Coupled effects of carbon dioxide and water vapor addition on soot formation in ethylene diffusion flames

An experimental and numerical investigation on the effects of the addition of CO2 and H2O on soot formation in ethylene counterflow diffusion flames was performed. Experimentally, axial profiles of soot volume fractions were measured using a light extinction technique. A numerical model, coupling a detailed gas-phase reaction mechanism and a polycyclic aromatic hydrocarbon (PAH)-based sectional soot model, was used to investigate the underlying mechanisms of the observed experimental trend. It was observed that addition of either CO2 or H2O led to a monotonic reduction of soot volume fraction. The results also showed that when added in the fuel stream, H2O was more effective than CO2 in inhibiting soot formation. However, CO2 became a more effective soot inhibitor than H2O when added in the oxidizer side of the flame. Simultaneous addition of H2O and CO2, in either the fuel or oxidizer side, retained the soot suppression benefits from H2O/CO2 individual addition, suggesting negligible chemical interactions between the dopants in terms of their effects on soot formation. The chemical suppressing effects of CO2 and H2O addition on soot formation were identified and attributed to reduced rates of PAH growth and soot inception. However, numerical analysis showed that the specific kinetic pathways responsible for soot inhibition were different for CO2 and H2O addition.