Analysis of a filtered flamelet approach for coarse DNS of premixed turbulent combustion

• Premixed combustion modeling regimes based on subgrid turbulence level.
• Flame reconstruction from 1-D laminar flame solution filtered at mesh width.
• A priori and a posteriori analysis of filtered flamelet approach for coarse DNS.

Nowadays numerical simulations of premixed turbulent combustion can be performed at high spatial resolutions due to the continuously increasing computational power. At a resolution of the order of the laminar flame thickness there is negligible turbulence in the subgrid level and a proper reconstruction of the filtered flame is adequate for accurate predictions. In this scenario, which can be termed as a coarse scale Direct Numerical Simulation (DNS), reconstruction of the filtered flame using a spatially filtered flamelet (1-D laminar flame) is analysed. First, DNS of a turbulent premixed slot burner is computed to generate a reference database. This database is filtered at different length scales of the order of laminar flame thickness for an a priori assessment of turbulent flame reconstruction using a filtered flamelet. The filtered DNS source term, molecular diffusion and subgrid convection of the reaction progress variable is compared with a flamelet convoluted with top-hat profiles of the same order. The source term and molecular diffusion are approximated well from the filtered flamelet while the unresolved convection shows deviation. Finally, the reconstruction of the turbulent flame is tested a posteriori on the same filter widths (mesh size) and the results are compared. It is found that a flamelet filtered spatially at the order of the computational mesh and tabulated using a single controlling variable i.e. the reaction progress variable is suitable for a coarse scale DNS of premixed turbulent combustion.