Nanoparticles and soot formation in co-flowing diffusion flames

It has been recently found that carbon nanoparticles smaller than 5nm different from soot particles are easily formed in rich combustion systems. A rather systematic characterization of these structures has been carried out in rich laminar premixed flames where it was possible to realize conditions where they are present even in absence of soot particles. Diffusion flames are more complex to characterize because they present spatial gradients and, in addition, the formation routes and the concentrations of the aromatic structures may be different from those of premixed combustion. The present paper deals with the characterization of the field of nanoparticles and soot in a laminar co-flowing diffusion flame of ethylene and air. Broadband UV-visible extinction measurements from 200nm to 500nm are carried out along the chords of the flames at different heights and then Abel-inverted in order to obtain the radial volume fractions of both nanoparticles and soot. Additional light scattering measurements are employed for the determination of the averaged size of the nanoparticles, in region where only UV absorption was present, and the d63 momentum of the polydispersion in the cases where the soot concentration was not negligible. The results are compared with those obtained by laser induced incandescence, employing a laser wavelength falling inside the absorption spectrum of the nanoparticles. Finally, transient thermocouple measurements are used to determine from the volume fractions and emissivity of nanoparticles and soot, according to the procedure previously employed in premixed flames