Flow characterization and dilution effects of N₂ and CO₂ on premixed CH₄/air flames in a swirl-stabilized combustor

Numerically-aided experimental studies are conducted on a swirl-stabilized combustor to investigate the dilution effects on flame stability, flame structure, and pollutant emissions of premixed CH₄/air flames. Our goal is to provide a systematic assessment on combustion characteristics in diluted regimes for its application to environmentally-friendly approaches such as biogas combustion and exhaust-gas recirculation technology. Two main diluting species, N₂ and CO₂, are tested at various dilution rates. The results obtained by means of optical diagnostics show that five main flame regimes can be observed for N₂-diluted flames by changing excess air and dilution rate. CO₂-diluted flames follow the same pattern evolution except that all the domains are shifted to lower excess air. Both N₂ and CO₂ dilution affect the lean blowout (LBO) limits negatively. This behavior can be counter-balanced by reactant preheating which is able to broaden the flammability domain of the diluted flames. Flame reactivity is degraded by increasing dilution rate. Meanwhile, flames are thickened in the presence of both diluting species. NO_x emissions are significantly reduced with dilution and proved to be relevant to flame stability diagrams: slight augmentation in NO_x emission profiles is related to transitional flame states where instability occurs. Although dilution results in increase in CO emissions at certain levels, optimal dilution rates can still be proposed to achieve an ideal compromise.