Effects of Secondary Air Injection on Turbulent Premixed CH ₄ /NH ₃ /Air Flames in a Two-Stage Swirl Combustor: An LES Study

This study numerically investigates the influence of secondary air injection on combustion performance and NO emissions in a two-stage swirl combustor using three-dimensional large-eddy simulations (LES). Turbulent CH4/NH3/air flames are examined under both single-stage combustion (SSC) and two-stage combustion (TSC) conditions, and the numerical results are validated against experimental data. For SSC, simulations are performed at equivalence ratios of ϕ=0.7 and 1.3. For TSC, secondary air is introduced at four downstream positions (Ls= 30, 50, 110, and 250 mm), maintaining an overall ϕ=0.7 while operating the primary jet at ϕ=1.3. The results show that secondary air injection reduces NO, CO, and unburned NH 3 emissions, with more effective NO suppression obtained at larger Ls (110 and 250 mm). In contrast, injection close to the primary flame (Ls=30 mm) forms a stratified flame structure that enhances NO formation through reactions between residual NH3 and secondary air. Beyond global emission trends and flame shape changes, the present study provides a novel regime-resolved analysis of premixed, stratified, and diffusion flame structures, together with N-element reaction pathway analysis within each flame regime. This framework reveals how the secondary air injection location modifies the relative roles of these flame regimes and thereby alters both NO formation and post-flame NO reduction chemistry. These findings identify Ls as a key design parameter for staged swirl combustors to control NO formation while maintaining favorable overall combustion performance.