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Buoyant Turbulent Jets and Flames. Part 2. Refractive Index, Extinction and Scattering Properties of Soot. Annual Report.


pdf icon Buoyant Turbulent Jets and Flames. Part 2. Refractive Index, Extinction and Scattering Properties of Soot. Annual Report. (6765 K)
Krishnan, S. S.; Faeth, G. M.

NIST GCR 00-796; GDL/GMF-99-02; 119 p. September 2000.

Sponsor:

National Institute of Standards and Technology, Gaithersburg, MD

Available from:

National Technical Information Service (NTIS), Technology Administration, U.S. Department of Commerce, Springfield, VA 22161.
Telephone: 1-800-553-6847 or 703-605-6000;
Fax: 703-605-6900; Rush Service (Telephone Orders Only) 800-553-6847;
Website: http://www.ntis.gov
Order number: PB2001-100005

Keywords:

turbulent jets; turbulent flames; buoyant plumes; buoyant flow; soot; extinction; diffusion flames

Abstract:

Extinction and scattering properties at wavelengths of 250-5200 nm were studied for soot emitted from large buoyant turbulent diffusion flames where soot properties are independent of position in the overfire region and characteristic flame residence time. Flames burning in still air and fueled with both gas (acetylene, ethylene, propane and propylene) and liquid (benzene, toluene, cyclohexane and n-heptane) hydrocarbon fuels were considered. Extinction and scattering measurements were interpreted to find soot optical properties using Rayleigh-Debye-Gans/polydisperse-fractal aggregate (RDG/PFA) theory after establishing that this theory provided good predictions of scattering patterns and ratios of total scattering/absorption cross sections. Effects of fuel type on soot optical properties were comparable to experimental uncertainties. Measured depolarization ratios were correlated with the primary particle size parameter, completing RDG/PFA methodology needed to make soot extinction and scattering predictions. Measurements of dimensionless extinction coefficients were in good agreement with earlier measurements for similar soot populations and were relatively independent of wavelength for wavelengths of 400-800 nm where a mean value of 8.4, averaged over fuel type and wavelength, was observed. The refractive index function for absorption was in good agreement with earlier reflectometry measurements in the visible but was larger than these measurements in the infrared. Similarly, present measurements of the refractive index function for scattering agreed with earlier reflectometry measurements for wavelengths of 400-550 nm but otherwise increased with increasing wavelength more rapidly than the rest. Finally, ratios of total scattering/absorption cross sections were relatively large in the visible and near-infrared, with maximum values as large as 0.9, suggesting greater potential for scattering from soot particles to affect flame radiation properties than previously thought.