Stochastic Modeling of Molecular Transport to an Evaporating Microdroplet in a Superheated Gas

The investigation summarized in this paper explored the use of a stochastic, direct-simulation Monte-Carlo scheme to model heat and mass transfer associated with the vaporization of liquid nitrogen microdroplets in superheated nitrogen gas. Two different stochastic models of the molecule–surface interactions were used in the particle simulation scheme. The first, which imposes conservation of mass at the droplet surface, is appropriate for a heated or cooled solid sphere. The second models the net generation of vapor at the surface of an evaporating droplet by allowing the molecular flux to adjust itself dynamically to balance the energy exchange. Predictions of the particle simulation model with the mass conservation surface treatment are found to agree favorably with heat transfer data for a solid sphere in a rarefied gas. The effects of noncontinuum behavior and interface vapor generation on transport during microdroplet evaporation are explored and a closed-form relation for the heat transfer coefficient is developed, which closely matches all our simulation heat transfer predictions for evaporating microdroplets. This relation apparently is the first to account for the simultaneous effects of interface vapor generation and noncontinuum behavior on heat transfer controlled microdroplet evaporation.