Air photolysis and recombination tracking: A new molecular tagging velocimetry scheme

A new scheme for molecular tagging velocimetry in unseeded airflows is presented. The method, called air photolysis and recombination tracking, is based on the photoinduced formation of nitric oxide (NO) in the waist region of a focused ArF excimer laser beam. The distribution of the formed NO molecules is imaged by planar laser-induced fluorescence in the gamma band, using a frequency-doubled dye laser beam. The role of N-2(+) ions in the NO formation process is discussed, and the lifetime of the NO molecules was determined to be at least 10 ms. The new method has been applied to a laminar and a pulsed airflow and a premixed methane/air flame. Velocities could be determined with an accuracy of 5% in the airflows (on a single-shot basis) and 13% in the flame. By the use of averaging over many laser pulses, velocities as low as 1 cm/s could be measured