Large eddy simulations of premixed CH4 bluff-body flames operating close to the lean limit using quasi-global chemistry and an algebraic chemiluminescence model

摘要

The present work describes the numerical study of unconfined turbulent premixed methane/air flames stabilized on an axisymmetric conical baffle under lean and ultra-lean, close to blow-off conditions. A finite-volume-based LES method, using the dynamic Smagorinsky subgrid model in combination with two turbulent combustion methodologies, the thickened flame model and the implicit LES (ILES) laminar reaction rate approach were employed in the investigation. Methane-air oxidation was modeled with a 14-species reduced scheme. OH* chemiluminescent species levels were also evaluated by post-processing quasi-steady-state-derived algebraic expressions, exploiting directly simulated species thus enabling direct comparisons with experimental images. The quality of the simulations was appraised against experimental data for velocity, turbulence, temperature, species mass fractions, heat release profiles as well as chemiluminescence images for conditions far and close to blow-off. Both turbulent combustion models followed closely several intrinsic trends and variations of the flame front anchoring and disposition close to the burner rim shear layers and along the reverse flow region as the fuel level was reduced toward the lean limit. The interaction of the bluff-body recirculation with the adjacent toroidal reacting shear layer and the impact of combustion and heat release on the development of the turbulent velocity and species fields in the near-wake recovery zone were adequately reproduced for both mixtures. Quantitative deviations between simulations and measurements, regarding heat release and OH species, increased for the near-LBO flame with an attendant extrapolated underprediction of the blow-off event by about 8% in terms of equivalence ratio.