EXPERIMENTAL AND COMPUTATIONAL ASSESSMENT OF WINDAGE LOSSES IN ROTATING MACHINERY

A combined experimental and computational investigation of the flow be tween a rotating cylinder and a fixed enclosure is presented The confi guration considered is related to the design of a centrifugal desalina tor, and includes the flow in the annular as well as in the axial gap regions. The computed flowfield shows significant variations in the ax ial distribution of the azimuthal shear stress due to the secondary fl ow associated with Taylor vortices. The averaged azimuthal shear stres s (or torque) is however nor very sensitive to the number of vortices. Computational results also show that, where the aspect ratio of the a nnulus (rotor length to radial gap width) is relatively small, this az imuthal variation results in a higher average azimuthal shear stress t han for the case where the aspect ratio of the annulus is relatively l arge. Various functional relations for windage torque of infinite cyli nders, developed by other researchers on the basis of power law, relat ions or assuming a log-law velocity distribution, are evaluated. Three such relations are found to be in good agreement with the experimenta l results when coupled with a correction for end effects. These relati ons are also the most useful with respect to the design of rotating eq uipment.