Experimental and kinetic modelling studies of laminar flame speed in mixtures of partially dissociated NH₃ in air

The laminar flame speed in mixtures of partially dissociated NH₃ in air under ambient temperature and pressure was studied via experimentation and kinetic modelling. The experiments were carried out using a vertical tube flame propagation method for low degrees of NH₃ dissociation and a Bunsen burner method equipped with Schlieren imaging technique for high degrees of NH₃ dissociation. Chemkin modelling incorporating three different reaction mechanisms, namely, the Okafor, Otomo, and Mathieu and Petersen mechanisms, respectively, was undertaken to predict the laminar flame speed. The degree of NH₃ dissociation varied from 0 to 50% (0–37.5 %v/v H₂ in the fuel mixture with a fixed H₂/N₂ ratio of 3) with equivalence ratios of 0.9–1.2. The experiments showed that a higher degree of NH₃ dissociation led to a larger initial flame kernel size upon ignition and brighter flames. Regardless of the equivalence ratio, the laminar flame speed increased with increasing the degree of NH₃ dissociation, from 0 to 50%, the maximum laminar flame speed increased from 7.8 to 22.7 cm·s⁻¹ at the equivalence ratio of 1.10. The Okafor mechanism offered the best predictions of the experimental data while the Otomo and Mathieu and Petersen mechanisms over-predicted the laminar flame speed. The production of key radicals including OH, H, O, and NH₂ was enhanced in the presence of H₂ and, thus the conversion of NH₂ to NH, HNO, and NNH, leading to a significant increase in the laminar flame speed.