Plasma-assisted NH3/air flame: Simultaneous LIF measurements of O and OH

Abstract

In the emerging field of plasma-assisted ammonia (NH₃) combustion, the evolution of key intermediate species has rarely been reported. This work establishes a simultaneous measurement system of laser-induced fluorescence (LIF) for hydroxyl (OH) and quantitative two-photon-absorption LIF for atomic oxygen (O), to explore the OH and O dynamics in an NH₃/air flame affected by a nanosecond (ns) pulsed plasma discharge. Firstly, with the plasma on, the molar fraction of O is quantified to reach 8.7 × 10^–3 in the burnt zone, about two orders of magnitude higher than that without plasma. In addition, the OH LIF signal intensity is four times higher, indicating a significant kinetic enhancement. Then, the spatial characteristics of OH and O are discussed and compared, showing remarkable discrepancy. The discrepancy between them indicates that O production is dominated by plasma kinetics, however, the OH production, primarily stemming from reactions between O and NH₃/H₂O, still depends on parameters associated with combustion kinetics. We further study the temporal dynamics of O and OH. It is concluded that O and OH peaks at 1.75 μs are mainly attributed to the pathway of quenching of the excited species. After that, O and OH start to decay but show significant differences between unburnt and burnt zones, which are characterized by a single-exponential decay and a bi-exponential decay, respectively. In the unburnt zone, the OH decay is much slower than the O decay due to the diverse pathways for OH production. In the burnt zone, the bi-exponential decay of O and OH can essentially be regarded as a process in which the NH₃/air reactive system reaches chemical equilibrium. At this stage, the impacts of the excited species from the plasma gradually diminish and combustion kinetics dominates alone.