Soot Oxidation in Hydrocarbon-free Flames

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There are high uncertainties in the existing models of soot oxidation rates. To ameliorate this, soot oxidation in flames was examined using a novel ternary flame system, advanced diagnostics, and a detailed examination of past studies. The ternary flame system comprises a coflowing propylene/air diffusion flame to generate a steady soot column that flows into a hydrogen ring flame. The soot is thereby oxidized in a region far separated from soot formation, which is unlike any past study of soot oxidation in diffusion flames. Nonintrusive optical diagnostics were developed using a digital color camera to measure temperature and soot volume fraction. These diagnostics were validated using a steady laminar ethylene/air diffusion flame and were then applied to the ternary flame. Also measured in the soot flame were velocity, soot primary particle diameter, and stable species concentrations along an axial distance of 45 mm. Temperatures were between 1500 to 1750 K, and O2 partial pressures were between 10-2 to 10-1 bar. The soot flame was found to be lean, and its OH (with partial pressures between 10-4 to 10-3 bar) was expected to be equilibrated owing to the catalyzed radical recombination in the presence of soot. Soot flux and soot oxidation rates (0.5 to 6 g/m2-s) were determined. Soot burnout was 90% at 55 mm height. New soot oxidation mechanisms for O2 and OH were developed from a large body of published soot oxidation measurements. The resulting O2 mechanism has an activation energy of 195 kJ/mol, and the OH mechanism has a collision efficiency of 0.10. Predictions using the new mechanisms are within ±80% of the present measurements in the ternary flame system.