Fuel blend and secondary air injection effects on the stability, morphology, and dynamics of two-stage NH3-CH4/H2 swirl flames

Ammonia (NH₃) has been identified as a potential carbon-free fuel to decarbonize power generation by gas turbines. However, challenges associated with flame stabilization and emissions must be solved. Two-stage, rich-lean combustion is a promising strategy that requires fine control of the secondary stage parameters, where air is injected to oxidize the remaining unburned fuel from the rich primary stage. This study investigates the effects of the fuel blend composition (NH₃-CH₄ and NH₃-H₂), NH₃vol fraction (X_NH3), primary (ϕ_primary) and global (ϕ_global) equivalence ratios, and geometry of the secondary air injection (number and diameter of holes) on the morphology of the lean secondary and rich-premixed primary flames, and primary flame stability and dynamics. Experiments are conducted with a lab-scale piloted burner inspired by the AE-T100’s micro gas turbine burner. By increasing the secondary air flow rate (Q_sec), ϕ_global varied from 0.91 down to a value that produces primary flame morphology changes (ϕ_global,FC), and then to a minimum value that eventually led to flame instability followed by blowout (ϕ_global,BO). These thresholds were found to depend on fuel composition and air injection geometry. High-speed chemiluminescence imaging of NH₂* combined with Dynamic Mode Decomposition (DMD) revealed distinct instability mechanisms: CH₄-blended flames exhibited longitudinal pulsations, while H₂-blended flames showed a rotating inner core that reignites upstream reactants. These instabilities are linked with a combustion regime transition for the secondary combustion zone, from diffusion-like to premixed-like (or partially premixed), indicative of an optimum ϕ_global for each geometry. Finally, increasing the number of secondary air holes (while keeping diameter constant) extended flame stability to leaner ϕ_global. Data showed that the geometry of the secondary air injection is more important for stability than other varied parameters, a valuable finding for the design of future two-stage, rich-lean NH₃ burners.