Photofragmentation laser-induced fluorescence imaging in premixed flames

Abstract

Two-dimensional measurements of primarily hydroperoxyl radicals (HO 2) are, for the first time, demonstrated in flames. The measurements are performed in different Bunsen-type premixed flames (H 2/O 2, CH 4/O 2, and CH 4/air) using photofragmentation laser-induced fluorescence (PF-LIF). Photofragmentation is done by laser radiation at 266 nm, and the generated OH photofragments are probed through fluorescence induced by a laser tuned to the Q 1(5) transition at 282.75 nm. The signal due to naturally occurring OH radicals, recorded by having the photolysis laser blocked, is subtracted, providing an image that reflects the concentration of OH fragments generated by photolysis, and hence the presence of primarily HO 2, but also smaller contributions from H 2O 2 and, for the methane flames, CH 3O 2. For the methane flames the measured radial profiles of OH photofragments and natural OH agree well with corresponding profiles calculated for laminar, one-dimensional, premixed flames using CHEMKIN-II with the Konnov detailed C/H/N/O reaction mechanism. An interfering signal contribution is observed in the product zone of the methane flames. It is concluded that the major source for the interference is most likely hot CO 2, from which O atoms are produced by photolysis, and OH is rapidly formed as the O atoms react with H 2O and H 2. This conclusion is supported by the fact that the interference is absent for the hydrogen flame, but appears when CO 2 is seeded into the flame. Another strong indication is that the Konnov mechanism predicts a similar buildup of OH after photolysis.