulent mixing for the liquid phase is dominated by this lateral flow. To investigate a driving force of such large scale lateral flow, the third experiment, the measurement of an instantaneous pressure differential between the subchannels, was made. The result showed that there is a close relationship between the liquid phase mixing rate and the magnitude of the pressure differential fluctuation.

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[in Japanese]

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