An experimental and kinetic study of OH(A²Σ⁺) formation and quenching in ammonia-hydrogen-air flames

This work provides a kinetic and experimental investigation of excited radical OH(²Σ⁺) (also referred to as OH*) in NH_3–H₂-air flames. A counterflow burner is used to stabilize laminar diffusion flames over wide ranges of ammonia fraction in the fuel blend (0.2 ≤ x_NH3 ≤ 0.8) and strain rate (40 ≤ a ≤ 200/s). Using an intensified camera or a spectrometer coupled to a Cassegrain optical system, spatially resolved and spatially integrated OH(A²Σ⁺-X²Π) chemiluminescence intensities are measured. These data are used to challenge a kinetic mechanism largely developed from existing literature schemes. Measurements and simulations show that two distinct OH* peaks exist in spatially resolved profiles for intermediate ammonia fractions in the fuel blend. Sensitivity analyses identified that reactions N₂O+H=N₂+OH* and H+O+M=OH*+M, respectively pertinent to NH₃ and H₂ oxidation routes, are responsible for the formation of OH*. The proposed kinetic mechanism gives a remarkable portrait of the empirical data recorded in diffusion flames as well as in premixed NH_3–H₂-air flames from the literature. Nevertheless, the contribution of reaction N₂O+H=N₂+OH* in the formation of OH* in the peak closest to the fuel side of diffusion flames is consistently overpredicted. Consequently, the frequency factor of the N₂O+H[dbnd]N₂+OH* reaction is adjusted to A = 1.35 × 10¹⁴ cm³mol⁻¹s⁻¹, which significantly improves predictions of spatially integrated and spatially resolved OH* intensities for all the diffusion and premixed flames examined.