QUANTITATIVE-ANALYSIS OF IN-SITU OPTICAL DIAGNOSTICS FOR INFERRING PARTICLE AGGREGATE PARAMETERS IN FLAMES - IMPLICATIONS FOR SOOT SURFACE GROWTH AND TOTAL EMISSIVITY/
Uo. Koylu, QUANTITATIVE-ANALYSIS OF IN-SITU OPTICAL DIAGNOSTICS FOR INFERRING PARTICLE AGGREGATE PARAMETERS IN FLAMES - IMPLICATIONS FOR SOOT SURFACE GROWTH AND TOTAL EMISSIVITY/, Combustion and flame, 109(3), 1997, pp. 488-500
An in situ particulate diagnostic/analysis technique is outlined based
on the Rayleigh-Debye-Gans polydisperse fractal aggregate (RDG/PFA) s
cattering interpretation of absolute angular light scattering and exti
nction measurements. Using proper particle refractive index, the propo
sed data analysis method can quantitatively yield all aggregate parame
ters (particle volume fraction, f(v), fractal dimension, D-f, primary
particle diameter, d(p), particle number density, n(p), and aggregate
size distribution, pdf(N)) without any prior knowledge about the parti
cle-laden environment. The present optical diagnostic/interpretation t
echnique was applied to two different soot-containing laminar and turb
ulent ethylene/air nonpremixed flames in order to assess its reliabili
ty. The aggregate interpretation of optical measurements yielded D-f,
d(p), and pdf(N) that are in excellent agreement with ex situ thermoph
oretic sampling/transmission electron microscope (TS/TEM) observations
within experimental uncertainties. However, volume-equivalent single
particle models (Rayleigh/Mie) overestimated d(p), by about a factor o
f 3, causing an order of magnitude underestimation in n(p). Consequent
ly, soot surface areas and growth rates were in error by a factor of 3
, emphasizing that aggregation effects need to be taken into account w
hen using optical diagnostics for a reliable understanding of soot for
mation/evolution mechanism in flames. The results also indicated that
total soot emissivities were generally underestimated using Rayleigh a
nalysis (up to 50%), mainly due to the uncertainties in soot refractiv
e indices at infrared wavelengths. This suggests that aggregate consid
erations may not be essential for reasonable radiation heat transfer p
redictions from luminous flames because of fortuitous error cancellati
on, resulting in typically a 10 to 30% net effect. (C) 1997 by The Com
bustion Institute.