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DOI: 10.1615/ICHMT.1995.RadTransfProc.200

Daniel W. Mackowski
Department of Mechanical Engineering, Auburn University, AL, 36849, USA


The Rayleigh-limit radiative absorption and emission properties of aggregated soot particles are investigated. It is assumed here that the size of the particle is significantly smaller than the radiation wavelength. For this condition, the absorption and scattering properties of the aggregates can be obtained from solution of Laplace's equation for a multiple sphere configuration. Calculations of soot absorption are performed for fractal-like configurations of spheres. It is shown that the independent sphere assumption for aggregate absorption does not hold when the real and imaginary parts of the primary sphere refractive index are relatively large. Rather, the absorption cross section of the aggregate can exceed the sum of the primary sphere cross sections by as much as 100%. Because of the spectral variation of carbonaceous soot refractive index this effect is expected to be most significant in the near to mid IR radiation wavelengths, and can lead to Planck mean absorption coefficients that are on the order of 20-50% greater than those predicted from spherical Rayleigh models at typical combustion temperatures. It is also shown that the Rayleigh-limit absorption properties of fractal aggregates scale according to the ratio of the primary sphere radius and the aggregate radius of gyration.

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