A COMPARATIVE CFD STUDY ON FLAMELET GENERATED MANIFOLD AND STEADY LAMINAR FLAMELET MODELING FOR TURBULENT FLAMES

摘要

Laminar Flamelet Model (LFM) [1-2] represents the turbulent flame brush using statistical averaging of laminar flamelets whose structure is not affected by turbulence. The chemical non-equilibrium effects considered in this model are due to local turbulent straining only. In contrast, Flamelet Generated Manifold (FGM) [3] model considers that the scalar evolution, the realized trajectories on the thermo-chemical manifold in a turbulent flame is approximated by the scalar evolution similar to that in a laminar flame. This model does not involve any assumption on flame structure. Therefore, it can be successfully used to model ignition, slow chemistry and quenching effects far away from the equilibrium. In FGM, 1D premixed flamelets are solved in reaction-progress space rather than physical space. This helps better solution convergence for the flamelets over the entire mixture fraction range, especially with large kinetic mechanisms at the flammability limits [4]. In the present work, a systematic comparative study of FGM model with LFM for four different turbulent diffusion/premixed flames is presented. First flame considered in this work is methane-air flame with dilution air at the downstream. Second and third flame considered are jet flames in a coaxial flow of hot combustion products from a lean premixed flame called Cabra lifted H_2 and CH_4 flames [5-6] where the reacting flow associated with the central jet exhibits similar chemical kinetics, heat transfer and molecular transport as recirculation burners without the complex recirculating fluid mechanic. The fourth flame considered is Sandia flame D [7], a piloted methane-air jet flame. It is observed that the simulation results predicted by FGM model are more physical and accurate compared to LFM in all the flames presented in this work.