Power Loss in High-Speed Micro Turbomachinery – An Experimental Study

Walton II, J.F., Heshmat, H. and Tomaszewski, M.J. “Power Loss in High-Speed Micro Turbomachinery – An Experimental Study” GT2012-69558, Proceedings of ASME Turbo Expo 2012: Power for Land, Sea and Air, June 11-15, 2012, Copenhagen, Denmark.

In the paper the authors present the results of a study to investigate the windage and foil bearing losses in modern high-speed micro-turbomachinery such as motors, turboexpanders and turbogenerator rotor systems. Areas investigated include clearance between rotor and static structure; surface roughness; and ambient pressure conditions. Comparisons between experiments and theoretical windage loss models show excellent correlation at high speeds. Testing also indicated that some optimum rotor stator gaps are possible to minimize windage loss and that surface roughness can significantly impact windage. It was also shown that for rotor systems with foil bearings, at lower speeds rapid deceleration occurs. This is most likely due to the reduction in film thickness and onset of asperity contact between rotor and bearing surfaces. As expected ambient pressure also impacts windage losses.

The theoretical rotor windage loss models, their range of applicability and key limitations of those loss models are discussed. While bearing losses are not included in theoretical windage loss models, it was shown that they do need to be considered, especially when developing very high speed motor/generators. Approaches to properly account for the wide array of losses in order to develop high power density power sources are recommended. 

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