Effect of hydrogen concentration on the vented explosion of hydrogen-air mixtures in a 5-m-long duct

A series of vented hydrogen-air explosion experiments were carried out in an end-vented rectangular tube, and the effect of hydrogen concentrations varying from 10% to 50% on the flame evolution and the maximum overpressure inside and outside the duct were investigated. Experimental results reveal that the flame behavior was strongly affected by hydrogen concentration: the flame speed at the vent exit first increased and then decreased as hydrogen concentration was increased; a tulip flame and two bright fireballs were respectively observed within and outside the duct in some tests. Two dominant pressure peaks and two types of oscillations could be distinguished in the internal pressure curves, corresponding to the rupture of the membrane, venting of the burned gas, Helmholtz, and acoustic oscillations, respectively. Under the current experimental conditions, the pressure peak with a low amplitude owing to the rupture of membrane dominated the external pressure-time histories for hydrogen concentrations less than 25%. However, when the hydrogen concentration ranged from 30% to 50%, the pressure peak caused by the second external explosion become dominant. Besides, the influence of the external explosions on the internal pressure peak structures was discussed. As hydrogen concentration was increased, the maximum internal overpressure first increased and then decreased, whereas the maximum external overpressure increased monotonically. In this study, the maximum reduced overpressures calculated by Molkov's best-fit model agreed well with the experimental data for near-stoichiometric and rich hydrogen-air mixtures.(c) 2022 Published by Elsevier Ltd on behalf of Institution of Chemical Engineers.