Evaluation of the chemical effect of carbon dioxide dilution to fuel on soot formation in co-flow laminar ethylene and propane diffusion flames

Abstract

In this study, the effects of carbon dioxide (CO2) addition on soot evolution were isolated systematically by adding 50% N2, 31% Ar+19% CO2, and 50% CO2 to the fuel side, respectively. Two series of diffusion flames, ethylene and propane, were experimentally investigated. Optical diagnostics, including laser-induced fluorescence (LIF), ratio pyrometry, and laser-induced incandescence (LII), were utilized to measure the spatial distributions of polycyclic aromatic hydrocarbons (PAHs), flame temperature, soot volume fraction fv, primary particle size dp, and number density of soot np. It was found that the flame height decreased gradually for the ethylene flame with adding N2/Ar + CO2/CO2. In contrast, the flame height remained essentially unchanged for propane-based flames. The results showed that adding N2/Ar + CO2/CO2 increased the temperature in the upper part of the ethylene flame, while the temperature change in the upper part of the propane flame was subtle and almost negligible. The total PAHs-LIF loading and the spatial distributions of fv/dp/np indicated that the chemical effect of CO2 added to ethylene flames in inhibiting the formation of large-size PAHs and retarding the soot inception stage was insignificant. However, it was not negligible for propane flames. The quantitative results revealed that the inhibition of total soot loading ST in propane flames was dominated by the chemical effect of CO2, while in ethylene flames, it was dominated by the thermal and dilution effect of CO2. The same conclusion was also found in the results of the maximum carbon conversion factor ηmax and the average particle size of soot. The specific growth rate of soot mass was almost unchanged regardless of the mixture on the fuel side. Adding CO2 had a more substantial chemical effect in inhibiting soot formation in propane flames than that in ethylene flames, which could mainly be attributed to a more effective reduction in the time for the increase of soot mass τg during soot evolution.