Flame quenching in front of a cold wall under two-step kinetics

Abstract The transient process of laminar flame quenching at a cold wall is studied. A plane flame is assumed that travels perpendicularly toward the wall. The kinetic model A + B → 2B and 2B → C consists of a chain branching and a chain breaking step. The theoretical results indicate that the history of the quenching process depends essentially on the magnitude of the intermediate species B that might be viewed as a radical. The radical level is a function of the ratio of the frequency factors of the chain breaking to the chain branching reaction. If the chain breaking reaction is fast, the radical level is low and the quenching process is rapid once it has been initiated. If the chain breaking reaction is slow, a large amount of radicals sustains the first reacton and the flame remains in front of the wall for a much longer time. During this period all the fuel in the quenching layer is finally consumed. The comparison with a calculation with a complete set of elementary reactions for the combustion of acetylene and methanol in air shows that high radical levels and complete fuel consumption are realistic features of real flames. This indicates that the two-step process chosen in this study is adequate to describe the flame quenching process at a cold wall in its essential features whereas a one-step global mechanism is not.