Abstract:
Transported probability density function (TPDF) methods are attractive for modeling turbulent flames as the highly nonlinear chemical reactions appear in closed form. However, challenges remain when applying TPDF methods to turbulent premixed flames, for which modeling molecular diffusion is difficult because the species local gradients are governed both by turbulence and chemical reactions. In this paper, recent progress on micromixing modelling for turbulent premixed flames is reviewed with particular focus on the model performance in different combustion regimes, the analysis and modelling of the scalar mixing timescales using direct numerical simulation (DNS) datasets, and the LES/TPDF simulations of near-limit premixed flames. For flames in the flamelet regime, localness in composition space for scalar mixing is essential and the mixing rate of reactive scalar is dominantly controlled by flame structure; for flames in the broken reaction zone regime, reactive scalar mixing is controlled by turbulent mixing. Then models for the mixing timescale of progress variable are discussed, with a focus on the recently developed hybrid mixing timescale model, which is applicable to different combustion regimes and has significant advantages over the existing constant mechanical-to-scalar timescale model. This work concludes with a discussion on the effects of grid resolution on LES/TPDF modelling of near-limit premixed flames.