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Research Papers

A Simple Model for Identifying the Flutter Bite of Fan Blades

[+] Author and Article Information
Fanzhou Zhao

Department of Mechanical Engineering,
Imperial College London,
London SW7 2AZ, UK
e-mail: fanzhou.zhao11@imperial.ac.uk

Nigel Smith

Rolls-Royce plc,
PO Box 31,
Derby DE24 8BJ, UK
e-mail: nigel.h.s.smith@rolls-royce.com

Mehdi Vahdati

Department of Mechanical Engineering,
Imperial College London,
London SW7 2AZ, UK
e-mail: m.vahdati@imperial.ac.uk

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received September 23, 2016; final manuscript received October 25, 2016; published online February 23, 2017. Editor: Kenneth Hall.

J. Turbomach 139(7), 071003 (Feb 23, 2017) (10 pages) Paper No: TURBO-16-1256; doi: 10.1115/1.4035567 History: Received September 23, 2016; Revised October 25, 2016

This paper presents the work on part-speed fan flutter due to acoustic reflections from the intake, commonly called “flutter bite.” A simple model for the prediction of the flutter bite is presented. In a previous work by the authors, it was shown that the acoustic effects of the intake are very important and need to be considered during the design of a fan blade. It was also shown that the contribution to blade aerodamping due to blade motion (for the isolated rotor in an infinitely long duct) and intake acoustics is independent and can be analyzed separately. The acoustic reflections from the intake change 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.

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References

Vahdati, M. , Simpson, G. , and Imregun, M. , 2011, “ Mechanisms for Wide-Chord Fan Blade Flutter,” ASME J. Turbomach., 133(4), p. 041029. [CrossRef]
Vahdati, M. , Smith, N. H. S. , and Zhao, F. , 2015, “ Influence of Intake on Fan Blade Flutter,” ASME J. Turbomach., 137(8), p. 081002. [CrossRef]
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Figures

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Fig. 1

Stability boundary for a fan blade

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Fig. 2

Illustration of acoustic reflection from the intake

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Fig. 3

Outline of the computational domain (downstream of the fan not shown)

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Fig. 4

Illustration of fan operating conditions used for flutter computation. WL: working line and HWL: high working line.

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Fig. 5

Aerodamping of the fan as a function of vibration frequency, L = 1.37D

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Fig. 6

Aerodamping of the fan as a function of intake length, ζ = 1.3

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Fig. 7

(a) Normalized amplitude and (b) phase of 2ND acoustic waves in the intake L = 1.37D and ζ = 1.3

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Fig. 8

Axial phase change through the intake duct as a function of vibration frequency, L = 1.37D. Upstream: δϕ− and downstream: δϕ+.

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Fig. 9

Axial phase change through the intake duct as a function of intake length, ζ = 1.3. Upstream: δϕ− and downstream: δϕ+.

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Fig. 10

Amplitude ratio and phase change of acoustic reflection from the intake highlight as a function of vibration frequency, L = 1.37D

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Fig. 11

Additional aerodamping due to acoustic reflections and flutter index as a function of vibration frequency, L = 1.37D

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Fig. 12

Additional aerodamping due to acoustic reflections and flutter index as a function of intake length, ζ = 1.3

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Fig. 13

Additional aerodamping due to acoustic reflections as a function of phase difference between the outgoing wave and the reflected wave at fan leading edge

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Fig. 14

Flutter index β contour for the intake L = 1.37D as a function of Mx and ωm*

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Fig. 15

Flutter index contour on the high working line (HWL) as a function of intake length L/D and shaft speed Ω

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Fig. 16

Aerodamping of the fan on the high working line (HWL), L = 1.37D

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Fig. 17

Additional aerodamping due to acoustic reflections and the predicted flutter index on the high working line (HWL), L = 1.37D

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Fig. 18

Flutter index β contour for the 1F/2ND mode of rig fan blades with a real intake L = 0.69D as a function of Mx and ωm

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