Hiroshige Kumamaru

Three-Dimensional Numerical Calculations on Liquid-Metal Magnetohydrodynamic Flow in Magnetic-Field Inlet-Region

Three-dimensional numerical calculations have been performed on liquid-metal magnetohydrodynamic (MHD) flow through a rectangular channel in the inlet region of the applied magnetic field, including a region upstream the magnetic field section. The continuity equation, the momentum equation including the Lorentz force term and the induction equation have been solved numerically. The induction equation is derived from Maxwells equations and Ohms law in electromagnetism. The discretization of the equations is carried out by the finite difference method, and the solution procedure follows the MAC method. Along the flow axis (i.e. the channel axis), the pressure decreases slightly as normal non-MHD flow, increases once, thereafter decreases sharply and finally decreases as fully-developed MHD flow. The sharp decrease in the pressure, resulting in a large pressure drop, in the inlet region is due to increase in the induced electric current in this region comparing with that in the fully-developed region. In the inlet region, the flow velocity distribution changes from a parabolic profile of a laminar non-MHD flow to a flat profile of a fully-developed MHD flow.

Sensitivity of the gradient oscillatory number to flow input waveform shapes

The sensitivity of the gradient oscillatory number (GON), which is a potential hemodynamic indicator for cerebral aneurysm initiation, to flow input waveform shapes was examined by performing computational fluid dynamics (CFD) simulations of an anatomical model of a human internal carotid artery under three different waveform shape conditions. The local absolute variation (standard deviation) and relative variation (coefficient of variation) of the GON calculations for three waveform shapes were computed to quantify the variation in GON due to waveform shape changes. For all waveform shapes, an elevated GON was evident at a known aneurysm site, albeit occurring at additional sites. No significant differences were observed among the qualitative GON distributions derived using the three different waveform shapes. These results suggest that the GON is largely insensitive to the variability in flow input waveform shapes. The quantitative analysis revealed that GON displays an improved relative variation over a relatively high GON range. We therefore conclude that it is reasonable to use assumed flow input waveform shapes as a substitute for individual real waveform shapes for the detection of possible GON elevations of individual clinical cases in large-scale studies, where the higher values of GON are of primary interest.

Internal-Shear Mode Instabilities on High-Speed Liquid Jet, (I) Characteristics of Linear Solutions

The instabilities on a high-speed liquid jet, induced by amplification of perturbations in the initially-laminar shear layer underneath the free surface (internal-shear mode instabilities), are investigated theoretically and experimentally. Part I of the present paper describes the characteristics of temporally and spatially unstable modes predicted by a linear perturbation equation where the shear layer velocity profile is represented by a single straight line. The analysis focuses on simulated liquid-metal beam target flow conditions characterized by high Froude numbers Fr Δ and moderate Weber numbers We Δ, both based on the velocity difference and the thickness of the shear layer. Temporally and spatially unstable modes are predicted for We Δ >1.8, for a certain range of wave number. For these spatiotemporal modes, the temporal growth rate tends to decrease with an increase in the spatial growth rate. The instabilities predicted by the present theory are all convective, and not absolute, for We Δ ranging up to 20.

Performance Investigation Into Supersonic Diffuser for a High Pressure Centrifugal Compressor

In high pressure centrifugal compressors, the overall stage performance is greatly influenced by its diffuser performance. Extremely complicated non-uniform and unsteady flow exists in the region between the impeller exit and the diffuser inlet. Furthermore, in the case of supersonic diffuser, shock waves can be observed near the diffuser inlet. These can cause aerodynamic losses. Therefore, it is essential to recognize such complicated flow to realize an appropriate diffuser design.An investigation into the performance of supersonic diffuser was carried out using a high pressure compressor test rig for a small industrial gas turbine with a high back swept impeller and a quasi pipe-shaped channel diffuser.In addition, 3D quasi-unsteady flow analyses of the entire compressor by a RANS code with Non Linear Harmonic method at several operating conditions between surge and choke were conducted to investigate the details of unsteady flow between the impeller exit and the diffuser exit.The results of the performance test and that of the 3D unsteady flow analyses have shown good agreement in the pressure rise and the isentropic efficiency at several operating conditions. These support high accuracy of the flow analyses and the performance measurements.Copyright

Three-Dimensional Numerical Calculations on Liquid-Metal Magneto-hydrodynamic Flow through Circular Pipe in Magnetic-Field Inlet-Region

Three-dimensional numerical calculations have been performed on liquid-metal magnetohydrodynamic (MHD) flows through a circular pipe in the inlet region of the applied magnetic field, including a sufficient calculation region upstream in the magnetic field section. The continuity equation, the momentum equation including the Lorentz force term, and the induction equation derived from basic equations in the electromagnetism have been solved numerically. Along the flow axis (i.e., the channel axis), the pressure decreases slightly as a normal non-MHD flow, increases once, thereafter, decreases sharply, and finally decreases as a fully-developed MHD flow. The sharp decrease in the pressure, resulting in a large pressure drop in the inlet region is due to the increase in the induced electric current in this region compared with that in the fully-developed region. The velocity distribution changes from a parabolic profile of a laminar non-MHD flow to a profile with peaks near the walls parallel to the magnetic...

Internal-shear mode instabilities on high-speed liquid jet, (II) Experimental analysis of curved target flow

The wave generation on high-speed liquid jets, induced by the instabilities of a laminar shear layer underneath the free surface (the internal-shear mode instabilities), are investigated theoretically and experimentally. Part II analyzes the jet curvature effect on the velocity profile and the surface configuration near the nozzle exit. The initial characteristics of the free-surface shear layer and the relaxation of the shear layer with the distance from the nozzle exit are also analyzed. Based on these results, the stability of the shear layer is analyzed for curved jet geometries simulating high-energy beam targets. The analytical results show that the gravity and the jet curvature have little effect on the stability of the shear layer, and that the shear layer stabilizes quickly with its relaxation. The predicted most-unstable wave numbers are in fair agreement with available experimental data for curved jets of both water and Li. This indicates that the internal-shear mode instabilities are responsible for the experimentally observed waves.

Numerical analyses on liquid-metal magnetohydrodynamic flow in sudden channel expansion

ABSTRACT Three-dimensional numerical calculations have been performed on the magnetohydrodynamic (MHD) flows through a rectangular channel with sudden expansion, particularly in order to estimate the pressure drop through the sudden expansion. The sudden expansion is in the directions both perpendicular and parallel to the applied magnetic field. The Hartmann number, the Reynolds number and the magnetic Reynolds number are set to ∼100, ∼1000 and ∼0.001, respectively, in simulating laboratory conditions. The continuity equation, the momentum equation and the induction equation were solved numerically by the finite difference method as discretization following the MAC method as solution procedure. On the whole, in the sudden expansion in the direction perpendicular to applied magnetic field, the loss coefficient is estimated to be nearly zero or small. In particular, the loss coefficient becomes negative for small aspect ratios. The reason of negative loss coefficient is attributable to decrease in the induced current just upstream of the expansion. On the other hand, in the sudden expansion in the direction of applied magnetic field, all the cases give positive and large loss coefficients, meaning that the pressure drop through the expansion becomes large. In particular, the loss coefficient becomes considerably large when the Hartmann number increases.

Experimental Studies on Micropumps Using Rotational/Reciprocating Motions of Magnetic Material Balls

In application of micropumps to new fields in chemistry, biology, medical science and others, smaller sizes are supposed to be important rather than higher pump performance. In this study, considering from such a view point, micropumps using rotational and reciprocating motions of magnetic material balls were proposed and studied experimentally. The pump performance, i.e. the relation between flow rate and pump head are measured from liquid level changes in two containers connected to the inlet and outlet of the micropump. For the rotational motion micropump, while the maximum flow rate obtained, ~2 mL/min, is large enough as a micropump, the maximum pump head achieved, ~15 mm, is small even for a micropump. It is desirable to increase the pump head furthermore for this micropump. For the reciprocating motion micropump, the maximum flow rate obtained and the maximum pump head achieved are ~7.5 mL/min and ~625 mm, respectively. These values of the pump performance are sufficient as a micropump. Both the micropumps can be incorporated into microfluidic devices (tips) and can pump arbitrary kind of liquid.

Model Calculations on Micropump Using Reciprocating Motion of Magnetic Material Ball

The authors are developing a micropump which combines reciprocating motion of a magnetic material ball in a pumping channel and four passive check valves. An additional experiment has been performed for one combination of the ball outer diameter and the channel inner diameter, and results of this experiment are presented in this paper. Including the previous experiments performed by the authors, the maximum pump head of ∼620 mm and the maximum flow rate of ∼7.5 mL/min have been obtained in the present micropump. Also, in this study, model calculations have been performed in order to predict the pump performance, i.e. the relation between pump head and flow rate. Calculated flow rates agree well with experimental data for larger gaps between the ball outer diameter and the pumping-channel inner diameter; however, calculated flow rates are larger than the experimental data for smaller gaps. Therefore, it is necessary to improve the calculation models, in particular by calculating leak flow rate in the pumping channel as a flow through a nozzle instead of that through an orifice.Copyright

Studies on Micropump/Minipump Using Rotational Motion of Magnetic Material Balls

Experiments and numerical analyses have been performed on micropumps/minipumps using rotational motion of magnetic material balls. In the pumps, magnetic material balls and nonmagnetic materials balls rotate in a closed channel loop, and a part of the balls acts as a piston and the remaining part of the balls serves as a valve. Experiments have been carried out on two pumps, i.e. a smaller pump and a larger pump with channel cross-sections of ∼1 mm and ∼2 mm inner diameter, respectively. The maximum flow rate achieved and the maximum pump head obtained are ∼500 μl/min and ∼70 Pa, respectively, for the smaller pump, and ∼2000 μl/min and ∼150 Pa, respectively, for the larger pump. Numerical analyses have been performed by dividing the pumping loop into a piston channel and a valve channel. The numerical analyses overestimate the flow rate obtained in the experiments, except for the region of larger pump heads in the larger pump.Copyright