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[Display omitted] • The well-designed rapidly-mixed swirl burner anchors compact ammonia/air flames. • Gliding arc discharges significantly extend the lean blow-off margin by repetitive ignition. • The flame structure and intermediate plasma chemistry are optically inspected. • Extremely lean ammonia flames assisted by plasma achieve NO x emissions below 100 ppm. Aiming at the potential development of ammonia as a carbon-free renewable fuel, this work investigates the flammability and NO x emission of swirling ammonia/air flames and utilizes plasma to enhance ammonia combustion. First, the well-designed rapidly-mixed swirl burner can anchor compact ammonia/air flames under a wide range of flow conditions, with the current maximum heat release of approximately 4.7 kW. The flame is progressively detached from the quartz confinement tube and goes blow-off when the equivalence ratio drops below approximately 0.7–0.8. Then, to alleviate the problem of low flammability, gliding arc discharges driven by a 12.5 kHz alternating current (AC) power supply are facilitated to extend the lean blow-off margin to approximately 0.3–0.4. The localized flame kernels surrounding the discharge column are detected by high-speed photography. The planar laser-induced fluorescence (PLIF) imaging of OH radicals, optical emission spectroscopy, and NH 2 * chemiluminescence measurement are performed to interpret the intermediate chemistry. Finally, the NO x emission of the swirling ammonia/air flame is measured by a flue gas analyzer. Results show that although the AC-powered gliding arc exhibits weak global effects on the NO x reduction of burner-stabilized ammonia/air flames prevailing at higher equivalence ratios (e.g. , φ > 0.75), leaner flames stabilized by discharges can achieve NO x emissions below 100 ppm due to the thermal DeNO x mechanism. [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]

• 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]

• Steady and complete CH 4 /O 2 /CO 2 combustion is achieved in an axial/tangential swirl jet burner. • Regimes for attached and lifted flames are determined in a wide operating range. • Detailed evolution from flame attachment to lift-off has been carefully examined. • Two linear fitting lines are obtained to predict the flame lift-off height. This work designs an axial/tangential swirl jet burner to establish stable CH 4 /O 2 /CO 2 combustion under a wide range of operating conditions. As the flow rate of the axial fuel stream increases, the flame is lifted and stabilized near the downstream wall. The lifted flame is robust and shows good combustion completeness. The critical fuel flow rate for flame lift-off is affected by the oxygen mole fraction of the oxidant flow. The diagram controlling flame attachment and lift-off is investigated experimentally to derive the critical Schmidt number. In addition, high-speed photography and planar laser-induced fluorescence (PLIF) techniques are employed to resolve instantaneous flame structures near lift-off. It is visualized that as the flame height increases at larger fuel flow rates, the flame layer encounters local extinction due to the stretching of the surrounding high-speed oxidant flow. The local stretch rate measured in cold flow agrees well with the theoretic extinction strain rate. Further, a fitting line is obtained to summarize the effect of flow conditions and burner geometry on flame lift-off height. Particularly, the normalized lift-off height shows a good linear relationship with the Reynolds number of the axial fuel jet. [ABSTRACT FROM AUTHOR]