Performance of a 500-kjoule MHD wind tunnel

Abstract

The operation of a pulsed MHD accelerator is described for a MHD wind-tunnel application. The accelerator is designed to provide a 50% velocity increase (from 15.5 kft/sec to 24 kft/sec), at constant enthalpy, of a gas at approximately atmospheric pressure. The accelerator gas source is a hot shot that heats potassium seeded nitrogen to 9000 °K and 1400 atm. The gas is expanded into a segmented electrode (94 pair) MHD accelerator channel. After tunnel startup, a 3-msec constant power pulse is switched on the electrodes so that current flow is approximately perpendicular to a quasi-steady 4-wb/m2 magnetic field. The power-on to power-off velocity ratio (=un/u/) at the accelerator exit is determined from changes in the stagnation pressure and open circuit induced potential. The corresponding enthalpy ratio (==hn/h/) is determined from the static pressure change and un/Uf. Results 0.5 msec after switch-on indicate un/Uf « 1.5 (un ~ 24 kft/sec) and hn/hf « 1.0. As a result of constant volumetric power addition and a decaying mass flow, un/u/ and hn/hf increase to 1.7 and 1.2, respectively, just prior to channel power switch-off. Although these results are essentially the design values, the detailed performance of the accelerator appears to be significantly influenced by spatial nonuniformities in the flow.