Hiroyuki Hirano

APPLICATION OF A PRECONDITIONING ITERATIVE METHOD TO THE COMPUTATION OF FLUID FLOW

In this paper, a new preconditioning iterative method is proposed. It consists of the Gauss–Seidel method with a preconditioning matrix which approximates the inverse matrix of A by a finite Neumann-type expansion. The convergence theorem of the method is given. The validity of the proposed method is illustrated by two numerical examples. We show that the proposed method can be applied to the computation of two-dimensional natural convection problem, and that it is more effective than other standard iterative methods.

The behavior of microscaled brownian particles in a cylinder under natural- and magnetic-convection flow field of air

ABSTRACT The behavior of carbon particles (mostly 1μm in diameter) in a natural-convection flow field of air and a magnetic field was studied numerically. One thousand particles were released randomly in a vertical cylinder whose height is equal to its radius. The bottom of the enclosure was heated and the side wall was cooled. A coil with electric current was set coaxially with the cylindrical enclosure. For the nano- and micro- order particles, Brownian motion was taken into consideration and the Langevin equation was solved for the particles. The results show that the behavior of 1-μm-sized particles depends mainly on the flow field. The magnetic field changes the flow mode of natural convection of air due to the paramagnetic susceptibility of oxygen and affects the behavior of the particles indirectly. The coil level, the aspect ratio of the cylinder, and the geometric arrangement of the hot and cold plates produce different flow fields, which makes the behavior of the particles different. The particles larger than 1μm circulate along the streak lines or cluster, but those smaller than 1μm are strongly driven by the Brownian motion.

VOF Modeling and Analysis of the Segmented Flow in Y-Shaped Microchannels for Microreactor Systems

Microscaled devices receive great attention in microreactor systems for producing high renewable energy due to higher surface-to-volume, higher transport rates (heat or/and mass transfer rates), and other advantages over conventional-size reactors. In this paper, the two-phase liquid-liquid flow in a microchannel with various Y-shaped junctions has been studied numerically. Two kinds of immiscible liquids were injected into a microchannel from the Y-shaped junctions to generate the segment flow mode. The segment length was studied. The volume of fluid (VOF) method was used to track the liquid-liquid interface and the piecewise-liner interface construction (PLIC) technique was adopted to get a sharp interface. The interfacial tension was simulated with continuum surface force (CSF) model and the wall adhesion boundary condition was taken into consideration. The simulated flow pattern presents consistence with our experimental one. The numerical results show that a segmented flow mode appears in the main channel. Under the same inlet velocities of two liquids, the segment lengths of the two liquids are the same and depend on the inclined angles of two lateral channels. The effect of inlet velocity is studied in a typical T-shaped microchannel. It is found that the ratio between the lengths of two liquids is almost equal to the ratio between their inlet velocities.

Water mist flow in a vertical bore of a superconducting magnet

The effect of magnetic field on water mist (diamagnetic) was investigated by both experimental and numerical methods. Water mist was produced by an ultrasonic atomizer and fed from the top of a cylinder that was located in a vertical bore of a superconducting magnet of 10 T or less. Water mist descends due to gravity at 0 T, but at 10 T it mostly levitates above the magnetic coil. In the numerical computation, water mist was simulated by 1000 water droplets 0.1−5μm in diameter d. Due to the small sizes of the water droplets, Brownian motion was considered and the Langevin equation was solved. At d<1μm, the particles are driven by the Brownian motion extensively and the magnetic and gravitational forces have almost no effect on them. At d⩾1μm, the trajectories of particles are greatly deformed by the magnetic field when they go through the magnetic coil. At γf=1000 (10.8 T for 9 cm cylinder diameter), the bulk of the water mist took an hourglass shape above the coil just as observed experimentally. At γf=2...

A PLIC-VOF-Based Simulation of Water-Organic Slug Flow Characteristics in a T-Shaped Microchannel

A water-organic slug flow in a T-shaped microchannel was numerically studied due to its importance in the microreactor system. Various factors affecting the flow mode were studied, for example, channel width, fluid viscosity, interfacial tension, and inlet velocity. The volume of fluid (VOF) method was used to track the liquid-liquid interface, and the piecewise-liner interface construction (PLIC) technique was adopted to get a sharp interface. The interfacial tension was simulated with continuum surface force (CSF), model and the wall adhesion boundary condition was taken into consideration. The results show that strong vortexes appear in both phases at the meeting sites of main and lateral channels where an organic slug is producing. Inlet velocity influences the slug length and flow mode greatly. The ratio between the slug lengths of two phases in the main channel is almost equal to the ratio between their inlet velocities. If the slug is produced, the interfacial tension and organic viscosity have less effect on the slug length for 200 μm microchannel. The slug producing rate is much higher in a narrow channel than that in a wide channel.

Numerical Experiment of Thermoset Particles in Surface Modification System with Discrete Element Method (Quantization of Cohesive Force Between Particles by Agglomerates Analysis)

Numerical simulations using the discrete element method with similar particle assembly model are performed for the surface modification process of thermoset particles. In the simulations, the cohesive force between particles and the effect of thermophoresis are examined numerically in a particle dispersion system. Simulations are also carried out for different gas dispersion velocities. By varying the dimensionless parameter associated with the cohesive force, average agglomerate diameter increases as the cohesive force between particles increases. To the end, it is demonstrated that the cohesive force between particles can be quantitatively determined if the average agglomerate diameter is experimentally or numerically obtained.

An attempt at numerical calculation of natural convection using preconditioned iterative methods

Preconditioning can change the degree of diagonal dominance and the eigenvalue of a coefficient matrix in the solution of a linear system of equations. Accordingly, an appropriate preconditioner can reduce the number of iterations, and further this may also reduce the number of arithmetic operations compared with those by the classical iterative methods without preconditioning. Consider the following preconditioned linear system of equations:

Epitaxial Growth of ZnTe Layers on ZnO Bulk Substrates by Metalorganic Vapor Phase Epitaxy

The crystallinity and surface roughness of ZnTe epilayers grown on (0001) ZnO bulk substrates by metal organic vapor phase epitaxy are investigated. X-ray diffraction, Raman spectra, photoluminescence, and atomic force microscopy analysis results prove that the crystallinity and surface roughness of ZnTe epilayers depend on epitaxial growth temperature. A high-crystal-quality (111) ZnTe heteroepitaxial layer, with near-band-edge emission at 549 nm, was obtained at a substrate temperature of 460 °C.

WATER MIST FLOW IN A SUPERCONDUCTING MAGNET INCLINED AT VARIOUS ANGLES

Water mist (diamagnetic) flow in a superconducting magnet of 10 T at various angles is studied experimentally and numerically. Water mist is produced by ultrasonic atomizers and fed into a cylindrical Plexiglas pipe (inner diameter, 90 mm) placed in a bore space of an inclined superconducting magnet. The water mist is found to stop at some locations in the magnet at inclined angles ψ ≤ π/6. At ψ ≥ π/4, the amount of mist flowing out of the other opening of the pipe increases with an increase in inclined angle. In the computation of this phenomenon, water mist is simulated with 1000 water droplets of 3 μm diameter. Brownian motion is considered and the Langevin equation is solved. The numerical results show that at ψ ≤ π/6, most of the water droplets accumulate above the magnetic coil. However, at ψ ≥ π/4, with an increase in inclined angle, the number of water droplets passing through the magnetic coil increases.

Experimental and computational studies on water mist flow in the horizontal bore of a superconducting magnet : Example of magnetic force application on micron-sized objects

The effect of a magnetic field on water mist (diamagnetic) is studied both experimentally and numerically. The water mist is produced by ultrasonic atomizers and is fed to a Plexiglas pipe (90 mm inner diameter and 1 m long) which is horizontally placed in a horizontal bore (d=100 mm) of a superconducting magnet (10 T at the magnet center). The water mist is found to flow out of the other end of the pipe opening when there is no magnetic field (b=0 T). At b=8 and 10 T, the mist is stopped at an intermediate location in the pipe and flows out from the inlet opening. In the computation, the water mist is simulated with 1000 particles of 0.01–5 µm in diameter. Brownian motion is considered and the Langevin equation is solved. Various magnitudes of magnetic strength, particle diameters and pressure gradients for air flow are numerically tested. The magnetic effect is obvious for particles with diameters larger than 1 µm. For example, for 3 µm particles, only a small amount of particles are able to pass through the weak magnetic field near the cylinder axis to the downstream and sediment over the downward pipe wall.