Gm. Faeth et Uo. Koylu, SOOT MORPHOLOGY AND OPTICAL-PROPERTIES IN NONPREMIXED TURBULENT FLAMEENVIRONMENTS, Combustion science and technology, 108(4-6), 1995, pp. 207-229
Motivated by the importance of soot to the emission of particulates an
d other pollutants from combustion processes, current understanding of
soot morphology and optical properties is reviewed, emphasizing nonpr
emixed flame environments. The understanding of soot morphology in fla
mes has grown rapidly in recent years due to the development of method
s of thermophoretic sampling and analysis by transmission electron mic
roscopy (TEM). The results show that soot consists of nearly spherical
primary particles, having diameters generally less than 60 nm, which
collect into open structured aggregates that are mass fractal objects.
Aggregates grow by cluster/cluster aggregation to yield broad aggrega
te size distributions with the largest aggregates containing thousands
of primary particles and reaching dimensions of several mu m. The opt
ical properties of soot aggregates generally are not suited for the Ra
yleigh and Mie scattering approximations which has led to the developm
ent of approximate Rayleigh-Debye-Gans (RDG) scattering models for pol
ydisperse fractal aggregate populations of soot. Evaluation of RDG mod
els for conditions where both soot structure and scattering properties
are known indicates encouraging agreement between predictions and mea
surements at both visible and infrared wavelengths, as well as reasona
ble accuracy for Rayleigh scattering theories in the infrared. Thus, t
here is potential for nonintrusive measurements of both soot concentra
tions and structure in flame environments, which should be helpful for
diagnosing problems of particulate and pollution emissions from combu
stion processes. However, additional work is needed to realize this po
tential, including: reliable solutions of the inverse problem, to yiel
d soot properties from scattering and extinction measurements, must be
developed; existing uncertainties about soot refractive indices, incl
uding effects of fuel type and flame conditions, must be resolved; and
more definitive assessment of the limitations of existing approximate
theories for soot optical properties must be obtained.