It is found that the oscillation of the external flame occurs nearly at the same time as the pressure oscillation in vessel, and the difference between their periods is not large. Thus, it can be deduced that the oscillation phenomenon of the external flame may be caused by the pressure oscillation in vessel. For pv> 0 kPa, when the internal pressure exceeds the ambient one after the appearance of the negative pressure phase, gas mixtures are discharged; as a result, the length of the external flame increases. On the contrary, if no pressure oscillation occurs in the vessel, then no oscillation of the external flame occurs, as in the case of pv 1⁄4 0 kPa. Fig. 10 also shows that Lmax is larger for pv 1⁄4 131 kPa than that for pv 1⁄4 20 kPa, but it is nearly independent of pv during the following oscillation phase.