Calculating Windage Losses: A Review

Windage is the effect of aerodynamic drag on the surfaces of a rotating system due to fluid shear effects. The fluid-friction losses that occur on the rotor of rotating machines often constitute a non-negligible drag on the system that must be estimated for proper sizing of the driving or driven element. This is especially true in high-pressure environments, such as hermetic compressors and turbines. Fluid-friction loss modeling is based on the size and rotation speed of the shaft, the density of the fluid, and an empirically-determined drag coefficient. The drag coefficient is generally a function of the Reynolds number but may also be dependent on the Taylor number. Several papers have provided empirical predictions for drag coefficients based on the Reynolds and Taylor numbers of the fluid, but other factors such as rotor shapes, assemblies, and surrounding fluid conditions can also affect the drag coefficient. There are two main geometries for a rotor: a face parallel to the axis of rotation, and a face that is perpendicular. The gap between the rotating component and the stationary housing also plays an important role in the drag coefficient. This review summarizes and compares these findings in a way that makes it easy for the reader to predict the total windage losses on a system for any rotor shape, speed, or operating condition. A quick reference table is presented in the conclusions section.