Numerical investigation on flow instability of sessile ethanol droplets evaporating in its pure vapor at low pressure

In order to understand the flow instability in the evaporation sessile droplet, a series of numerical simulations are carried out on the ethanol droplet evaporating in its pure vapor at low pressure. The contact radius of sessile droplet on the substrate is 2.5mm. The temperature of the ethanol vapor is fixed at 298K, and the corresponding saturated vapor pressure is 7615Pa. Results show that with the increase of the substrate temperature, different types of the thermal patterns appear in sequence. They are the steady axisymmetric pattern or multi-cell pattern driven by tangential temperature gradient, Benard-Marangoni cells mainly induced by vertical temperature gradient, and longitudinal rolls caused by inclined temperature gradient. The relation between the evaporation rate and the contact angle is non-monotonic, which depends on the combined effects of Marangoni flow intensity, evaporating surface area as well as the length of heat transfer path from substrate to evaporation surface. The type of flow instability is independent on gravity. However, buoyancy convection changes the transition Marangoni number of the thermal patterns. (C) 2020 Elsevier Ltd. All rights reserved.