Effects of oxygen enrichment on laminar burning velocities and Markstein lengths of CH₄/O₂/N₂ flames at elevated pressures

Laminar burning velocities and Markstein lengths of CH₄/O₂/N₂ mixtures were measured in a constant volume combustion chamber over a wide range of O₂ contents and equivalence ratios at elevated pressures. The laminar burning velocities were extracted using an accurate nonlinear methodology with properly chosen flame radii to reduce the systematic uncertainty. Chemical kinetic simulation was conducted using three different chemical reaction mechanisms with the PREMIX code. In general, numerical results agree well with the experimental data under most conditions. However, GRI Mech 3.0 and Aramco Mech 1.3 slightly under-predict the laminar burning velocities at high O₂ content, and Han Mech overestimates the experimental data at low O₂ content. With oxygen enrichment, the laminar burning velocity is dramatically enhanced by thermal effect, and diffusion effect makes a small positive contribution, while kinetic effect is negligible. Furthermore, Markstein length decreases with the oxygen enrichment ratio because of the combined effects of the density ratio and flame thickness. Moreover, for high O₂ content mixtures, the flame becomes unstable mainly due to the effect of hydrodynamic instability.