Detonation attenuation and quenching in hydrogen mixtures after the interaction with cylinders

The attenuation and quenching of H_2/O_2 detonations transmitted across a column of cylinders were studied experimentally and analytically at sub-atmospheric pressures. Two distinct transmission regimes were observed: successful transmission and complete quenching. The transition between the two regimes was found to correlate with the ratio of inter-cylinder separation distance (b) to a characteristic detonation scale for large blockage ratios (BRs), with critical limits comparable with those previously reported for detonation diffraction from slots. Based on available cell size measurements, the critical transmission limit was b/λ=4.5±3. The proposed theoretical model based on Whitham's geometric shock dynamics confirmed the equivalence between the detonation diffraction at abrupt area changes and around cylinders with large BRs. Complete quenching observed experimentally was accounted for by the weak transmitted shock strength upon detonation failure. For the tested BRs, the transmitted shock speed ranged between 50 the Chapman-Jouguet detonation speed. As a result, the post-shock temperatures fell below the cross-over regime for hydrogen ignition, leading to very long ignition delay times (t_i) despite the temperature rise induced by Mach reflection. The long t_i suppressed auto-ignition, while the high isentropic exponent prevented further convective mixing required for re-initiation. This explained the fundamental difference between arresting hydrogen and hydrocarbon detonations, for which transmitted fast flames punctuated by auto-ignition events were always observed. The transmitted shock strength was found to be well-predicted by our previous self-similar multiple discontinuity gas-dynamic model. Over a very narrow range near the critical conditions, both hot spot re-ignition and detonation re-initiation from Mach shock reflections were observed.