Chemiluminescence spectra investigation of ammonia flame over a wide-range equivalence ratios in a rapid compression machine

Abstract

This work investigates the chemiluminescence spectra of ammonia flame on a rapid compression machine using central spark ignition, with the equivalence ratios ranging from 0.4 to 1.5, pressure at the end of compression of 30 bar, and temperature at the end of compression of 900 K. For the first time, the color of an ammonia flame was found to vary with the equivalence ratio under engine-related conditions from images observed by a high-speed camera: orange at the equivalence ratio of 0.4, light pinkish purple at 0.7, blue at 1, and golden yellow for the flame front while dark blue for internal flame at 1.5. The chemiluminescence emissions of OH∗, NH∗, and NH₂ ∗ were observed from the spectra. Integral intensity ratios NH∗/OH ∗ and Red/Blue are two indicators of extreme lean combustion, OH∗/Violet, NH∗/Red, and WB3/WB1 can identify conditions closer to stoichiometric condition by comparation, and WB3/WB2 has the potential to serve as an indicator for determining other parameters independently of the equivalence ratio. The intensities of the R, G, and B channels in combustion images exhibit distinct trends over time across different equivalence ratios. The ratio of R value to B value is smallest under stoichiometric condition and decreases over time under different equivalence ratios. Chemical analysis indicates that under lean combustion conditions, the path of NH₂ to H₂NO is enhanced due to the increased oxygen, while under rich combustion conditions, the direct dehydrogenation of NH₂ to NH is enhanced as a result of decreased oxygen.