Structure of the soot growth region of laminar premixed methane/oxygen flames

The structure of the soot growth region of laminar premixed methane/oxygen flames (fuel-equivalence ratios of 1.60-2.77) was studied both experimental ly and computationally. Measurements were carried out in flames stabilized on a hat flame burner operated at standard temperature and pressure, and in cluded velocities by laser velocimetry, soot volume fractions by laser exti nction, soot temperatures by multiline emission, gas temperatures (where so ot was absent) by corrected fine-wire thermocouples, major gas species conc entrations by sampling and gas chromatography, and hydrogen atom concentrat ions by the Li/LiOH technique in conjunction with atomic absorption to find the proportion of free lithium in the flames. The measured concentrations of major gas species were in reasonably good agreement with predictions bas ed on the detailed mechanisms of Leung and Lindstedt, and Frenklach and cow orkers. The measurements also confirmed predictions of both these mechanism s that H-atom concentrations are in local thermodynamic equilibrium through out the soot growth region even through the concentrations of major gas spe cies are not. Thus, present findings support recent evaluations of the hydr ogen-abstraction/carbon-addition (HACA) soot growth mechanism in similar fl ames, where the approximation that H-atom concentrations were in local ther modynamic equilibrium was adopted, based on predictions using the two mecha nisms, due to the absence of direct H-atom concentration measurements. (C) 2000 by The Combustion Institute.