Ammonia is a promising carbon-free fuel, but its low reactivity limits stable combustion. A study by researchers in the Department of Mechanical Engineering at the National University of Singapore and Université Orléans, INSA CVL, PRISME, examines oxidizer-side enhancement strategies, that is O₂ enrichment and N₂O substitution, with detailed numerical simulations. The focus is firstly on the linear stability of laminar premixed NH₃/O₂/N₂ and NH₃/N₂O/N₂ planar flames, quantified via dispersion relations.

For both flames, decreasing the equivalence ratio (ꬾ) generally enhances flame instability due to the reduced effective Lewis number (Le). The O₂-supported flames are only weakly influenced by the oxidizer molar fraction (X_oxid), while the N2O-supported flames show strong sensitivity to X_oxid. In addition, pressure variations up to 10 atm produce negligible changes in the instability characteristics, consistent with the weak pressure dependence of the thermal expansion ratio ꬾ and the product Ze (Le-1). Here, Ze denotes the Zeldovich number.

Nonlinear evolutions are further examined for lean NH₃/O₂/N₂ and NH₃/N₂O/N₂ flames at ꬾ = 0.6. For all cases, the global flame speed enhancement is governed primarily by increased flame surface area, with the stretch enhancement factor remaining close to unity. This interpretation is supported by the statistics of heat release, curvature, and displacement speed, which show that most flame elements are only weakly curved and retain a local thermo-chemical structure close to that of the corresponding 1D unstretched planar flame.

This work presents the first numerical study on the instabilities of premixed ammonia flames enhanced by both N₂₊O -enrichment and N₂O +H-substitution strategies. The novelty lies in systematic quantification of dispersion relations across a broad range of mixture compositions and pressures for linear stability, and large-domain, long-time simulations of nonlinear flame evolution.

Significantly, this research elucidates how different oxidizer strategies modulate hydrodynamic and thermo-diffusive mechanisms to dictate flame stability. Furthermore, by demonstrating that global flame speed enhancement in these mixtures is driven primarily by increased flame surface area rather than local stretch effects, the findings establish a robust foundation for further turbulent modeling.

Ultimately, these fundamental insights offer essential quantitative guidance for designing stable, high-efficiency ammonia combustors for clean-energy and propulsion applications.