Effects of ignition position on the explosion of methane-air mixtures with concentration gradients

Abstract

To investigate the effects of ignition position on the explosion of non-uniform methane-air mixtures, experiments were carried out in a closed pipe of 1 m length and 0.3m × 0.3 m cross-section. Methane is uniformly charged at the top of the pipe and then diffused freely, creating a concentration gradient. The concentration gradient of the methane-air mixture depends on the diffusion time. The effects of the three ignition positions – top (IP1), center (IP2) and bottom (IP3) - on the flame propagation at different ignition delay time (tig) were investigated. The ignition position and tig are the key factors influencing the success of ignition. For IP2 and IP3, the flame propagation velocity increases with the increase of tig, but the flame propagation velocity at IP1 does not change significantly with an increase in tig. When tig ≤ 25min, the temperature (Tmax) and maximum overpressure (Pmax) in the pipe at IP1 are significantly higher than those at IP2 and IP3. When the methane-air mixtures are nearly uniform, the Tmax and Pmax at different ignition locations are almost identical. The maximum pressure rise rate, (dP/dt)max, was affected by the flame pipe collision and different flame shapes caused by the ignition position.