Jun Ishimoto

Boiling two-phase flows of magnetic fluid in a non-uniform magnetic field

The effects of a magnetic field on the characteristics of boiling two-phase pipe flow of a temperature-sensitive magnetic fluid are clarified in detail both theoretically and experimentally. Firstly, the governing equations of boiling two-phase flow of a magnetic fluid are presented and numerically solved. Secondly, the aspect of boiling two-phase flow is visualized with ultrasonic wave echo in the region of non-uniform magnetic field and various distributions of two-phase flow characteristics are obtained. Finally, the effect of a magnetic field on the stability of boiling two-phase pipe flow of a magnetic fluid under a non-uniform magnetic field is investigated both theoretically and experimentally. These fundamental studies show that the precise control and stabilization of boiling two-phase flow of a magnetic fluid are possible by using the magnetic force of the fluid effectively.

Computational Prediction of the Effect of Microcavitation on an Atomization Mechanism in a Gasoline Injector Nozzle

The effect of microcavitation on the 3D structure of the liquid atomization process in a gasoline injector nozzle was numerically investigated and visualized by a new integrated computational fluid dynamics (CFD) technique for application in the automobile industry. The present CFD analysis focused on the primary breakup phenomenon of liquid atomization which is closely related to microcavitation, the consecutive formation of liquid film, and the generation of droplets by a lateral flow in the outlet section of the nozzle. Governing equations for a high-speed lateral atomizing injector nozzle flow taking into account the microcavitation generation based on the barotropic large eddy simulation-volume of fluid model in conjunction with the continuum surface force model were developed, and then an integrated parallel computation was performed to clarify the detailed atomization process coincident with the microcavitation of a high-speed nozzle flow. Furthermore, data on such factors as the volume fraction of microcavities, atomization length, liquid core shapes, droplet-size distribution, spray angle, and droplet velocity profiles, which are difficult to confirm by experiment, were acquired. According to the present analysis, the atomization rate and the droplets-gas atomizing flow characteristics were found to be controlled by the generation of microcavitation coincident with the primary breakup caused by the turbulence perturbation upstream of the injector nozzle, hydrodynamic instabilities at the gas-liquid interface, and shear stresses between the liquid core and periphery of the jet. Furthermore, it was found that the energy of vorticity close to the gas-liquid interface was converted to energy for microcavity generation or droplet atomization.

Numerical simulation of cavitating flow of liquid helium in venturi channel

Abstract The fundamental characteristics of the two-dimensional cavitating flow of liquid helium through a venturi channel near the lambda point are numerically investigated to realize the further development and high performance of new multi-phase superfluid cooling systems. First, the governing equations of the cavitating flow of liquid helium based on the unsteady thermal nonequilibrium multi-fluid model with generalized curvilinear coordinates system are presented, and several flow characteristics are numerically calculated, taking into account the effect of superfluidity. Based on the numerical results, the two-dimensional structure of the cavitating flow of liquid helium though venturi channel is shown in detail, and it is also found that the generation of superfluid counterflow against normal fluid flow based on the thermomechanical effect is conspicuous in the large gas phase volume fraction region where the liquid-to-gas phase change actively occurs. Furthermore, it is clarified that the mechanism of the He I to He II phase transition caused by the temperature decrease is due to the deprivation of latent heat for vaporization from the liquid phase.

PRESSURE DROP REDUCTION OF SLUSH NITROGEN IN TURBULENT PIPE FLOWS

Slush fluid such as slush hydrogen and slush nitrogen is a two-phase (solid-liquid) single-component cryogenic fluid containing solid particles in liquid, and consequently its density and refrigerant capacity are greater than for liquid state fluid. Experimental tests were performed with slush nitrogen to obtain the frictional pressure drop flowing in a 15 mm internal diameter, 400 mm long, horizontal, stainless steel pipe. The primary objective of this study was to investigate the pressure drop reduction phenomenon with changes in velocity and solid fraction. From the experimental results, the pressure drop correlation between the friction factor and the Reynolds number was obtained and an empirical correlation was derived. Flow patterns for slush nitrogen inside a pipe and the behavior of solid particles were also observed using a high speed camera.

Numerical study of cavitating flow characteristics of liquid helium in a pipe

The fundamental characteristics of the two-dimensional cavitating flow of liquid helium in a vertical pipe near the lambda point are numerically investigated to realize the further development and high performance of new cryogenic superfluid cooling systems. It is found that the phase transition of the normal fluid to the superfluid and the generation of superfluid counterflow against normal fluid flow based on the thermomechanical effect is conspicuous in the large gas phase volume fraction region where the liquid to gas phase change with cavitation actively occurs. Furthermore, it is clarified that the mechanism of the He I to He II phase transition caused by the temperature decrease is due to the deprivation of latent heat for vaporization from the liquid phase.

Numerical Analysis of Cavitating Flow of Liquid Helium in a Converging-Diverging Nozzle

The fundamental characteristics of the two-dimensional cavitating flow of liquid helium through a horizontal converging-diverging nozzle near the lambda point are numerically investigated to realize the further development and high performance of new multiphase superfluid cooling systems. First, the governing equations of the cavitating flow of liquid helium based on the unsteady thermal nonequilibrium multifluid model with generalized curvilinear coordinates system are presented, and several flow characteristics are numerically calculated, taking into account the effect of superfluidity. Based on the numerical results, the two-dimensional structure of the cavitating flow of liquid helium though a horizontal converging-diverging nozzle is shown in detail, and it is also found that the generation of superfluid counterflow against normal fluid flow based on the thermomechanical effect is conspicuous in the large gas phase volume fraction region where the liquid to gas phase change actively occurs. Furthermore, it is clarified that the mechanism of the He I to He II phase transition caused by the temperature decrease is due to the deprivation of latent heat for vaporization from the liquid phase

Numerical Analysis of Two-Phase Pipe Flow of Liquid Helium Using Multi-Fluid Model

The two-dimensional characteristics of the vapor-liquid two-phase flow of liquid helium in a pipe are numerically investigated to realize the further development and high performance of new cryogenic engineering applications. First, the governing equations of the two-phase flow of liquid helium based on the unsteady thermal nonequilibrium multi-fluid model are presented and several flow characteristics are numerically calculated, taking into account the effect of superfluidity. Based on the numerical results, the two-dimensional structure of the two-phase flow of liquid helium is shown in detail, and it is also found that the phase transition of the normal fluid to the superfluid and the generation of superfluid counterflow against normal fluid flow are conspicuous in the large gas phase volume fraction region where the liquid to gas phase change actively occurs. Furthermore, it is clarified that the mechanism of the He I to He II phase transition caused by the temperature decrease is due to the deprivation of latent heat for vaporization from the liquid phase. According to these theoretical results, the fundamental characteristics of the cryogenic two-phase flow are predicted. The numerical results obtained should contribute to the realization of advanced cryogenic industrial applications.

Wave propagation in two-phase flow of magnetic fluid under a nonuniform magnetic field

Abstract The effect of nonuniform magnetic field on the linear and nonlinear wave propagation phenomena in two-phase pipe flow of magnetic fluid is investigated theoretically to realize the effective energy conversion system using boiling two-phase flow of magnetic fluid. Firstly, the governing equations of two-phase flow based on the unsteady thermal nonequilibrium two-fluid model are presented and the linear void wave propagation phenomena in boiling two-phase flow are numerically analyzed by using the finite volume method. Next, the nonlinear pressure wave propagation in gas-liquid two-phase flow is numerically analyzed by using the finite different method. According to these theoretical studies on the wave propagation phenomena in two-phase flow of magnetic fluid, it seems to be a reasonable proposal that the precise control of the wave propagation in two-phase flow is possible by effective use of the magnetic force.

Computational study of the dynamics of two interacting bubbles in a megasonic field

Clarification of the mechanism of particle removal by megasonic cleaning and control of cavitation bubbles in the megasonic field are essential for cleaning of nanodevices without pattern damage. Multiple bubble interactions complicate the mechanism of particle removal. Therefore, it is important to understand multiple bubble dynamics to clarify the mechanism of particle removal by megasonic cleaning. In the present study, the dynamics of two bubbles in a megasonic field with several initial radii and initial separation distances were simulated by numerical analysis using a compressible locally homogeneous model of a gas-liquid two-phase medium. The present numerical method simulated the various complex behaviors of two bubbles, which are repulsive motion, coalescence, periodic and stable motion of the separation distance, and bubble breakup. The initial separation distance strongly affected the behavior of the two bubbles because the effect of the secondary pressure induced by the oscillation of one bubble on the other bubble depends on the separation distance. In particular, when the equilibrium radii are larger than the resonant radius and the radius of one or both bubbles is close to the resonant radius, the bubbles can show characteristic behaviors, such as periodic and stable motion of the separation distance.

Basic Study on Two‐Phase Flow Characteristics of Slush Nitrogen in a Pipe

The fundamental two‐phase flow characteristics of slush nitrogen in a pipe are numerically and experimentally investigated to develop effective cooling performance for long distance superconducting cables. First, the governing equations of the two‐phase slush nitrogen flow based on the unsteady thermal nonequilibrium two‐fluid model are presented and several flow characteristics are numerically calculated taking into account the effects of slush volume fraction, particle diameter, and duct shape. Furthermore, in experimental study, minute slush is generated by the slush atomization nozzle and the unsteady pressure and temperature distributions of two‐phase slush nitrogen flow along the longitudinal direction of pipe are measured. According to the numerical and experimental results, it is found that the reduction of the pressure loss by using slush is possible in the case of long distance pipe lengths. Also, optimized thermal flow conditions for cryogenic two‐phase slush nitrogen with practical use of latent heat of slush melting are predicted for the development of new type superconducting cooling systems.