Experimental Study on Coaxial Swirling Flows
The experimental investigation of a swirling jet and an annular swirling stream issuing from coaxial cylinders is presented. The objective is to contribute to the research on the combined flow field close to vortex breakdown conditions. The two swirling water streams are interacting in the extension of the outer cylinder. Swirl is generated by two rotating impellers, located in the inner tube and the annular duct, just before the merging of the two streams. Controlled flow parameters comprise the flow rates of the streams and the angular velocities of the impellers. The flow field is monitored by means of Stereoscopic 3D-PIV, providing the velocity components on an axial, central plane. Four typical test cases were investigated comprising four combinations of inlet conditions. Two dimensionless numbers were utilized to interpret the experimental results, a modified Rossby number and the velocity ratio ζ, along with the Reynolds numbers of the internal and annular stream, respectively. The most important coherent structure developed, is a recirculation region formed downstream of the exit of the internal swirl nozzle. A bubble type vortex breakdown occurs when the appropriate flow conditions are applied. The alterations of the flow field were discussed with respect to the changes of the inlet conditions. The flowrates of the two streams and the combined swirl strength applied through the rotating impellers appear to be crucial for the onset of the vortex breakdown. Comparisons were drawn with previous work.
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