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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]

• A 2D/3D model was developed to simulate composite propellant burning under wide pressure range. • Flame structure and standoff distance for multiple flame were examined in detail. • Heat flux determined by AP monopropellant, premixed binder and primary diffusion flames controls the burning rate. • Effect of primary diffusion flame on the burning rate alleviates with increasing AP particle size. • Relationship between cone-angle of final diffusion flame and flow velocity was elucidated. This study developed a combustion model for ammonium perchlorate (AP)/hydroxyl–terminated polybutadiene (HTPB) composite propellant to capture the flame structure and elucidate the mechanism governing burning rate at different pressures, AP sizes and surface morphologies. A numerical model was proposed to characterize gas domain by solving Navier-Stokes equations of reacting flow field with 3-step and 12-species global kinetics; and the condensed domain was coupled by considering interfacial energy balance. The present model was validated with experimental results of both pure AP and AP/HTPB combustion, in which the mean absolute percentage error of burning rate for all cases was less than 5%. The standoff distances of AP monopropellant, premixed binder and primary diffusion flames were determined as 14.2, 12.1 and 25.8 μm at 2 MPa, respectively, reducing with pressure. The front and height of final diffusion flame were determined by axially varied radial gradient of temperature and its height was obtained as 152 μm at 2 MPa, which rises as the pressure or AP particle size increases. The increase of burning rate with pressure is attributed to the raise of surface heat flux dominated by standoff distances of AP monopropellant, premixed binder and primary diffusion flames, in which the effect of primary diffusion flame enhances as the AP size decreases resulting from enlarged lateral diffusion of species. Finally, the effects of protrusive, recessive AP surfaces and their combination with planar AP surface on the flow and flame properties were examined. [ABSTRACT FROM AUTHOR]