IGNITION SIMULATION OF METHANE HYDROGEN MIXTURES IN A SUPERSONIC MIXING LAYER/

Ignition of methane/hydrogen and air streams in a supersonic mixing la yer was investigated numerically with the C-1 chemistry for interests in aerospace application. Attention was paid to ignition delay times a nd ignition processes with the addition of methane to hydrogen and the addition of hydrogen to methane involving elementary reactions. In th e first case, results showed that the addition of methane to hydrogen dramatically affected the ignition time. Three stages, an chain-branch ing inhibition stage, a transition stage and a reaction competition st age, were identified. During the first stage, endothermic reaction, CH 4 --> CH3 + H, resulted in a slow development of temperature; and reac tion, H + CH4 --> CH3 + H-2, scavenged radical H. As a result, ignitio n time increased quickly. During the second stage, production of new H radical through reaction CH4 --> CH3 + H, leveled off this rapid incr ease of ignition time. During the third stage, the increase of concent ration of H radicals induced a fast procession of H + CH4 --> CH3 + H- 2, which suppressed the two key reactions, H + O-2 --> OH + O and H O-2 + M --> HO2 + M, and then led to a second rapid increase of igniti on time. In the second case, results showed that the characteristic ti me of chemical heat release of methane/air was much longer than that o f its chain-branching process. It was found that the addition of hydro gen to methane significantly accelerated the ignition of methane/air. Furthermore, this ignition enhancement was shown to be proportional to the amount of hydrogen added to methane.