Numerical study on the ignition and combustion of a CH4 jet flame in diluted and undiluted coflows

Abstract

This study numerically investigates the ignition and evolution of CH4 jet flames in a hot coflow (JHC) using large eddy simulation (LES). Three coflow oxygen fractions are carefully selected to compare moderate or intense low-oxygen dilution (MILD, YO2,C = 6 % and 9 %) and high temperature combustion (HTC, YO2,C = 23.3 %). Systematic investigations are performed on flame ignition, lift-off, reaction progress, and stabilization mechanisms. The results show that for the MILD cases, both the ignition process and final stable flame are controlled by turbulent mixing and chemistry in a state far from chemical equilibrium. In contrast, the reactions are much stronger for HTC, and the final flame is controlled by chemistry through equilibrium reactions. Moreover, for all three cases, the reactions always progress more rapidly at the lean side than at the rich side; thus, autoignition dominates the former, whereas flame propagation is more important for the latter. In addition, the premixing of fuel and oxidant prior to main reactions is crucial for realizing MILD combustion; therefore, the flame is lifted and stabilized by the autoignition of partially premixed reactants in the fuel-lean region. In contrast, the final HTC flame attaches to the burner exit and is stabilized by both the lean-side autoignition and rich-side flame propagation. For all three cases, the lean-side and stoichiometric flames adopt the nonpremixed mode, whereas the rich-side reactions are in the premixed mode, except for the flame base.