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In the emerging field of plasma-assisted ammonia (NH 3) 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 NH 3 /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 NH 3 /H 2 O, 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 NH 3 /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. [ABSTRACT FROM AUTHOR]

• The flame and flow dynamics under air flow pulsations are resolved by performing validated LES and simultaneous OH-PLIF/PIV. • Temporal variations of stretch rate and heat release rate can be used as a criterion for flame extinction/reignition. • A novel decoupled simulation which integrates the plasma source terms and the CFD model is developed. • Thermal and kinetic effects of microsecond repetitively pulsed discharge on combustion are quantitatively interpreted. This paper intensively investigates the inlet pulsation-induced extinction of a premixed swirl flame (PSF) and its plasma-assisted stabilization using the microsecond repetitively pulsed (MRP) discharge. A well-designed low-frequency flow pulsation is applied to the air feedline for the mimicking of transient operations/disturbances in practical engines, which exhibits significant deterioration on the lean blowout (LBO) limit. The MRP discharge is utilized to improve the stability of perturbed flames. It extends the LBO limit at a proper time delay between the air flow pulsation and the discharge, with discharge power less than 1% of the combustion power. Further, the validated large-eddy simulations (LES) and the simultaneous OH planar laser-induced fluorescence/particle imaging velocimetry (OH-PLIF/PIV) measurements are performed to capture the unsteady evolution of flames approaching lean blowout. The nonlinear flame response to the flow pulsation quantitatively reveals the phase difference between the maximum local stretch rates (κ max) and volumetric heat release rates (q ̇ c). A combined effect of the excessive stretch and the reduction of heat release due to flow pulsation can be used to interpret the flame extinction. Finally, a novel LES-ZDPlasKin combined approach, which decouples the discharge and combustion processes, is dexterously developed to simulate plasma-assisted combustion behaviors. The reignition and stabilization due to the plasma effects are well reproduced in the predictive model, pronouncing the indispensable and synergistic thermal and kinetic effects of the MRP discharge on flame stabilization. [ABSTRACT FROM AUTHOR]

This work seeks to achieve stable combustion of swirling premixed flows using gliding arc discharges and understand how plasma alters flame dynamics near blow-off. Gliding arc discharges powered by 7 kHz and 25 kHz alternating current (AC) supplies are demonstrated to significantly extend the flammability and stability limits of swirling premixed flames either confined or unconfined within a quartz tube, with time-averaged discharge power close to 1% of the power typically released by a stable flame. Side visualization of OH profiles using the planar laser-induced fluorescence (PLIF) technique illustrates the unsteady evolution of flames approaching lean blow-off, confirming the positive effect of gliding arcs on the ultra-lean residual flames. Contrary to the unconfined flame anchored near the nozzle exit or the bluff body, the confined flame detached from the wall progressively presents different unstable morphologies, e.g., oscillating flames and lifted helicon flames. The activation of gliding arcs provides the additional anchoring mechanism for the lean swirling flame and particularly suppresses the natural flame oscillation with heat-release rate fluctuations of above 30%. The mechanism controlling flame responses to gliding arc includes a nonlocal effect through the externally applied AC electric field displacing flame and a local effect of spark discharge triggering flame kernels by producing heat and active species. The external sub-breakdown electric field can enlarge the spreading angle of OH-PLIF layers in the unconfined flame, but demonstrates few positive effects on the blow-off limit. Regarding the local effects within or near the arc column, top infrared imaging provides visible evidence for flame kernels initiated upstream of the inner recirculation zone, yielding repetitive ignition in the fresh reactants. [ABSTRACT FROM AUTHOR]