Determination of aerodynamic damping during torsional vibrations of turbine blades

Abstract Steam turbine blade is a very complex structure. It has geometric complexities like variation of twist, taper, width and thickness along its length. Most of the time these variations are not uniform. Apart from these geometric complexities, the blades are coupled by means of lacing wire, lacing rod or shroud. Blades are attached to a flexible disc which contributes to the dynamic behavior of the blade. Root fixity also plays an important role in this behavior. There is a considerable variation in the frequencies of blades of newly assembled turbine and frequencies after some hours of running. Again because of manufacturing tolerances there can be some variation in the blade to blade frequencies. Determination of natural frequencies of the blade is therefore a very critical job. Problems associated with typical industrial turbine bladed discs of a 235 MW steam turbine are highlighted in this paper.

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Determination of Aerodynamic Damping at High Reduced Frequencies

Volume 7C: Structures and Dynamics ◽

10.1115/gt2018-75294 ◽

2018 ◽

Author(s):

Minghao Pan ◽

Paul Petrie-Repar ◽

Hans Mårtensson ◽

Tianrui Sun ◽

Tobias Gezork

Keyword(s):

Acoustic Resonance ◽

Forced Response ◽

Economic Consequences ◽

Navier Stokes ◽

Flow Solver ◽

Aerodynamic Damping ◽

Acoustic Modes ◽

Reduced Frequency ◽

Standard Configuration

In turbomachines, forced response of blades is blade vibrations due to external aerodynamic excitations and it can lead to blade failures which can have fatal or severe economic consequences. The estimation of the level of vibration due to forced response is dependent on the determination of aerodynamic damping. The most critical cases for forced response occur at high reduced frequencies. This paper investigates the determination of aerodynamic damping at high reduced frequencies. The aerodynamic damping was calculated by a linearized Navier-Stokes flow solver with exact 3D non-reflecting boundary conditions. The method was validated using Standard Configuration 8, a two-dimensional flat plate. Good agreement with the reference data at reduced frequency 2.0 was achieved and grid converged solutions with reduced frequency up to 16.0 were obtained. It was concluded that at least 20 cells per wavelength is required. A 3D profile was also investigated: an aeroelastic turbine rig (AETR) which is a subsonic turbine case. In the AETR case, the first bending mode with reduced frequency 2.0 was studied. The 3D acoustic modes were calculated at the far-fields and the propagating amplitude was plotted as a function of circumferential mode index and radial order. This plot identified six acoustic resonance points which included two points corresponding to the first radial modes. The aerodynamic damping as a function of nodal diameter was also calculated and plotted. There were six distinct peaks which occurred in the damping curve and these peaks correspond to the six resonance points. This demonstrates for the first time that acoustic resonances due to higher order radial acoustic modes can affect the aerodynamic damping at high reduced frequencies.

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Aerodynamic Damping Characteristics of a Transonic Turbine Cascade

Volume 4: Manufacturing Materials and Metallurgy; Ceramics; Structures and Dynamics; Controls, Diagnostics and Instrumentation; Education; IGTI Scholar Award; General ◽

10.1115/99-gt-050 ◽

1999 ◽

Cited By ~ 1

Author(s):

Mizuho Aotsuka ◽

Toshinori Watanabe ◽

Yasuo Machida

Keyword(s):

Phase Angle ◽

Aerodynamic Force ◽

Turbine Blades ◽

Aerodynamic Characteristics ◽

Influence Coefficient ◽

Suction Surface ◽

Aerodynamic Damping ◽

Unsteady Aerodynamic ◽

Design Case ◽

Oscillation Instability

The unsteady aerodynamic characteristics of oscillating thin turbine blades were studied both experimentally and numerically to obtain the comprehensive knowledge on the aerodynamic damping of the blades operating in transonic flows. The experiment was carried out in a linear cascade tunnel by use of the influence coefficient method. The two flow conditions were adopted, namely, a near-design condition and an off-design condition with a higher back pressure. In the results for the near-design case, a strong vibration instability was observed in the positive side of the interblade phase angle. In the off-design case, however, the instability did not appear for almost all the interblade phase angles. A drastic change was found in the phase angle of unsteady aerodynamic force between the two cases, which change was a governing factor for the oscillation instability. Numerical simulation based on 2-D Euler equation revealed that the phase change came from the change in phase of the unsteady surface pressure across the shock impingement point on the blade suction surface in the off-design case. The numerical results also showed that the aerodynamic damping increased with increasing reduced frequency, and that the oscillation instability disappeared.

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Transfer Matrix Eigensolutions for Damped Torsional Systems

Journal of Vibration and Acoustics ◽

10.1115/1.3274703 ◽

1985 ◽

Vol 107 (1) ◽

pp. 128-132 ◽

Cited By ~ 8

Author(s):

S. Doughty ◽

G. Vafaee

Keyword(s):

General Solution ◽

Transfer Matrix ◽

Transfer Matrix Method ◽

Matrix Method ◽

Solution Method ◽

The Other ◽

Torsional Vibrations

A transfer matrix method is presented for the determination of complex eigensolutions associated with the damped torsional vibrations of single shaft machine trains. The system is described and the natures of the eigenvalues are discussed. The general solution method is developed, and the method is applied to two example problems. One of the examples is quite simple, while the other is entirely realistic.

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TEMPERATURE DETERMINATION OF INTERNALLY COOLED, MODEL TURBINE BLADES OF DS IN 792 SUPERALLOY FOR CALCULATION OF THE STRESS- AND STRAIN BEHAVIOUR

Local Strain and Temperature Measurement ◽

10.1016/b978-1-85573-424-1.50032-0 ◽

1999 ◽

pp. 274-282

Author(s):

D. FRANK ◽

F. SCHUBERT

Keyword(s):

Turbine Blades ◽

Stress And Strain ◽

Temperature Determination ◽

Strain Behaviour ◽

Internally Cooled

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Empirical determination of severe trauma in seals from collisions with tidal turbine blades

Journal of Applied Ecology ◽

10.1111/1365-2664.13388 ◽

2019 ◽

Vol 56 (7) ◽

pp. 1712-1724 ◽

Cited By ~ 5

Author(s):

Joe Onoufriou ◽

Andrew Brownlow ◽

Simon Moss ◽

Gordon Hastie ◽

Dave Thompson

Keyword(s):

Turbine Blades ◽

Severe Trauma ◽

Empirical Determination ◽

Tidal Turbine ◽

Tidal Turbine Blades

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Vibrations of a Three-Bladed Wind Turbine Rotor Due to Classical Flutter

ASME 2002 Wind Energy Symposium ◽

10.1115/wind2002-48 ◽

2002 ◽

Cited By ~ 2

Author(s):

M. H. Hansen

Keyword(s):

Wind Turbine ◽

Critical Frequency ◽

Turbine Blades ◽

Turbine Rotor ◽

Normal Operation ◽

Aerodynamic Damping ◽

Operation Conditions ◽

Torsional Frequency ◽

Classical Flutter ◽

Wind Turbine Rotor

The aeroelastic stability of a three-bladed wind turbine is considered with respect to classical flutter. Previous studies have shown that the risk of stall-induced vibrations of turbine blades is related to the dynamics of the complete turbine, for example does the aerodynamic damping of a rotor whirling mode depend highly on the tower stiffness. The results of this paper indicate that the turbine dynamics also affect the risk of flutter. The study is based on an eigenvalue analysis of a linear aeroelastic turbine model. In an example of a MW sized turbine, the critical frequency of the first torsional blade mode is determined for which flutter can occur under normal operation conditions. It is shown that this critical torsional frequency is higher when the blades are interacting through the hub with the remaining turbine, than when all blades are rigidly clamped at the root. Thus, the dynamics of the turbine has increased the risk of flutter.

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