A Filtered Tabulated Chemistry model for LES of stratified flames

The model called Filtered Tabulated Chemistry for LES (F-TACLES), recently developed for turbulent premixed combustion [1], is extended in the present article to stratified flames where the equivalence ratio is not spatially uniform. The principle is to generate a look-up table where thermo-chemical variables, computed from a set of filtered laminar unstrained premixed flamelets, are stored as a function of the filtered progress variable Y˜c, the filtered mixture fraction c˜, and the mixture fraction subgrid scale variance z″2∼ for a given value of the LES filter size Δ. From this database and using a classical subgrid scale flame surface wrinkling model, a closure model for the filtered progress variable balance equation is then proposed. As only three balance equations (respectively for Y˜c, z˜ and z″2∼) are added to the Navier–Stokes system, this turbulent combustion model is straightforward to implement and is not expensive in terms of CPU time. After preliminary tests on 1-D planar flames, the proposed strategy is applied to a 3-D simulation of a swirled turbulent stratified flame. The agreements observed between numerical and measured data validate the modeling strategy. Finally, it is observed that the flame position cannot be captured without accounting for the subgrid scale mixture fraction heterogeneities that impact on the flame propagation. This result highlights the importance of the mixture fraction filtered density function on the filtered chemical reaction rate.