A numerical study on side-wall quenching of premixed laminar flames : An analysis of ammonia/hydrogen/air mixtures

It is well known that low reactivity of ammonia (NH₃) is enhanced when blended with hydrogen (H₂). This blending could affect the interactions between ammonia-based flames and walls. In the present study, the side-wall quenching (SWQ) of two-dimensional (2-D) premixed laminar ammonia/hydrogen/air flames is examined using detailed chemistry and the mixture-averaged transport model. Six test cases are designed to assess the SWQ characteristics of such flames by systematically varying the equivalence ratio from 0.6 to 1.2 and the blending ratio (molar ratio of hydrogen to ammonia/hydrogen in the mixture) from 0.35 to 0.45 under a fixed wall temperature of 500 K at atmospheric pressure. The results show that the quenching distance (maximum wall heat flux) decreases (increases) as the equivalence ratio and the blending ratio increase. In addition, the results indicate that, for the same fuel mixture, the ratio of the quenching Peclet number for the SWQ to the corresponding value for the head-on quenching (HOQ) is in the range of 1.2 to 1.35. The results reveal that the N₂ pathway is the dominant mechanism of NO formation near the wall at the quenching region. Within this pathway, R76 (NH₂ + NO ⇔ N₂ + H₂O) is the leading reaction converting NO to N₂. Furthermore, the significant role of R85 (NH + NO ⇔ N₂O + H) in converting NO to N₂O is highlighted near the wall. Moreover, the contribution of radical recombination reactions to the total heat release rate is highlighted near the wall, and their significant reactions are identified. These reactions get more prominent as the equivalence ratio and the blending ratio increase. Novelty and significance Using ammonia enriched with hydrogen as a carbon-free fuel mixture results in several technical issues in combustion devices. The side-wall quenching (SWQ) of such fuel mixtures is one of the less explored areas within the combustion community, which needs further attention. The present study is one of the first to provide a detailed numerical analysis of the SWQ of such fuel blends. The novelty of this work is that, for the first time, (1) the formation of pollutants, in particular NO and N₂O, is investigated in the vicinity of the wall in the context of the SWQ and (2) the significant role of radical recombination reactions is highlighted near the wall. It should be emphasized that understanding the underlying phenomena of the interactions between ammonia/hydrogen/air flames and walls is an important step toward the development of near wall combustion models.