Effect of Fuels and Oxygen Indices on the Morphology of Soot Generated in Laminar Coflow Diffusion Flames

Abstract

The morphological characteristics of soot are of primary importance to quantify its effect on climate forcing and human health and also to interpret the signals acquired in optically based soot diagnostics. In the present study, the morphology of soot particles produced in laminar coflow diffusion flames was investigated under different fuel and oxidizer conditions. Particles were sampled thermophoretically at the centerline at different heights above the burner in laminar diffusion flames of three common fuels, namely, ethylene, propane, and butane, at two sooting statuses, i.e., under the smoke point and at the smoke point. The oxygen content in the oxidizer stream was systematically varied. Different morphological parameters of mature soot, including primary particle number, aggregate radius of gyration, fractal dimension and prefactor, and overlapping, were obtained based on analysis of transmission electron microscopy images. The diameter and number of monomers were calculated by using automated methods based on the Euclidean distance mapping and the relative optical density methods, respectively. The other parameters were derived from these two parameters and some empirical correlations. The fractal dimension under different fuel and oxidizer conditions falls in a relatively narrow range from 1.68 to 2.05, while the derived fractal prefactor varies significantly from 0.24 to 2.66. An exhaustive comparison of the morphological parameters obtained in this study with the literature data suggests that our results fall within a range similar to those reported in previous studies. The present results suggest that the soot morphology does not display a clear trend of variation with the change in oxygen index for all three hydrocarbon fuels studied, at least for the fractal dimension and primary particle diameters obtained in this study.

Soot-PCF: Pore condensation and freezing framework for soot aggregates

“…Wentzel et al (2003) found values between 0.1 and 0.29 for soot aggregates from diesel emissions. As another example, Cortés et al (2018) reported values of 0.04 ≤ Cov ≤ 0.18 when using TEM to study soot aggregates generated in co-flow diffusion flames using different fuels (ethylene, propane and butane), which is in good agreement with the values (0.11-0.17) of soot aggregates derived from diffusion flames reported in . The overlap of primary particles in soot aggregates has gained attention because it impacts the radiative properties of the aggregates (e.g.…”

“…Delhaye et al, 2017;Liati et al, 2014Liati et al, , 2019Marhaba et al, 2019;Smekens et al, 2005), and commercial carbon blacks and/or soot generated from controlled flames in the laboratory (e.g. Clague et al, 1999;Cortés et al, 2018;Ferraro et al, 2016;Megaridis and Dobbins, 1990;Ouf et al, 2016Ouf et al, , 2019. For instance, Liati et al (2014) used TEM to investigate the size of primary particles of a turbofan engine (type CFM 56-7B26/3), frequently used in Boeing 737 aircrafts that was operated with standard Jet A-1 fuel.…”

“…In these burners the combustion conditions can be varied in a controllable manner to generate soot with primary particle diameters between approximately 5 nm to 50 nm (e.g. Cortés et al, 2018;Megaridis and Dobbins, 1990; and references therein), i.e. covering the size range relevant for ambient soot.…”

Investigating Soot Parameters in an Ethane/Air Counterflow Diffusion Flame at Elevated Pressures

“…Both intrusive and non-intrusive diagnostics have been used for many years in laboratory flames at atmospheric pressure to study soot (Sorensen et al, 1992, Koylu, 1997, Hu et al, 2003, Fang et al, 1998, De Iuliis et al, 1998a, De Iuliis et al, 2011, Cortés et al, 2018, Ferraro et al, 2016. Laser diagnostics are favored for soot studies in flames due to their non-perturbing nature.…”

Formation of Soot in Oxygen-Enriched Turbulent Propane Flames at the Technical Scale

“…Oxygen enrichment, which may be used to increase the peak-flame temperature, decrease flue gas losses, or reduce equipment size, is applied in some industrial applications (e.g., glass manufacturing and aluminum recycling) and in oxy-fuel combustion for carbon capture. Several studies have investigated oxygen-enriched combustion for co-flowing diffusion flames experimentally [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33], and many have noted a significant increase in soot formation when the oxygen content in the oxidizer was increased (due to higher temperatures and reaction rates). As for the impact of pressure on soot formation in laminar flames, the soot volume fraction appears to scale in proportion to the total pressure in terms of a power-law, i.e., ∝ P n .…”