The mechanism of propagation of NH₃/air and NH₃/H₂/air laminar premixed flame fronts

The mechanism of flame front propagation in NH₃/air and NH₃/H₂/air steady, laminar premixed flames is examined. Since the process is characterised by a state of chemical non-equilibrium, the analysis focuses on the explosive mode that is introduced by chemical kinetics. The chemistry expressed in this mode is the one that tends to lead the system away from equilibrium and sustains the chemical non-equilibrium state. The algorithmic tools of Computational Singular Perturbation method are employed, so the analysis is not hindered by the size of the detailed chemical kinetics mechanism employed. Under engine-relevant conditions and a stoichiometric mixture, it is shown that in the NH₃/air case the flame front propagation is driven by reaction [Formula presented] far from the front and by reaction [Formula presented] closer to the front; the latter assisted by reaction [Formula presented] . These reactions are mainly responsible for the heat released, by effectively feeding the most exothermic reactions, which are OH-consuming. The ensuing chemical activity in the neighbourhood of maximum heat release rate generates upstream diffusion of heat, NH₂, NO, H and H₂, which initiate the chemical activity ahead of the flame front. This mechanism of front propagation is promoted by H₂ addition in the mixture, by reinforcing the action of these three reactions and by activating another OH-producing reaction [Formula presented] . A preliminary investigation of lean mixtures indicated that this flame front propagation mechanism is also present in the case of a pure ammonia fuel. However, when H₂ is present in the initial mixture, significant changes are observed that relate to the prevailing lower temperatures and the decreased upstream diffusion of heat. These findings provide novel insights with direct implications for controlling and optimising NH₃ and NH₃/H₂ flames planned for engine applications. The approach proposed here can also be extended for analysing flame propagation mechanisms across a more diverse spectrum of fuel mixtures and flame configurations, offering invaluable support to technologies pivotal in the ongoing energy transition efforts.