Prediction of Windage Losses of an Enclosed High Speed Composite Rotor in Low Air Pressure Environments

Abstract

The frictional windage losses associated with non-ventilated airflows in the air gaps between the rotor and stator of a high speed rotating machine can greatly influence the rotor outer and stator inner surface temperatures. The characteristics of the radial and axial air-gap flows have been of general interest in many engineering applications. A rotating air gap flow is very complex, and in general, can be categorized as a continuum flow, slip flow, and free molecule flow, depending on the ratio of its mean free path to the air gap dimension. For a continuum flow between concentric rotating cylinders, secondary flow of rows of circumferential Taylor vortices in the air gap due to centrifugal flow instability of a curved flow at relatively high rotating speeds will typically be formed. As the air pressure in the air gap drops significantly, rarefied gas flow, departure from continuum flow, occurs when the mean free path becomes relatively large compared to the air gap dimension. This paper has developed and summarized an analytical approach to predict high speed windage losses (rotor tip velocities up to 900 m/s) at low rotor cavity air pressures (0.1 torr to 10 torr). The predicted transient windage losses at various air pressures and high rotor speeds are compared with measured windage losses generated in continuum and slip flow regimes. The agreements between the predicted and measured windage losses are relatively well.

Investigation of windage splits in an enclosed test fixture having a high speed composite rotor in low air pressure environments

“…The intent of these windage tests was not only to validate the analytical windage predictions in different flow regimes, but also to investigate how the windage losses were distributed between the rotor and stator at the selected rotor peak velocities. The spin tests provided good windage drag measurements and the analytically predicted transient windage losses agreed well with the measured windage losses at all three pressure levels [7].…”

“…The University of Texas at Austin Center for Electromechanics (UT-CEM) has designed and conducted a series of composite rotor spin tests to measure the windage losses and temperature distributions of a test setup at high rotor speeds and low air pressures [5,6]. The intent of the windage tests is to validate the windage loss predictions [7] and investigate how the air-gap windage is distributed between the rotor and stator under the test conditions. The findings of the composite rotor spin tests will then be used to perform integrated rotor, stator, and air-gap windage thermal analyses of high speed rotating electrical machines.…”

Transient 3-D thermal analysis for an air-cooled induction motor

“…An analytical approach has been developed to predict high speed windage losses (rotor tip velocities up to 900 m/s) at low rotor cavity air pressures (0.1 torr to 10 torr) with windage tests conducted in both the continuum-flow and slip-flow regimes [2].…”

Low Cost Flywheel Energy Storage for a Fuel Cell Powered Transit Bus

“…Aside from greater aerodynamic losses, operation at a higher pressure, such as 0.1 to 1 Torr, could be detrimental to the stator windings and result in an insulation breakdown. The windage calculations for the flywheel are based on a method that characterizes the slip flow regime and has been supported by test data [11]. At 18,000 r/min with slip flow conditions, the total windage loss is 95.3 W.…”