AIR CONDENSATION IN A HYPERSONIC WIND TUNNEL

Abstract

Air condensation studies were conducted in a 3-in. hypersonic wind tunnel where the flow Mach numbers ranged from 9.5 to 17.0. The stream pitot and static pressures both were found to be affected strongly by the presence of condensation. Large amounts of apparent supersaturation, which increased with increasing Mach number, were found to exist. It was shown that hypersonic wind tunnels may be operated with stagnation temperatures much lower than those temperatures required for avoiding saturation conditions in the air stream, and single-phase flow still may be maintained. C^OR the past two decades the problems arising from the -•- occurrence of air condensation in hypersonic nozzles have been of great concern to the operators and designers of hypersonic wind tunnels. When condensation occurs, isentropicity is lost, and meaningful data no longer are obtained. This phenomenon of air condensation may occur when the decreasing pressure and temperature of the expanding flow reach or exceed the air saturation point conditions. At this point the air may begin to condense or it may continue to expand into a supersaturated state. To avoid condensation, the air may be heated to stagnation temperatures sufficiently high that the static air temperatures throughout the expansion are greater than the saturation temperature. However, at high Mach numbers this approach becomes impractical because of the prohibitively high stagnation temperatures required. A number of early studies19 of air condensation in wind tunnels were made in the period of 1948 to 1953 in the Mach number range of about 5 to 7. Most of these studies showed that condensation occurred at, or very soon after, the point where the stream reached saturation conditions. More recently, in 1960, Dayman10 at the Jet Propulsion Laboratory reported that appreciable levels of supersaturati on could be achieved at Mach numbers up to 9.5. Subsequently, Lee and Gregorek at Ohio State University showed with unpublished data that, at Mach numbers greater than 10, large, significant amounts of supersaturati on apparently could be obtained. The present study was undertaken for the purpose of extending and gaining more complete knowledge of the phenomenon of air condensation. In particular, the objectives were to determine the influence of air condensation on the pressure characteristics of the flow and to establish the operational limits of the facility for maintaining condensation-free flow in terms of Mach number and stagnation pressure and temperature.