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 was developed to identify the limitations of single mixture fraction FPV models that presume a single compositionally uniform fuel stream. The revised model was employed in Large Eddy Simulation of the modified University of Sydney turbulent jet burner with compositionally inhomogeneous inlet conditions. The first mixture fraction characterized the mixing between the CH₄/air mixture issuing from the burner and the surrounding coflow air. The second mixture fraction tracked mixing of CH₄ and air regardless of the air's point of origin. Also a fuel premixing fraction was defined that corresponded to the fuel side boundary condition for the solution of the one-dimensional flamelet equations in terms of the first mixture fraction. Comparison of temperature and species predictions with experimental data showed that the structure of the flame with inhomogeneous inlets could not be predicted by a single mixture fraction model while this was successfully achieved with the two mixture fraction models.