Experimental study of the effect of a cavity on propagation behavior of premixed methane–air flame

In this study, experiments were performed in a closed duct with a cavity to reveal the effect of the cavity on the flame propagation behavior. A high-speed camera and three pressure sensors were used to measure the premixed flame evolution and pressure history, respectively. The results indicate that the cavity can prompt flame front propagation in the cavity region and at a certain distance downstream of the cavity via two mechanisms. The first mechanism involves an increase in the flame surface area owing to the vortex growth and turbulence evolution. In the second mechanism, the expansion flow from the delayed burning of the unburned gas within the vortex and cavity accelerates the flame front. The presence of the cavity causes the flame-tip velocity to fluctuate, and the frequency of rises and falls highly depends on the length scale of the cavity. For cases with a similarly scaled cavity depth (H/D), the maximum pressure (P_max) increased approximately linearly with an increase in the scaled cavity length (L/D). When the L/D was kept constant, as the H/D increased, the P_max increased for cases with L/D > 1, however, P_max decreased for cases with L/D < 1. This trend resulted from the combined effect of the time required to attain the maximum pressure and the ratio of the volume to the surface area on the pressure reduction due to heat loss.