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 electron transpiration cooling 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 bias and enable stable operation at surface temperatures below 2000 K.

Quantification of plasma enabled surface cooling by electron emission from high temperature materials

“…Additionally, it should be noted that the reported cooling capacity is not a limitation of the cooling method. A low work function surface such as a cesium-wetted material [32] should be able to further improve the cooling capability of the method, and a cesium seeded self-sustained thermal plasma may remove any need for external bias voltage sources. Cesium evaporation will provide additional cooling on the surface.…”

“…Cesium evaporation will provide additional cooling on the surface. However, active cesium transpiration would be required to achieve an optimum cesium coverage of the surface [32]. Such a cesium-fed thermionic cooling concept will be explored in future work.…”

“…A recently proposed engineering application of thermionic cooling is for thermal protection of leading edges of hypersonic platforms [29]. Several theoretical and numerical studies have shown that thermionic cooling at leading edge temperatures approaching 2000 K can provide considerable cooling capacity [30][31][32]. However, no experimental demonstration has yet been made, and the attainable cooling capacity remains uncertain.…”