Condensation of air and nitrogen in hypersonic wind tunnels.

Abstract

A previous experimental study of air condensation occurring in the nozzle of hypersonic wind tunnels has been extended toward lower static pressures and higher Mach numbers. The results indicated that in the region of lower pressures, where large amounts of supercooling are regularly obtained, air behaves essentially as pure nitrogen, indicating that it is the spontaneous condensation of nitrogen which triggers the onset phenomenon at the low pressures. A theoretical analysis, based on established drop growth and self-nucleation concepts, supported the experimental findings. The ratio of the local expansion rate and static pressure, a theoretically derived correlation parameter, was found to be effective in correlating the present experimental condensation onset data as well as all other pertinent data available from the literature. The analysis also indicated that in the realm of higher stream static pressures, where little or no air supercooling is obtained experimentally, the condensed particles of the normal CO2 and H^O content in air are sufficient for causing condensation of nitrogen and oxygen through a seeding action. A new method is presented for determining a conservative value of the minimum stagnation temperature which insures condensationfree flow when operating a hypersonic wind tunnel with either air or nitrogen.