Direct numerical simulation of turbulence–radiation interactions in homogeneous nonpremixed combustion systems

Abstract

An important issue in chemically reacting turbulent flows is the interaction between turbulence and radiation (turbulence–radiation interactions—TRI). TRI arises from highly nonlinear coupling between fluctuations in temperature and species composition with the fluctuations of radiative intensity. Here direct numerical simulation (DNS) has been employed to investigate TRI in a statistically homogeneous nonpremixed system. A photon Monte Carlo method has been used to solve the radiative transfer equation (RTE). Radiation properties correspond to a nonscattering fictitious gray gas with a Planck-mean absorption coefficient that mimics that of typical hydrocarbon-air combustion products. Individual contributions of emission and absorption TRI have been isolated and quantified. The temperature self-correlation, the absorption coefficient-Planck function correlation, and the absorption coefficient-intensity correlation have been examined for intermediate-to-large values of the optical thickness, and contributions from all three correlations have been found to be significant. Emission TRI has also been estimated analytically using a β PDF model. The β PDF results are in good agreement with DNS results. This suggests that relatively simple models for emission TRI in nonpremixed systems might be developed based on a modeled equation for mixture fraction variance.