During the combustion of carbonaceous fuels under rich conditions, incomplete combustion will lead to a fraction of fuel’s carbon being converted into carbonaceous particulate known as soot. The particle inception process is believed to occur at the transition from gas-phase to solid-phase species and is known to involve polyaromatic hydrocarbons (PAH). While parts of the soot formation mechanism are well-understood, this particle inception process lacks the same level of experimental rigor. A combination of in situ and ex situ optical spectroscopy methods have allowed us to fill this experimental void. Tauc/Davis-Mott analysis using a supercontinuum light source was used to map out the experimentally determined optical band gap through a range of laminar diffusion flames including both gas and liquid phase fuels. The average optical band gap observed across all of the flames was 2.06 ±0.03eV. Semi-empirical (ZINDO/S) and quantum mechanical (DFT) computations were performed to relate the experimentally determined optical band gap to the HOMO-LUMO gap in a variety of PAH. The average PAH sizes in these flame systems corresponded to structures of 11 ± 4 rings and conjugation lengths between 0.85 ± 0.11 nm. Raman spectroscopy of thermophoretically sampled soot supported these results concluded finding a consistent conjugation length of 0.86 ± 0.10 nm was observed throughout the entire flame system . FTIR spectroscopy of thermophoretically sampled soot in the same flame system found the prevalence of both aromatic and aliphatic compounds throughout the entire flame, but was not able to conclude the nature of the aliphatic species. The conclusions from this work indicate that the particle inception occurs with structures about the size of ovalene and this size seems to be consistent across of a variety of fuels and sooting levels. Additionally, the consistency of conjugation lengths and optical band gaps determined throughout the flame systems, with increasing soot volume fractions indicates that further mass growth of the soot particulates occurs through aggregation processes.
|Advisor:||Miller, J. Houston|
|Commitee:||Chen, Hanning, Dowd, Cynthia, Lighty, JoAnn, Vertes, Akos|
|School:||The George Washington University|
|School Location:||United States -- District of Columbia|
|Source:||DAI-B 77/08(E), Dissertation Abstracts International|
|Keywords:||Laser diagnostics, Polyaromatic hydrocarbons, Soot, Supercontinuum light source|
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