Experimental and Numerical Investigation into the Effects of Unsaturated Carbon Bonds of Hydrocarbon Fuels on Soot Formation in Laminar Diffusion Flames

Abstract

ABSTRACT Fuel structure is the main factor controlling soot behavior. Compared to the carbon number, the unsaturated carbon bonds have a greater effect on soot formation. This work analyzed the mechanism of C = C and C ≡ C on soot by adding ethylene and acetylene to the fuel stream of methane laminar diffusion flame experimentally and numerically. The results showed that the blending of C2H4 results in an increase of flame height without causing smoking, while the C2H2 addition up to 40% results in soot wings. The effect of C ≡ C bond on the flame structure and natural luminosity is much higher than that of C = C bond. The peak soot volume fraction (SVF) increases with the increasing blending ratio of C2H4 and C2H2, but the latter is one order of magnitude higher. The promotion effect of C ≡ C on soot growth attributes to the synergistic increase of HACA rate and PAHs condensation rate, while that of C = C is mainly due to the increased HACA rate. With the increase of unsaturated carbon bond concentration, the primary particle number density increases, the soot inception process is advanced. This trend is further enhanced with the increase of carbon bond unsaturation due to the shortening of the precursor formation pathway and the acceleration of reaction rate. With the increase of C2H4 blending ratio, the peak O2 oxidation rate increases linearly, while with the increase of C2H2 ratio, peak O2 oxidation rate increase first and then decrease, peaks at 40% ratio. Compared with O2 oxidation, the OH oxidation rate is not sensitive to carbon bond unsaturation and unsaturated carbon bond concentration.