S. Kabashima

Comparative studies of the performance of closed cycle disk MHD generators using argon, helium and an argon-helium mixture

Abstract Comparative studies of the performance of closed cycle disk MHD generators using argon, helium and an argon-helium mixture are performed under the concept of fully ionized seed. Calculations show that either pure argon or pure helium is desirable as an inert gas in closed cycle MHD generators compared to the mixture of argon and helium and that a typical MHD channel for helium becomes compact. In typical disk generators, it is found that partial load operation with high efficiency can be established, and that fluctuations of the seed fraction are permitted within the limits of about ±3% and ±6% for the disk generator using argon and helium as inert gases, respectively. Furthermore, it is shown that a stable plasma can be maintained for a wide range of gas temperature by adjusting the seed fraction.

Magnetohydrodynamic power generation experiments with Fuji-1 blowdown facility

Experimental results of nonequilibrium plasma magnetohydrodynamic (MHD) power generation from the Fuji-1 blowdown facility are presented. The Fuji-1 experiments have been conducted since 1983, and the disk-shaped MHD generators called Disk-F3a, F3r, and F4 have been tested during the last decade. In the experiment with the newest Disk-F4 generator, which was designed based on the results of previous experiments with the Disk-F3a and F3r generators, an output power of 506 kW and an enthalpy extraction ratio of 18.4% for thermal input of 2.75 MW were simultaneously obtained. During the experiment, however, the deposition of seed material on the electrodes and the insulator walls was revealed, and large amount of impurity (water vapor) contamination in the working gas was detected. In the last experiment the stainless-steel-coated anodes were used instead of copper anodes to prevent the seed material from depositing on the generator walls, and the impurity contamination was reduced by increasing the bottom temperature of the heat exchanger. Consequently, an output power of 544 kW for thermal input of 3.38 MW and an enthalpy extraction ratio of 18.9% for thermal input of 2.17 MW were successfully demonstrated. Each value is the highest among those achieved with the Fuji-1 facility.

High enthalpy extraction experiments with Fuji-1 MHD blow-down facility

Abstract Recent experimental results of closed cycle MHD electrical power generation with the “Fuji-1” blow-down facility are presented. In the experiment with Disk-F4 MHD generator, which was conducted with a modified seed injection system in 1997, an enthalpy extraction ratio of 18.4% was successfully demonstrated with a large output power of 506 kW. This enthalpy extraction ratio is the highest among those achieved with the Fuji-1 facility. The experimental results also revealed the electrical characteristics of the generator installed in the blow-down facility. The decline in the output power and its recovery were observed at the early stage of the power generation run. This fact could be attributed to the attachment of seed material to the generator walls and to its detachment, related to the relatively slow rise in temperature on the wall surface. It was verified for the first time in the Fuji-1 experiment that the reduction of impurity contamination resulted in improvement in the generator performance.

Unsteady Discharge and Fluid Flow in a Closed-Cycle Disk MHD Generator

Unsteady electrical discharges in a nonequilibrium disk MHD generator and its effects on the flowfield are studied theoretically. Numerical results obtained from time-dependent and one-dimensional MHD equations have shown a periodic motion of the discharge, which induces fluctuations of Hall current and flowfield. Repetitional frequency of the discharge becomes higher with increasing seed fraction and external load resistance. When Joule heating is large, the unsteady discharge becomes limited only near the anode, and a steady discharge dominates over the disk generator. 17 references.

Boundary layer effects in He and Ar driven disk MHD generators

Abstract A two-dimensional compressible turbulent boundary layer model is adopted to predict the boundary layer characteristics of the 100 MW thermal input supersonic disk MHD generator walls for helium and argon under various enthalpy extraction conditions. The results show that the boundary layer thickness increases downstream in the channel for both argon and helium, and for helium, there occurs a boundary layer separation under high enthalpy extraction conditions. A slow variation of the Faraday current is observed through the boundary layer, while the Hall current decreases sharply, and the direction of the Hall current gets reversed in the vicinity of the insulator wall at the channel downstream end. This effect is observed to be more prominent for helium than for argon. The percentage of pressure loss through friction is found to be more for argon than for helium, while the heat loss is observed to be more for helium than for argon.

Dynamic properties of nonequilibrium plasma in disk MHD generator

The realization of the stable plasma due to the fully ionized seed has been dynamically confirmed in a channel of the closed cycle disk generator by solving time-dependent MHD equations. The relaxation process is examined near the inlet of the disk generator. In the stable state, as the load resistance increases, the width of the relaxation region becomes narrower and, as a result, the uniformity of the plasma in a whole channel becomes better. The influences of the inlet conditions on changes of the relaxation region are also examined.

Effects of diffuser shape and back pressure on the performance of disk MHD generators

The effects of diffuser shape and back pressure on the performance of closed-cycle disk MHD generators are investigated with two-dimensional numerical simulations taking account of the boundary layer. The enthalpy extraction ratio decreases and fluctuates periodically with increase in the back pressure. The amplitude of power output fluctuation and the period depend on the volume of the inverse flow region. For the reduced height diffuser, the inverse flow region is suppressed and the fluctuation becomes smaller than that for the constant height diffuser. When operated with a back pressure lower than an optimum value, the diffuser does not work because of reacceleration in the diffuser. For the higher back pressure, on the contrary, the inverse flow region propagates into the MHD channel, then the enthalpy extraction ratio is reduced and the Mach number at the exit of the MHD channel becomes lower than unity. For the optimum back pressure, high enthalpy extraction ratio is achieved and the adiabatic efficiency at the diffuser exit is decreased by several percent from that evaluated at the MHD channel exit.

Absorption cross section of the B−X band system of K2

Abstract The absorption cross section of the B1∏u−X1Σ+g band system (610–720 nm) of K2 has been investigated experimentally in K-Ar mixtures. Absorption spectra of the band system were measured at a gas temperature of 773 K and gas pressures from 10 to 1000 kPa. The number density of K was measured using the total absorption of the resonance lines and the number density of K2 was determined from the equilibrium relation. It has been found that the absorption coefficient of the band system is smoothed by collision broadening of the lines in the band system and becomes effectively constant at pressures greater than 200 kPa. The effect of the gas temperature on the absorption cross section is evaluated theoretically.

Design of helium-driven MHD disk generators with high performance

Abstract A design method for helium-driven MHD disk generator channels including boundary-layer behavior is proposed. In this method, the main flow in the channels is one-dimensionally treated, taking account of the concept of effective cross-sectional area of the channels. This effective area is determined by solving two-dimensional boundary-layer equations. Using this method, the generator channels on various scales are numerically designed under the condition of fully ionized seed. Furthermore, the scale effects of the boundary layer on the area ratio, heat loss and adiabatic efficiency of the generators are examined. It is found from this study that a generator having a thermal input of more than 100 MW is required to achieve high enthalpy extraction with high adiabatic efficiency (more than 65%).

Superconducting magnet for a helium-driven disk MHD generator

Design studies on superconducting magnets coupled with a full-scale (1000 MWth) closed-cycle disk MHD generator are described. Special attention is given to a hew supporting structure that can sustain a large electromagnetic force induced between the windings in Helmholtz-type magnets; and the feasibility of this structure is examined. It is shown that the magnet is expected to become considerably compact.