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

Abstract

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.