Technical Report 261, c4e-Preprint Series, Cambridge

Authors: Yong Ren Tan, Maurin Salamanca, Jiaru Bai, Jethro Akroyd, and Markus Kraft*

Reference: Technical Report 261, c4e-Preprint Series, Cambridge, 2020

Associated Themes:
 

• Soot formation studied in laminar diffusion flames doped with C3 oxygenated fuels
• Ethylene flame doped with up to 20% IPA, DMM or DMC at constant total carbon flow
• 5% doping increased soot volume fraction in the order IPA > DMM > DMC
• DMM and DMC decreased soot volume fraction at ≥ 15% doping
• Effect explained in terms of molecular structure of oxygenated fuels

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 (SVF). IPA caused a strong increase in SVF, whereas DMM and DMC both caused an initial increase followed by a progressive decrease in SVF 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 SVF 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 SVF at 5% doping. The difference in SVF 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. The difference between the DMC versus DMM flames is 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.

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