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Laser Imaging of Chemistry-Flowfield Interactions: Enhanced Soot Formation in Time-Varying Diffusion Flames.


pdf icon 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.