On preferential diffusion of binary component liquid droplets evaporating in a two-phase mixing layer

Numerical simulations of a two-phase binary component droplet laden temporally developing mixing layer are conducted in order to investigate the influence of preferential diffusion of liquid species. A multicomponent evaporation model based on classical rapid mixing vaporization and Raoult’s law is applied to the Lagrangian description for individual droplets. Results are first presented for single isolated binary component droplets under the same conditions used in the final mixing layer simulations in order to illustrate the models behavior and ability to account for preferential vaporization of the more volatile species. Single droplet simulations are performed for a variety of binary component species pairs having varying properties; including heptane, decane, hexane, hexane–trichloroacetane (TCA), and water. The results illustrate the importance of the latent heat of vaporization, in addition to the traditionally cited boiling temperature, in determining the correct relative volatilities of the liquid species. The results illustrate that species can exhibit strong preferential diffusion effects even with equal boiling temperatures when their latent heats vary substantially. A Lagrangian droplet model is then coupled with the compressible form of the continuity, momentum, energy and species transport equations governing the carrier gas phase. High resolution simulations are conducted of a two-dimensional temporally developing mixing layer with one stream laden with binary component evaporating droplets. Preferential vaporization is found to significantly affect the evaporated species concentrations and distributions within the mixing layer. Evaporative flow saturation is observed, wherein the laden stream becomes saturated before the evaporation is complete. Resulting species concentration distributions within the mixing layer are determined by the coupled effects of preferential vaporization of the more volatile species, and by preferential concentration of droplets within the flow.