Second-order conditional moment closure for turbulent jet diffusion flames

The limited success in predicting nitric oxide concentrations in turbulent hydrogen-air diffusion flames using conditional first-moment closure indicates the importance of the correlations of the conditional fluctuations. Hence, a second-order closure for the chemical reaction rate term is suggested. Correction terms that account for moments of higher order are introduced. They are derived from a global one-step mechanism for hydrogen-air combustion. Assuming the radicals O, H and OH to be in partial equilibrium and the minor radicals such as HO2 and H2O2 in steady state, chemical reaction rates can be represented by a two-variable formalism for a given enthalpy. Then: second-order moments of combined variables can be used to find more accurate solutions of the conditionally averaged chemical source terms. The model is complemented by a conditional variance transport equation. The validity of the model has been verified by comparison with experimental data, and good to excellent predictions of nitric oxide levels are achieved in undiluted and helium-diluted hydrogen jet flames.