Effect of methane addition to hydrogen–air mixtures on the transition to detonation due to shock wave focusing in a \(90^{\circ }\) wedge

Abstract

This work presents the experimental investigation of the influence of methane addition to \({\hbox {CH}}_{{4}}\)–\(\hbox {H}_{{2}}\)–air mixture (\(\varphi = 0.8\)–1.6) on the critical conditions for transition to detonation in a \(90^{\circ }\) wedge. Similar to \(\hbox {H}_{{2}}\)–air mixtures investigated previously, for \({\hbox {CH}}_{{4}}\)–\({\hbox {H}}_{{2}}\)–air mixtures results showed three ignition modes: (i) flame ignition with ignition delay time longer than 1 µs, (ii) strong ignition with instantaneous transition to detonation, and (iii) weak ignition with delayed transition to detonation. In a stoichiometric mixture with 5% \({\hbox {CH}}_{{4}}\) (i.e., 95% \({\hbox {H}}_{{2}}\) in fuel), the transition to detonation corresponds to a shock velocity of roughly 752 m/s (an increase of 37 m/s from that obtained in \(\hbox {H}_{{2}}\)–air) corresponding to \(M = 1.89\). In general, 5% \(\hbox {CH}_{{4}}\) addition caused an increase of 3.25–5.03% in the critical shock wave velocity necessary for transition to detonation for all lean and rich mixtures considered. Also, similar to that found for \({\hbox {H}}_{{2}}\)–air mixtures, the transition-to-detonation velocity increased for a leaner and richer mixture. Moreover, it was observed that methane addition in general increased the pressure limit at the wedge tip necessary for the transition to detonation.