Filtered Density Function Simulations of a Near-Limit Turbulent Lean Premixed Flame

The predictive numerical simulation of near-limit turbulent premixed combustion, in which the turbulent intensity is high and the fuel/air mixture is near the flammability limit, remains challenging. In this study, large-eddy simulation (LES)/filtered density function (FDF) simulations of a high-speed piloted premixed jet burner flame are performed to illustrate the importance of the resolved level of reactive scalars and micromixing modeling on predicting the overall combustion process. The simulations with increased resolved levels of reactive scalars yield notably improved predictions throughout the extinction–reignition region of this flame. The sources of grid sensitivity are thoroughly examined through the quantification of the resolved levels of the velocity and scalar fields, as well as the mixing-reaction budgets. Subgrid mixing is identified as the most prominent factor for grid sensitivity. The effects of mixing timescale modeling are investigated and quantified through a parametric study of the mixing rate parameter and the development of an augmented hybrid mixing timescale model in the LES context. As far as the mixing formulation is concerned, the Euclidean minimum spanning tree model shows a lower level of grid dependence than the interaction by exchange with the mean model due to the enforcement of localness in composition space.