MEASUREMENT OF SOOT MORPHOLOGY, CHEMISTRY, AND OPTICAL PROPERTIES IN THE VISIBLE AND NEAR-INFRARED SPECTRUM IN THE FLAME ZONE AND OVERFIRE REGION OF LARGE JP-8 POOL FIRES

Abstract

The dimensionless extinction coefficient, K e , was measured for soot produced in 2 m JP-8 pool fires. Light extinction and gravimetric sampling measurements were performed simultaneously at 635 and 1310 nm wavelengths at three heights in the flame zone and in the overfire region. Measured average K e values of 8.4 ± 1.2 at 635 nm and 8.7 ± 1.1 at 1310 nm in the overfire region agree well with values from 8–10 recently reported for different fuels and flame conditions. The overfire K e values are also relatively independent of wavelength, in agreement with recent findings for JP-8 soot in smaller flames. K e was nearly constant at 635 nm for all sampling locations in the large fires. However, at 1310 nm, the overfire K e was higher than in the flame zone. Chemical analysis of physically sampled soot shows variations in carbon-to-hydrogen (C/H) ratio and polycyclic aromatic hydrocarbon (PAH) concentration that may account for the smaller K e values measured in the flame zone. Rayleigh–Debye–Gans theory of scattering for polydisperse fractal aggregate (RDG-PFA) was applied to measured aggregate fractal dimensions and found to under-predict the extinction coefficient by 17–30% at 635 nm using commonly accepted refractive indices of soot, and agreed well with the experiments using the more recently published refractive index of 1.99–0.89i. This study represents the first measurements of soot chemistry, morphology, and optical properties in the flame zone of large, fully-turbulent pool fires, and emphasizes the importance of accurate measurements of optical properties both in the flame zone and overfire regions for models of radiative transport and interpretation of laser-based diagnostics of soot volume fraction and temperature.