Effects of air humidity in supersonic wind tunnels

Abstract

The available theoretical and experimental information on condensation of water vapour in the supersonic flow of air is reviewed and the influence of condensation on operation of supersonic tunnels is considered. The mechanism of condensation in supersonic flow is of molecular nature and does not depend on the presence of solid condensation nuclei in the air. As estimated by Oswatitsch and confirmed by experimental results, the condensation in supersonic flow of air is primarily a function of the adiabatic supercooling DeltaT h to ad (defined in Fig. l), which determines the conditions at which the condensation shock occurs. For medium-sized supersonic tunnels (say 1-ft square working section) the adiabatic supercooling is of the order of 50 deg C. For most test purposes it is essential to eliminate the detrimental effects of condensation, on flow distribution in the tunnel working section. The usual method is to use highly dried air, and the question of the required dryness is considered. It is shown that by increasing stagnation temperature condensation can be avoided usually only at Mach numbers smaller than 1.5. Alternatively, condensation can be eliminated from the tunnel nozzle by pre-expansion in an auxiliary nozzle, as verified experimentally.