Elsevier

Combustion and Flame

Volume 219, September 2020, Pages 13-19
Combustion and Flame

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

https://doi.org/10.1016/j.combustflame.2020.04.008Get rights and content

Abstract

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, La, 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.

Introduction

Soot nanoparticles are known to have detrimental effects on human health, climate, and air quality [1], [2], [3]. As a consequence of these concerns, significant research efforts have been put into understanding and explaining the chemical and physical processes behind the mechanisms of soot formation in fuel-rich flames. Although the majority of existing combustion technologies operate at pressure conditions ranging from moderate to high, most of our understanding of the soot formation process relies on studies performed at atmospheric condition. Pressure is known to strongly affect flame burning properties and soot formation [4]. It is established that pressure significantly increases soot loading [4]; as the pressure is increased in a combustion chamber, more soot is formed and released. However the amount of soot in an exhaust also depends on the extent of oxidation in the chamber. Nevertheless, soot concentration is not the only parameter to be considered. Soot particle size distribution, morphology, and nanostructure are also important parameters since they directly affect the soot oxidation rate as well as particle optical properties [5,6]. However, probing and characterizing soot from high pressure environments is not a trivial task, and only a few attempts have been made so far [7], [8], [9], [10], [11], [12], [13], [14]. Therefore, a clear understanding of the effect of pressure on particle size/morphology and nanostructure is far from being established firmly.

Recently, in an attempt to probe the dependence of soot nanostructure on pressure, we used Raman spectroscopy to characterize soot particles collected from several methane laminar diffusion flames operated under a variety of pressure conditions [10]. It was demonstrated that soot nanostructure was primarily affected by soot residence times rather than pressure [10]. Soot particles were shown to graphitize at an increasing rate when collected at increasing distances from the burner rim, while no differences were observable by changing the pressure of the system [10].

Compared to methane flames, ethylene flames are known to display significant differences in terms of soot formation and properties. In addition to the considerably different soot propensities, the dissimilar chemical structures of the two fuels may also affect their soot characteristics, such as particle size, morphology and structure [9,11,[15], [16], [17]. Thus, to complement our previous work [10] and achieve a better understanding of the effect of pressure on soot nanostructures, we carried out Raman spectroscopy measurements in this work on soot samples extracted from ethylene diffusion flames at various pressures.

Section snippets

Experimental

Soot samples were collected thermophoretically from a set of nitrogen diluted ethylene laminar diffusion flames operated under various pressures. A high pressure combustion chamber capable of stabilizing laminar diffusion flames on a coflow burner was used. The high-pressure combustion chamber, housing the burner and the integrated thermophoretic soot sampling system has been thoroughly described elsewhere [8], [9], [10], [11], [12]. Briefly, the burner, installed in the high-pressure chamber,

Results and discussion

To track the evolution of soot particles, the ethylene laminar diffusion flames operated at several pressures, i.e., 3, 6 and 10 bar, have been characterized by SSE measurements. Two-dimensional depictions of the soot volume fraction (SVF) measurements for flames operated under these three pressures are reported in Fig. 2.

Soot volume fractions increase significantly with increasing pressures, which is also illustrated by the trend of increasing maximum soot yields shown in Fig. 3. For all the

Conclusions

In this study, we investigated nanostructure of soot particles collected in a set of ethylene laminar diffusion flames at different pressures from 1 to 10 bar. Soot volume fraction and temperature measurements by spectral soot emission diagnostic have been performed for flames operated under different pressures. The flame conditions corresponding to soot inception and maximum volume fraction were selected, and the corresponding soot particle nanostructure is analyzed by Raman spectroscopy.

The

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by Natural Sciences and Engineering Research Council of Canada (RGPIN-2017-06063) and by the “Accordo di Programma CNR-MSE Ricerca di Sistema Elettrico” under the project “MIcro co/tri generazione di Bioenergia Efficiente e Stabile (Mi-Best)”.

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