Scalar mixing and dissipation rate in large-eddy simulations of non-premixed turbulent combustion

Predictions of scalar mixing and the scalar dissipation rate from large-eddy simulations of a piloted non-premixed methane/air diffusion flame (Sandia flame D) using the Lagrangian-type flamelet model are presented. The results obtained for the unconditionally filtered scalar dissipation rate are qualitatively compared with general observations of scalar mixing from experiments in non-reactive and reactive jets. In agreement with experimental data provided the reaction zone has an inward direction, regions of high scalar dissipation rate are organized in layerlike structures, inwardly inclined to the mean flow and aligned with the instantaneous reaction zone. The analysis of single-point time records of the mixture fraction reveals ramplike structures, which have also been observed experimentally and are believed to indicate large-scale turbulent structures. The probability density function (pdf) of the instantaneous resolved scalar dissipation rate at stoichiometric mixture evaluated at cross sections normal to the nozzle axis shown to be described accurately by a lognormal pdf with sigma = 1. A new model for the conditionally averaged scalar dissipation rate has been proposed and is shown to account for local deviations from the simple mixing layer structure. The stabilizing effect of the pilot flame in the present configuration is also discussed. Finally, the influence of the resolved fluctuations of the scalar dissipation rate on the flame structure is investigated, revealing only a weal influence on temperature and nitric oxide predictions. However, the model requires further refinement for situations in which local extinction events become important.