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, N-2 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 N-2-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 N-2 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. NOx emissions are significantly reduced with dilution and proved to be relevant to flame stability diagrams: slight augmentation in NOx 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.