QUANTITATIVE-ANALYSIS OF IN-SITU OPTICAL DIAGNOSTICS FOR INFERRING PARTICLE AGGREGATE PARAMETERS IN FLAMES - IMPLICATIONS FOR SOOT SURFACE GROWTH AND TOTAL EMISSIVITY/

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.