Research Papers: Gas Turbines: Turbomachinery

Aeroelastic Instability in Transonic Fans

[+] Author and Article Information
Mehdi Vahdati

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

Nick Cumpsty

Mechanical Engineering Department,
Imperial College London,
Exhibition Road,
London SW7 2AZ, UK
e-mail: n.cumpsty@imperial.ac.uk

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 13, 2015; final manuscript received July 21, 2015; published online September 7, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(2), 022604 (Sep 07, 2015) (14 pages) Paper No: GTP-15-1257; doi: 10.1115/1.4031225 History: Received July 13, 2015; Revised July 21, 2015

This paper describes stall flutter, which can occur at part speed operating conditions near the stall boundary. Although it is called stall flutter, this phenomenon does not require the stalling of the fan blade in the sense that it can occur when the slope of the pressure rise characteristic is still negative. This type of flutter occurs with low nodal diameter forward traveling waves and it occurs for the first flap (1F) mode of blade vibration. For this paper, a computational fluid dynamics (CFD) code has been applied to a real fan of contemporary design; the code has been found to be reliable in predicting mean flow and aeroelastic behavior. When the mass flow is reduced, the flow becomes unstable, resulting in flutter or in stall (the stall perhaps leading to surge). When the relative tip speed into the fan rotor is close to sonic, it is found (by measurement and by computation) that the instability for the fan blade considered in this work results in flutter. The CFD has been used like an experimental technique, varying parameters to understand what controls the instability behavior. It is found that the flutter for this fan requires a separated region on the suction surface. It is also found that the acoustic pressure field associated with the blade vibration must be cut-on upstream of the rotor and cut-off downstream of the rotor if flutter instability is to occur. The difference in cut off conditions upstream and downstream is largely produced by the mean swirl velocity introduced by the fan rotor in imparting work and pressure rise to the air. The conditions for instability therefore require a three-dimensional geometric description and blades with finite mean loading. The third parameter that governs the flutter stability of the blade is the ratio of the twisting motion to the plunging motion of the 1F mode shape, which determines the ratio of leading edge (LE) displacement to the trailing edge (TE) displacement. It will be shown that as this ratio increases the onset of flutter moves to a lower mass flow.

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Fig. 1

Performance plot for the model fan used in this investigation

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Fig. 2

Measured and computer distribution of stagnation pressure (a), efficiency (b) and computed Mach downstream of fan at conditions for stable and unstable operation (mref = 0.97 and 0.93).

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Fig. 3

Computed steady static pressure on the suction surface of the fan blade (surface streamlines superimposed) and downstream entropy in wake; mref = 0.97 (left) and 0.93 (right)

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Fig. 4

Contours of Mach number at 90% span for mref = 0.93 (left) and 0.97 (right), relative tip velocity close to sonic, Ω = 0.89

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Fig. 5

Contours of blade vibration: (a) is the total motion, (b) is the plunging motion, and (c) is the twisting motion. Contour levels smaller for twisting.

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Fig. 6

Schematic of the components of blade vibration with the twist on the left and plunge on the right. The blade second from the top is at zero phase with maximum negative velocity.

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Fig. 7

Computed damping coefficients for the fan with a flight intake and with an infinite (nonreflecting) upstream intake plotted against rotation speed; mref = 0.93

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Fig. 8

Computed damping versus span for a range of mass flow rates mref. Ω = 0.89, μ = 2, F = 0.67.

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Fig. 9

Comparison of radial profiles of slope of characteristic against measured data

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Fig. 10

Computed components of damping coefficient as a function of mass flow rate, Ω = 0.89, μ = 2, F = 0.67

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Fig. 11

Computed damping coefficient for full blade and for section at 90% span as a function of frequency and mass flow rate, Ω = 0.89, μ = 2

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Fig. 12

Damping coefficient as a function of frequency for μ = 1, 2, and 3. Frequency nondimensionalized in different ways: (a) versus reduced frequency fr, (b) versus F = 2πrtf/μa0, and (c) as f/fcut-on.

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Fig. 13

Unsteady pressure contours at 90% span, mref = 0.93, Ω = 0.89, μ = 2

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Fig. 14

Amplitude and phase of pressure just upstream and downstream of blade at 90% span, mref = 0.93, Ω = 0.89, μ = 2

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Fig. 15

Twist and plunge contributions to amplitude and phase of pressure upstream (left) and downstream (right) of blade at 90% span, mref = 0.93, Ω = 0.89, μ = 2

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Fig. 16

Cut-on frequencies upstream and downstream of the blade as a function of fan rotational speed, constant working line, and μ = 2

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Fig. 17

Main regions of the flow

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Fig. 18

Variations of phase difference between axial velocity and static pressure in three regions; mref = 0.93, Ω = 0.89, 1F mode, μ = 2

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Fig. 19

Computations at choked condition, (a) steady Mach no. at 90% height for mref = 1.2, Ω = 0.89. (b) Phase difference between axial velocity and static pressure for 1F mode, μ = 2.

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Fig. 20

Variations of upstream and downstream pressure and axial velocity as a function of mass flow Ω = 0.89, 1F mode, F = 0.67, μ = 2

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Fig. 21

Variations of axial velocity at three instants of time—mref = 0.93, Ω = 0.89, 1F mode, F = 0.67, μ = 2

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Fig. 22

Radial profiles of pressure and axial velocity phase, Ω = 0.89, 1F mode, F = 0.67, μ = 2

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Fig. 23

Variations of TL, PL, and blade displacement as a function of blade number—for μ = 2, mref = 0.93. Quasi-steady analysis.

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Fig. 24

Variations of TL, PL as a function of μ, mref = 0.93, fr = 0.086. Quasi-steady analysis.




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