Modeling and Simulation of Single Ethanol/Water Droplet Evaporation in Dry and Humid Air

The modeling and simulation of the heating and evaporation of a spherically symmetric single, bi-component ethanol/water droplet in convective air is studied which allows for an improved understanding of the different heating and evaporation characteristics of the two components of the bi-component droplet. The concentration inside the droplet interior is considered to be uniform and the temperature is modeled with the distillation-limit model. The Abramzon–Sirignano model is used for describing the convective droplet heating and evaporation, and Brenn's extension for multicomponent droplets is used for modeling the ethanol/water droplet. The UNIFAC method is employed to describe the activity coefficients to account for the non-ideal behavior of the ethanol/water mixture. The numerical results corresponding to non-ideal and ideal mixing are compared with experimental data from the literature. Moreover, a parameter study of the ethanol/water droplet evaporation in both dry and humid air for combustion conditions is performed where both the initial gas temperature and velocity and the initial droplet size and composition are varied. The evolution of the droplet surface area, the mass fractions, the mass evaporation rate, and the surface temperature are analyzed for the different conditions. In general, the UNIFAC method initially leads to a somewhat faster evaporation rate and slows down after the higher volatile component has almost completely evaporated; however, its effect on the droplet lifetime is small. Humidity strongly affects the bi-component droplet heating and evaporation because of the condensation of water on the droplet surface which retards the water evaporation but enhances the initial ethanol evaporation for high relative humidity. Also, in humid conditions, the droplet lifetime is strongly increased compared to evaporation in dry air. The parametric dependencies of droplet lifetime and the evaporation characteristics of the bi-component ethanol/water droplet evaporation in both dry and humid convective air may be used in more complex simulations of non-reacting and reacting spray flows.