Effects of carbon dioxide addition on soot dynamics in ethylene/air inverse diffusion flames: An experimental and computational analysis

The effects of carbon dioxide (CO₂) addition to ethylene (C₂H₄)/air inverse diffusion flames (IDFs) to the air stream on soot formation characteristics are investigated with the addition ratio of 0–13.64 %. The planar laser-induced fluorescence (PLIF) and planar laser-induced incandescence (PLII) techniques, in conjunction with CoFlame and Chemkin code simulations were utilized to assess the distributions of Polycyclic Aromatic Hydrocarbons (PAHs) and Soot Volume Fraction (SVF). The findings indicate that increasing CO₂ addition results in a gradual decrease in the mole fraction of hydroxyl (OH) radicals and flame temperature, accompanied by a reduction of approximately 15 % in the reaction zone height in experimental observations and 19 % in simulations. The inhibition of soot formation is evident through a consistent decline in the normalized total SVF, a decrease in the peak volume fraction of radial soot distribution, and reduced total SVFs observed across different flame sections at varying heights. In the meanwhile, increasing the CO₂ doping ratio significantly reduces the peak signal intensity of PAHs, particularly affecting high molecular weight PAHs (A3-A4, A2-A3) with reductions of up to 75.5 %. Furthermore, reductions are noted in the rates of soot inception and subsequent surface growth, accompanied by an upward displacement of the initial inception and growth location. The condensation of PAHs controls the soot surface growth. The thermal and chemical effects of CO₂ were differentiated by employing the virtual substance FCO₂. The results suggest that the thermal effect of CO₂ lowers flame temperature, reduces combustion intensity, and consequently inhibits soot nucleation. The chemical effect of CO₂ competes for H radicals through the reverse reaction of CO + OH ≤> CO₂+H. This process suppresses the formation and growth of PAHs, consequently leading to a reduction in soot production.