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Computational Evaluation of Approximate Rayleigh-Debye-Gas/Fractal-Aggregate Theory for the Absorption and Scattering Properties of Soot.

Computational Evaluation of Approximate
Rayleigh-Debye-Gas/Fractal-Aggregate Theory for the
Absorption and Scattering Properties of Soot.
(1029 K)

Farias, T. L.; Carvalho, M. G.; Koylu, U. O.; Faeth, G.
M.

Journal of Heat Transfer, Vol. 117, No. 1, 152-159,
February 1995.

### Sponsor:

National Aeronautics and Space Administration,
Washington, DC

### Keywords:

soot; scattering coefficient; optical properties;
aggregates; refractive index; evaluation; soot
aggregates; scattering cross sections

### Abstract:

*
A computational evaluation of an approximate theory for
the optical properties of soot is described, emphasizing
the small-angle (Guinier) regime. The approximate
theory (denoted RDG-FA theory) is based on the
Rayleigh-Debye-Gans scattering approximation while
treating soot as mass-fractal aggregates of spherical
primary particles that have constant diameters and
refractive indices. The approximate theory was
evaluated by more exact predictions from the solution of
the volume integral equation formulation of the
governing equations, using the method of moments, and
based on the ICP algorithm of Iskander et al. (1989).
Numerical simulations were used to construct
statistically significant populations of soot aggregates
having appropriate fractal properties and prescribed
numbers of primary particles per aggregate. Optical
properties considered included absorption, differential
scattering, and total scattering cross sections for
conditions typical of soot within flame environments at
wavelengths in the visible and the infrared. Specific
ranges of aggregate properties were as follows: primary
particle optical size parameters up to 0.4, numbers of
primary particles per aggregate up to 512, mean fractal
dimensions of 1.75, mean fractal prefactors of 8.0, and
refractive indices typical of soot. Over the range of
the evaluation, ICP and RDG-FA predictions generally
agreed within numerical uncertainties (ca. 10 percent)
within the Guinier regime, complementing similar
performance of RDG-FA theory in the power-law regime
based on recent experiments. Thus, the use of
approximate RDG-FA theory to estimate the optical
properties of soot appears to be acceptable -
particularly in view of the significant uncertainties
about soot optical properties due to current
uncertainties about soot refractive indices.
*