Prediction of finite chemistry effects using large eddy simulation

A large eddy simulation (LES) in three dimensions is applied to study flow, mixing, and combustion in a highly turbulent jet flame. Turbulence chemistry interactions, including finite rate chemistry effects, are investigated. The hydrogen fuel has been diluted with nitrogen to allow for both accurate numerical and accurate experimental investigation. In the numerical method, fluctuations of density in time and space are considered to depend only on the chemical state, not on pressure. This low-Mach assumption greatly improves the efficiency of the code. Mixing and the effects of heat release are included by means of the mixture-fraction formulation. To model subgrid scale stresses and scalar fluxes, the Smagorinsky model is used since the dynamic Germano procedure did not show any particular advantage for this flame. To relate mixture fraction to density, temperature, and species concentrations, a steady flamelet model is used. To evaluate the performance of LES with steady flamelet chemistry, a comparison has been made to experimental data, as well as to the results of a probability density function simulation, with a five-step mechanism considering differential diffusion effects. This is done in terms of averaged quantities, scatter plots, and conditional averages. The LES results were found to be in good agreement with the existing data. For this stable flame, the influence of differential diffusion (inherent to hydrogen flames) seems to be negligible.