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A Simple Model for Identifying the ‘Flutter Bite’ of Fan Blades

[+] Author Affiliations
Fanzhou Zhao, Mehdi Vahdati

Imperial College London, London, UK

Nigel Smith

Rolls-Royce plc, Derby, UK

Paper No. GT2016-56336, pp. V07BT34A004; 13 pages
doi:10.1115/GT2016-56336
From:
  • ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition
  • Volume 7B: Structures and Dynamics
  • Seoul, South Korea, June 13–17, 2016
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 978-0-7918-4984-2
  • Copyright © 2016 by Rolls-Royce plc

abstract

This paper presents a simple model for the prediction of the ‘flutter bite’ of fan blades due to acoustic reflections from the intake. In a previous work by the authors, it was shown that the acoustic effects of the intake is very important and needs to be considered during the design of a fan blade. It was also shown that the damping due to blade motion and intake acoustics are independent and can be analysed separately. The acoustic reflections from the intake changes the damping of the blade by modifying the phase and amplitude of the unsteady pressure at the leading edge of the fan. It will be shown in the paper that, for a given intake, the phase and amplitude of the reflected acoustic waves can be evaluated analytically based on established theories independent of the fan design. The proposed model requires only the design intent of the fan blade and the geometry of the intake, which are available in the early design stages of a new engine, and can predict the operating conditions at which fan flutter is likely to occur. The proposed simple model can be used in two ways:

(i) For a particular intake design, the flutter bite speed can be determined based on the fan operating line. This type of analysis can be very useful before the experimental testing stages of an engine, where possible ‘flutter bite’ regions must be identified prior to the experiments so as to avoid potential damages to the engine. Moreover, whole annulus unsteady CFD flutter computations are usually performed for a ‘fan plus intake’ design prior to manufacture. However due to the high demands on both computational time and resources, these computations cannot be carried out over the entire fan speed range in the early design stages. The proposed model can predict the fan speeds at which fan flutter is likely to occur, so that CFD investigations can be carried out accordingly.

(ii) For a particular fan design, the effects of different intakes on the fan flutter stability can be determined so as to devise an optimal intake design which minimises the chance of potential flutter bite.

Copyright © 2016 by Rolls-Royce plc

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