Tailoring reduced mechanisms for predicting flame propagation and ignition characteristics in ammonia and ammonia/hydrogen mixtures

Ammonia, as a carbon-free energy carrier, has been of emerging interest to reduce carbon footprint in the propulsion and power sectors. An accurate understanding and modeling of ammonia combustion are of fundamental and practical significance to its application implementation. In consideration of the complexity of ammonia-based chemical mechanisms, this work develops 4 reduced mechanisms based on a detailed one without modifying the rate coefficients. The applicability of these mechanisms is widely validated against experimental data in the literature. The overall agreement of model predictions with experimental values is satisfactory. For NH3/air flames, all mechanisms well describe the laminar burning velocity at elevated inlet temperatures, but the Mech20 presents a slight over-estimation, signifying the relative significance of reactions involving N2H3 and N2H4. Meanwhile, these reactions are extremely important in capturing propagation characteristics of NH3/H2/air mixtures. Both the original and reduced mechanisms over-predict the ignition delay time in NH3/air flames to some extent, which deserves further optimization. Sensitivity analyses identify the top 10 sensitive reactions regarding flame propagation and ignition characteristics, which vary significantly with kinetic mechanisms, especially in NH3/H2/air flames. Generally, the mechanisms developed applies to the ammonia combustion for most operating conditions considered and provide insights into the fundamental knowledge.