Experimental and chemical kinetics analysis on the combustion behaviors and flame instabilities of diluted methane/air mixtures

The evolutions of spherical flame of methane/air/diluent mixtures in a cylindrical chamber were observed by setting different equivalence ratios and diluent volume fraction V dilutions , and the relationship of combustion pressure vs. chemical parameters was improved. Moreover, the influences of on the methane combustion characteristics and flame instability were explored by chemical pathways. Based on the results obtained, the spherical flame exhibits asymmetric propagation and generates a concavity in its lower flame front that closely resembles the "Tulip flame" at higher V dilutions . The dominant mechanism responsible for this phenomenon is the pure hydrodynamics influenced by stronger buoyancy effects (i.e., Froude number <= 0.016). Moreover, a linear correlation of laminar burning velocity (LBV) with the maximum combustion pressure p max modified by flame thickness delta, which explains that p max is primarily determined by the heating rate and combustion velocity. Results from the sensitivity analysis of LBV reveal that simplifying the mechanisms for CH4-rich 4-rich and CH4-lean 4-lean mixtures must be differentiated to reduce the calculated quantity and enhance calculated accuracy. Furthermore, CO2/H2O 2 /H 2 O chemical effects only contribute to the inhibition of LBV under the extremely CH4-lean 4-lean conditions due to the influences of on CO2/H2O 2 /H 2 O chemical pathways. In addition, the thermal diffusion and flame thickness are sensitive to the chemical pathways associated with OH and H radicals, respectively. Therefore, CO2/H2O 2 /H 2 O chemical effects both weaken the influences of Thermal-diffusion and Darrieus-Landau instabilities on methane flames by affecting the productions of H and OH radicals, and H2O 2 O chemical effect can relieve flame concavity due to the improvement of the sensitivity of flame front on stretch effect.