Research Papers

Influence of Intake on Fan Blade Flutter

[+] Author and Article Information
Mehdi Vahdati

Department of Mechanical Engineering,
Rolls-Royce Vibration University
Technology Centre,
Imperial College London,
London SW7 2AZ, UK
e-mail: m.vahdati@imperial.ac.uk

Nigel Smith

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

Fanzhou Zhao

Department of Mechanical Engineering,
Rolls-Royce Vibration University
Technology Centre,
Imperial College London,
London SW7 2AZ, UK
e-mail: fanzhou.zhao11@imperial.ac.uk

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received October 22, 2014; final manuscript received October 31, 2014; published online January 28, 2015. Editor: Kenneth C. Hall.

J. Turbomach 137(8), 081002 (Aug 01, 2015) (10 pages) Paper No: TURBO-14-1276; doi: 10.1115/1.4029240 History: Received October 22, 2014; Revised October 31, 2014; Online January 28, 2015

The main aim of this paper is to study the influence of upstream reflections on flutter of a fan blade. To achieve this goal, flutter analysis of a complete fan assembly with an intake duct and the downstream outlet guide vanes (OGVs) (whole low pressure (LP) domain) is undertaken using a validated computational fluid dynamics (CFD) model. The computed results show good correlation with measured data. Due to space constraints, only upstream (intake) reflections are analyzed in this paper. It will be shown that the correct prediction of flutter boundary for a fan blade requires modeling of the intake and different intakes would produce different flutter boundaries for the same fan blade. However, the “blade only” and intake damping are independent and the total damping can be obtained from the sum of the two contributions. In order to gain further insight into the physics of the problem, the pressure waves created by blade vibration are split into an upstream and a downstream traveling wave in the intake. The splitting of the pressure wave allows one to establish a relationship between the phase and amplitude of the reflected waves and flutter stability of the blade. By using this approach, a simple reflection model can be used to model the intake effects.

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Fig. 1

Stability boundary for a fan blade

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Fig. 2

Model used for flutter computations

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Fig. 3

Pressure ratio versus nondimensional mass flow

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Fig. 4

Domain used for flutter computations with bump

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Fig. 5

Aerodamping as a function of bump distance

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Fig. 6

Contours of unsteady pressure on the cylinder outer wall with bump: complete signal (top), upstream (middle), and downstream travelling (bottom) components

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Fig. 7

Normalized amplitude and phase of reflected wave

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Fig. 8

(a) Phase difference between upstream and reflected waves; (b) damping as a function of phase difference

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Fig. 9

Instantaneous variations of leading/trailing edge unsteady pressure at 90% span for two different bump positions: (a) no bump, (b) bump x = 0.4, and (c) bump x = 0.7.

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Fig. 10

Comparison of Δp and unsteady lift at 90% height

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Fig. 11

Amplitude and phase of pressure just upstream and downstream of blade at 90% span

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Fig. 12

(a) Damping and (b) phase of LE, TE pressure wave as a function of mass flow, and (c) damping as a function of phase difference

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Fig. 13

Mach number profiles at different fan speeds

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Fig. 14

Computed damping on HWK operating line against nondimensional tip speed Ω

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Fig. 15

Phase and amplitude of pressure wave in intake

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Fig. 16

Cut-on reduced frequency, phase, and amplitude of pressure wave at fan face against tip speed Ω

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Fig. 18

Effective damping of intake for 3ND

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Fig. 19

Profiles of intakes used in this study

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Fig. 20

Phase and amplitude of pressure wave and damping for different intake length



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