Planar KrF laser-induced OH fluorescence imaging in a supersonic combustion tunnel

Planar fluorescence images of OH were obtained in a continuous-flow electrically heated high-enthalpy hydrogen-air combustion tunnel using a tunable KrF laser. These images were compared to previously recorded fluorescence images produced using a doubled-dye laser under similar conditions. For the detection configuration used, the images of doubled-eye laser-induced fluorescence demonstrated a severe distortion as a result of laser beam absorption and fluorescence trapping. By contrast, images of the fluorescence induced by the tunable KrF laser retained the symmetry properties of the flow. Based on signal-to-noise ratio measurements, the field of the fluorescence obtained with the doubled-eye laser is considerably larger than the fluorescence field induced by the KrF laser. Based on these results, tunable KrF laser systems are recommended for quantitative OH imaging in facilities where the product of the optical path length for either fluorescence excitation or collection and the average OH concentration along that path is greater than 10 exp 16 cm/cu cm. (Author)