Study on the influence of ignition position on the explosion characteristics of methane-air premix in a semi-closed pipeline

Flammable gas explosion frequently occurs in the actual oil and gas pipeline transportation or mining operations. Given this problem, this paper studies the effect of ignition positions on the explosion characteristics of the methane-air mixture using a semi-closed transparent gas explosion pipe. According to the results, regardless of ignition position, the overpressure curve of each measuring point presents a process of rising-falling to negative-rising-stabilizing, and the flame propagation to the open end (left flame) evolves from spherical to finger shape. As the ignition position is farther away from the closed end, the overpressure at the closed end and the average flame speed on the right side of the ignition source climb gradually. In contrast, the average flame speed on the left side declines gradually. Besides, When the ignition position（IP3 and IP4）are near the open end, P1（Overpressure at the closed end）overpressure curves under both working conditions oscillate obviously before reaching the peak. The flame front undergoes a repeated compression-stretch-recompression, and the extreme point of the oscillation overpressure waveform always corresponds to the extreme point of the flame position. During this period, the amplitude of the P1 curve grows linearly and parabolically with time. According to the research, the overpressure oscillation is triggered by the reverse pressure wave after the left-side flame rushes out of the pipeline under such working conditions. The growing amplitude during the oscillation can be caused by various factors, including the thermoacoustic instability and the interaction between the flame front of the closed end and the unburned gas column. The research results deepen the understanding of the combustion and explosion characteristics of flammable gas, better the explosion safety protection in its storage, transportation, and processing, and provide a theoretical basis for the safety protection of detonation devices.