Existence and chemistry of stretched ammonia/hydrogen weak flames at elevated pressures

Recently, a two-stage ignition phenomenon of NH₃/H₂ mixtures was experimentally observed in a rapid compression machine, which is closely linked to the concept of flame bifurcations and cool flames. One interesting question may arise: do similar flame bifurcations exist in NH₃/H₂ mixtures? To answer this, this study employs the premixed counterflow configuration and examines the potential bifurcations of the NH₃/H₂/air flame with one-dimensional simulations. For the first time, a novel weak combustion mode of NH₃/H₂ mixtures is observed and termed as the weak flame in the following. Unlike the conventional hot flame, the weak flame exhibits significantly lower flame temperatures (1300–1500 K) and a mere 1 % of the heat release rate (∼10⁹ J/m³/s) associated with hot flames. Within the weak flame, H₂ is entirely oxidized to H₂O, whereas only a portion of NH₃ is partially oxidized, resulting in the formation of H₂O, N₂, N₂O, and NO. Further reaction path analyses reveal that the NH₃ (+OH) → NH₂ (+NO₂) → H₂NO (+NH₂, +HO₂, +O₂, +NO₂) → HNO (+O₂) → NO (+HO₂) → NO₂ → N₂O pathway is the primary oxidation route of ammonia in the weak flame. Furthermore, the effects of pressure, hydrogen content, and equivalence ratio are systematically assessed to explore the operation conditions of the ammonia/hydrogen weak flame. The study reveals that the weak flame is promoted at elevated pressures, and exists with a moderate hydrogen addition, i.e., $x_{H2}$ = 0.02–0.4. A regime diagram is further proposed to summarize the combined influences of hydrogen molar fraction and equivalence ratio. In the end, the impacts of chemical mechanisms are tested and the dominant ammonia oxidization path in the weak flame persists for models capable of predicting weak flames.