Effect of film thickness and methane fraction on explosion characteristics of biogas/air mixture in a duct

Abstract In order to evaluate the explosion characteristics of biogas (i.e., the mixture of CH4 and CO2), experiments were conducted in a duct with a length-to-width ratio of 10. The ignition was activated at the closed end while the opposite end was sealed by a PVC film of varying thickness. The effect of PVC film thickness and methane fraction in biogas was studied. The results show that there were three possible pressure peaks (Pb, Pmfa, Pext), due to the film failure, the attainment of maximum flame area and the external explosion, respectively. The pressure profile pattern was dependent on the combination of film thickness and methane fraction. Two pressure peaks Pb and Pmfa were observed for the PVC film thickness δ ≤ 0.033 mm. However, the pressure peak Pmfa was absent for the film thickness δ ≥ 0.055 mm because the PVC film did not rupture in these cases when the flame had touched the sidewall. In other words, the maximum flame area was achieved prior to the moment of venting. It signifies that in the absence of explosion venting, the flame quenching due to the sidewalls alone was insufficient to induce an individual pressure peak Pmfa. The third pressure peak Pext was low for the biogas explosion in the current configuration. The film thickness exerted a greater impact on bursting pressure Pb than did the methane fraction, at least for the range of film thicknesses and methane concentrations considered. Additionally, the individual effect of the film thickness or the methane fraction was less influenced by each other. The existing theories were accurate to predict the characteristic times of the flame propagation for δ = 0.011 mm but not for δ ≥ 0.033 mm. The PVC film thickness affected the explosion overpressure more, whereas the CH4 fraction in biogas affected the flame front velocity more remarkably.