Experimental and modeling study on the molecular weight distribution and properties of carbon particles in premixed sooting flames

The evolution of the molecular weight (MW) distribution and structural properties of carbon particulate formed in methane, ethylene and benzene fuel-rich premixed flames, burning in similar conditions of maximum flame temperature, was experimentally measured and modeled. Both solubility and chromatographic separation of the carbon particulate allowed to follow the variation of molecular weight distributions of the different fractions. Structural properties of the carbon particulate in terms of H/C ratio and UV-visible absorption have been determined from particles inception to their maturation as a function of fuel identity. The experimental data were numerically predicted using a detailed gas-phase chemical kinetic mechanism coupled with a discrete sectional approach for the modeling of the molecule-to-particle process. The model was able to follow both particle concentration and structural properties for different fuels. In the benzene flame high molecular mass aromatic compounds appeared very early in the oxidation region and were no longer detected at the end of flame whereas aliphatic fuels showed both aromatic molecules and freshly-nucleated soot particles persisting downstream of the flame. Elemental analysis and UV-visible properties of the carbon particulate showed that also the graphitization process is earlier and much faster in the benzene flame whereas in aliphatic flames the final ordered structure is produced later on the flame and with a slower rate. Detailed modeling of the carbon particulate yields and properties confirms the differences in the evolution of carbon particulate and its structure in benzene and aliphatic flames, also individuating the classes of species: molecules and clusters of molecules, responsible for soot inception.