Effect of concentration and ignition position on vented methane-air explosions

Experiments were conducted in a 1 m(3) vessel with a top vent to investigate the effect of methane concentration and ignition position on pressure buildup and flame behavior. Three pressure peaks (p(1), p(2), and P-ext) and two types of pressure oscillations (Helmholtz and acoustic oscillations) were observed. The rupture of vent cover results in p(1) that is insensitive to methane concentration and ignition position. Owing to the interaction between acoustic wave and the flame, p(2) forms in the central and top ignition explosions when the methane-air mixture is near-stoichiometric. When the methane-air mixture is centrally ignited, p(2) first increases and then decreases with an increase in the methane concentration. The external explosion-induced P-ext is observed only in the bottom ignition explosions with an amplitude of several kilopascals. Under the current experimental conditions, flame-acoustic interaction leads to the most serious explosions in central ignition tests. Methane concentration and ignition position have little effect on the frequency of Helmholtz and acoustic oscillations; however, the Helmholtz oscillation lasts longer and first decreases and then increases as the methane concentration increases for top ignition cases. The ignition position significantly affects the Taylor instability of the flame front resulting from the Helmholtz oscillation.