Behavior of a large nonequilibrium MHD generator

Abstract

A large nonequilibrium MHD generator of the linear, segmented-electrode, Faraday type has been studied both experimentally and theoretically. Experiments have been run at the following nominal operating conditions: generator Mach number of 2.2 with a mass flow of 0.4 kg/sec1 of helium seeded with about 0.3% cesium; stagnation conditions were 2000°K and about 5-atm pressure. The loaded generator exhibited several unusual features, including a very slow increase in the load currents axially and very large anode drops. The Hall voltage was a small fraction of its ideal value. At short circuit, load currents at least five times those possible with frozen ionization were drawn; but these currents were still far below those predicted by one-dimensional theory. At open circuit the transverse voltages were usually very low near the inlet and exit but as large as 0.6 of the ideal value near the channel center; negative electrode drops were measured on both cathodes and anodes. A theoretical model is proposed to explain this behavior. It is hypothesized that highly conducting layers exist along the electrode walls which short the generator through the end regions at open circuit. These layers exist because the transverse current is impeded at the insulator segments of the electrode wall, leading to a larger conductivity there than in the main flow. Under load these layers interact with a two-dimensional inlet relaxation front to couple the generator end-toend, producing the very slow axial build-up of the load current. The proposed model is shown to explain the major features of the experimental results.