Non-isothermal droplet evaporation and condensation in the near-continuum regime

The problem of non-isothermal quasi-steady state evaporation and condensati on of aerosol spheres is examined to determine the rates of simultaneous he at and mass transport in the Knudsen (transition) regime. Now expressions f or the mass and heat fluxes are obtained that show explicitly the dependenc e of the rate processes on the Knudsen number, the accommodation coefficien ts for mass and energy transport and on the molecular weight ratio of the v apor and gas molecules. The analysis, based on the solution of the Boltzman n equation by the method of Grad for Maxwellian molecules, is shown to yiel d results in the continuum regime (Kn much less than 1) in reasonable agree ment with classical methods based on continuum theory and with measured wat er droplet evaporation rates in dry air. Computations of heat and mass tran sport rates for ice sublimation for upper tropospheric and stratospheric co nditions for sizes that correspond to the continuum and transition regimes show that the process is very nearly isothermal. Parametric studies explore the effects of temperature, humidity and accommodation coefficients on the heat and mass transport processes. Although the method of Grad is known to fail in the free-molecule regime, the results agree with more rigorous the oretical solutions for isothermal processes in the near-continuum regime an d with an earlier solution for hard sphere molecules in the near-continuum regime. It is shown that flux-matching or resistance models used for the tr ansition regime do not show the correct dependence on the Knudsen number an d other parameters. (C) 2001 Elsevier Science Ltd. All rights reserved.