Analysis of micro-mixing and chemical kinetic effects of ultra-lean methane-air mixture ignited by turbulent jets

Abstract

The Equilibrium-PaSR and the Constant-volume reactor framework were used to construct the TJI combustion system with or without turbulence effects. Numerical investigations on micro-mixing and chemical kinetic effects of ultra-lean methane-air mixture ignited by turbulent jets were conducted. The results show that main chamber combustion (MCC) is commonly affected by the jet reactive stages and fuel mixed time. The higher mixing time leads to faster heat transfer and less temperature rise, which ultimately limits the fuel conversion. So PaSR1 fuel reactivity has a stronger effect on t50 (time of 50 % fuel conversion in MCC) than the micro-mixing effect. Besides, the methane content in the jet is particularly important for the overall combustion process in MCC. For the analysis of the chemical kinetic effects, it is known that the ignition of MCC at lower temperatures is more strongly affected by the chemical effects of the combustion products from the pre-combustion chamber (PCC). As the temperature increases, the chemical effect decreases slightly, and the physical effect presents an increasing trend. The low-carbon intermediate CH3/C2H2 radicals in the cold jet have the most significant effect on shortening the ignition delay time (IDT). The ignition ability of the hot jet is closely related to the chemical effects of H/O/OH radicals, and the chemical effects of important low-carbon intermediate species are significantly reduced. For flame instability analysis, the combustion products can reduce the flame thickness of the mixture in MCC to increase the intensity of hydrodynamic instability. With the increase of [O + OH], the destabilizing effect of the thermal-diffusion instability of methane-air premixing gradually increases.