Comparison of the premixed flame dynamics of CH4/O2/CO2 mixtures in closed and half-open ducts

Abstract

The premixed flame dynamics of CH4/O2/CO2 mixtures ignited in two duct configurations, i.e., both ends closed and half-open (open at one end and closed at the opposite end), were investigated. The influence of the combustion-generated gas compression on the chemical kinetics was analyzed. The results show that CH4/O2/CO2 mixtures formed thrice distorted tulip flames (DTF) in the closed duct. The shortened time scale β impairs the interaction between the flame front and the Rayleigh-Taylor instability, resulting in the gradual reduction of the flame distortion degree. Moreover, the deeper primary cusps generated by the stronger Darrieus-Landau instability is favorable for forming a distorted tulip flame with more times of distortions. The duct configuration exerts a significant influence on the relationship between the maximum flame velocity (Vmax) and pressure (Pmax) versus the peak mole fraction [H + O + OH]max. The higher flow velocity of unburned gas induced by the duct-venting and the expansion waves generated by the collapses of secondary cusps as an additional contribution promote the flame velocity for the half-open duct and the pressure rise for the closed duct, respectively. In addition, R1: H + O2 = O + OH exhibits the most positive effect on the laminar burning velocity Sl. R97: CH3 + H (+M) <=> CH4 (+M) and R39: HCO + OH <=> H2O + CO show the most negative influence on Sl for the oxygen fraction γ ≤ 0.45 and γ > 0.45, respectively. The gas compression owing to the pressure waves especially discernible in the closed duct inhibits [H + O + OH]max, and is further enhanced by the oxygen enrichment. This entails to a certain extent a much lower flame velocity in the closed duct as compared to in the half-open duct.