Filtered Rayleigh scattering thermometry in vortex-strained and sooting flames

Filtered Rayleigh Scattering (FRS) has been used to map two-dimensional temperature fields in a nonsooting vortex-strained, CH₄-air diffusion flame and in a sooting, premixed C₂H₄-air flame. The FRS technique provides quantitative, spatially correlated temperature data without the flow intrusion or time lag associated with physical probes. Use of a molecular iodine filter for FRS imaging relaxes the requirement for clean, particulate-free flow fields and offers the potential for imaging near walls, test section windows and in sooty flames, all of which are precluded in conventional Rayleigh imaging. FRS is combined with Raman imaging of the CH ₄ fuel molecule and a flamelet-based model to obtain cross-section-corrected thermometry in a periodically forced slot diffusion flame, whose structure has been reported previously by Mueller and Schefer. The slot diffusion flame is forced at 90 Hz where two consecutive fuel-side vortices interact to produce a repeatable strain-induced extinction event. The joint FRS-Raman imaging approach reveals the time-evolution of the diffusion-flame structure during flame quenching. Our FRS instrument is then applied to a sooting flame, where conventional Rayleigh imaging often fails as a result of intense soot Mie-scattering interference. We present both filtered and nonfiltered Rayleigh images from a premixed C₂H₄-air flame stabilized on a McKenna burner. The images reveal the significant amount of attenuation of soot Mie scattering interference when using FRS. FRS-measured flame temperatures in the combustion-product region of this flickering premixed flame range from 1500 to 1800 K for equivalence ratios, φ, from 2.10 to 2.29. Laser-induced incandescence (LII) measurements of soot volume fraction reveal that the FRS measurements are possible in this flame up to soot volume fractions of order 0.1 ppm.