Laser Imaging of Chemistry-Flowfield Interactions: Enhanced Soot Formation in Time-Varying Diffusion Flames.
Laser Imaging of Chemistry-Flowfield Interactions:
Enhanced Soot Formation in Time-Varying Diffusion
Flames.
(993 K)
Harrington, J. E.; Shaddix, C. R.; Smyth, K. C.
Society of Photo-Optical Instrumentation Engineers
(SPIE). Laser Techniques for State-Selected and
State-to-State Chemistry II. Session 5. Laser
Diagnostics for Combustion. Volume 2124. January
27-29, 1994, Los Angeles, CA, SPIE, Bellingham, WA,
Editor(s), 1-14 pp, 1994.
Keywords:
lasers; flow fields; soot formation; diffusion flames;
extinction; incandescence; flame luminosity; polycyclic
aromatic hydrocarbons; methane
Abstract:
Models of detailed flame chemistry and soot formation
are based upon experimental results obtained in steady,
laminar flames. For successful application of these
descriptions to turbulent combustion, it is instructive
to test predictions against measurements in time-varying
flowfields. This paper reports the use of optical
methods to examine soot production and oxidation
processes in a co-flowing, axisymmetric CH4/air
diffusion flame in which the fuel flow rate is
acoustically forced to create a time-varying flowfield.
For a particular forcing condition in which tip clipping
occurs (0.75 V loudspeaker excitation), elastic
scattering of vertically polarized light from the soot
particles increases by nearly an order of magnitude with
respect to that observed for a steady flame with the
same mean fuel flow rate. The visible flame luminosity
and laser-induced fluorescence attributed to polycyclic
aromatic hydrocarbons (PAH) are also enhanced. Peak
soot volume fractions, as measured by time-resolved
laser extinction/tomography at 632.8 and 454.5 nm and
calibrated laser-induced incandescene (LII), show a
factor of 4-5 enhancement in this flickering flame. The
LII method is found to track the soot volume fraction
closely and to give better signal-to-noise than the
extinction measurements in both the steady and
time-varying flowfields. A Mie analysis suggests that
most of the enhanced soot production results from the
formation of larger particles in the time-varying
flowfield.
