Effect of Maturity on the Measurements of Soot Volume Fraction by the Auto-compensating Laser-induced Incandescence Technique

Abstract

International audienceSoot is a major byproduct of incomplete combustion of hydrocarbon fuels and biomass burning. Soot is also commonly known as black carbon after emissions. Accurate measurements of soot concentrations at the exhaust of combustion systems are of great importance for emission control and assessing the effectiveness of combustion and fuel technologies on soot emission reduction. Laser-induced incandescence (LII) has been found widespread use for measurements of carbonaceous and metal nanoparticles in many different applications.The auto-compensating LII (AC-LII) technique has become one of the popular in-situ real-time optical method for both in-flame and exhaust soot measurement. In AC-LII, the absolute LII signals at two different spectral bands in the visible to near-infrared are detected and the laser-heated soot particle temperature is determined through the principle of two-color pyrometry. Then the soot volume fraction can be inferred from the soot temperature and one of the two LII signal. In the current AC-LII data analysis, it is implicitly assumed that the soot to be measured is mature and the absorption functions at the two detection wavelengths are equal. Recent studies have revealed that soot emitted from difference combustion sources displays size-dependent absorption property, an indication that soot emitted from different sources may have different maturity. Soot maturity can be characterized by the Angstrom absorption exponent and less mature soot displays a wavelength-dependent soot absorption function. Neglect of the wavelength-dependent soot absorption function results in overestimation of soot temperature and hence underestimation of soot volume fraction.In this study, the effect of soot maturity on AC-LII measured soot volume fraction was analyzed and quantified by comparing the AC-LII measurements with light extinction measurements in a laminar coflow diffusion flame