Cesium seeding for effective electron transpiration cooling in hypersonic flows

Abstract

The concept of electron transpiration cooling (ETC) uses thermionic emission of electrons from a low work function material to cool surfaces in hypersonic flight. A theoretical estimate of the emission current is given by the Richardson–Dushman equation. In hypersonic flights, the emission current can deviate from this estimate as the ambient air is partially ionized and a plasma sheath forms near the surface. Depending on the sheath structure, the emission current can be enhanced by the Schottky effect, or could be reduced by the space charge effects. In this study, we present a theoretical analysis of ETC of the leading-edge surface of a hypersonic vehicle, considering the transpiration of liquid cesium through a porous tungsten material. A part of the transpired cesium is adsorbed on the surface, which lowers the emitter work function, while the rest is evaporated due to high surface temperatures. Both the effects provide substantial cooling. The evaporated cesium is ionized in the ambient air, which alters the plasma conductivity and reduces space charge effects. The effect of individual fields of ionized species near the surface is found to be negligible. Cesium transpiration is found to eliminate the requirement for an applied surface potential and enable stable operation at surface temperatures below 2000 K.