Fig. 2 shows a schematic of the experimental apparatus. Methane-air mixtures were ignited at the center of the vented vessel via electric spark with an ignition energy of approximately500 mJ. The schlieren system combined with high-speed camera 1 was used to visualize the internal explosion flames. High-speed camera 2 was used to record the flame behaviors inside and outside of the vented vessel. The frame rate of the high-speed cameras was 1000 fps. Two piezoelectric pressure transducers were used to determine the pressure histories during the venting process. One (PT1) was installed in the vessel wall opposite to the vent, and the other (PT2) was fitted in the wall of a neck 2 cm from the vent, as shown in Figs. 1 and 2. Both pressure transducers were coated with a thin layer of silicon grease to avoid thermal effects on pressure measurements.