Exploring the influence of swirl intensity on stability, emissions, and flame structure in non-premixed NH3/CH4 swirling flames

Abstract

Ammonia's appeal as a clean, carbon-free fuel is marred by its low reactivity and high NOx emissions, limiting its practical use. Understanding swirl intensity's influence on ammonia flame stabilization and emissions remains a gap. This study endeavors to explore the impact of swirl intensity on stabilization, NO/NH emissions, and the spatial distribution of NO and OH radicals in non-premixed NH/CH swirling flames. The burner consists of a central fuel jet (with a fuel jet velocity, U) surrounded by swirling airflow (of an air velocity, U). A blend of NH/CH, with ammonia mole fraction (: 0 to 0.85), was supplied to the central jet. The stability map, comparing U versus U, revealed that at the same swirl number (S), as U increased, a higher U became necessary to extinguish the flame, while, the higher levels amplified the sensitivity of U to U. For ≤ 0.25, an increase in S elevated the U required for the flame to blowout. However, this impact of S diminishes as increases until it starts to adversely reduce the blowout limits at = 0.85. At ≥ 0.75, adjusting Φ (the global mixture equivalence ratio) towards stoichiometry alongside increased S effectively lowers NO emissions and limits NH slip into the exhaust. The OH/NO-PLIF measurements highlight conical OH/NO layers near the burner exit. This conical NO layer transforms into a broader and more uniformly downstream, where the intensity of NO within this area likely mirrors the NO levels measured in the exhaust. Increasing at low S flames elongates both the OH/NO structure and intensifies the NO gradient in downstream distances. Conversely, an increase in S enhances fuel/air mixing and diminishes the impact of increased and Φ on the flame structure. A reduced sensitivity of NO emissions to Φ is observed with increasing S.