Theoretical and Numerical Study of Evaporation Effects in Spray Flamelet Modeling

Abstract

Even though laminar spray flames differ considerably from their gaseous counterpart, 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 flamelet. In this work, a combined theoretical and numerical study of the impact of spray evaporation on the structure of laminar spray flames is presented. Numerical simulations of an axisymmetric laminar mono-disperse ethanol/air counterflow spray flame are performed in order to evaluate the influence of spray evaporation on flame characteristics. Flame structures for different initial droplet radii and strain rates are considered. Special emphasis is given to the effects of the spray on extinction and on different local combustion regimes. Moreover, the classical flamelet equations are reinvestigated, and the derivation of spray flamelet equations is presented, where additional terms caused by spray evaporation are identified—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 presented and discussed for the numerical spray flame structures. 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 and regimes of the flame 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.