Soot formation, composition, and morphology

A complete understanding of soot particle formation is critical for accurate combustion models. Limited details are known about the polycyclic aromatic hydrocarbon (PAH) molecules that comprise soot mass, the physical processes involved in nascent soot formation, particle morphology transformation in high temperature environments, and the influence of products of combustion in the soot formation zone. These phenomena have been studied using a variety of fuels in the University of Michigan Rapid Compression Facility at high temperature (1600--2000 K) and high-pressure (10 atm) environments that are similar to those found in advanced combustion strategies. Details of soot particle nanostructure were identified using a high-resolution transmission electron microscope (HRTEM).;This study discovered PAH molecule size distributions and particle morphologies from HRTEM images of nascent and mature soot particles. Molecules deposited on both nascent and aged soot particles had a constant size distribution. At low temperatures, amorphous nascent particles were created. At high temperatures, the nascent particles became comprised of agglomerated 2--8 nm diameter particles. Shortly after inception, the small particles no longer attached as growth species. A coagulation model replicated the experimental finding and relates small particle agglomeration to the PAH molecules rate of production.;Particle morphologies were found to change as nascent particles mature. Transformations were also induced in HRTEM by irradiating nascent particles. Following irradiation, particle morphologies were very similar to mature particle morphologies created at high temperatures. HRTEM images of the transformation showed tangential molecular alignment at the particle surface, merging of adjacent particles, and epitaxial alignment of interior molecules with the surface molecules.;Finally, it was discovered that CO2 enhanced soot formation rate in an oxygen deficient reaction zone when combined with acetylene fuel but had an undetectable affect on methane fuel. Gas chromatography confirmed the CO2 reactivity. A chemical kinetics investigation discovered that OH radical production from CO2 caused the enhancement in soot formation rate. OH radicals were less important in the methane fuels due to more prevalent reactions with H and CH3 radicals and a longer reaction pathway between the fuel and the aromatic molecules.