Experiments and modeling of incipient soot formation and growth in flames

Soot emitted from combustion processes has raised significant environmental and health concerns. The formation and growth of incipient soot particles have not been well understood due to a lack of suitable instrumentation in the critical particle size range of 1 to 10 nm.; In this work, a scanning mobility particle sizer with a sample probe subsystem was developed and extensively characterized. Using this experimental technique, soot formation and growth in a premixed ethylene-oxygen-argon flame was studied. Bimodal behavior was captured. For smaller particle sizes the size distribution can be described by a power-law decay function and a lognormal size distribution is observed for larger particle sizes. Through computational analysis, it was found that the bimodal behavior was a result of the competition between persistent soot inception and coagulation. Furthermore the bimodality is consistent with the notion of soot inception following the 2nd order nucleation kinetics. Two series of flames were designed to investigate the influence of flame temperature on particle size distribution functions. It was found that soot favors a bimodal size distribution at low flame temperatures and the bimodal disappears when the flame temperature reaches above 1850 K. The characteristic decay of simulated benzene concentration profiles in the post flame parallels the disappearance of bimodality.; Using the same experimental technique, particle-particle coagulation inside sampling lines was investigated to verify the existence of reported transparent soot particle in flames below critical sooting limit. No particle was observed in nonsooting flames. It was estimated that transparent soot particles, if they exist, has a number density 5 orders lower than that reported in the literature.; The effect of ferrocene doping on critical sooting limit (C/O)c was also studied. It was found the (C/O)c was lowered by a notable amount with just 20 ppm of ferrocene doping. The sooting tendency for both low- and high-temperature flames increases with increasing ferrocene doping, and the sooting temperature window is broadened as a result of ferrocene addition. The increased sooting tendency is likely the result of iron-oxide induced nucleation.