On the modeling of scalar mixing timescale in filtered density function simulation of turbulent premixed flames

A new closure of the scalar mixing timescale is formulated to enhance the predictability of large eddy simulation (LES)/filtered density function (FDF) simulations for turbulent premixed flames. Specifically, the new model integrates a dynamic closure for turbulence-induced mixing with a closure for reaction-enhanced mixing, such that the model explicitly accounts for the subgrid mixing due to turbulence and reaction. The model adaptively adjusts the relative contribution from these two aspects according to the local state of combustion and requires no tuning for the mixing rate parameter (CM). To evaluate the model performance, LES/FDF simulations are carried out for the Sydney piloted premixed jet burner flames PM1-50 and PM1-150. Compared with the constant CM model with the baseline CM = 2, the proposed model notably improved the prediction of the overall combustion progress of both flames. The relative importance of the reaction-enhanced mixing in comparison with the turbulence-induced mixing is further investigated. For flame PM1-50, the reaction-enhanced mixing has a prominent impact throughout the combustion progress, resulting in a large variation in CM in the progress variable space. This illustrates the advantage of the proposed model for the flame close to the flamelet regime. For flame PM1-150, the variation in CM during the combustion progress is relatively small owing to the relatively weak reaction-enhanced mixing compared to PM1-50. However, this desired CM is much larger than the order of unity. Therefore, the proposed model also has its advantage for the flame close to the broken-reaction zones regime. © 2020 Author(s).