Unstructured LES-CMC modelling of turbulent premixed bluff body flames close to blow-off

Abstract

A finite volume Large Eddy Simulation-Conditional Moment Closure (LES-CMC) formulation is applied to turbulent premixed bluff body methane-air flames at conditions far from (A1) and close to (A4) blow-off. The unstructured topology of the CMC grid allows refinement in regions where turbulence inhomogeneity is expected, providing an improved description of the turbulence-chemistry interaction phenomenon, non-negligible at conditions investigated in this study. Subgrid scale (SGS) progress variable variance and scalar dissipation rate are closed with models associated with the SGS combustion, turbulence and molecular diffusion processes of premixed flames using detailed kinetics (GRI-3). The simulations effectively reproduce the general trends, especially considering the challenging conditions at very lean mixtures (down to φ=0.64): the characteristic ‘M’-shaped morphology of flame A4 and the significant increase in flame brush thickness compared to flame A1 is accurately replicated, although overall flame heights are under-predicted. Formaldehyde and OH distributions are compared with PLIF measurements and excellent agreement is reported supporting previous experimental observations: for flame A1, OH is seen throughout the recirculation zone while CH2O is present only close to the shear layer along which heat release occurs in a thin, unbroken region of CH2O/OH overlap, disturbed only occasionally by vortex-like structures. For flame A4 close to blow-off, the calculation generally shows appreciable quantities of CH2O throughout the recirculation zone with isolated pockets of OH, surrounded by high heat release, in agreement with experimental findings. The simulation further evidences that in regions experimentally void of both CH2O and OH, large quantities of partially reacted fluid are present which entered the recirculation zone from the top. Quantitative comparisons of flame surface density and local flame curvature statistics are also calculated which agree well with experimental data, suggesting a comprehensive description of the physico-chemical processes of the proposed modelling strategy.