Kinetic interactions in NH₃+H₂+H₂O oxidation: Broad-range shock tube ȮH laser diagnostics and modeling

Co-firing low-reactivity NH₃ with H₂ enhances its practical utility, while steam injection mitigates H₂-induced reactivity overshoot and suppresses NO_x emissions. Given steam's abundance in exhaust streams and its recirculation potential, understanding high-temperature H₂O-fuel interactions is critical. This study employs reflected shock waves with UV laser absorption spectroscopy to measure in situ ȮH radical time-histories during NH₃/H₂/H₂O/O₂/Ar oxidation at pressures of 1.5–15.5 atm, temperatures of 1566–2485 K with variable H₂ blending and H₂O addition ratios. Thirteen kinetic models were evaluated against measurements using error function analysis. The Mei-2019 model showed minimal overall error yet exhibited notable deviation in ȮH-ignition delay times (IDTs) and profile shapes. The model was refined by updating rate constants of 16 ȮH-sensitive reactions, informed by critical reassessment of experimental and theoretical literature. Virtual additive analysis revealed that under low-enthalpy-change conditions, both H₂ and H₂O primarily influence OH kinetics via direct chemical participation, not third-body collision or thermal effects. Higher H₂ fraction intensify H₂-related pathways, expanding H/ȮH/Ö radical pools and accelerating NH₃ consumption via Ḣ-abstraction. Pathway-specific interrogation further identified key H₂O-involved reactions governing hydrogen-rich ammonia combustion: H₂ + ȮH = Ḣ + H₂O, NH₃ + ȮH = NH₂ + H₂O, Ö + H₂O = 2ȮH.