Bifurcation and multi-zone heat release in NH3/NO-air counterflow diffusion flames due to NO addition

This study investigates the extinction behavior, steady solution branching, and internal reaction structure of NH 3 /NO-air diffusion counterflow flames. Ammonia is the fuel and NO/air mixtures serve as the oxidizer, with the NO mole fraction in the oxidizer ( X NO ) varied from 0.0 to 1.0. The evolution of S-curves, extinction scalar dissipation rates, and flame structure is examined as a function of X NO . The extinction scalar dissipation rate exhibits a non-monotonic dependence on X NO , increasing markedly at intermediate NO levels and decreasing when NO becomes dominant. Additional calculations with fixed oxidizer-side O 2 fraction show that NO addition itself monotonically promotes extinction resistance, indicating that the non-monotonic trend in the original formulation arises from the competition between NO-promoted radical chemistry and the simultaneous weakening of O 2 -supported branching as NO replaces air. Sensitivity and chemical-pathway analyses show that extinction control shifts from conventional O 2 -driven branching in NH 3 /air flames to NHx-NO coupling at elevated X NO , with reactions, e.g., NH 2 + NO ⇌ NNH + OH playing a key promoting role. At high NO fractions ( X NO >0.6), folded S-curves emerge, yielding a high-temperature strong-flame branch characterized by temperatures exceeding 2,600 K, elevated radical levels, and multiple heat release rate (HRR) peaks. The strong flame is characterized by higher temperatures and radical concentrations, enhanced H/O/OH–N coupling, faster NH₃–NHₓ conversion, and stronger N/NO-driven heat release than the normal flame. Moreover, the primary HRR peak is governed by NH 3 consumption by H-abstraction, whereas the secondary and tertiary peaks arise from NO-related oxidation-reduction chemistry. As the scalar dissipation rate changes and temperature decreases, the multi-zone HRR structure collapses sequentially. Additional radiative calculations show that radiation mainly affects the low-strain extinction branch by introducing weak-flame turning points. Multi-mechanism comparisons and robustness analysis further confirm that the key observations are not artifacts of a particular chemical mechanism or numerical setup.