Dynamics of atomic oxygen production in an NH3/air flame assisted by a nanosecond pulsed plasma discharge

Atomic oxygen (O) is vital in plasma-assisted combustion, but its quantitative measurement is challenging and therefore rarely reported. This work performs femtosecond two-photon-absorption laser-induced fluorescence (fs-TALIF) imaging to quantify the dynamics of atomic oxygen production by a single nanosecond (ns) pulsed plasma discharge in an ammonia/air flame for the first time. First of all, the plasma kinetic enhancement is quantified by the plasma-produced O with a molar fraction of up to 2.8 x 10-3, which is at least one order higher than that in the flame without plasma. Then, the spatial distribution of O demonstrates significant differences between the unburnt and burnt zones, revealing the distinct mechanisms of O production in unburnt and burnt NH3/air mixtures. Specifically, a new O region is formed in the burnt zone of the lean flame, which is probably mainly caused by the pathway of NO dissociation into O by the ns plasma. Furthermore, the temporal dynamics of O in different zones are discussed in-depth. In the unburnt zone, from the continuous increment of O after hundreds of ns following the ns discharge initiation, it can be seen that the relaxation of the excited nitrogen plays an equally important role in O production as the direct electron impact. Subsequently, O atoms in the unburnt zone exhibit an exponential decay, which is attributed to the formation of O3 from O. Whereas, in the burnt zone, a bi-exponential decay of O is found for the first time. Such decay mainly results from the reaction between O and H2O, which importantly, is a reaction to form OH radicals. The transition between decays is due to the formation of the O region in the unburnt zone and the dominance of the combustion kinetics in the postplasma stage.