Evaporation of a suspended binary mixture droplet in a heated flowing gas stream

Abstract

Studying vaporization of binary mixture liquid droplets as a model system provides useful information for understanding several engineering applications involving multicomponent systems. The present study investigated vaporization behaviour of both polar and non-polar binary droplet systems in hot convective environment using a mathematical model and experiments. A lumped parameter rapid mixing evaporation model accounting for non-ideal solution behaviour and temperature dependent physical properties was presented. The model p redicted temporal variation in droplet diameter and temperature were validated against the experimental data for heptane-decane and water-glycerol mixture. Experiments involving the water-glycerol system involved both size and temperature measurement of millimetre-size droplets at constant gas temperature ~353 K and at free stream gas velocity ~4.3 m/s. In general, model predictions were in good agreement with the experiments however some deviations in the model prediction were noted at the transition stage specifically for the water-glycerol system which was attributed to the liquid phase diffusional resistance not accounted in the present modelling framework. Inclusion of activity coefficient into the model was shown to have insignificant effect on the evaporation rate in both non-polar and polar systems. Internal motions induced by the external shear flow and internal density difference were quantified. Time scales based on heat transfer (convective heat transfer and thermal diffusion) and mass diffusion accounting for the internal motion were estimated. It was shown that the unsteady heating duration of the droplet fall within the two limits set by the heat transfer and mass diffusion time scales.