Soot Formation in Laminar Partially Premixed Coflow Flames by Thermophoretic Particle Densitometry

The formation of soot particles in combustion depends on several factors that include: pressure, temperature, fuel chemical structure and extent of premixing. An improved version of the thermophoretic particle densitometry (TPD) method, has been used in this study for quantitative and qualitative characterization of soot particles generated in laminar partially-premixed co-flow flames having different equivalence ratios. To this aim, the dependence of thermocouple temperature response on particle concentration and properties of collected material has been exploited. A variety of thermal emissivity values are measured for flame-formed carbonaceous particles, ranging from ε ≈ 0.4 – 0.5 for freshly nucleated particles up to the value of ε = 0.95, typical of a mature soot particle. The correct determination of ε is necessary to accurately evaluate the particle volume fraction at the early stage of the soot formation, where particle concentration measurement is indeed particularly challenging. Data also evidence that oxidation has a severe role in affecting both soot emissivity and soot concentration determination by TPD. However, an attempt to include soot oxidation in the TPD volume fraction procedure is also illustrated resulting in a good agreement of the data obtained with other techniques. Moreover, a modeling analysis based on a sectional approach has been performed in order to corroborate the experimental findings.