Droplet evaporation and condensation in the near-continuum regime

The problem of quasi-steady state evaporation and condensation of aerosol d roplets is re-examined to determine the effect of the molecular interaction model, on the predicted mass transfer rates in the Knudsen regime. A new e xpression for the mass flux is obtained that contains explicitly the depend ence of the rate process on the accommodation coefficient and on the molecu lar weight ratio of the vapor and gas molecules. The analysis, based on the solution of the Boltzmann equation by the method of Grad for Maxwellian mo lecules, is shown to yield results in the near-continuum regime (Kn < 1) ve ry close to a number of previous theoretical analyses based on hard sphere molecules and semi-theoretical correlations, including the Fuchs-Sutugin eq uation. These results indicate that the theoretical predictions are not sen sitive to the molecular interaction model used, but depend strongly on the method of solution in the near-free-molecule regime where the method of Gra d fails. As the continuum regime is approached, the solution becomes indepe ndent of the accommodation coefficient. Theoretical predictions agree with previously published evaporation data for isothermal evaporation of dibutyl phthalate (DBP) in air and dibutyl sebacate (DBS) in nitrogen using an acc ommodation coefficient of 1.0 for DBP and 0.9 for DBS. (C) 2001 Elsevier Sc ience Ltd. All rights reserved.