Deflagration evolution of premixed H₂/CH₄/air mixture undergoing a sudden expansion near the ignition source in a half-open duct

Experiments were carried out in a half-open duct to investigate the deflagration evolution of premixed CH4/H-2/ air mixtures undergoing a sudden expansion. Obstacles were located over and under the ignition source to simulate a long and narrow flat gap, and when the flame shot through the gap into the main body of the explosion duct, it would undergo a process of sudden expansion. By adjusting the spacings (S) and widths (W) of the gap, a series of sudden expansion conditions were considered, and the flame propagation behaviors and explosion characteristics were investigated. The results showed that different extent of flame front inversion was observed under different size of gap conditions, including inclined flat flame, vertical flat flame and tulip flame. Besides, results indicated that the vortex pairs near the obstacles play an essential role in the flame front inversion phenomenon. In addition, it was also found that the maximum overpressure in the sudden expansion duct was much lower than that in the smooth duct or with obstacles in the middle of duct. In addition, the maximum pressure (P-max) exhibits a concave function with the increase of W, and the minimum value of P-max was obtained at W = 150 mm, S = 30 mm, which was closely related to the combined effect of venting and combustion expansion.