Influence of evaporation on spray flamelet structures

The structure of laminar spray flames considerably differs from their gaseous counterpart. However, most often flamelet models employed in the simulation of turbulent spray combustion are based on laminar gas flame structures neglecting the influence of spray evaporation in the laminar spray flamelets. In this work, a combined theoretical and numerical study of the impact of spray evaporation on the structure of laminar spray flames is presented. Spray flamelet equations are derived, which explicitly take into account evaporation effects – the classical gas flamelet equations are recovered for non-evaporating conditions. Two new terms accounting for evaporation and for combined mixing and evaporation, respectively, are identified, and their relative importance is evaluated by means of numerical simulations of an axisymmetric laminar mono-disperse ethanol/air counterflow spray flame. The results show that the distribution of the spray evaporation rate plays a key role in the characterization of the spray flame structure. The new source terms overweigh the dissipation term of the gas phase in most situations even for non-evaporating species. Therefore, spray evaporation should always be considered. The relevance of the present formulation for turbulent spray modeling is evaluated and discussed, and a novel spray flamelet formulation is suggested.