Laser Induced Fluorescence
LIF targets specific chemical species within the flame, allows for the qualitative visualization of the flame front. The OH PLIF system consists of a Quantel TDL-90 dye laser pumped by the 532 nm output of a Spectra Physics GCR 170-10 Nd:YAG laser operating at 10 Hz. The output beam of the TDL-90 was tuned to ∼283 nm to excite the (1,0) band of the OH radical, while the resulting fluorescence signal was captured through the use of a CCD camera (Dantec HiSense Mk II, 1.3 megapixel resolution) coupled with an image intensifier (Hamamatsu C9546-C03L), 78 mm focal length UV lens (Pentax C91698, f/3.8), and narrow bandpass filter (300–330 nm).
The UV output beam from the dye laser is directed through a set of sheet-forming optics to provide a laser sheet ∼25 mm in width and ∼2 mm thick. The laser sheet entered the flame volume through a side window of the HPOC with the ICCD camera focused through the top window of the HPOC, 90° relative to the laser sheet and axial fluid flow. For all experimental cases, the laser sheet was fixed at the burner exit so as to allow for imaging of the flame stabilization location. Each image is 1024 × 1344 pixels in the axial and radial directions respectively, with a resolution of 13.6 pixels/mm yielding a field of view of approximately 75 mm × 100 mm.
A calibration target was utilized for image scaling, focusing the camera on the central plane of the burner exit nozzle, and to evaluate the influence of image distortion caused by the cylindrical quartz tube, with images skewed 0.67 1.11 mm in the radial direction (0.07–0.14 mm in the axial direction) at ±40 mm from the burner centerline. In proceeding images, r = 0 mm represents the burner exit nozzle centreline and y = 0 mm represents the edge of the burner exit nozzle. The PLIF system produces pulse energies of ∼10–12 mJ/pulse at 283 nm. Given the short lifetime of the OH radical fluorescence signal and the potential influence of quenching effects, the gate timing of the image intensifier was set to 100 ns, triggered by a BNC 575-8C pulse generator to ensure that signal capture was appropriately timed with the dye laser excitation light pulse. The image intensifier gain was held constant throughout.
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