Laminar burning velocity of NH₃ + N₂ + O₂ and NH₃ + Ar + O₂ flames at ultra-lean and rich conditions

Ammonia (NH₃) has received increasing attention as a carbon-free alternative fuel. The laminar burning velocities (S_L) of NH₃-containing flames have been extensively investigated, resulting in the development, updating, or optimization of many mechanisms. In this study, we aimed to test the validity of the available mechanisms by measuring the S_L of ammonia flames at ultra-lean (ϕ = 0.3–0.6) and rich (ϕ = 1.5–1.8) conditions, which were not covered by literature experiments and thus outside the validation targets of the literature mechanisms. The measurements were conducted using the heat flux method at 1 atm and 298 K, with careful evaluation of uncertainties in laminar burning velocities, oxygen ratios, and equivalence ratios. In addition to NH₃ + O₂ + N₂, we also tested NH₃ + O₂ + Ar flames to provide data at broader conditions. Simulations were carried out using eleven kinetic mechanisms that have been validated against the NH₃ laminar burning velocities when first published, and none of them reproduces well the present experimental data with mixed behavior observed, indicating the necessity of S_L measurements with different oxygen ratios and diluent types, as well as that there is still space for mechanism updating. A-factor reaction sensitivity analyses were also conducted using the eleven mechanisms, providing uncertainty ranges of rate constants based on their variations in the different mechanisms. Our analyses suggest that future mechanism updating should focus on especially on the temperature dependences of the sensitive reactions with large rate constant uncertainties. In addition, mechanism updating targeting fuel-rich side data will be more challenging than the lean side due to a larger number of large-uncertainty reactions.