Predicting evaporation dynamics of a multicomponent gasoline/ethanol droplet and spray using non-ideal vapour-liquid equilibrium models

Abstract

The present work investigates the effects of more realistic evaporation models based on non-ideal vapour-liquid equilibrium (VLE) on evaporation dynamics of a multicomponent droplet and spray. To achieve this goal, the UNIFAC model [1,2] has been used to determine the activity coefficients γ, accounting for the non-ideality of vapour-liquid equilibrium. The phase diagram of an ethanol/isooctane mixture determined by using the UNIFAC model shows good agreement with measurement. For evaporation of an ethanol/isooctane droplet, two compositions, i.e. 36 and 78 vol.% of ethanol (E36 and E78), have been studied. While Raoult’s law can reasonably approximate the evaporation process of the E36 droplet, non-ideal vapour-liquid equilibrium models which take into account the activity of liquid components must be used to properly predict the evaporation of the E78 droplet. With UNIFAC, the evaporation dynamics of a two-component isooctane/ethanol spray also show good agreement with published data. Evaporation dynamics of a four-component ethanol/gasoline spray was finally investigated. A ternary mixture composed of isooctane, n-pentane and n-decane has been used as a more realistic surrogate for gasoline. It was found that the non-ideality of the evaporation model significantly affects the vapour composition during spray evaporation. Therefore, to properly predict multi and many-component droplet and spray evaporation, which largely determines spray combustion and emission dynamics, non-ideal vapour-liquid equilibrium models should be used. Compared to other approaches, the group contribution method UNIFAC shows unique advantages in that it only requires properties of constituent functional groups of each component, which is particularly useful when there is no VLE data available for binary-component subsystems of the multicomponent mixture. The extension of its use towards modelling evaporation of a many-component liquid mixture is therefore straightforward.