Evaporation of droplets into a background gas: Kinetic modelling

A new kinetic model for droplet evaporation into a high pressure background gas, approximated by air, is described. Two regions above the surface of the evaporating droplet are considered. These are the kinetic region, where the analysis is based on the Boltzmann equation, and the hydrodynamic region. It is assumed that the mass fluxes leaving the kinetic region and the corresponding diffusion fluxes in the hydrodynamic region are matched. A modified version of the previously developed method of direct numerical solution of the Boltzmann equation is used. It is assumed that the mass flux leaving the droplet's surface is the maximal one (evaporation coefficient is equal to 1). The model and numerical algorithm allowed us to calculate the value of the net evaporation coefficient, defined as the ratio of the actual mass flux leaving the kinetic region and the maximal possible mass flux. The values of this coefficient for diesel fuel (approximated by n-dodecane) were shown to be much less than I for droplet surface temperatures less than 650 K. For these droplets, the kinetic effects predicted by the new model turned out to be negligible when the contribution of air in the kinetic region was ignored. These effects, however, appear to be noticeable, and larger than those predicted by the approximate analysis, if the contribution of air in the kinetic region is taken into account. It is recommended that the kinetic effects are taken into account when accurate analysis of diesel fuel droplet evaporation is essential. (C) 2007 Elsevier Ltd. All rights reserved.