Simultaneous soot temperature and volume fraction measurements in axis-symmetric flames by a two-dimensional modulated absorption/emission technique

The original contribution of the present paper is to present a joint theoretical and experimental approach to implement the modulated absorption/emission technique. Two-dimensional fields of soot temperature and volume fraction can then be measured simultaneously in a reference steady laminar coflow axis-symmetric non-premixed ethylene flame established over the Santoro burner. The spontaneous flame emission is captured by two CMOS cameras that allow the measurements of the flame emission intensity at two different wavelengths, i.e., 645. nm and 785. nm in this study. Concomitantly, two 70. mm diameter laser beams are used to pass through the flame, enabling the spectral line-of-sight attenuation to be imaged at the two wavelengths by the CMOS sensors. In these spectral ranges, both absorption and emission phenomena are dominated by soot. The local spectral absorption coefficient and spectral emission rate at both wavelengths are obtained by the onion-peeling method with Tikhonov regularization. The soot volume fraction map is inferred from the spectral absorption coefficient field. Mapping soot temperature does not require any model correlating soot volume fraction and local spectral absorption coefficient. Only the measurements of the latter at both wavelengths are required to enable the self-calibration of the technique and infer soot temperature from the ratio of the local spectral emission rates. Thus, the issue of the large discrepancies in the wavelength-dependent soot refractive index reported in the literature does not arise. Within the region of high soot temperatures, the results obtained by the methodology are in good agreement with numerical and experimental data available in the literature. The use of continuous wave lasers as the light sources enables future investigations in flickering flames where the phenomenon of intermittent soot release through the flame tip still needs to be better understood. \ua9 2015 The Combustion Institute.Peer reviewed: YesNRC publication: Ye