Experimental and Numerical Study of the Effect of CO2 Replacing Part of N2 present in Air on CH4 Premixed Flame Characteristics Using a Reduced Mechanism

The effect of CO2, which replaces part of N-2 present in air, on flame stability, laminar burning velocities (LBVs), and intermediate radicals (O OH) of CH4/O-2/N-2/CO2 premixed flames has been analyzed using detailed experiments and numerical studies. The numerical simulations were conducted using the PREMIX code with a detailed chemical reaction mechanism (GRI-Mech 3.0) and a reduced mechanism (39 species and 205 reactions) based on GRI-Mech 3.0 over a wide range of equivalence ratios (Phi = 0.7-1.3) and CO2 substitution ratios (0-30%). The reduced mechanism showed a good agreement with the other detailed mechanisms and experimental data. The experimental and numerical results showed that the substitution of CO2 diminishes the stability of the flame, and the flame blow-out speed is significantly reduced (the substitution ratio is 0-30%, and the corresponding flame blow-out velocity is 5.2-2.5 m/s). In addition, CO2 inhibits the LBV of the flame owing to the decrease of O and OH mole fractions. It not only accelerates the consumption of these two free radicals but also inhibits the generation of these two free radicals. Further analysis concluded that the substituted CO2 has the greatest influence on the LBV sensitivity coefficient of the HO2 + CH3 = OH + CH3O reaction.