Precession and Recirculation in Turbulent Swirling Isothermal Jets

A burner with a hybrid bluff body swirl design, incorporating a central jet surrounded by a swirling annulus, is applied to the investigation of turbulent isothermal swirling jets. Nonswirling flows stabilized on this burner form only one stagnation zone on the face of the bluff body. Such a far upstream recirculation zone is typical of those formed behind solid obstructions placed in strong axial streams. The addition of swirl leads to more complex flow patterns, which may include a second recirculation zone (in more downstream axial locations), flow instabilities, and precession. Both of these effects are investigated. Laser Doppler Velocimetry is used to ascertain the presence of downstream recirculation. The seed- (alumina-) laden central jets in these flows are visualized using a combination of laser sheet forming optics and high-speed CCD camera. Postprocessing of the imaging data succeeds in resolving the precession frequency of the jet despite the variable seed density in the experiments. Precession frequency appears to be a function of swirl number as well as the Reynolds number of both the central jet and the swirling annulus.