Large eddy simulation of dilute reacting sprays: Droplet evaporation and scalar mixing

Large eddy simulation (LES) of turbulent reacting sprays is performed to investigate the interactions of droplet evaporation and subfilter scalar mixing processes. A stochastic method is proposed to generate the subfilter fluctuations in gas-phase reactive scalars in the framework of a mixture-fraction-based combustion model. The subfilter fluctuations of the gas-phase temperature and composition, seen by droplets, are used to refine the estimates of the interphase heat/mass transfer rates. Gas-phase combustion is described by the flamelet progress variable approach. The effects of droplet evaporation on subfilter scalar mixing are considered by solving the transport equation for the subfilter mixture fraction variance. The mixture fraction that is valid instantaneously in both the gas and the liquid phases is adopted and its implication on the modeling of the evaporation source term in the subfilter variance equation is discussed. The modeling approach has been applied to the Sydney dilute reacting sprays. To account for uncertainty in the modeling of scalar dissipation rates in spray flames, a parametric study is performed for the constant in the scalar dissipation model for the subfilter variance equation. The effects of subfilter fluctuations on droplet evaporation are found to depend on the inflow condition of the pre-evaporated gas-phase mixture and the liquid injection rate in the present dilute reacting sprays.