Direct numerical simulation study of evaporation effects in combustion suppression by inert droplets

Direct numerical simulation (DNS) is performed to investigate the evaporation effects of inert droplets on turbulent micromixing and combustion in a three-dimensional temporally-developing turbulent reacting mixing layer laden with more than 107 Lagrangian droplets/particles initiated in the fuel stream. Different characteristic evaporation times of droplets are obtained by varying the latent heat of vapourisation. Dilution and thermal cooling effects are then studied in the context of combustion suppression. Since the conventional mixture fraction for a two-scalar mixing and reactive system is inappropriate for a three-scalar (fuel, oxidiser and vapour) system as in the present study, the instantaneous local equivalence ratio is proposed instead to represent the mixture status. The evaporation effects on turbulent micromixing are studied using the conditionally averaged scalar dissipation rate, which consists of six components in the present three-scalar mixing problem. Thermal cooling and mixture dilution by vapour are found to have different influence on the six scalar dissipation rates.