The existence of young soot in the exhaust of inverse diffusion flames

Knowledge of the chemical and physical structure of early soot is useful in the development of soot particle inception models. This paper examines the hypothesis that soot exiting an inverse diffusion flame is similar in chemical and morphological structure to (1) soot precursor particles and (2) soot that exits underventilated flames. Experiments involving soot collection from the exhaust of laminar ethylene inverse diffusion flames were performed. Soot samples were analyzed for morphology using transmission electron microscopy, for carbon-to-hydrogen ratio using elemental analysis, for organic fraction using thermal-optical analysis, and for polycyclic aromatic hydrocarbon content using laser microprobe mass spectrometry and gas chromatography/mass spectrometry. Results of these analyses support the validity of the above hypothesis. This finding is significant because exhaust collection from the inverse flame provides an opportunity to gather large samples of young soot without invading the flame with an intrusive probe (a necessary task when collecting precursors low in the center of a normal diffusion flame). Larger samples can then be subjected to more detailed analysis than previously possible. An identification of specific polycyclic aromatic hydrocarbon isomers present in young soot from diffusion flames is reported. The data are available for comparison with polycyclic aromatic hydrocarbon growth, soot inception, and soot growth models.