Evaporation dynamics of a binary mixture droplet subjected to forced convection

Abstract

Evaporation of binary mixture droplets (BMDs) is a ubiquitous natural phenomenon with numerous industrial applications. In this study, a theoretical model of BMD evaporation under forced convection is established by considering the influence of evaporative cooling, thermal and Marangoni effects, convection, and a Stefan flow. The dynamic evaporation of a binary ethanol–water droplet on a heated substrate is simulated, and the internal and external flow structures of the droplets and their interactions are investigated. The influence of temperature-dependent physical properties on the evaporation dynamics is analyzed, and the effect of the forced convection intensity on the exclusion distance and Marangoni instability is explored. Our findings reveal that, during the stable flow stage, a single vortex flow pattern prevails, characterized by a circulating zone with low ethanol concentration within the droplets. However, in the Marangoni instability-driven flow (MIF) stage, a complex multi-vortex flow appears inside the droplets, with a heterogeneous ethanol distribution. Under the action of the Stefan flow, external forced convection cannot directly affect the flow inside the droplets through viscous shear but indirectly impacts the internal flow through heat and mass transfer. The temperature-dependence of physical properties significantly influences the internal flow and delays the onset of the MIF stage. Forced convection affects the heat and mass transfer by changing the thickness of the thermal and concentration boundary layers. Compared with BMD evaporation under natural convection, the heat and mass transfer rate are significantly higher under forced convection, particularly in the MIF stage.