Experiments on duct-vented explosion of hydrogen–methane–air mixtures: Effects of equivalence ratio

Abstract In this study, experiments on the duct-vented deflagrations of hydrogen–methane–air mixtures were performed in a cylindrical vessel connected with a relief duct, at an initial pressure of 101 kPa and an initial temperature of 290 K, to investigate the effects of equivalence ratio ( ϕ ), in the range of 0.6–1.8, on flame evolution and pressure buildup within and outside the venting configuration. Experimental results reveal that secondary explosion in the relief duct occurred and a pressure peak with amplitude higher than the overpressure in the vented chamber formed, which resulted in the gases in the relief duct flowing reversely to the vented chamber (reverse flow). The secondary explosion in the relief duct reduced the venting efficiency in the vented chamber and created a pressure peak there, which dominated the pressure–time histories in the vented cylindrical vessel for ϕ ranging from 0.8 to 1.4. Acoustic oscillations of the overpressure in the cylindrical vessel could be easily distinguished for ϕ  ≤ 1.0, and the pressure peak owing to acoustic enhanced combustion was dominant at ϕ  = 0.6. The maximum overpressure in the cylindrical vessel first increased and thereafter decreased as ϕ was increased from 0.6 to 1.5 and was independent of ϕ for richer mixtures. A shock wave was visualized in the tests with ϕ in a range of 1.0–1.2 when the combustible cloud outside the venting configuration was ignited, which resulted in a pressure spike in the external pressure–time histories. The maximum overpressure outside the relief duct first increased and thereafter decreased as ϕ was increased.