Laminar burning rates and markstein lengths of CH₄/O₂/inert mixtures at high pressures

Outwardly-propagating spherical flames at constant pressure for methane/oxygen/inert mixtures were experimentally studied for pressures up to 40 atmospheres. The flames were imaged with Schlieren cinematography in a constant, high-pressure combustion apparatus. Results substantiate previous observations of the propensity of cell formation over the flame surface due to hydrodynamic instability, and provide convincing evidence that wrinkled flame is the preferred mode of propagation in methane/air mixtures in environments above only a few atmospheres. It is further shown that, by using helium as a diluent, and by reducing the oxygen concentration of the combustible, thermal-diffusional instability can be mostly suppressed and the hydrodynamic instability delayed. Stretch-free laminar flame speeds and Markstein lengths as functions of equivalence ratio were subsequently determined for such smooth flames up to 40 atm, and were compared with the calculated values allowing for detailed chemistry and transport.