Masaaki Okubo

PERFORMANCES OF DISK CCMHD GENERATOR WITH SMALL AREA RATIO

Performances of disk MHD generator with small area ratio were studied experimentally. The experiments were carried out using shock-tube facility, and cesium seeded argon was used as a working fluid. In the present experiments, two inlet stagnation gas pressure of 0.21MPa and 0.29MPa were selected. The highest adiabatic efficiency(=45.9%) was achieved at higher stagnation pressure. High enthalpy extraction of 17.2% remained obtained at the same time. At lower stagnation pressure, adiabatic efficiency remained lower(=39.7%) although almost the same enthalpy extraction of 17.3% was achieved. The difference in adiabatic efficiency between two stagnation pressures is due to the difference in stagnation pressure ratio between channel exit and inlet. It is considered that these differences are caused by differences in friction loss and/or electrical efficiency. Furthermore, a simple analysis suggests that positive swirl in the channel results in higher electrical efficiency. Therefore an introduction of inlet swirl will be useful for further improvement of disk MHD generator performances.

Interfacial waves of the magnetic fluid in vertical alternating magnetic fields

Abstract The stability of the surface of a magnetic fluid layer under the influences of vertical alternating magnetic fields was investigated experimentally and analytically. Two kinds of waves were excited on the surface and the critical conditions for the onset and the wavelength of these waves were obtained.

Coherent structure in the turbulent wake behind a circular cylinder 2. Numerical simulation using the vortex filament model

In the previous paper the authors reported observing the formation of a spoon-shaped vortex chain in a wake behind a circular cylinder as a coherent structure in turbulence. In this report numerical simulation is carried out based on the assumption that the structure is formed by deformation of the Kannan vortices. The basic equation is the localized induction equation for a single vortex filament with an influence of the background mean flow. The vortex filament is given an initial deformation within a plane at an angle θ to the x–z plane (x is the mean flow direct and z the spanwise direction) with the width Zw, and the further deformation process of the filament is numerically traced. The first calculation is made with fixed Zw and various values of θ. The result shows that the vortex filament finally reaches a structure lying on a plane with a constant angle of 30° ~ 45° to the x-z plane irrespective of the initial values of θ. The second calculation is made with fixed θ and various values of Zw. In this case the final spanwise scale of the deformed region of the filament has almost constant values of about 4d–6d (d is diameter of the cylinder). These results indicate that the final structure of the vortex filament is stable and definite irrespective of the initial disturbances.

Performances of closed cycle disk MHD generator with Ar/Cs

Recent experimental results of shock-tube driven disk MHD generators with Ar/Cs are reported. Experiments were carried out under various load conditions, two different stagnation pressures and various seed fractions. Under the present experimental conditions, effects of the load resistance at the nozzle on enthalpy extractions were small. Enthalpy extractions remained small for the single load resistance. Effects of the stagnation pressure on enthalpy extractions were also found to be small. By optimizing seed fractions, enthalpy extractions higher than 20% were achieved for the segmented load resistance. About 35-45% of the inlet stagnation pressure was lost, even in the nozzle, by Joule dissipation and differences of the total pressure loss at each load conditions were small. The pressure loss due to the shock wave was found to be small owing to the low incident Mach number of the flow. Different kinds of discharge structures were found. A circular luminous discharge due to shock waves appeared, even in the nozzle, under open circuit nozzle conditions. For a single load resistance, spoke instability appeared in the nozzle, this being due to the low electrical conductivity of the plasma in the nozzle.

Numerical prediction on performance of closed cycle disk MHD generator

Numerical calculations were made on nonequilibrium MHD flows in a disk MHD generator in a strong magnetic field. The main objectives of the present study are to develop a numerical simulator for the authors present closed-cycle disk MHD generator and to obtain knowledge for the design of a new disk MHD generator with high performance-especially high adiabatic efficiency. The basic equations were quasi one-dimensional equations which govern the behavior of nonequilibrium two-temperature plasma MHD flow. The TVD MacCormack method was first applied to the present system. Steady and stable solutions in the relatively wide range of operating conditions of the generator were obtained due to the high resolution and stable TVD scheme. The distribution of several physical quantities and the basic performances agreed well with the authors experimental data. A shock wave was induced in the nozzle where ionization proceeded rapidly. As a result, the stagnation pressure loss reached 45/spl sim/55% at the inlet of the MHD channel.

Coherent structure in the turbulent wake behind a circular cylinder 3. Flow visualization and hot wire measurements

In the previous papers the authors have reported that the two-dimensional Karman vortices behind a circular cylinder are deformed until they form chains of spoon-shaped vortex couples whose spanwise scale is about 8d, which is a new type of coherent structure. In this report experimental evidence of this structure is presented. Formation process of the structure and the turbulence in it were investigated for the wake behind a circular cylinder with Re = 2100 and 4200 by means of the flow visualization technique, simultaneous hot wire measurements, spanwise correlation measurements, construction of instantaneous velocity field by the conditional sampling method, etc.