Direct numerical simulation of evaporating droplets in turbulent flows for prediction of mixture fraction fluctuations:: application to combustion simulations

The source term owing to droplets evaporation and the scalar dissipation rate are two correlations appearing in the mixture fraction variance equation that need closures in Reynolds Averaged Navier-Stokes formulation. They are investigated using direct simulation of statistically homogeneous two-phase flow where isooctane droplets evaporate in hot air and are tracked with a Lagrangian method. As droplet diameters are much smaller than integral length scales, scale separation between wake production and classical turbulence can be assumed, which implies that both source and dissipation terms can be decomposed into several contributions. It is proposed to keep only a contribution called effective for the variance source term. The scalar dissipation rate is estimated with a transport equation that exhibits other unclosed correlations. It is proposed to adapt classical closures.