Soot surface growth and oxidation in laminar diffusion flames at pressures of 0.1–1.0 atm

Abstract

The flame and soot structure and the soot surface growth and oxidation properties of round laminar jet diffusion flames were studied experimentally at pressures of 0.1–1.0 atm. Measurements were made along the axes of flames fueled with acetylene–nitrogen mixtures and burning in coflowing air with the reactants at normal temperature (300 K). The following properties were measured as a function of distance from the burner exit: soot concentrations by deconvoluted laser extinction, soot temperatures by deconvoluted multiline emission, soot structure by thermophoretic sampling and analysis using Transmission Electron Microscopy (TEM), concentrations of major stable gas species (N2, Ar, H2O, H2, O2, CO, CO2, CH4, C2H2, C2H4, C2H6, C3H6, C3H8, and C6H6) by isokinetic sampling and gas chromatography, concentrations of some radial species (H, OH, O) by deconvoluted Li/LiOH atomic absorption, and flow velocities by laser velocimetry. These measurements yielded local soot surface growth and oxidation rates, as well as local flame properties that are thought to affect these rates. Present measurements of soot surface growth rates (corrected for soot surface oxidation) in laminar diffusion flames at various subatmospheric pressures were consistent with earlier measurements of soot surface growth rates in laminar premixed and diffusion flames involving a variety of hydrocarbons at atmospheric pressure; in addition, rates from all the available flames were in good agreement with each other and with existing Hydrogen-Abstraction/Carbon-Addition (HACA) soot surface growth mechanisms in the literature for values of steric factors in these mechanisms on the order of unity, as expected. Similarly, present measurements of early soot surface oxidation rates (corrected for soot surface growth and prior to consumption of 70% of the maximum mass of the primary soot particles) for fuel-rich and near-stoichiometric conditions in laminar diffusion flames at pressures of 0.1–1.0 atm were consistent with earlier measurements of rates of early soot surface oxidation in laminar premixed and diffusion flames involving a variety of hydrocarbons at atmospheric pressure; in addition, rates from all available flames could be explained by reaction with OH, as proposed by K.G. Neoh et al. (in: Particulate Carbon, Plenum, New York, 1980, p. 261), having a collision efficiency of 0.13 and supplemented to only a minor degree by direct soot surface oxidation by O2.