Structural effects of C3 oxygenated fuels on soot formation in ethylene coflow diffusion flames

Abstract

This paper investigates how the structure of three C3 oxygenated fuels: dimethyl carbonate (DMC), dimethoxymethane (DMM) and isopropanol (IPA) influences soot formation when the fuels are blended with ethylene in laminar coflow diffusion flames. Up to 20% of the total carbon was substituted with oxygenated fuel. Colour-ratio pyrometry was used to measure the soot volume fraction (fv). IPA caused a strong increase in fv, whereas DMM and DMC both caused an initial increase followed by a progressive decrease in fv as the proportion of oxygenated fuel was increased. Differential mobility spectrometry and thermocouple probes were used to measure the particle size distribution and gas temperature in the flames at 5% blend strength. The hottest region of the 5% flames was consistently about 100 K cooler than the corresponding region of the ethylene flame, indicating a thermal effect of the doping. The 5% flames showed an increase in the maximum centre-line average particle size and fv versus the ethylene flame, with the IPA showing the largest increase. The evolution of the centre-line particle size distributions showed that the 5% flames experienced earlier particle growth compared to the ethylene flame. Consideration of the role of the chemical pathways towards benzene formation suggests that methyl radicals from the decomposition of the oxygenated fuels are responsible for the increase in fv at 5% doping. The difference in fv between the IPA versus DMM and DMC flames is thought to be due to the additional presence of C3 species originating from the carbon-carbon bonded backbone of IPA. Meanwhile, the differences between the DMC versus DMM flames are thought to arise from CO2 produced during the decomposition of DMC, and a corresponding thermal effect where the pyrolysis region of the 5% DMC flame was observed to be about 50 K cooler than the other flames.