Experimental investigation on ignition characteristic of NH3/O2/N2 mixtures: High pressure effects and model comparison

Abstract

To gain a deep insight into ammonia combustion under real powertrain operating conditions, the oxidation of NH₃/O₂/N₂ mixtures is measured using a rapid compression machine at high pressures of 40−100 bar, temperature of 1000−1200 K, equivalence ratios of 0.25−1.0, and dilution ratio N₂/O₂ of 3.76−8.52. Four representative ammonia oxidation mechanisms, including Glarborg−2024, POLIMI−2023, NUIG−2024, and KAUST−2023, are validated against the new high-pressure auto-ignition data and the results reveal that the performance of these mechanisms varies depending on the experimental conditions where POLIMI−2023 shows a relatively better prediction on the current data. Twelve high sensitivity coefficient reactions in four mechanisms are selected to compare the rate constant, indicating the necessity to unify the rate coefficients of important reactions and update old values. Rate coefficients even differ by an order of magnitude or more, including NH₂+HO₂=OH+H₂NO, H₂NO+NH₂=HNO+NH₃, and H₂NO+HO₂=HNO+H₂O₂. The kinetic analysis indicates that the reduced sensitivity coefficients of inhibiting reactions NH₃+O₂=NH₂+HO₂, NH₂+NO=N₂+H₂O and NH₂+NO₂ =N₂O+H₂O contribute to the high reactivity of ammonia mixtures at high pressures. The NH₂ radicals also participate less in the oxidation of species including NO, N₂H₂, N₂H₄ and H₂NO as the pressure rises. The experimental data qualitatively reveals the temperature effect on the pre-ignition phenomenon. The measured pressure profiles under different temperatures show that pre-ignition diminishes gradually as the temperature rises. A large amount of heat accumulation from exothermic reactions including NH₂+NO=N₂+H₂O, NH₃+OH=H₂O+NH_2, and NH₂+NO₂ =N₂O+H₂O contribute to slow pressure rise during the pre-ignition. One possible reason for the pre-ignition should be facility-related factors such as little lubricating oil attached to the wall.