Impact of pilot fuel-staging ratio on combustion and emission characteristics in a centrally-staged swirling combustor

Experimental and numerical studies are conducted to investigate the impact of pilot fuel-staging ratio on combustion and emissions characteristics in a centrally-staged low-emission combustor, since it is a critical parameter for combustion organization. Flow field, fuel distribution, flame structure, temperature distribution, and emission characteristics are analyzed. Experimental results show that as the pilot fuel-staging ratio increases from 10 % to 50 %, NOx emissions rise from 72.72 to 154.05 ppm. Numerical results reveal that the flow field initially contains both primary and corner recirculation zones, while a front recirculation zone progressively develops. The interstice between main and pilot fuel distribution zones narrows, while the distribution cone angles of two-stage fuels are enlarged. The mixing distance of pilot fuel increases, while that of main fuel decreases. The two-stage flames exhibit a distinct stratification phenomenon. The primary high-temperature zones, exceeding 2100 K, exhibit a dual-branch V-shaped distribution. The combustion reaction and high-temperature zones shift toward the combustor outlet. NOx emissions are chiefly produced in the primary high-temperature zones, predominantly through thermal mechanisms, and rise significantly with the increased pilot fuel-staging ratio. The entrainment effect of the front recirculation zone leads to a high NOx concentration in the relatively low-temperature zone it occupies.