Spectrally Resolved Measurement of Flame Radiation to Determine Soot Temperature and Concentration

TLDR

The study discusses the key optical parameters relevant to soot radiation measurement. The authors develop a multi‑wavelength flame emission technique to determine soot temperature and volume fraction with high spatial resolution in axisymmetric laminar diffusion flames. Horizontal line‑integrated spectra over 500–945 nm are collected and inverted via one‑dimensional tomography using a three‑point Abel inversion to obtain radial soot radiation distributions, from which temperature profiles are extracted; absolute calibration then yields absorption coefficients proportional to soot volume fraction, and an emission correction scheme is outlined for heavily sooting flames. Uniform cross‑sectional sampling is essential, as variations in sampling area cause inconsistencies, especially at flame edges; emission attenuation has minor influence on temperature and volume fraction for the tested loading; the technique’s results agree closely with coherent anti‑Stokes Raman thermometry and two‑dimensional soot extinction, indicating the spectral response slope of the refractive index absorption function is between 0 and 20 %.

Abstract

A multiwavelength flame emission technique is developed for high spatial resolution determination of soot temperature and soot volume fraction in axisymmetric laminar diffusion flames. Horizontal scans of line-integrated spectra are collected over a spectral range of 500-945 nm. Inversion of these data through one-dimensional tomography using a three-point Abel inversion yields radial distributions of the soot radiation from which temperature profiles are extracted. From an absolute calibration of the flame emission and by use of these temperature data, absorption coefficients are calculated, which are directly proportional to the soot volume fractions. The important optical parameters are discussed. It is shown that a uniform sampling cross section through the flame must be maintained and that variations in sampling area produce inconsistencies between measurements and theory, which cannot be interpreted as spatial averaging of the property field. The variations in cross-sectional sampling area have the largest influence on the measurements at the edges of the flame, where the highest resolution is required. Emission attenuation by soot has been shown to have minor influence on the soot temperature and soot volume fraction for the soot loading of the axisymmetric flame tested. An emission correction scheme is outlined, which could be used for more heavily sooting flames. For a refractive index absorption function E(m) = Im[(m 2 - 1)/(m 2 + 2)] that is independent of wavelength, the soot temperatures and soot volume fractions measured with this technique are in excellent agreement with data obtained by coherent anti-Stokes Raman scattering nitrogen thermometry and two-dimensional soot extinction in the same ethylene coflow diffusion flame. The agreement of the results suggests a limit of the slope of the spectral response of E(m) to be between 0 and 20% over the spectral range examined.