Extinction and Scattering Properties of Soot Emitted From Buoyant Turbulent Diffusion Flames

Abstract

Extinction and scattering properties at wavelengths of 250–5200 nm were studied for 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 flame residence times. Flames burning in still air and fueled with gas (acetylene, ethylene, propane, and propylene) and liquid (benzene, toluene, cyclohexane, and n-heptane) hydrocarbon fuels were considered. Measured scattering patterns and ratios of total scattering/absorption cross sections were in good agreement with predictions based on the Rayleigh-Debye-Gans (RDG) scattering approximation in the visible. Measured depolarization ratios were roughly correlated by primary particle size parameter, suggesting potential for completing RDG methodology needed to make soot scattering predictions as well as providing a nonintrusive way to measure primary soot particle diameters. Measurements of dimensionless extinction coefficients were in good agreement with earlier measurements for similar soot populations and were independent of fuel type and wavelength except for reduced values as the near ultraviolet was approached. The ratios of the scattering/absorption refractive index functions were independent of fuel type within experimental uncertainties and were in good agreement with earlier measurements. The refractive index function for absorption was similarly independent of fuel type but was larger than earlier reflectometry measurements in the infrared. Ratios of total scattering/absorption cross sections were relatively large in the visible and near infrared, with maximum values as large as 0.9 and with values as large as 0.2 at 2000 nm, suggesting greater potential for scattering from soot particles to affect flame radiation properties than previously thought.

High spatial resolution laser cavity extinction and laser-induced incandescence in low-soot-producing flames

“…However, soot particles are formed of aggregated and transformed primary particles, an effect that becomes more significant under high soot loading. Krishnan et al [53] found that the ratio of scattering to extinction cross section could be as large as 0.47 in the visible range for heavy sooting diffusion flames fuelled with n-heptane, benzene and toluene. Nevertheless, Liu et al investigated the signal trapping effect in LII measurements using the Rayleigh-Debye-Gans model to show that the contribution of scattering of soot particles in laminar diffusion flames should be negligible [52].…”

Understanding and predicting soot generation in turbulent non-premixed jet flames.

“…Optical extinction by soot nominally follows a  -1 dependence at visible and near-infrared wavelengths [94]. To minimize the influence of optical extinction on the LII measurements of soot concentration, an excitation wavelength of 1064 nm (YAG fundamental) was used in this project, which limits the extinction of the laser beam itself and also allows detection of the LII signals at wavelengths through the visible region, limiting the extinction of the LII signal in comparison to typical LII signal detection around 400 nm.…”

Experimental and theoretical comparison of spatially resolved laser-induced incandescence (LII) signals of soot in backward and right-angle configuration

“…The values readily available from literature at this wavelength range to the authors knowledge from around 0.20 to 0.42 [27,31]. Choosing 0.42 [27] instead of 0.296 would increase the absorption term with 42% and hence lead to higher degree of mass loss.…”

“…The absorption function must also be used for calculating the LII signal, and since broad-band detection was used, the wavelengthdependence of the E(m) is required. Since it is generally believed that the E(m) varies very little with wavelength in the visible region (See for example [31]), the parameter was treated as constant in this work, and as a constant the relative shape of the calculated signal is unaffected by the exact choice of E(m). For simplicity the E(m) was set to 0.296 also when evaluating the signal response.…”