Study on premixed flame dynamics of CH4/O2/CO2 mixtures

Abstract

In order to study the flame dynamics characteristics of premixed CH4/O2/CO2 mixtures, a series of experiments were carried out in an elongated square duct. The equivalence ratio ϕ was chosen as 0.6, 0.8, 1.0, 1.2 and 1.4, and the oxygen fraction γ in the O2/CO2 oxidizer varied from 0.20 to 0.60 with the step of 0.05. The flame structural evolution in time, the flame front dynamics and the pressure dynamics are discussed. The results show that the flame regime can be classified into buoyancy-dominated, tulip and non-tulip flames, depending on the reactivity (i.e., the laminar burning velocity, LBV) of the CH4/O2/CO2 mixture. In addition, the inclined, axisymmetric and T-shaped tulip flames were observed. The flame velocity profiles versus the flame front position heavily relied on the flame regime, and the flame front position at which the inflection point in velocity occurred was in good agreement with the predicted one through Bychkov's model. The LBVs of CH4/O2/CO2 mixtures were found to show a good linear dependence on the O2 to CO2 molar ratio for the conditions considered. The maximum flame velocity (Vmax) is well correlated with the LBV, and its logarithm is linearly related to the reciprocal of the adiabatic flame temperature (Tad). For high-speed flames (LBV ≥ 65.51 cm/s or Ma ≥ 0.437), gas compression will moderate the flame acceleration noticeably. Unimodal and bimodal pressure dynamics were initiated depending on the flame regime. For both off-stoichiometric and low oxygen-fraction mixtures featuring low reactivity, the maximum pressure (Pmax) was less affected by oxygen fraction. Furthermore, the Pmax was continuously growing with the increasing LBV, indicating that the LBV shows its superior potential as a single indicator to predict the explosion pressure in the elongated duct.