Ignition simulation of methane/hydrogen mixtures in a supersonic mixing layer

Ignition of methane/hydrogen and air streams in a supersonic mixing layer was investigated numerically with the C-1 chemistry for interests in aerospace application. Attention was paid to ignition delay times and ignition processes with the addition of methane to hydrogen and the addition of hydrogen to methane involving elementary reactions. In the first case, results showed that the addition of methane to hydrogen dramatically affected the ignition time. Three stages, an chain-branching inhibition stage, a transition stage and a reaction competition stage, were identified. During the first stage, endothermic reaction, CH₄ → CH₃ + H, resulted in a slow development of temperature; and reaction, H + CH₄ → CH₃ + H₂, scavenged radical H. As a result, ignition time increased quickly. During the second stage, production of new H radical through reaction CH₄ → CH₃ + H, leveled off this rapid increase of ignition time. During the third stage, the increase of concentration of H radicals induced a fast procession of H + CH₄ → CH₃ + H₂, which suppressed the two key reactions, H + O₂ → OH + O and H + O₂ + M → HO₂ + M, and then led to a second rapid increase of ignition time. In the second case, results showed that the characteristic time of chemical heat release of methane/air was much longer than that of its chain-branching process. It was found that the addition of hydrogen 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.