Improved Pollutant Predictions in Large-eddy Simulations of Turbulent Non-premixed Combustion by Considering Scalar Dissipation Rate Fluctuations

In this study a new formulation of the unsteady flamelet model is derived to account for the locally resolved distribution of the scalar dissipation rate obtained from large-eddy simulations (LES). Starting from the unsteady flamelet equations, a transformation leads to an Eulerian flamelet model, in which the scalar dissipation rate appears as function of time, space, and mixture fraction. In previous work, it has been shown that LES provides most of the fluctuations of the scalar dissipation rate. Therefore, the present model can be solved within an LES using a local fluctuating scalar dissipation rate. The model is applied to the Sandia flame D, which is a partially premixed, piloted jet diffusion flame. Previously, we have investigated this flame with an unsteady flamelet model, in which only conditionally averaged values for the scalar dissipation rate have been used. Compared to this simulation, accounting for scalar dissipation rate fluctuations leads to improved predictions of the flame structure. In particular, a region of heat release in the rich region of the flame, which is caused by the partial premixing of the fuel with air, does not occur if scalar dissipation rate fluctuations are considered, which is in agreement with the experimental data. This also leads to strongly improved predictions of the mass fractions of stable intermediate chemical species, such as CO and H2.