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Prediction of destabilizing blade tip forces for shrouded and unshrouded turbinesThe effect of a nonuniform flow field on the Alford force calculation is investigated. The ideas used here are based on those developed by Horlock and Greitzer. It is shown that the nonuniformity of the flow field does contribute to the Alford force calculation. An attempt is also made to include the effect of whirl speed. The values predicted by the model are compared with those obtained experimentally by Urlicks and Wohlrab. The possibility of using existing turbine tip loss correlations to predict beta is also exploited. The nonuniform flow field induced by the tip clearnance variation tends to increase the resultant destabilizing force over and above what would be predicted on the basis of the local variation of efficiency. On the one hand, the pressure force due to the nonuniform inlet and exit pressure also plays a part even for unshrouded blades, and this counteracts the flow field effects, so that the simple Alford prediction remains a reasonable approximation. Once the efficiency variation with clearance is known, the presented model gives a slightly overpredicted, but reasonably accurate destabilizing force. In the absence of efficiency vs. clearance data, an empirical tip loss coefficient can be used to give a reasonable prediction of destabilizing force. To a first approximation, the whirl does have a damping effect, but only of small magnitude, and thus it can be ignored for some purposes.
Document ID
19860020711
Document Type
Conference Paper
Authors
Qiu, Y. J. (Massachusetts Inst. of Tech. Cambridge, MA, United States)
Martinezsanchez, M. (Massachusetts Inst. of Tech. Cambridge, MA, United States)
Date Acquired
August 12, 2013
Publication Date
December 1, 1985
Publication Information
Publication: NASA. Lewis Research Center Instability in Rotating Machinery
Subject Category
MECHANICAL ENGINEERING
Funding Number(s)
CONTRACT_GRANT: NAS8-35018
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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