An exploratory modelling study of chemiluminescence in ammonia-fuelled flames. Part 2

Abstract

The detailed kinetic mechanism of the author was extended by reactions describing formation and consumption of excited species which are formed in NH3+CH4+air flames, complementing the modelling efforts presented in Part 1. Currently the model includes the following excited species: O(1D), OH*, O2*, CH*, CH2(1), NO2*, NO(A), NH*, N2(A), NH2*, C2*, CO2*, CH2O*, and CN*, among which many were observed in chemiluminescence signatures of ammonia-fuelled flames. The new model predictions were compared with the experimental data obtained in laminar premixed counterflow NH3+CH4+air flames (Combust. Flame 231 (2021) 111508). The overall agreement between the measurements and calculations was not as good as it was observed for NO(A), OH* and NH* in NH3+H2+air flames presented in Part 1. It was argued that both unquantified experimental uncertainties and remaining deficiencies of the model could contribute to the discrepancies found. Nevertheless, for OH*, NH*, CN*, CO2* and CH*, as well as for several ratios of chemiluminescence intensity of different excited species the predicted trends both in terms of their variation with equivalence ratio and the amounts of ammonia in the fuel are in qualitative agreement with the measurements. The most important inconsistency between the experiments and modelling is found for NO(A), which is the only species in NH3+CH4+air flames forming, according to the present mechanism, by the energy transfer from N2(A). This indicates that either formation of N2(A) precursors, namely NH and N, is not accurate due to missing interaction of nitrogen and hydrocarbon chemistry, or reactive quenching of N2(A) is incomplete and requires further development.