Measurement and Kinetics Prediction of Undiluted Methane-Oxygen Laminar Flame Speeds

The extremely high energy release of CH4/O2 combustion has been a hurdle to safely measuring its fundamental combustion properties, as evidenced by the dearth of experimental data in the literature. To address this need, this study presents new CH4/O2 laminar flame speed data for a wide range of equivalence ratios (ϕ = 0.2 to 2.4) at low pressure (0.5 atm) and multiple initial pressures (0.5 to 3 atm) at an equivalence ratio far from stoichiometry (ϕ = 2). All experiments were conducted at an initial temperature of 298 K using a constant-volume, spherically expanding flame vessel. At an initial pressure of 0.5 atm, the laminar flame speed reached a maximum of 411 cm/s near ϕ = 0.9, indicating that peak flame speed is achieved with a slightly fuel-lean mixture. Far from stoichiometry, flame speeds were approximately 72 cm/s and 34 cm/s at ϕ = 0.2 and 2.4, respectively. Laminar flame speed predictions were computed using several commonly cited chemical kinetics mechanisms for methane combustion to evaluate mechanism performance, with performance approximately the same for all mechanisms. Every mechanism underpredicted laminar flame speed at 0.5 atm for 0.3 < ϕ < 1.9, with performance especially poor near stoichiometry (approx. 100 cm/s underprediction near the peak). For ϕ = 2.0, experiments were conducted for pressures from 0.5 to 3.0 atm, showing that the flame speed varies approximately according to the power law S_L ~ P^(-0.1). The mechanisms captured this pressure dependency, with GRI-Mech 3.0 performing especially well. Sensitivity analysis shows that many reaction adjustments are necessary to solve the underprediction near stoichiometry, with CH3+OH⇌CH2O+H potentially a reaction of interest.