Soot is a dominant feature of hydrocarbon/air flames, affecting their reaction mechanisms and structure. Motivated by these observations, the present investigation extended past experimental studies of soot processes in laminar premixed and diffusion flames in this laboratory [1-9], to consider soot oxidation in laminar diffusion flames using similar methods. Soot oxidation in diffusion flame environments have been studied as follows: in methane/air flames by Garo et al. [10,11], in ethylene/air flames by Puri et as. [12,13], and in ethylene-nitrogen/oxygen-argon flames by Haudiquert et al. [14]. These studies support the dominant role of OH in soot oxidation at near-stoichiometric conditions in diffusion flames but yielded OH collision efficiencies that varied widely among the three studies and were not in good agreement with the results on Neoh et al. [8,9] for premixed flames. One explanation for the disagreement was that optical scattering and extinction measurements were used to infer soot structure properties during the diffusion flame studies that were based on models that have not been very successful for representing soot optical properties [15,16]. Difficulties with radical measurements using LIF methods, due to PAH fluorescence in soot-containing environments, offer another explanation for the problematical collision efficiencies for soot oxidation by OH found in Refs. 10-14. The present investigation sought to resolve problems of soot oxidation in diffusion flames by measuring soot and flame structure in a variety of soot-containing diffusion flames and exploiting the measurements to establish diffusion flame soot oxidation mechanisms.
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