Heating and evaporation of ethanol/iso-octane droplets considering non-ideal mixtures and finite rate transfer

Abstract

In this paper, the heating and evaporation process of ethanol/iso-octane droplets is deeply investigated through analytical solutions and considering thermodynamic models for non-ideal mixtures. The study is based on the Effective Thermal Conductivity Model (ECM) and Effective Diffusivity Model (EDM), which take into account the finite thermal/mass diffusivities inside the droplets and also consider the effect of droplet internal recirculation. Three sets of cases are considered: ideal liquid and gas phases, non-ideal liquid and gas phases and ideal gas phase together with the non-consideration of the Poynting factor for the liquid phase. The activity coefficients for the non-ideal liquid phase are calculated through the UNIFAC model and the fugacity coefficients for the non-ideal gas phase are calculated from the Virial equation of state. A parametric study is performed with focus on relevant conditions to direct injection internal combustion engines, within an ambient pressure range from 1 to 20 bar, initial ambient gas temperature range from 453 K to 673 K and initial ethanol volume fraction inside the droplet varying from 0 to 1. The thermodynamic analysis of the mixture, the time evolution of the droplet surface temperature and mass fraction together with the droplet lifetimes are presented for several different conditions. The obtained results show that the effect of the non-ideality of both phases has a significant impact on the results when analyzing ethanol/iso-octane mixtures. From cases analyzed considering both phases as ideal mixtures the deviation of the droplet lifetime reached maximum values (in absolute value) of 15.6% at 1 bar, 10.5% at 10 bar and 14.9% at 20 bar and for the ideal gas approach (non-ideal liquid) the deviation reached maximum values (in absolute value) of 3.9% at 10 bar and 9.4% at 20 bar, when compared to the non-ideal approach. These deviations certainly can lead to significant deviations in the prediction of the mixture formation in direct injection engines.