Carrier gas effects on homogeneous nucleation of water vapor in a shock tube

Abstract

Condensation of water vapor was studied in the expansion wave of the driver section of a shock tube by light scattering and pressure measurements. By arranging the onset of condensation to occur at the terminating characteristic of the unsteady expansion wave, the gasdynamic analysis could be simplified. The composition of the test gas was varied from pure vapor (steam) to small mole fractions of H2O in different carrier gases, yielding onset conditions in the temperature and pressure ranges of 225 to 282 °K and 2 to 62 Torr, respectivly. Steam condensation is well predicted by the classical rate expression for homogeneous nucleation, modified to account for nonisothermal effects and a variable amount of carrier gases. Results for the transition region from the pure vapor to small mole fractions of water vapor in argon, helium, and air reveal a dependence of the computed nucleation rates of several orders of magnitude on the mole fraction of the carrier gas, which is not explained by the nonisothermal theory. Possible reasons for this effect are discussed.