Measurement of the dimensionless extinction coefficient of soot within laminar diffusion flames

Abstract

The dimensionless extinction coefficient ( K e) of soot must be known to quantify laser extinction measurements of soot concentration and to predict optical attenuation through smoke clouds. Previous investigations have measured K e for post-flame soot emitted from laminar and turbulent diffusion flames and smoking laminar premixed flames. This paper presents the first measurements of soot K e from within laminar diffusion flames, using a small extractive probe to withdraw the soot from the flame. To measure K e, two laser sources (635 nm and 1310 nm) were coupled to a transmission cell, followed by gravimetric sampling. Coannular diffusion flames of methane, ethylene and nitrogen-diluted kerosene burning in air were studied, together with slot flames of methane and ethylene. K e was measured at the radial location of maximum soot volume fraction at several heights for each flame. Results for K e at both 635 nm and 1310 nm for ethylene and kerosene coannular flames were in the range of 9–10, consistent with the results from previous studies of post-flame soot. The ethylene slot flame and the methane flames have lower K e values, in some cases as low as 2.0. These lower values of K e are found to result from the contributions of (a) the condensation of PAH species during the sampling of soot, (b) the wavelength-dependent absorptivity of soot precursor particles, and, in the case of methane, (c) the negligible contribution of soot scattering to the extinction coefficient. RDG calculations of soot scattering, in combination with the measured K e values, imply that the soot refractive index is in the vicinity of 1.75–1.03 i at 635 nm.