Investigation on the NO formation of ammonia oxidation in a shock tube applying tunable diode laser absorption spectroscopy

Abstract

The time-resolved nitric oxide (NO) measurements of NH3/O2/Ar mixtures using tunable diode laser absorption spectroscopy (TDLAS) were performed in a shock tube covering the temperature range of 1581-2720 K with an average pressure of 3.08 bar and three equivalence ratios of 0.5, 1.0 and 1.5. Considerable differences in the shape, amplitude and phase of NO profiles were observed for different temperatures and equivalence ratios. Two characteristic parameters (NOmax: maximum NO concentration; Pos_half_NOmax: the time from time zero (P5, T5) to the time corresponding to the position of half NOmax) were extracted for quantitative analysis. Generally, lower temperatures and higher equivalence ratios lead to lower NOmax. Pos_half_NOmax is retarded with lower temperatures, but it is not sensitive to equivalence ratios. The comparisons between experimental and predicted results by nine literature kinetic models reveal that no mechanism can well reproduce the NO time histories for all cases. The performance of each mechanism was quantitatively evaluated by the methodology proposed in this study. The Glarborg et al. mechanism (Prog. Energy Combust. Sci., 2018) shows the best performance in predicting NOmax and complete NO profiles while the Shrestha et al. mechanism (Proc. Combust. Inst., 2021) shows the best performance in Pos_half_NOmax prediction. NH3→NH2→NH→HNO→NO is identified as the main reaction pathway for NO formation, where NH3→NH2→H2NO→(HNOH)→NH3 and NH3→NH2(→NH)→N2H2→NH3 dominate in high (> 2300 K) and low temperature (< 1900 K) regimes, respectively. The NO consumption observed at temperatures over 2300 K and in rich mixture at temperatures below 1900 K were found to be attributed to N + NO <=> N2 + O and excessive NH/NH2 radicals, respectively. Such kind of consumption disappear in lean and stoichiometric mixtures (<1900 K) due to decomposition of HONO through HONO (+M) <=> NO + OH (+M). The Glarborg et al. mechanism was modified by updating the rate coefficients of NH3 + H <=> NH2 + H2 from the Shrestha et al. mechanism. The modified mechanism shows improved accuracy in Pos_half_NOmax prediction and becomes the best-performed model both in predicting the amplitude and phase of the NO profiles among all ten mechanisms.