A two mixture fraction flamelet model for large eddy simulation of turbulent flames with inhomogeneous inlets

A revised flamelet/progress variable (FPV) model in which two mixture fractions are defined has been developed to address the limitations of single mixture fraction FPV models that presume a single, compositionally uniform fuel stream. The revised model is applied in Large Eddy Simulation of the modified University of Sydney turbulent jet burner with compositionally inhomogeneous inlet conditions. The first mixture fraction (Z) characterizes the mixing between the methane/air mixture issuing from the burner and the surrounding coflow air. The second mixture fraction (Z*) tracks mixing of methane and air without regard to the point of origin of the air. Additionally, a fuel premixing fraction (F) has been defined that corresponds to the fuel side boundary condition for the solution of the 1D flamelet equations in terms of the first mixture fraction. Two methods of characterizing the thermochemical state are considered, one using Z and Z* and the other using Z and F. Comparison of temperature and species predictions with experimental data shows that the structure of the flame with inhomogeneous inlets cannot be predicted by a single mixture fraction model while this is successfully achieved with the two mixture fraction models. Even for a near-homogeneous inlet with only minor variations in composition, the two mixture fraction model predictions provide significant improvements over a single mixture fraction model.