Influence of the soot property uncertainties in temperature and volume-fraction measurements by two-colour pyrometry

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Abstract
A sensitivity analysis of the two-colour pyrometry technique for calculation of soot temperature T and volume fraction fv is presented. Two different combustion environments are exemplified for high (fv approximately=10-5) and low (fv approximately=10-7) soot concentration regimes. The calculations are carried out taking into account particle shape, different expressions for monochromatic emissivity, optical property variations, size distribution of soot particles and a finite light-detection bandwidth. The results indicate which spectral range, for the choice of the ( lambda 1, lambda 2) wavelengths employed in the emission measurements, and computing algorithm, for the reckoning of the T values, can be appropriated to guarantee a lower a priori uncertainty in the T and fv calculated values. Results stress the evidence that T and fv values are critically sensitive to the complex refractive index, m=n-ik, and that they are irrelevantly influenced by the probability distribution of particle size. The critical theoretical influence of the type of fuel, characterized by a different soot hydrogen/carbon ratio, upon the inferred fv values is assessed. Determinations of temperature values with ( lambda 1, lambda 2) measurement wavelengths above 1.2 mu m is demonstrated to be invalidated by an uncertainty contribution of at least 20 K in the regime fv approximately=10-5 and 100 K in the regime fv approximately=10-7.