The extinction limits and the radical index of non-premixed counterflow flames of methane/ammonia/nitrogen versus high-temperature air

The extinction limits of non-premixed counterflow flames of methane (CH₄)/ammonia (NH₃)/nitrogen (N₂) versus high-temperature air (T_Air = 700 K and 1000 K) were investigated both experimentally and numerically. Extinction stretch rates of non-premixed counterflow flames of CH₄/NH₃ mixtures decreased greatly as the ammonia mixing ratio increased. Recent chemical kinetic models could well predict measured extinction limits of non-premixed counterflow flames of CH₄/NH₃ mixtures, especially for T_Air = 1000 K. Chemical kinetic analyses indicated that the nature of NH₃ consuming active radicals but not regenerating them through its oxidation is the primary reason for the drastic decreases in extinction stretch rates of non-premixed counterflow flames of CH₄/NH₃ mixtures. Furthermore, the combined metric of the transport weighted enthalpy (TWE) and the radical index (RI) is introduced for non-premixed counterflow flames of CH₄/NH₃ mixtures. The OH-radical index (RI_OH), previously used in the combined metric for extinction limits of non-premixed counterflow flames of large hydrocarbons, expresses linear relationships with extinction limits for both T_Air conditions. According to further investigations on heat releases from individual reactions, the contribution to heat releases from a reaction involving O radicals, i.e., CH₃ + O ⇄ CH₂O + H, becomes large in addition to reactions involving OH radicals, i.e., CO + OH ⇄ CO₂ + H and H₂ + OH ⇄ H + H₂O. Based on the analyses, a new radical index based on the combination of OH and O radicals (RI_OH&O) is proposed. The RI_OH&O better expresses the linear relationship between extinction limits and the combined metric of RI_OH&O and TWE.