Infrared Imaging of Large-Amplitude, Low-Frequency Disturbances on a Planar Jet

The purpose of this study is to demonstrate that periodic, large-amplitude, low-frequency disturbances on a planar jet can increase spreading and mixing of a planar jet with the surrounding fluid. This is achieved by enhancing the natural instabilities of the jet, resulting in the formation of large vortical structures in the mixing layers. The jet had a Reynolds number of about 7.2 × l0³, an aspect ratio of 47, and Strouhal numbers, based on the nozzle width and the disturbance frequency, up to 0.324. A relatively new technique, full-field infrared imaging, was used to determine jet behavior and mixing. This technique provides a significant advantage over conventional temperature measurement techniques. Over 68,000 data points can be monitored as often as 30 times each second. Contour plots of the isotherms of the infrared images showed that the disturbances increased the spread rate of the jet and accelerated the transition of the planar jet into an axisymmetric regime. A relative mixing efficiency was defined and measured. The mixing that occurred within the thermal half-width of the jet when pulsed at Str = 0.168 was about 32% greater at seven nozzle widths than that of the natural jet.