Exploring multi-physics field coupling strategies to improve NOx control in swirling combustion systems

This study explores how stratification influences NOx emissions in two ammonia–hydrogen swirling vortex-tube combustors: the Stratified Vortex-tube Combustor (SVC) and the Rapidly Mixed Vortex-tube Combustor (RMVC). The SVC employs a stratified combustion strategy, whereas the RMVC operates in a rapidly mixed mode. Under identical operating conditions and evaluation criteria, the SVC consistently achieves lower peak and volume-averaged NOx levels than the RMVC across a wide range of equivalence ratios and hydrogen contents, while both combustors maintain combustion efficiencies above 80%, improving further with increasing hydrogen enrichment. A key finding is that, in the SVC, the synergistic coupling between the swirling flow field and the stratified species field not only intensifies the combustion process—as evidenced by elevated peak heat release rates—but also leads to a more compact flame structure. This coupling effectively reduces the spatial extent of high-temperature regions and lowers the volume-averaged temperature, thereby suppressing NOx formation. Meanwhile, species stratification promotes higher concentrations of reductive radicals (H₂, NH₂, NH, and H), enhancing NOx reduction pathways. With increasing hydrogen content, these synergistic effects become more pronounced, underscoring the critical role of configuration-driven flow–species-field interaction in achieving clean, efficient, and robust ammonia combustion.