Spectroscopic and Chemical Characterization of Soot Inception Processes in Premixed Laminar Flames at Atmospheric Pressure

Absorption, laser induced fluorescence and laser light scattering measurements have been performed in a slightly-sooting methane/oxygen flame at atmospheric pressure. Sampling and analysis has been also carried out with the specific aim of measuring and characterizing all of the condensable material formed during the combustion process. The combined use of scattering and extinction measurements in the ultra-violet (266 nm) and in the visible (532 nm) allowed to determine simultaneously the volume fraction, the average size and the number density of high molecular weight soot precursors and soot particles. The results of the optical measurements were in good agreement with the concentration measured by the sampling procedure. The formation of "particles" early in the flame with typical size of about 3-4 nm with no absorbtion and fluorescence in the visible could be shown. These "transparent" particles were considered to be precursors of soot particles on the basis of their decreasing concentration during soot inception. The results presented suggest that there is an initial fast polymerization process the building bricks of which are aromatic compounds with few condensed rings (not more than 2-3 rings) connected by aliphatic and oxygen bonds. Particle inception is primarily controlled by the internal arrangement of these polymers leading to structures with more condensed aromatic rings and more compact three dimensional shape thus forming the first soot nuclei.