The role of preferential evaporation on the ignition of multicomponent fuels in a homogeneous spray/air mixture

This work examines the importance of preferential evaporation and liquid species diffusion on the ignition of multicomponent surrogate fuels in homogeneous mixtures. To this end, a model is developed that considers the evaporation of a monodisperse droplet cloud in a homogeneous and isobaric gas-phase environment. The mathematical model accounts for physico-chemical relevant processes, involving (i) species diffusion in the liquid phase, (ii) two-phase thermodynamic equilibrium and (iii) gas-phase chemistry. After evaluating the accuracy of individual model components against available experimental data, the model is applied to investigate the ignition of a surrogate mixture for a Jet-A fuel (POSF 4658) at conditions of interest to gas turbine engines. A parametric study is performed to examine effects on the interaction between multicomponent evaporation and ignition. It is found that the maximum reactivity occurs for conditions at which the ignition time in the gas phase is comparable to the evaporation time. At these conditions, the mixture composition is significantly affected by preferential evaporation, which in turn affects the reactivity of the gas phase. It is shown that intradroplet diffusion plays a major role on the radial droplet composition, thereby conditioning the evaporation rates of single species. A comparison with a zero-diffusivity model for the treatment of droplet internal diffusion emphasizes the importance of liquid diffusion on the autoignition time, and confirms the need for properly accounting for a detailed description of multicomponent evaporation effects.