Optical measurements of temperature fields in sooting flames: influence of soot self-absorption

Regular pyrometry techniques have been extensively used to infer the temperature field in sooting flames from soot luminosity. However, correction for soot self-absorption along the line-of-sight needs to be considered. The original contribution of the present paper is to assess both numerical and experimental uncertainties that can be attributed to the soot self-absorption effect on the soot temperature field measured by the two-color Modulated Absorption/Emission (2C-MAE) technique. Unlike for regular pyrometry techniques, the design of the 2C-MAE technique actually enables the direct measurement of the local spectral absorption coefficient field. The proportion of flame emission trapping caused by the soot along the line-of-sight is first simulated for different levels of soot loading ranges. The retrieved temperature error when self-absorption is neglected can then be quantified as a function of the level of soot loading and the detection spectral ranges of the technique. As a result, it is found that the proportion of the flame emission attenuation due to self-absorption is directly proportional to the soot volume fraction fv and hardly depends on the temperature field to be retrieved. These trends are emphasized as the lower spectral range of detection is shifted towards the smaller wavelength. In addition, a linear correlation between the absolute temperature error and the peak fv in the flame can be extracted and confirmed by experimental results. Eventually, it is also found that selecting detection spectral ranges centered at 645nm and 785nm offer the minimum temperature deviation when soot self-absorption is neglected for any conditions of soot loading level within the three studied spectral combinations. This finding is especially relevant for the identification of the optimal operating conditions required by regular 2C-pyrometry for the sooting flames considered.