Aspects of soot dynamics as revealed by measurements of broadband fluorescence and flame luminosity in flickering diffusion flames

Numerous investigators have attributed laser-induced broadband fluorescence observed in both rich, premixed flames and in diffusion flames to small polycyclic aromatic hydrocarbons (PAH). However, the wide variety of experimental flame conditions and excitation/detection wavelengths have clouded the interpretation of such measurements, for example, in terms of indicating either the presence of soot precursors or unreactive by-products (or both). This paper presents PAH fluorescence measurements excited at 283.5 nm and detected at 400–447 nm in a series of steady and flickering methane, propane, and ethylene diffusion flames burning at atmospheric pressure in an axisymmetric, coflow configuration. In the flickering flame experiments, acoustic forcing of the fuel rate is used to phase lock the periodic flame flicker close to the natural flame flicker frequency caused by buoyancy-induced instabilities. When compared to our earlier measurements of soot concentrations in the same flames, soot inception in the annular region is found to occur at the interface between the fluorescing PAH and the region of high radical concentrations. Although the peak PAH fluorescence signals and maximum soot concentrations do not occur at the same spatial locations, indirect evidence is presented that the species responsible for PAH fluorescence participate in either soot inception or growth. In contrast to prior suggestions that PAH fluorescence intensities scale with soot concentrations, the relative peak PAH fluorescence signals are observed to be 1.0:9.8:5.4 for the steady methane, propane, and ethylene flames, respectively, whereas the maximum soot levels follow a different trend of 1.0:19:39. Similar results are observed in the flickering flames, all of which exhibit enhanced PAH fluorescence signals compared to the steady flames. PAH fluorescence excited at 560.3 nm in the steady flames is also strongest for propane. Measurements of flame radiation arising from soot particles have also been made, with detection at 395–547 nm and to a limited degree at 833–900 nm. Visible flame emission is particularly sensitive to the local soot temperature. Comparison of the luminosity images with those of OH· fluorescence and soot scattering shows that the luminosity is strongest where the hydroxyl radicals and soot layers overlap, i.e., in regions of active soot oxidation.