Flame propagation and spectrum characteristics of CH₄-air gas mixtures in a vertical pressure relief pipeline

The explosion characteristics, flame propagation mechanism, and spectrum characteristics of five free radicals (C₂·, CHO·, CH·, O·, and OH·), formaldehyde (CH₂O), and one reagent (O₂) under various concentrations (w) of methane were investigated using a vertical rectangular pressure relief pipeline. The experimental results indicated that the instantaneous flame propagation speed (v), maximum flame temperature (T_ex), maximum explosion pressure, and relative spectrum intensity (I) first increased and subsequently decreased with methane concentration. The maximum instantaneous flame propagation speed (v_max), maximum flame temperature (T_max), maximum explosion pressure (P_max), and maximum relative spectrum intensity (I_max) were achieved, and the time taken to form a tulip-shaped flame was the shortest at w = 10 vol%. The experiment also revealed that T_ex and w had a polynomial relationship. In the explosion process, positive correlations were noted among the T, P, and I. Furthermore, the spectrum signal characteristic times of the intermediate species was considerably influenced by v. When the methane concentration was 9.5 vol%, the I of the seven intermediate generation rates exhibited two distinct trends. This finding contributes to the analysis and validation of the microscopic mechanism underlying gas explosion flame propagation. Further, a reference for selecting new gas explosion suppressants based upon the attenuation level of free radical spectrum intensity.