EFFECT OF CARRIER GAS-PRESSURE ON CONDENSATION IN A SUPERSONIC NOZZLE

Supersonic nozzle experiments were performed with a fixed water or eth anol vapor pressure and varying amounts of nitrogen to test the hypoth esis that carrier gas pressure affects the onset of condensation. Such an effect might occur if nonisothermal nucleation were important unde r conditions of excess carrier gas in the atmospheric pressure range, as has been suggested by Ford and Clement [J. Phys. A 22, 4007 (1989)] . Although a small increase was observed in the condensation onset tem perature as the stagnation pressure was reduced from 3 to 0.5 atm, the se changes cannot be attributed to any nonisothermal effects. The puls ed nozzle experiments also exhibited two interesting anomalies: (1) th e density profiles for the water and ethanol mixtures were shifted in opposite directions from the dry N2 profile; (2) a long transient peri od was required before the nozzle showed good pulse-to-pulse repeatabi lity for condensible vapor mixtures. To theoretically simulate the obs erved onset behavior, calculations of nucleation and droplet growth in the nozzle were performed that took into account two principal effect s of varying the carrier gas pressure: (1) the change in nozzle shape due to boundary layer effects and (2) the variation in the heat capaci ty of the flowing gas. Energy transfer limitations were neglected in c alculating the nucleation rates. The trend of the calculated results m atched that of the experimental results very well. Thus, heat capacity and boundary layer effects are sufficient to explain the experimental onset behavior without invoking energy transfer limited nucleation. T he conclusions about the rate of nucleation are consistent with those obtained recently using an expansion cloud chamber, but are at odds wi th results from thermal diffusion cloud chamber measurements.