Outward propagation, burning velocities, and chemical effects of methane flames up to 60 ATM

Using a specially designed high- and constant-pressure combustion chamber, the propagation and morphology of spark-ignited expanding spherical methane flames were imaged using schlieren cinematography and a high-speed digital camera. Stretch-free laminar burning velocities were subsequently determined for methane/air flames up to 20 atm and methane/oxygen/helium flames up to 60 atm. Computational simulation using GRI-MECH 3.0 showed satisfactory agreement with the experimental data up to 20 atm, and moderate deviation for pressures above 40 atm. Markstein lengths, global activation energies, and overall reaction orders were also determined as functions of pressure, with the latter two parameters exhibiting non-monotonic behavior caused by the changeover from H-O₂ to HO₂ chemistry similar to that of the explosion limits of homogeneous hydrogen/oxygen mixtures.