Zensaku Kawara

Two-phase flow in microchannels

Abstract Gas–liquid two-phase flow patterns are visualized with a microscope for air–water flow in circular tubes of 20, 25 and 100 μm i.d. and for steam–water flow in a 50 μm i.d. circular tube. The superficial velocities cover a broad range of J L =0.003–17.52 m/s and J G =0.0012–295.3 m/s for air–water flows. Several distinctive flow patterns, namely, dispersed bubbly flow, gas slug flow, liquid ring flow, liquid lump flow, annular flow, frothy or wispy annular flow, rivulet flow, liquid droplets flow and a special type of flow pattern are identified both in air–water and steam–water systems, and their special features are described. It has been confirmed that two-phase flow patterns are sensitive to the surface conditions of the inner wall of the test tube. It has been evidenced that a stable annular flow and gas slug formation with partially stable thin liquid film formed between the tube wall and gas slugs appeared at high velocities under carefully treated clean surface conditions. At lower velocities, dry and wet areas exist between gas slug and the tube wall. The cross-sectional average void fraction was also calculated from photographs, showing a good agreement with the Armand correlation for air–water flow in lager tubes.

Turbulent heat transport in a horizontal fluid layer heated internally and from below

Abstract Measurements were taken of the fluid velocity and temperature fluctuations with turbulent thermal convection in a horizontal layer of water with uniform volumetric energy sources and a constant rate of bottom heating. The experimental data were compared with a simple mixing-length analysis and a correlation equation was derived for the turbulent velocity at the midplane of the fluid layer. Turbulent heat flux, which was obtained from the measured values of velocity and temperature fluctuations, was lower than the total heat flux predicted from both internal and external heating rates.

Consideration of Heat Transfer Enhancement Mechanism of Nano- and Micro-Scale Porous Layer via Flow Visualization

A convective heat transfer enhancement using nano- and micro-scale porous layer surface was discovered by the authors in 2004. Heat transfer experiments, analytical considerations, and flow visualization near the porous layer were performed to grasp the heat transfer enhancement mechanism. The heat transfer experiments revealed the porous layers were able to enhance heat transfer by 20–25% in net energy compared to the bare plate, independent of substrate materials. In order to understand the mechanism, one-dimensional unsteady heat conduction analysis was performed for a liquid column in the pore. It was found that the temperature recovery of the porous layer was incapable of catching up with the very fast fluctuation, so that the porous layer might be a thermal resistance when the main flow was strongly turbulent. The vestige visualized by the tracer particles of around 0.85 μm in diameter showed a fluid behavior like “squirt” from the porous layer. From the observation of the porous layer surface, the porous layer has some micro-scale bubbles inside its own pore-connecting structure in spite of the good wetting feature. These bubbles could be a main contributor to this heat transfer enhancement. To discuss this postulation, observations of bubble behavior in a microchannel have been carried out.

Study on Thermal Mixing of MHD Liquid Metal Free-Surface Film Flow

Abstract In this study, the mixing of temperature-stratified liquid metal free-surface flow by a delta-wing obstacle installed on the channel bottom has been experimentally and numerically investigated in the presence of a transverse magnetic field. The surface temperature distribution of the channel was measured by using 25 thermocouples (TCs) embedded in the channel bottom, downstream of the obstacle, which was located upstream of the heater installed at the free-surface. The experiments were conducted for the turbulent flow region where Re = 12,000 and in the range of N = 0–5.02 in the presence of the transverse magnetic field. As for the laminar flow region, it is difficult to carry out the experiment, so the numerical simulations were conducted using Re = 2,300 and in the range of N = 0–10. According to the comparison of numerical results with and without the delta-wing obstacle in laminar flow region, the entire temperature distribution with the obstacle was warmer than that without the obstacle. This was consistent with the expectation that a delta-wing obstacle would increase thermal mixing.

Study on Flow Characteristics of Micro-Bubble Two-Phase Flow

Experimental study was carried out on micro-bubble containing bubbly two-phase flow in a circular pipe. Flow resistance in a pipe reduces relative to single-phase flow, especially in the transient region from laminar to turbulent flow. Local velocity profile shows increase of liquid velocity near the wall, and overall velocity profile is planarized with it. Visual observation with high-speed imaging system shows that comparatively large bubbles with slip velocity tend to be observed more frequently near the wall, and this suggests the existence of the interaction between coherent turbulent structure and micro-bubbles.

Application of Very High Speed Camera in Measurement of Liquid Film Flow on Nuclear Rod Bundle in Micro-Scale

The annular liquid film flow on nuclear rod bundle is the complicated phenomena with very fine scales on time and space, so existing measuring techniques have many difficulties to investigate this phenomenon. In this work, to avoid these problems, a very high speed camera with long-ranged microscope optical system has been introduced to measure the flow on the simulated 3 × 3 rod bundle made of transparent acrylic resin. The results include the real-time visualization of important processes of entrainment and deposition with high spatiotemporal resolution and the relationship between velocity and diameter of each droplet. These initial data have approved the advantage of high speed camera technique in the studying of annular two phase flow.

Development of Surface-Volume Tracking Method Based on MARS

The MARS (Multi-interfaces Advection and Reconstruction Solver) [1] is one of the direct numerical methods for multiphase flow solvers with a volume tracking procedure for free surface or interface deformation. The main feature of the MARS is 1) a precise conservation of volume of fluid (VOF), 2) a surface-volume tracking procedure with a precise linear interface calculation and 3) a representation of the interface/surface within one or two control volumes. Since this method has been developed on a staggered structuregrid system, it is difficult to perform the computation with high accuracy in a complicated computational domain. Therefore, it is necessary to develop a new version of the MARS on the unstructured grid system. In this study, a new version of the MARS based on collocated structure-grid system has been developed. In order to validate this procedure, the well-known as the “Dam breaking problem” was chosen and unmerically solved. On the other hand, the experiments of this problem with the same configuration of the solution domain as the numerical simulation were conducted, and the numerical and experimental results were compared with each other.

Enhancement of MSF Using Microbubbles

Abstract Multi Stage Flash (MSF) distillation plants are widely used in saline water desalination. In order to enhance MSF, it is important to increase an evaporation rate in the flashing stage. A spray flash method, in which superheated water jets are injected through nozzles into a depressurized environment to increase the gas/liquid interface area, is a promising technique to make the increase of evaporation rate, which leads directly to the reduction of energy consumption and cost of the MSF plant. In this paper, the introduction of microbubbles into the spray jet as the nucleation sites to increase the evaporation rate of the spray flash is proposed. The spray flash behaviours with/without microbubbles at outside/inside of the nozzle-inside were observed by means of a high speed camera to investigate the mechanism of enhancement of spray flash due to microbubbles. Moreover, the number densities of droplets and bubble volume increase were obtained from visualized images in order to discuss quantitatively on the effects of introduction of microbubbles.

Fundamental Study of Wave Propagation on Liquid Surface Related IFE Mirror

A laser fusion reactor needs an optical mirror in its final optical system. This optical mirror is exposed to the neutron produced with fusion reaction. It is pointed out that the exposure to neutron will produce hydrogen gas in the mirror and cause swelling deformation of mirror. To avoid this swelling of mirror, a liquid-metal mirror is promising. High energy laser shots on the liquid mirror will cause the surface wave. These waves must be damped to under 1/10 of the laser wavelength in 250 ms or less. In this study, the hydrodynamic behavior of the liquid surface was investigated by experiment with water as surrogate liquid, the computational evaluation for the wave propagation with the MARS (Multi-interface Advection and Reconstruction Solver, 2001) was carried out, and the design window for the optical mirror based on the water experiment was discussed.

Development of Microbubble Generation Method

In microbubble drag-reducing technology, the energy consumption in microbubble generation is often too large to achieve a high efficient energy conversion. The purpose of this research is to improve the efficiency of the microbubble generation method in practical applications. In this paper, microbubbles are generated by a special designed nozzle, whose principle is a converging flow. In water, it is possible to generate bubbles, whose diameter ranges from 1.5 to 4 mm, by using this nozzle. If a very little amount of Cetyltrimethylammonium Chloride (CTAC) surfactant is added to the water at T = 20°C, the average diameter of bubbles of 46.7–48.6μm can be generated because of the reduction of the surface tension of the bubbles. It was found that the amount of air injected showed weak influence on the generated bubble size. It was also found that the counter-ion sodium salicylate (NaSal) could assist to enlarge the bubble and reduced the bubble dispersion due to the change of fluid property. After adding NaSal to the CTAC solution, the average bubble diameter generated was enlarged to 122 μm, however, this bubble size is still effective to reduce the turbulence drag in practical application.