MULTI-LINE FLUORESCENCE IMAGING OF THE ROTATIONAL TEMPERATURE-FIELD IN A SHOCK-TUNNEL FREE JET

A quasi-steady, highly underexpanded free jet of argon seeded with nit ric oxide (NO) was generated at the exit of a converging, axisymmetric nozzle supplied by a shock-tunnel reservoir at 4200 K and 3.0 atm. Du ring each run of the facility, an isolated transition in the A(2) Sigm a <-- X(2) Pi (0, 0) band of NO at similar to 226 nm was pumped with a pulse of frequency-doubled dye laser light formed into a thin sheet a nd directed perpendicularly through the axis of the jet. The red-shift ed components of the resulting fluorescence at 90 degrees with respect to the laser were imaged onto an intensified, charge-coupled device a rray. A ratio of images obtained by exciting lines originating from tw o different rotational states could be used to infer the mean rotation al temperature field. However, because of the extreme variations in te mperature and density present in the free jet, no single pair of lines simultaneously provided adequate signal levels and temperature sensit ivity over the flow's entire temperature range (i.e., similar to 100-3 100 K). Instead, a combination of images obtained with four different transitions was used. Excellent agreement was observed between multi-l ine temperature evaluations from single-shot and frame-averaged images and a numerical simulation of the flow performed by the method of cha racteristics.