The evolution of soot particles in premixed and diffusion flames by thermophoretic particle densitometry

In this study, an improved version of the thermophoretic particle densitometry (TPD) method, introduced by McEnally et al. (1997) has been used for quantitative and qualitative characterization of soot particles generated in both premixed and diffusion flames. To this aim, the dependence of thermocouple temperature response on particle concentration and properties of collected material has been exploited. A variety of values for the thermal emissivity of flame-formed carbonaceous particles are measured, ranging from ? ? 0.4-0.5 for freshly nucleated particles up to the value of ? = 0.95, typical of a mature soot. The data demonstrate that a correct determination of ? 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. Raman spectroscopy analysis of the carbon particles has been conducted with the aim to better understand and to support the observed variation in the thermal emissivity of the carbon particles. In particular, the variation of thermal emissivity is showed to take up with a variation of hydrogen percentage and optical bandgap. Data also evidence that oxidation has a severe role in affecting both the determination by TPD of soot emissivity and soot concentration. In flame regions where soot oxidation occurs, the present formulation of TPD technique severely underestimate soot concentration.