Numerical investigation of evaporation-induced flow around sessile droplets using a sharp-interface algebraic VOF approach

Abstract

In this study, we numerically investigate the external gaseous flow around an evaporating sessile droplet, especially the interaction between different transport mechanisms of the evaporation-induced flow and evaporation dynamics. A sharp-interface algebraic volume-of-fluid (SA-VOF) solver which incorporates the Boussinesq approximation to deal with thermal and solutal buoyancy has been developed for direct numerical simulation of evaporating droplets in natural convection. Evaporation of a heptane droplet is first simulated to validate the developed solver, and the predicted evaporation rates show good agreement with experimental data. An evaporating sessile droplet on a heated substrate is then simulated through three distinct models to account for contributions from different transport mechanisms of the evaporation-induced flow, i.e. diffusion, the Stefan flow and natural convection. Extensive cases have been simulated at different gravity levels and environmental conditions to clarify the separate role of each mechanism. Results show that the convective flow can enhance evaporation of sessile droplets considerably at elevated substrate temperatures, accounting for up to 40.4% of total evaporation at the substrate temperature of 60 °C and the humidity of 36%. The contribution of the Stefan flow or natural convection in the evaporation process increases with the substrate temperature while stays almost independent of the environmental humidity. Additionally, minute details of the evaporation-induced flow at zero and normal gravity, including flow field, gas mixture density, vapor fraction and temperature distribution, are comparatively studied to reveal the complex characteristics of momentum, mass and heat transfer associated with droplet evaporation in natural convection.