Examination of mixing and differential molecular diffusion in DNS of a high-Karlovitz number turbulent premixed jet flame

DNS of a high-Karlovitz number turbulent premixed jet flame has been reported recently (Wang et al., Proc. Combust. Inst., 2017, 36, 2045-2053). The DNS flame features an intense interaction between the turbulence and flame structures in the broken reaction zone regime as suggested by the DNS dimensionless parameters. In this work, we analyze the DNS results to gain insights into the effect of sub-filter scale mixing and molecular diffusion in the context of large-eddy simulations (LES) and probability density function (PDF) method. First, a sub-filter scale mixing time scale is analyzed with respect to the filter size to examine the validity of a power-law scaling model for the mixing time scale in LES/PDF. The results show remarkable agreement with a simple power-law scaling when the filter size is sufficiently large. Second, the conditional diffusion velocities in the composition space are explored by using the DNS data for the purpose of understanding the unclosed term in the PDF method. Third, the effect of differential molecular diffusion in the DNS flame is examined and quantified. All these results are expected to have implications for LES/PDF modeling of turbulent premixed combustion under extreme conditions.