Exploring the Effects of Swirl Intensity on NO Emission in Ammonia-Methane-Air Premixed Swirling Flames

The high emission of nitrogen oxides (NOx) is one of the major obstacles to the practical application of ammonia as a carbon-free fuel. Improving the flow distribution and structure has been demonstrated to achieve low NOx emissions. However, in comparison to hydrocarbon flames, the influence of swirl intensities on ammonia NOx emissions is still not well understood. This study builds upon previous research by further exploring the effects of swirl intensity on NO production in ammonia-methane-air premixed swirling flames. A new adjustable axial swirler was designed to achieve a wide range of swirl numbers. We measured flame morphology, NO emissions, and NO and OH planar laser-induced fluorescence (PLIF) images over extensive ranges of equivalence ratio and ammonia fraction. The study found that increasing the swirl number from 0.6 to 1.0 resulted in a more compact flame, with enhanced reactions in the corner recirculation zone. Varying the swirl number significantly alters the NO concentration in the exhaust gas. The concentration of NO was significantly reduced at an equivalence ratio of 0.90 and an ammonia fraction greater than 80%. NO/OH-PLIF indicated that NO was primarily formed in the main reaction zone, with NO-PLIF intensity in the post-flame zone almost remaining constant at different heights. The integrated intensities of NO and OH-PLIF were obtained at different heights above the nozzle. A positive linear correlation was observed between NO-PLIF plateau intensity and NO mole fraction. The increased heat loss to the wall at larger swirl intensities reduces the flame temperature in the main reaction zone, which inhibit the formation of OH radicals, ultimately resulting in low NO emissions.