Experimental and computational assessment of windage losses in rotating machinery

A combined experimental and computational investigation of the flow between a rotating cylinder and a fixed enclosure is presented The configuration considered is related to the design of a centrifugal desalinator, and includes the flow in the annular as well as in the axial gap regions. The computed flowfield shows significant variations in the axial distribution of the azimuthal shear stress due to the secondary flow associated with Taylor vortices. The averaged azimuthal shear stress (or torque) is however nor very sensitive to the number of vortices. Computational results also show that, where the aspect ratio of the annulus (rotor length to radial gap width) is relatively small, this azimuthal variation results in a higher average azimuthal shear stress than for the case where the aspect ratio of the annulus is relatively large. Various functional relations for windage torque of infinite cylinders, developed by other researchers on the basis of power law, relations or assuming a log-law velocity distribution, are evaluated. Three such relations are found to be in good agreement with the experimental results when coupled with a correction for end effects. These relations are also the most useful with respect to the design of rotating equipment.