On the effect of pressure on soot nanostructure: A Raman spectroscopy investigation

Although the majority of existing combustion devices operate at high pressure conditions, most of our understanding of the soot formation process and soot physicochemical properties rely on studies performed at atmospheric pressure. Pressure is known to have nonlinear effects on combustion processes and a significant influence on soot formation; soot loading increases with increasing combustion pressure. Soot characteristics directly affect soot oxidation and optical/radiative properties, and it is desirable to have a better insight into them under high-pressure conditions. Due to scarcity of information on soot primary particles, aggregate morphology, and soot nanostructure relevant to high-pressure combustion, there are challenges in predicting soot oxidation and radiation, particularly at engine-relevant conditions. In this study, we perform Raman spectroscopy measurements on soot sampled from a set of laminar diffusion flames of ethylene at various pressures, ranging from atmospheric pressure to 12 bar. Our results show an increase in soot maturity as the pressure increases within the range of investigated pressures. In the examined co-flow flames, pressure seems to have an indirect influence on soot nanostructure through an earlier inception of soot, resulting in longer residence times of the carbon soot particles in the hot and reactive flame environment. It is found that soot maturity, tracked through the size of graphitic domains, L-a, increases linearly with residence times. The longer residence time of soot in high-pressure flames could be the main cause of the higher degree of graphitization observed, which suggests a greater resistance to oxidation with increasing pressure. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.