Flame behaviors of vented inhomogeneous hydrogen deflagrations in an enclosure: Effects of the ignition position

The effect of ignition position on vented inhomogeneous hydrogen deflagrations in a 7 m3 enclosure was investigated. For bottom ignition IG1, 1/3-height ignition IG2 and center ignition IG3, the initial flame bubble appears as an ellipsoid and extends vertically within the hydrogen release plume. Subsequently, a coupled flame structure involving both the jet fire and flame bubble propagates towards the vent. The flame bubble retains a shape in which the upper part is wider than the lower part during its expansion. For 2/3-height ignition and top ignition, a coupled flame structure is generated involving the horizontal propagation of a layer of flame below the ceiling and expansion of the flame bubble. For ignitions at IG1, IG2 and IG3, the speed of the rightward flame front rapidly decreases from its initially substantial value to a small value as the horizontal flame front moves away from the jet centerline. Moreover, the double flame accelerations are recorded. Three overpressure peaks are observed, namely Popen, Phel and Pvib, which result from the vent opening, Helmholtz-type oscillations and thermo-acoustic oscillations, respectively. The maximum overpressure Pmax is close to each other by ignitions at the same non-dimensional height of 0.82. However, a larger Pmax is induced by ignition at a lower height. The maximum Pmax is caused by bottom ignition. The overpressure oscillation with high-frequency and wideband is enhanced for bottom ignition, and the maximum frequency of overpressure oscillations during the thermo-acoustic coupling is 1041 Hz.