A filtered-laminar-flame PDF sub-grid-scale closure for LES of premixed turbulent flames: II. Application to a stratified bluff-body burner

Large eddy simulation of the two stratified nonswirling configurations of the Cambridge burner studied by Sweeney et al. (2012) is presented. The sub-grid-scale combustion closure relies on a physical space filtering operation with a filter size determined locally depending on the resolved and sub-grid-scale flame properties, which is discussed in a companion paper. Similarly to the premixed configuration of the same burner, the modeling reproduces the differential diffusion effects leading to accumulation of carbon and an enhancement of mixture fraction in the recirculation zone, an effect that is less pronounced than in the fully lean premixed case, because of the modification of the topology of the reaction zone that is induced by the mixture stratification. The study of the LES combustion regimes shows that the reaction zones develop under a quite large range of flame topologies, from wrinkled flamelets up to thin reaction zones. Instantaneous and time-averaged LES data were analyzed to extract information concerning the degree of stratification and the orientation of flame and mixing vectors. A decomposition of the flame response into premixed, diffusion, and partially premixed flamelets is performed, to conclude that the premixed mode dominates close to the burner, with a partially premixed burning regime further downstream. Overall, the length scales associated with stratification were found to be much larger than that of the reaction zone and flame, resulting in a quasi-homogeneous propagation, predominantly in a back supported stratified combustion regime. Overall good agreement between simulation and measurements was obtained for either configurations.