Shock-tube laser absorption measurements of N2O time histories during ammonia oxidation

The oxidation of ammonia was studied experimentally by monitoring the time history of the intermediate N₂O species using laser absorption spectroscopy. Experiments were conducted in a shock tube for mixtures of NH₃/O₂ diluted in ∼96.7% Ar for equivalence ratios of 0.54, 1.03, and 1.84. The equivalence ratios were determined accurately using spectroscopic measurements of NH₃ with another laser before each experiment. Experiments were performed at an average pressure of 1.2 atm and covered a temperature range of 1829 to 2198 K. For the same temperature, experiments revealed that increasing the equivalence ratio leads to less N₂O formation. The time-history profiles showed that N₂O is formed at the beginning of the experiments, mainly from the formed NO, until reaching a peak. The N₂O is then fully consumed, mainly via its reaction with H-atom. Characteristic parameters, such as the N₂O peak time and mole fraction, were extracted from the N₂O profiles and compared with 15 recent NH₃ kinetics models. The comparison revealed that none of the existing kinetics models were able to correctly predict both the peak N₂O time and mole fraction together. Two of the models were selected to perform a chemical analysis, and an improvement of the predictive capability of one model is proposed. The N₂O profiles reported herein are excellent validation targets that offer stringent constraints for the improvement of future NH₃ kinetics models.