Chemical mechanism analysis on flame propagation properties of methane/ air deflagration mix-diluted by CO₂ + N₂

To investigate the inhibitive behaviors methane/air deflagration using mix-dilutions (i.e., CO2 + N2), a series of deflagration experiments were conducted in a cylindrical chamber. Furthermore, various proportions of CO2 were employed in the 10% and 20% mix-dilutions to assess the requisite conditions for the CO2 chemical effect to effectively restrain methane deflagration. Additionally, the inhibition mechanism of methane deflagration by mix-diluents was elucidated by isolating the CO2 chemical effect and analyzing the chemical kinetics involved. The results show that an inversion directly forms in the lower flame front of spherical flame without undergoing the four evolution stages of "Tulip flame", and buoyancy effect is the dominant factor leading to the inversion by theoretical calculations that exclude the influence of the intrinsic flame instabilities. Moreover, the flame inversion created by the buoyancy effect can reduce the chamber destruction by weakening the quench expansion wave. Additionally, the increasing CO2 proportion in 20% and 10% mix-dilutions exhibits significant differences in its inhibitory efficiency on methane detonation pressure, indicating the requirement of certain critical conditions for the CO2 chemical inhibition to take effect. Therefore, chemical pathway analysis reveals that the CO2 chemical effect significantly inhibits the rate of R99 through the reduction in the generation of H radicals only under thicker flame front conditions. Hence, the chemical inhibition of methane deflagration by CO2 necessitates the provision of temporal scales by reducing the temperature rate and the thickening the flame thickness through inerting effect.