Soot suppression and radiation enhancement in CO2 diluted laminar inverse diffusion flames

Inverse diffusion flame (IDF) configuration is widely employed in high-temperature industrial processes. While oxy-fuel combustion is considered a promising technology for CO₂ capture, the high CO₂ concentrations involved can significantly suppress soot formation and potentially reduce radiative heat transfer. In this study, ethylene IDFs with varying levels of CO₂ dilution in the oxidizer are investigated through high-fidelity numerical simulations. Detailed soot kinetic models and non-gray radiative property models are employed to ensure close agreement with experimental measurements, including flame temperature, flame height, and soot volume fraction. A fictitious species strategy is employed to isolate the thermal, radiative, transport, and chemical effects of CO₂ on soot formation. Additionally, the individual radiative contributions of CO₂, H₂O, CO, and soot particles are quantitatively evaluated. Results reveal that the thermal and chemical effects of CO₂ are the most significant in suppressing soot formation, primarily by lowering flame temperature and reducing the concentration of soot-forming species. The chemical effect is dominant at a 50% dilution level, while the thermal effect becomes more important at 70%. The transport effect of CO₂ primarily leads to an increase in flame height, but has a negligible impact on peak soot volume fraction. Moreover, the overall radiative capability of CO₂-diluted flames is consistently higher than that of N₂-diluted flames at equivalent dilution levels, with the difference becoming more pronounced at higher dilution levels. In N₂-diluted flames, soot radiation dominates but decreases sharply with increasing dilution. In contrast, CO₂-diluted flames exhibit dominant CO₂ radiation, which remains largely unaffected by further dilution.