Optical Properties of Overfire Soot in Buoyant Turbulent Diffusion Flames at Long Residence Times.
Optical Properties of Overfire Soot in Buoyant Turbulent
Diffusion Flames at Long Residence Times.
Koylu, U. O.; Faeth, G. M.
Journal of Heat Transfer, Vol. 116, No. 1, 152-159,
Sponsor:National Institute of Standards and Technology,
soot; turbulent flames; diffusion flames; flame
radiation; scattering coefficient; extinction; soot
The optical properties of soot were studied for the
fuel-lean (overfire) region of buoyant turbulent
diffusion flames in still air. Results were limited to
the long residence time regime where soot structure is
indpendent of position in the overfire region and
residence time for a particular fuel. Measurements
included scattering, absorption, and extinction cross
sections at 514.5 nm and extinction cross sections at
632.8 and 1152 nm for flames fueled with acetylene,
propylene, ethylene, and propane. The measurements were
used to evaluate scattering predictions based on the
Rayleigh-Debye-Gans (RDG) approximation for randomly
oriented polydisperse fractal aggregates of spherical
primary soot particles having constant diameters. The
present soot aggregates exhibited significant departures
from Rayleigh-scattering behavior at 514.5 nm, with
forward scattering roughly 100 times larger than
wide-angle scattering and ratios of scattering to
absorption cross sections in the range 0.22-0.41,
increasing with increasing propensity of the fuel to
soot. The approximate RDG theory generally provided an
acceptable basis to treat the optical properties of the
present overfire soot aggregates, although additional
measurements in the Guinier (small angle) regime are
needed for a definitive evaluation of model performance.