The distortion of the flame bubble occurs much later for pv20 kPa. As shown in Fig. 6, the flame propagates towards and away from the vent at an oscillatory rate of approximately 1.5e2.5 m/s in approximately the first 30 ms after ignition, and the flame bubble remains nearly spherical, as shown in Fig. 5b. Note that the flame speed of 1.5e2.5 m/s is much larger than the laminar burning rate because of self-acceleration (Ferrara et al., 2008). As the internal flame approaches the entrance of the neck, the flame propagating towards the vent is accelerated due to the abrupt decrease in passage area from vessel to neck, as shown in Fig. 6.