Flame propagation dynamic characteristics and mechanisms of methane-syngas-air mixtures in a semi-enclosed duct

The composition of syngas is complex, often leading to the generation of methane, which impacts the explosive characteristics of syngas. The analysis focused on the kinetic characteristics and mechanisms of flame propagation in methane-syngas-air mixtures in a semi-enclosed duct under various equivalence ratios (phi = 0.8-1.4) and methane volume ratios (R = 0 %-70 %) under ambient temperature and pressure. The results indicated that the flame speed and pressure propagation of methane-syngas-air mixtures could be divided into three stages: a stable stage, an oscillatory increase stage, and an oscillatory decrease stage. In the stable stage, as the methane proportion in the premixed gas increased, the oscillations in the flame and pressure decreased. Turbulent flow of the mixed gas was observed due to restrictions and obstructions within the duct. This led to an increase in the peak pressure and flame speed of the mixed gas, thereby resulting in an oscillatory increase stage. Distorted tulip-shaped flames were also formed in the semi-enclosed duct. The flame of the pure syngas/air mixture exhibited a hemispherical shape and a finger-shaped and slope-shaped propagation structure. When 30 %, 50 %, or 70 % of the methane in the methane-syngas-air mixtures was added, the flame exhibited a propagation structure characterized by a hemispherical shape, finger shape, slope shape, flat shape, tulip shape, or distorted tulip shape. The flame front evolved with time from a linear growth stage to a nonlinear growth stage. The flame front speed of the methane-syngas-air mixtures was closely related to the pressure and flame structure. With the increase in the methane proportion in the methane-syngas-air mixtures, the concentration of key radicals and the rate of OH* generation and consumption gradually decreased. Sensitivity analysis revealed that with increasing methane proportion in the premixed gas, the most important reaction dominating OH* consumption changed from R15 (H + O-2(+M) <=> HO2(+M)) to R138 (CH4 + OH <=> CH3 + H2O) and then to R112 (2CH(3)(+M) <=> C2H6(+M)); moreover, the most important reaction dominating OH* generation remained R1 (H + O-2 <=> O + OH). The addition of methane interrupted the hydrogen-oxygen chain reaction of the methane-syngas-air explosion process.