混相流 Vol. 31(2017) No. 2

So far, needs for nuclear safety have much encouraged multiphase flow research, which, in return, has contributed to enhance nuclear safety. In view of this, the author has been conducting his research on gas-liquid two-phase flow related to nuclear reactor safety. This paper reviews such research history and gives some future prospects.

Gas-liquid two-phase flows exist in a wide range of applications and enabling technologies. If, however, “understanding” we mean that the phenomenon in two-phase flow system can be predicted in terms of governing parameter, it should be concluded that two-phase flow is poorly understood item, and many questions are still open. A thorough theoretical approach with such microscopic experimental results as three-dimensional and high spatiotemporal measurements is needed to assess the performance of gas-liquid two-phase flow. My study of gas-liquid two-phase flows at microgravity conditions and in small diameter pipe is outlined in this report, especially for which regarding to interfacial area transport and drift flux. Research on two-phase at micro- or reduced-gravity conditions and in small pipe is expected to be conducted more widely in the future, shedding light on two-phase flow phenomena in piping system.

Several types of air pollution control technologies about waste combustion system were developed on the last quarter century. If those new technologies have come into wide use, we ought to explain several new technology convincingly to customers or users. Therefore I will take a few examples of explanations about those new technologies.

A progress of slurry transport is described in this report. In around 1980, slurry transport was extensively studied. Papers on pressure losses of slurry flows, transition velocity, measurement of in-situ concentration, snow/ice-water slurry flows were published by several researchers. Recently, the study on slurry transport is not so active, but solid-liquid flows can be seen in many fields. It is anticipated that studies on solid-liquid flows in the fields of disaster site restoration and next-generation resources and energy development will be progressed.

The turbulence modulation, that means influences of dispersed particles to the fluid turbulence, is one of the challenging subjects in the multiphase flow research. In recent decades, experimental and numerical researches have been conducted extensively. Especially the direct numerical simulation (DNS) became a powerful means to reveal the multiphase turbulence phenomena. But DNS cannot become a practical tool for multiphase flows in industry or in nature. In this review article, an example of particle-laden flow by the immersed solid method and our recent progress for the application to the two-phase heat transfer are shown. Then, considering the current status, it is pointed out that some moderately-averaged equation, which includes momentum exchange and residual stress terms, is essential for semi-DNS including finite-sized particles (e.g., Kolmogorov scale particle) to deal with large-scale multiphase flow fields.

Although numerous studies for fish swimming have been reported, there are many problems or mysteries that remain unsolved, for example, high propulsive efficiency of actual living fishes compared with fish robots, and the aim or advantages of fish jumping through water surface. In order to discuss influence of fish shape and swimming depth on its swimming velocity near the water surface, flow around self-propelled fish models with various shapes is analyzed numerically by using the level set method and the modified body-force type immersed boundary method. The swimming velocity of the fish was greatly changed by a ratio of the projected area in the propulsive direction to the projected area in the moving direction of the tail fin. Among various shape models that the ratio is constant, the smaller the projected area in the propulsive direction became, the higher the terminal swimming velocity became. This result did not depend on the swimming depth.

To evaluate effects of the diameter on countercurrent flow limitation (CCFL) in vertical pipes, in our previous study, we classified CCFL into CCFL-L at the sharp-edged lower end, CCFL-U at the sharp-edged upper end, and CCFL-P inside the vertical pipe with round-edged upper and lower ends, and we found that the characteristic length in the Wallis parameter, w = D^(1-β)L^β (where D and L respectively denote the diameter and the Laplace capillary length), is respectively β = 0, 1 and β ≈ 0.5 for CCFL-L, CCFL-U and CCFL-P. In this study, we evaluated effects of fluid properties on CCFL in vertical pipes by using existing CCFL data. The most effective fluid properties is the liquid viscosity and we selected the viscosity ratio of gas and liquid (μ_G/μ_L) as a dimensionless parameter. From effects of μ_G/μ_L on the slope m and constant C in the Wallis correlation, we classified three regions of large positive interrelations of m and C with μ_G/μ_L for small μ_L (room-temperature to high-temperature water), small interrelations of m and C with μ_G/μ_L for medium μ_L (low-concentration glycerol water solution), and the large negative interrelation of m with μ_G/μ_L for large μ_L (high-concentration glycerol water solution). We obtained exponent functions of μ_G/μ_L for m and C in the case of the small μ_L (room-temperature to high-temperature water) for CCFL-L and CCFL-U except CCFL-P without high-temperature CCFL data.

Seawater was injected into reactor cores in the accident at the Fukushima Daiichi Nuclear Power Station. This study intended to provide base data to consider reactor core cooling by seawater. Pool nucleate boiling heat transfer experiments and vertical upward forced-convective boiling heat transfer experiments were conducted by using water and artificial seawater. In the pool nucleate boiling experiments of seawater, slow surface temperature excursion was initiated at some heat flux that was considerably lower than the critical heat flux although the surface heating rate was kept almost constant. The initiation of the surface temperature excursion coincided with the initiation of the deposition of calcium sulfate on the heat transfer surface. The temperature excursion was caused by the heat conduction resistance increase with an increase in deposition layer thickness. It was suggested that the deposition of calcium sulfate on the heat transfer surface starts when the seawater concentration at the vicinity of the heat transfer surface becomes lower than 11 wt%. In the forced-convective boiling heat transfer, even if the seawater concentration at the inlet was low, the sea salt concentration was enriched because of evaporation as flow proceeded, which resulted in the initiation of the surface temperature excursion.

A theoretical and experimental study was conducted to investigate the influence of side wall on the dryout of liquid film flowing on an inclined plate. The film surface forms a meniscus having a minimum thickness near the side wall. The thin film causes the decline of dynamic pressure of flow and the dryout may happen at higher flow rate. In this study, the meniscus profile near the side wall was theoretically analyzed from the energy minimum principle under the condition of force balance between gravitational and viscous forces. Here we considered the system energy contributed from the work related to wetting behavior, the work resulted from the increase of gas-liquid interfacial area, the kinetic energy of film flow, as well as the liquid potential energy. The calculated results of meniscus radius and minimum film thickness roughly agree with the results of experimental measurement and CFD.