Mixing and Radiation Properties of Buoyant Luminous Flame Environments. Part 1. Self-Preserving Plumes. Final Report.
Mixing and Radiation Properties of Buoyant Luminous
Flame Environments. Part 1. Self-Preserving Plumes.
Final Report.
(9601 K)
Dai, Z.; Sangras, R.; Tseng, L. K.; Faeth, G. M.
NIST GCR 99-769; GDL/GMF-98-02; 128 p. March 1999.
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.
Website: http://www.ntis.gov
Order number: PB99-140535
Keywords:
fire research; flame research; flame structure; optical
properties; refractive index; soot; soot aggregates
Abstract:
An investigation of the structure and optical properties
of soot is described, motivated by the need to develop
nonintrusive methods for measuring soot properties and
to estimate the continuum radiation properties of soot
for flame environments. The study included evaluation of
Rayleigh-Debye-Gans/polydisperse-fractal-aggregate
(RDG-PFA) approximate scattering theory for the optical
properties of soot aggregates, the determination of the
structure and fractal properties of soot that are needed
to make predictions with RDG-PFA theory, and the use of
RDG-PFA theory to measure the refractive index and
related optical properties of soot. Measured soot
optical properties were in good agreement with the
predictions of RDG-PFA theory over a broad test range,
as follows: soot from both the fuel-rich and fuel-lean
regions of buoyant diffusion flames, wavelengths of
350-800 nm, primary particle size parameters as large as
0.46, and soot formed in diffusion flames burning in air
for a variety of gaseous and liquid hydrocarbon fuels
(acetylene, ethylene, propylene, butadiene, benzene,
cyclohexane, toluene and n-heptane). Evaluation of the
structure and fractal properties of soot was carried out
for a similar range of wavelengths and soot properties
showing that soot aggregate fractal properties are
relatively independent of flame conditions, yielding a
fractal dimension of 1.82 and a fractal prefactor of
8.5, with experimental uncertainties (95% confidence) of
0.08 and 0.5, respectively. The research also yields
some helpful relationships between actual and projected
soot aggregate properties. Evaluation of the refractive
index properties of soot was limited to soot emitted
from buoyant turbulent diffusion flames in the long
residence time regime (where soot properties are
independent of position in the overfire region and
characteristic residence time) for the same range of
wavelengths and fuels as before. Major findings
concerning soot refractive index and related properties
are as folows: dimensionless extinction coefficients
were relatively independent of fuel type and wavelength
and agreed with results from earlier studies within
experimental uncertainties; in agreement with recent
studies, there was no evidence of a resonance condition
in the near uv for soot, similar to graphite; the
refractive index functions relevant to absorption and
emission, E(m), were relatively independent of fuel type
and wavelength and agreed with earlier work for
wavelengths greater than 400 nm; and the refractive
index functions relevant to scattering, F(m), were also
relatively independent of fuel type but increased faster
with increasing wavelength than observed during earlier
studies.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899