Aspects of Soot Dynamics as Revealed by Measurements of Broadband Fluorescence and Flame Luminosity in Flickering Diffusion Flames.
Aspects of Soot Dynamics as Revealed by Measurements of
Broadband Fluorescence and Flame Luminosity in
Flickering Diffusion Flames.
(3207 K)
Smyth, K. C.; Shaddix, C. R.; Everest, D. A.
Combustion and Flame, Vol. 111, 185-207, 1997.
Keywords:
diffusion flames; soot; ethylene; fluorescence;
luminosity; methane; hydroxyl radicals; polycyclic
aromatic hydrocarbons; propane; flame flicker
Abstract:
Numerous investigations 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, and 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 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 of 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.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899