Toward the Development of an RRQL System—Part I: Swirl Pattern Effect on Exhaust Emissions and Chemiluminescence Distribution for NH3–Air Premixed Swirl Flames

Abstract Ammonia (NH3) is a carbon–free energy source and hydrogen carrier, but its fuel–bound nitrogen can lead to significant nitrogen oxides (NOx) emissions. Staged combustion strategies, such as rich–quench–lean, can achieve low NOx emissions. However, improper design may result in high NOx levels when operating with a rich head end without a sufficient post–flame relaxation time. Previous work has shown that a rich–relaxation–quench–lean (RRQL) configuration can minimize NOx emissions in the rich head end (Cole et al., 2024, “Rich Ammonia Flame Shapes and NO Relaxation: Facility Development and Characterization,” ASME Paper No. GT2024–122369). This study focuses on how different swirl geometries affect exhaust emissions and flame morphology in rich, premixed NH3–air flames, for the design of a rich relaxation head end of an RRQL combustor. Experiments were conducted using a modular swirl burner and measuring NOx, N2O, and NH3 emissions, and recording natural flame luminosity and NH2* and OH* chemiluminescence images. Swirler design affects emissions by influencing flame length (with shorter flames allowing for more postflame relaxation time) and flame–wall interactions (influencing NH3 and N2O emissions). A compact flame with minimal wall interactions allows for increased NOx relaxation, while minimal wall interactions prevent heat losses and instabilities, which minimize NH3 and N2O emissions at a richer equivalence ratio, potentially enhancing H2 production during the relaxation phase without increasing overall primary stage NOx emissions, thereby improving system efficiency. Detailed spatial evolution of NOx, N2O, and NH3 emissions further supports the RRQL as a promising combustor design for NH3 combustion, if the rich head end is designed properly.