After vent failure, the burnt gas mixtures in the flame bubble rush out of the vented vessel quickly, causing the whole flame bubble to be pushed to the vent and then bounced back, as shown in Fig. 5b and Fig. 6. This bounce phenomenon of the whole flame bubble is known as Helmholtz oscillation (McCan et al., 1985; Cooper et al., 1986). Fig. 5b also shows that when the flame bubble is bounced back from 58 ms, more cracks appear on the flame surface, and some gases are sucked into the vessel from 60 ms to62 ms; this behavior implies that a negative pressure phase exists in the vessel. Later, the flame bubble continues to expand, but the oscillation of the flame speed damps and the flame speed decreases gradually. As shown in Fig. 6, the flame speed remains nearly 0.5 m/s when the flame approaches the vessel wall opposite to the vent. Note that the bounce phenomenon of the whole flame bubble is not observed for pv 1⁄4 0 kPa in current study.