Numerical investigation of natural convection effects on sessile and pendant evaporating drops

This study investigates the effect of natural convection on the evaporation of sessile and pendant drops, accounting for buoyancy forces arising from variations in gas density due to non-uniform distributions of vapor concentration and temperature in the surrounding gas mixture. The governing conservation equations are solved using the simulation software Ansys Fluent on two-dimensional axisymmetric grids under steady-state conditions and assuming that the drops are spherical caps. A range of drop sizes is considered to span Grashof numbers relevant to typical applications. Three evaporating fluids (n-octane, ethanol, and water) with molar masses higher or lower than that of air are analyzed, and helium as the inert species was used to illustrate the effect of buoyancy in evaporation. The impact of surface wettability is examined by considering a range of contact angles between 30° and 150°. The influence of natural convection on sessile and pendant drops is quantified, highlighting the role of gas mixture composition. A novel correlation is derived, with the scope to correct the evaporation rate predicted by the diffusion-driven analytical model, and to account for the buoyancy force in the case of both sessile and pendant drops vaporizing on substrates with different wettability.