Bifurcations and negative propagation speeds of methane/air premixed flames with repetitive extinction and ignition in a heated microchannel

Detailed behaviors of ignition kernel(s) in a uniform stoichiometric methane/air mixture under the temperature gradient were investigated numerically by using a fundamental system of microcombustion. Bifurcation of the heat release rate peak in an ignition phase at the high wall temperature side, which has been observed in previous experimental and theoretical studies, was successfully reproduced by the present computation. The bifurcated heat release rate peak exhibited negative propagation speed relative to the local flow velocity by consuming a separated methane/air mixture in the downstream side of the boundary zone between an incoming fresh mixture and burned gas. CH4 was completely consumed at the main peak, whereas CO remained unreacted in the wide region behind the main peak. In a weak reaction phase at the low wall temperature side, two bifurcations of heat release rate peak were newly captured. By the two bifurcations, three heat release rate peaks, namely, a main and two bifurcated peaks appeared. The two bifurcations were caused by remaining intermediates such as CH3, CO, H, and OH in the downstream side of the boundary zone. The main and one bifurcated peak disappeared, whereas the other bifurcated peak remained and flowed downstream. The main and two bifurcated peaks exhibited negative propagation speeds relative to local flow velocity by consuming the remaining intermediates. CO which formed in the middle of the boundary zone in the weak reaction phase remained unreacted and kept on flowing downstream, but did not flow out since the next cycle of the ignition phase was initiated there.