Simultaneous CH planar laser-induced fluorescence and particle imaging velocimetry in turbulent nonpremixed flames

We report the simultaneous measurement of the flamefront location, using single-shot CH planar laserinduced fluorescence (PLIF), and the velocity field, using two-color digital particle imaging velocimetry (PIV), in nonpremixed turbulent flames. To minimize the influence of particle scattering on the CH PLIF images, we pump a CH B-X(0,0) transition at 390 nm and detect A-X fluorescence at 420–440 nm, employing Schott glass filters to reject the strong particle scattering. The PIV images are recorded on a high-resolution (2036× 3060 pixels) color CCD camera, and the velocities are derived from 64or 128-pixel-square interrogation regions. We demonstrate this technique in a nonsooting, permanently blue nonpremixed turbulent jet flame (Rejet = 18 600). Here, PLIF images reveal a CH layer of thickness typically < 1 mm from flame base to tip. Furthermore, in these permanently blue flames, we observe instantaneous flamefront strain rates – derived from the PIV data – in excess of ±104 s−1 without flame extinction. PACS: 07.60; 42.80; 82.40 Recently, combustion researchers have begun to apply simultaneous particle imaging velocimetry (PIV) and planar laserinduced fluorescence (PLIF) to study turbulent flames. Frank et al. [1] demonstrated the feasibility of PIV and PLIF imaging of biacetyl (as a marker of the reactants) in a premixed flame. Hasselbrink et al. [2] used simultaneous PIV and PLIF of the OH radical to investigate a nonpremixed CH4-air flame. OH PLIF has the significant advantage of high signal strength, due to the abundance of superequilibrium OH near the flamefront; however, the use of high OH concentration as a marker of the primary reaction zone can be misleading. That is, persistence of the OH due to the slow three-body recombination reactions can indicate broad reaction zones, especially in the far field of jet diffusion flames. While these broad zones are indicative of regions of recombination reactions, they may not accurately mark the primary hydrocarbon reaction zone. ∗ Corresponding author While CH is effective at marking this layer of hydrocarbon reactions, it is usually found in very small concentrations (tens of ppm or less as compared to 103 to 104 ppm for OH); interfering fluorescence from polycyclic aromatic hydrocarbons (PAH), a soot precursor, further complicates single-shot planar imaging of CH. Nonetheless, under conditions where one can suppress soot formation without also suppressing CH formation, the coupling of PIV and CH PLIF is feasible. We have applied this combined technique to the study of 1) lifted, nonpremixed CH4-air flames [3] – making measurements at the flame base, prior to the formation of PAH – and 2) nonsooting, permanently blue nonpremixed turbulent jet flames, where the fuel is diluted with N2 and the oxidizer is pure O2. In this communication, we illustrate this combined technique with measurements from a permanently blue flame. For the CH PLIF, we tuned an Nd:YAG-pumped dye laser system to the Q1(7.5) transition of the B2Σ−–X2Π (v′ = 0, v" = 0) band (λ = 390.30 nm) and detected fluorescence from the A-X and B-X(0,1) bands, λ = 420–440 nm [4, 5]. Subsequent to laser excitation, the A state, v′ = 1 and 0, is populated via fast electronic energy transfer (EET). Garland and Crosley [6] estimate a ratio of EET to B-state electronic quenching of 0.2; thus, the net fluorescence yield with this approach is significantly greater than that obtained by pumping an A-X(0,0) transition and detecting fluorescence from the A-X(0,1) band, where the ratio of the Einstein emission coefficients for the (0,1) and (0,0) bands is ∼ 0.018 [7]. Because of the strength of the B-X(0,0) mainbranch transitions [8], we observed strong saturation effects. While saturation has some benefits, e.g., reducing the dependence of the image on the laser-sheet irradiance distribution, it also has the disadvantage of increasing the apparent thickness of the laser sheet. Furthermore, at high laser irradiance, ∼ 2×108 W/cm2, a small but finite LIF signal was observed in the fuel core of a laminar nonpremixed flame. As the laser irradiance was decreased, the ratio of fuel-core signal to peak CH signal also decreased; as a consequence, we operated with a maximum probe-volume irradiance of ∼ 0.5×108 W/cm2(15 mJ/pulse). We generated the 390-nm laser radiation by wavelength mixing the output of a dye laser operating at 616.4 nm