Aerosol Evaporation in the Transition Regime

ABSTRACT

This paper attempts to provide a definitive study of microdroplet evaporation over a range of Knudsen numbers extending from the continuum regime to the free-molecule regime. By electrodynamically levitating a droplet of dibutyl phthalate (DBP) in a temperature-controlled vacuum chamber, by following the evaporation by means of angular scattering measurements and by recording morphology-dependent resonance spectra, precise evaporation rate data were obtained for various pressures and temperatures. The DBP/air system was selected because of the large amount of data available in the literature for DBP and because the molecular mass ratio m _DBP/m _air, corresponds to the solution of the linearized Boltzmann equation reported by Loyalka et al. (1989). The evaporative flux data for DBP in air are shown to be in agreement with Loyalka's analysis, with the interpolation formula of Loyalka (1983) and with the empirical Fuchs-Sutugin (1970) equation. The free-molecule data are shown to be consistent with an evaporation coefficient close to unity. The theory of Sitarski and Nowakowski (1979) based on Grad's (1949) thirteen moment method of solution of the Boltzmann equation is shown to agree with the data for Kn < 0.2.