Shapes of Soot Particles, Both Charged and Uncharged, after Molecular Beam Sampling a Premixed Oxyacetylene Flame, Burning at Atmospheric Pressure

• Published in: Combustion science and technology Vol. 185; no. 12; pp. 1762 - 1776
• Main Authors: Fialkov, A. B., Hayhurst, A. N., Taylor, S. G., Newcomb, S. B.
• Format: Journal Article
• Language: English
• Published: Philadelphia, PA Taylor & Francis Group 02.12.2013; Taylor & Francis; Taylor & Francis Ltd
• Subjects: Applied sciences; Atmospheric pressure; Charged soot particles; Combustion. Flame; Crystals; Electron microscopy of soot; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Laminar flames; Molecular beam sampling; Molecules; Morphology; Soot; Theoretical studies. Data and constants. Metering; Transmission electron microscopy; Laminar flame; Soot; Molecular beam sampling; Premixed flame; Electron microscopy of soot; Combustion; Molecular beam; Electron microscopy; Flame propagation; Charged soot particles; Laminar flames; Flame structure; Sampling
• ISSN: 0010-2202; 1563-521X
• DOI: 10.1080/00102202.2013.833612

Three types of soot particle (positively charged, negatively charged, and uncharged) have been collected simultaneously using molecular beam sampling from different points along the axis of a weakly sooting, premixed flame of C 2 H 2  + O 2  + Ar, burning at atmospheric pressure and with a final temperature of ∼2300 K. Particles were examined using transmission electron microscopy (TEM). Most of them had expected shapes, but some had quite unusual configurations and were surprisingly large (1-2 µm); these bigger particles often had amorphous, liquid-like structures, although some had crystalline exteriors. There was no significant difference in size or morphology between uncharged particles and those charged either positively or negatively. Also, it was clear that the concentration of neutral soot particles always exceeded that of the positively charged ones, which in turn were more abundant than those with negative charges. Some surprisingly large particles of soot were found leaving the flame's reaction zone; these were ascribed to this premixed flame having an exaggerated pyrolysis zone just before its reaction zone. Some particles grew to ∼2 µm after a further residence time of 2 ms in the visibly sooting region. No evidence was found for molecular beam sampling falsifying these TEM images of young soot particles. However, it does appear that during the supersonic expansion, immediately after the flame was sampled, soot particles reached a terminal velocity, which was less than the final velocity of the gas molecules leaving the expansion.