Effect of methane addition to hydrogen-air mixtures on the transition to detonation due to shock wave focusing in a 90° wedge

This work presents the experimental investigation of the influence of methane addition to CH4-H2-air mixture (phi = 0.8-1.6) on the critical conditions for transition to detonation in a 90. wedge. Similar to H2-air mixtures investigated previously, for CH4-H2-air mixtures results showed three ignition modes: (i) flame ignition with ignition delay time longer than 1 mu s, (ii) strong ignition with instantaneous transition to detonation, and (iii) weak ignition with delayed transition to detonation. In a stoichiometric mixture with 5% CH4 (i.e., 95% H2 in fuel), the transition to detonation corresponds to a shock velocity of roughly 752 m/s (an increase of 37m/s from that obtained in H2-air) corresponding to M = 1.89. In general, 5% CH4 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 H2-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.