High-Speed OH* and CH* Chemiluminescence Imaging and OH Planar Laser-Induced Fluorescence (PLIF) in Spherically Expanding Flames
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2019 by German Aerospace Center (DLR). Published by the American Institute of Aeronautics and Astronautics, Inc. This work presents a chemical-species-specific investigation of laminar, spherically expanding flames in a constant volume combustion vessel using high-speed imaging of chemilumines-cence of electronically excited flame radicals as well as single-shot, hydroxyl radical planar laser-induced fluorescence (OH PLIF). Chemiluminescence images were acquired at 2 kHz using a high-speed, CMOS camera coupled with a high-speed, intensified-relay optical (IRO) device. Phase-locked OH PLIF imaging was performed using a 10 Hz, nanosecond, ND:YAG laser pumping a tunable dye laser, and the images were acquired using an ultraviolet, intensi-fied, charge-coupled device (ICCD) camera. A series of chemiluminescence images is shown at three wavelength bands: broadband ultraviolet (~200-700 nm), OH* (315 15 nm), and CH* (434 17 nm) emissions. The reaction zone is represented by the distribution of OH* emission, and CH* emission is a marker of the flame front. The full width at half maximum of the chemiluminescence profile is used to define the radical zone thickness. A radical zone thickness image processing algorithm was used to obtain a flame thickness of approximately 4.9 mm for OH* image in a = 1.2 flame. A series of images in a = 1.2 flame are used to evaluate the time-evolution of the OH* and CH* zone thicknesses. Subsequently, a series of OH PLIF images acquired in sequential experiments shows the time-evolution of flame diameter and ground-state OH distribution from the spontaneous emission following the Q1 (5) excitation transition. The current measurements will be used in subsequent studies for calculating laminar flame speeds and comparing it with existing data, as well as for detailed chemical kinetics studies of laminar and turbulent, spherically expanding flames.
