Numerical study of droplet evaporation under an external flow condition

Numerical simulation was performed for sessile droplet evaporation under an external flow condition. The conservation equations of mass, momentum and vapor concentration are solved by employing the level-set method which is modified to include the effect of phase change. One of the major difficulties in computing the whole period of droplet evaporation arises from the fact that the surface tension term in the momentum equation causes a serious time step restriction for stable computation. In this work, the time step constraint is avoided by simplifying the droplet shape as a spherical cap whose curvature is spatially uniform so that the surface tension term is treated as a part of the pressure term. The droplet interface is calculated geometrically and then the level-set function is reconstructed from the droplet interface. The numerical result for sessile droplet evaporation in stationary air showed good agreement with the experimental data reported in the literature. The numerical method was applied to droplet evaporation under an external flow condition, which is a three-dimensional case requiring huge computing time without the present simplification. The effects of external flow temperature and velocity on the droplet evaporation rate are quantified.