Direct Numerical Simulation of Inert Droplet Effects on Scalar Dissipation Rate in Turbulent Reacting and Non-Reacting Shear Layers

Three-dimensional direct numerical simulation has been performed to investigate the effects of inert evaporating droplets on scalar dissipation rate chi in temporally-developing turbulent reacting and non-reacting mixing layers with the Reynolds number based on the vorticity thickness up to 8000 and the number of traced Lagrangian droplets up to 10(7). The detailed instantaneous field analysis and ensemble-averaged statistics reveal complex interactions among combustion, droplet dynamics and evaporation, all of which have a considerable influence on chi. The presence of inert evaporating droplets promotes chi in both non-reacting and reacting mixing layers. In the latter, combustion reduces chi, so when combustion is suppressed by evaporating droplets, chi is enhanced. The transport equation of chi has been analyzed to investigate the various effects on chi in detail. The terms in the equation contain explicitly the evaporation rate and its spatial derivative, acting as a sink and a source for chi, respectively. On the whole, the net effect of the evaporation-rate terms is to promote chi. However, the production and dissipation terms are the dominant source and sink terms, respectively.