A rapid expansion of moist air or steam in a supersonic nozzle gives rise to non-equilibrium condensation. If the latent heat released by condensation exceeds a certain quanity, the flow becomes unstable and a periodic flow oscillation occurs. In the present study, a numerical study of moist air flows in a supersonic nozzle was carried out using a special short duration supersomic wind tunnel, called a Ludwieg tube. The effect of the initial relative humidity of the moist air on static pressure, condensate mass fraction and nucleation rate and total pressure loss has been clarified in case without non-equilibrium condensation upstream of nozzle. The results obtained clearly show that the total pressure loss due to non-equilibrium condensation in the Ludwieg tube should not be neglected even for a very low initial relative humidity and it is resulted from the periodic excursions of the condensation shock wave.
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