Comparative analysis of five NH$_3$/air oxidation mechanisms

TL;DR

This paper compares five ammonia oxidation mechanisms to understand their differences in predicting ignition delay times, focusing on the reactions and variables driving the explosive mode during autoignition.

Contribution

It introduces a systematic analysis of ammonia oxidation mechanisms using the Computational Singular Perturbation algorithm to identify key reactions and variables influencing ignition delay.

Findings

Differences in chemical and thermal runaway durations affect ignition delay predictions.

Reactions involving species with two nitrogen atoms are active only in some mechanisms.

OH-producing and OH-consuming reactions are central to thermal and chemical activity.

Abstract

Five recently developed chemical kinetics mechanisms for ammonia oxidation are analysed and compared, in the context of homogeneous adiabatic autoignition. The analysis focuses on the ignition delay and is based on the explosive mode that is shown to drive the process. Using algorithmic tools based on the Computational Singular Perturbation algorithm, the reactions responsible for the generation of the explosive mode are identified, along with the variables (species mass fractions and temperature) that associate the most to this mode. Comparison of these sets of reactions and variables, obtained for each mechanism, allows to correlate the differences in the predictive outcomes from the mechanisms with specific reactions. The major differences identified, which lead to different ignition delay times, relate to (i) the relative duration of chemical and thermal runaways (a sizeable chemical runaway develops only in some mechanisms) and (ii) the dominant chemistry during the chemical runaway (chemistry involving species with two nitrogen atoms is active only in some mechanisms). The major similarities identified refer to the thermal runaway and in particular to (i) the chemical activity, which is supported mainly by OH-producing reactions and by reactions producing their reactants and (ii) the thermal activity, which is dominated by strongly exothermic OH-consuming reactions.