Effects of vent burst pressure on hydrogen-methane-air deflagration in a vented duct

The effect of the static burst pressure of the vent cover (${p}_{{\rm{v}}}$) on the flame evolution of hydrogen–methane–air deflagration and pressure buildup inside and outside the duct was studied. A series of vented deflagration experiments of hydrogen-methane-air mixtures were carried out in a 300 mm long, 300 mm wide, and 1000 mm high duct with a 250 mm × 250 mm vent at the top. Stoichiometric hythane–air mixtures were prepared according to Dalton’s law of partial pressure. The mixed gas was ignited at the vessel center by an electric spark. The explosion flames were recorded by a high-speed camera at 2000 frames per second. Piezoresistive pressure sensors were used to record the internal and external overpressures. Experimental results reveal that ${p}_{{\rm{v}}}$ significantly affects the pressure-time histories and the flame propagation in the duct. Helmholtz-type oscillations of the internal pressure at the lower flame front were observed in all tests, with the oscillation frequency increasing with ${p}_{{\rm{v}}}$. Acoustic oscillations with a ~1200-Hz frequency occur when ${p}_{{\rm{v}}}$≥12 kPa. The maximum internal overpressure increases with the distance to the vent. The maximum internal overpressures near the vent and at the duct center increase almost linearly with ${p}_{{\rm{v}}}$. However, the maximum internal overpressure at the duct bottom is not increasing monotonically with ${p}_{{\rm{v}}}$. The pressure peak resulting from the external explosion, which increases with ${p}_{{\rm{v}}},$ always dominates the external pressure–time histories. In addition, external explosion affects the venting process in all tests, but the effect is significantly weakened when ${p}_{{\rm{v}}}$ ≥ 31 kPa.