Numerical study of effect of CO₂ addition in flame structure and NO_x formation of CH₄-air counterflow diffusion flames

Numerical study with detailed chemistry has been conducted to investigate the effect of CO₂ addition on flame structure and NO_x formation in CH₄-air counterflow diffusion flame. Radiation effect is found to be dominant especially at low strain rates. The addition of CO₂ makes radiation effect more remarkable even at high-strain rates. It is, as a result, seen that flame structure is determined by the competition between the radiation and strain rate effects. The important role of CO₂ addition is addressed to thermal and chemical reaction effects, which can be precisely specified through the introduction of an imaginary species. Thermal effect contributes to the changes in flame structure and NO formation mainly, but the effect of chemical reaction cannot be neglected. It is noted that flame structure is changed considerably due to the addition of CO₂, in such a manner, that the path of methane oxidation prefers to take CH₄ → CH₃ → C₂H₆ → C₂H₅ instead of CH₄ → CH₃ → CH₂ → CH.