Study of deposit morphology in a propane diffusion-flame under fuel-rich conditions

The morphologies of deposits on 15-μm diameter SiC fibres have been investigated with a scanning electron-microscope and compared with UV-excited laser-induced broadband fluorescences in a confined co-flowing, propane/air laminar diffusion-flame under a fuel-rich condition. A double structure of polycyclic aromatic hydrocarbon (PAH) fluorescence, having a hollow region between the central and outer fluorescence images, was observed in confined flames under extremely fuel-rich conditions. A comparison of the PAH fluorescence with the morphologies of the deposits indicates that the “dark” hollow zone is caused by a decreased number density of large grown-up droplet-like condensed-phase molecules, and the outer weak fluorescence zone is caused by the diffusion of gas-phase small ones. The evolution of soot precursors is stopped at the earlier stage of transition, to soot from condensed-phase large molecules in the non-sooting diffusion flame, under a fuel-rich condition.