Sessile droplet evaporation in the atmosphere of different gases under forced convection

Abstract

The phenomenon of evaporation from the surface of a liquid droplet into a neutral noncondensible gas was numerically studied by taking forced convection gaseous flow into account. The mathematical model considers the effects of surface tension, gravitational force, viscosity of both liquid and gaseous media, as well as the Stefan flow from the droplet surface, possible free gravitational convection, and the Marangoni convection in droplets, and it is designed to describe diffusion-limited evaporation. We consider the diffusion-limited evaporation process when the diffusive gas flux to the droplet surface is compensated by the convective Stefan flow from the surface. The results indicate an interaction of the liquid and gaseous media. Convective gas flows cause the liquid to move and a vortex to occur in the droplet. The flow velocities in a vortex are 103 times less than the characteristic velocity of forced convection flow in air. The droplet surrounded by gaseous flow changes its shape and oscillates, which causes a gas-density wave. Calculations have shown that the diffusion-limited evaporation rate does not change in the presence of forced convection, which contradicts most of the known experimental works. The possible reason for this discrepancy is the presence of non-equilibrium conditions at the liquid–gas interface in experiments. This leads to a consequent change of the evaporation mode to non-diffusive, while the numerical model postulates the Stefan condition and diffusion-limited evaporation.