Fatigue Damage Prevention on Turbine Blades: Study of Underplatform Damper Shape

2007 ◽  
Vol 347 ◽  
pp. 159-164 ◽  
Author(s):  
Teresa Berruti ◽  
Christian M. Firrone ◽  
M. Pizzolante ◽  
Muzio M. Gola
Keyword(s):  
Turbine Blades ◽  
Forced Vibrations ◽  
Forced Response ◽  
Numerical Code ◽  
Vibration Energy ◽  
Test Rig ◽  
Blade Vibration ◽  
Static Test ◽  
Underplatform Damper ◽  
Contact Parameters

Forced vibrations can lead to an irreparable damage of a blade array. Devices called “underplatform damper” that dissipate the vibration energy are employed in order to reduce blade vibration amplitude. The present paper deals with the design of the underplatform damper. A numerical code to calculate the forced response of a blade array with dampers has been previously purposely developed. A method is here proposed for the estimation of the unknown contact parameters demanded by the code. The computation results are here validated by means of comparison with experimental results on a static test rig. Three dampers with different shape are tested.

A Resonance Tracking Algorithm for the Prediction of Turbine Forced Response With Friction Dampers

2000 ◽  
Author(s):  
C. Bréard ◽  
J. S. Green ◽  
M. Vahdati ◽  
M. Imregun
Keyword(s):  
Rotor Blade ◽  
Turbine Blades ◽  
Turbine Rotor ◽  
Forced Response ◽  
Rotor Blades ◽  
Test Case ◽  
Friction Damper ◽  
Blade Vibration ◽  
Frequency Shifts ◽  
Friction Dampers

This paper presents an iterative method for determining the resonant speed shift when non-linear friction dampers are included in turbine blade roots. Such a need arises when conducting response calculations for turbine blades where the unsteady aerodynamic excitation must be computed at the exact resonant speed of interest. The inclusion of friction dampers is known to raise the resonant frequencies by up to 20% from the standard assembly frequencies. The iterative procedure uses a viscous, time-accurate flow representation for determining the aerodynamic forcing, a look-up table for evaluating the aerodynamic boundary conditions at any speed, and a time-domain friction damping module for resonance tracking. The methodology was applied to an HP turbine rotor test case where the resonances of interest were due to the 1T and 2F blade modes under 40 engine-order excitation. The forced response computations were conducted using a multi-stage approach in order to avoid errors associated with “linking” single stage computations since the spacing between the two bladerows was relatively small. Three friction damper elements were used for each rotor blade. To improve the computational efficiency, the number of rotor blades was decreased by 2 to 90 in order to obtain a stator/rotor blade ratio of 4/9. However, the blade geometry was skewed in order to match the capacity (mass flow rate) of the components and the condition being analysed. Frequency shifts of 3.2% and 20.0% were predicted for the 1T/40EO and 2F/40EO resonances in about 3 iterations. The predicted frequency shifts and the dynamic behaviour of the friction dampers were found to be within the expected range. Furthermore, the measured and predicted blade vibration amplitudes showed a good agreement, indicating that the methodology can be applied to industrial problems.

Range of Variability in the Dynamics of Semi-Cylindrical Friction Dampers for Turbine Blades

2008 ◽  
Author(s):  
Stefano Zucca ◽  
Daniele Botto ◽  
Muzio M. Gola
Keyword(s):  
Degrees Of Freedom ◽  
Turbine Blades ◽  
Numerical Test ◽  
Forced Response ◽  
Numerical Code ◽  
Test Case ◽  
Force Coefficient ◽  
System A ◽  
Flat Side ◽  
Fatigue Failures

Under-platform dampers are used to reduce resonant stresses in turbine blades to avoid high cycle fatigue failures. In this paper a model of semi-cylindrical under-platform damper (i.e. with one flat side and one curved side) for turbine blades is described. The damper kinematics is characterized by three degrees of freedom (DOFs): in-plane translations and rotation. Static normal loads acting on the damper sides are computed using the three static balance equations of the damper. Non-uniqueness of normal pre-loads acting on the damper sides is highlighted. Implementation of the model in a numerical code for the forced response calculation of turbine blades with under-platform dampers shows that non-uniqueness of normal pre-loads leads to non-uniqueness of the forced response of the system. A numerical test case is presented to show the capabilities of the model and to analyze the effect of the main system parameters (damper mass, excitation force, coefficient of friction and damper rotation) on the damper behavior and on the system dynamics.

Alternate Mistuning of Turbine Bladings Coupled by Underplatform Dampers

2012 ◽  
Author(s):  
S. Tatzko ◽  
L. Panning-von Scheidt ◽  
J. Wallaschek ◽  
A. Kayser
Keyword(s):  
Turbine Blades ◽  
Steady State Solution ◽  
Computational Effort ◽  
Forced Response ◽  
Nonlinear Coupling ◽  
Cyclic Symmetry ◽  
Periodic Excitation ◽  
Blade Vibration ◽  
Mode Localization ◽  
Bladed Disk

In turbo machinery design it is important to avoid vibrations that can destroy the turbine in the last resort. The rotating structure is exposed to periodic excitation forces. Two main types of periodic excitation can be distinguished. Flutter is the effect when mass flow forces couple with a natural vibration mode. The result is a negative damping coefficient and amplitudes will rise up to malfunction of the structure. The engine order excitation is a periodic excitation where the force signal is directly related to the speed of the rotor. A forced response calculation gives information about the blade vibration. Nonlinear coupling, i.e. friction coupling, between blades is used to increase damping of the bladed disk. Dynamic analysis of turbine blades with nonlinear coupling is a complex task and computer simulations are inevitable. Various techniques have been developed to reduce computational effort. The cyclic symmetry approach assumes each blade around the disk to be identical. Thus only one sector of the disk is sufficient to compute the steady state solution of the whole turbine blading. However, it has been observed that mistuning of blades reduces the flutter instability. On the other hand statistical mistuning can lead to dangerously high forced response amplitudes due to mode localization. A compromise is intentional mistuning. The simplest approach is alternate mistuning with every other blade exhibiting identical mechanical properties. This work explains in detail how a turbine bladed disk can be modeled when alternate mistuning is applied intentionally. Cyclic symmetry is used and each sector comprises two blades. This untypical choice of the sector size has significant impact on results of a cyclic modal analysis. Simulation results show the influence of alternate mistuned turbine bladings which are coupled by underplatform damper elements.

Transient Forced Response Analysis of Mistuned Steam Turbine Blades During Startup and Coastdown

2016 ◽  
Vol 139 (1) ◽  
Author(s):  
Christian Siewert ◽  
Heinrich Stüer
Keyword(s):  
Mechanical Model ◽  
Turbine Blades ◽  
Forced Vibrations ◽  
Forced Response ◽  
The Self ◽  
Response Analysis ◽  
Computationally Efficient ◽  
Bladed Disk ◽  
Steam Turbine Blades ◽  
Number Of Publications

It is well known that the vibrational behavior of a mistuned bladed disk differs strongly from that of a tuned bladed disk. A large number of publications dealing with the dynamics of mistuned bladed disks are available in the literature. The vibrational phenomena analyzed in these publications are either forced vibrations or self-excited flutter vibrations. Nearly, all published literature on the forced vibrations of mistuned blades disks considers harmonic, i.e., steady-state, vibrations, whereas the self-excited flutter vibrations are analyzed by the evaluation of the margin against instabilities by means of a modal, or rather than eigenvalue, analysis. The transient forced response of mistuned bladed disk is not analyzed in detail so far. In this paper, a computationally efficient mechanical model of a mistuned bladed disk to compute the transient forced response is presented. This model is based on the well-known fundamental model of mistuning (FMM). With this model, the statistics of the transient forced response of a mistuned bladed disk is analyzed and compared to the results of harmonic forced response analysis.

Modelling a Friction Damper: Analysis of the Experimental Data and Comparison With Numerical Results

2006 ◽  
Author(s):  
Christian M. Firrone ◽  
Daniele Botto ◽  
Muzio M. Gola
Keyword(s):  
Experimental Data ◽  
Dry Friction ◽  
Negative Impact ◽  
Real Life ◽  
Forced Response ◽  
Nonlinear Frequency ◽  
Friction Damper ◽  
Typical Application ◽  
Static Test ◽  
Underplatform Damper

High cycle fatigue is one of the main causes of failure of blades in turbomachinery. The negative impact of HCF on turbomachinery blades can be reduced by dry friction vibration damping. A typical application of dry friction damping in gas turbine is the so called “underplatform damper”. In this work a ‘real life’ asymmetric underplatform damper is experimentally tested with two real blades. A static test rig is used to obtain the nonlinear frequency response function of a mock-up made with two real blades with an underplatform damper between them. This paper addresses an underplatform damper model taking into account damper rotation. The proper mathematical formulations have been developed and forced response calculation of the system have been performed. Comparison with experimental data are carried out for different values of excitation forces and for pre-load similar to real centrifugal force values.

Transient Forced Response Analysis of Mistuned Steam Turbine Blades During Start-Up and Coast-Down

2014 ◽  
Author(s):  
Christian Siewert ◽  
Heinrich Stüer
Keyword(s):  
Mechanical Model ◽  
Turbine Blades ◽  
Forced Vibrations ◽  
Forced Response ◽  
Response Analysis ◽  
Computationally Efficient ◽  
Bladed Disk ◽  
Start Up ◽  
Steam Turbine Blades ◽  
Number Of Publications

It is well-known that the vibrational behavior of a mistuned bladed disk differs strongly from that of a tuned bladed disk. A large number of publications dealing with the dynamics of mistuned bladed disks is available in the literature. The vibrational phenomena analyzed in these publications are either forced vibrations or self-excited flutter vibrations. Nearly all published literature on the forced vibrations of mistuned blades disks considers harmonic, i. e. steady-state, vibrations, whereas the self-excited flutter vibrations are analyzed by the evaluation of the margin against instabilities by means of a modal, or rather than eigenvalue, analysis. The transient forced response of mistuned bladed disk is not analyzed in detail so far. In this paper, a computationally efficient mechanical model of a mistuned bladed disk to compute the transient forced response is presented. This model is based on the well-known Fundamental Model of Mistuning. With this model, the statistics of the transient forced response of a mistuned bladed disk is analyzed and compared to the results of harmonic forced response analysis.

Measured and Simulated Forced Response of a Rotating Turbine Disk With Asymmetric and Cylindrical Underplatform Dampers

2020 ◽  
Vol 142 (5) ◽  
Author(s):  
Thomas Hoffmann ◽  
Lars Panning-von Scheidt ◽  
Jörg Wallaschek
Keyword(s):  
Large Scale ◽  
Turbine Blades ◽  
Forced Response ◽  
Turbine Disk ◽  
Nonlinear Frequency ◽  
Test Rig ◽  
Nonlinear Frequency Response ◽  
Turbine Disks ◽  
Small Test ◽  
Good Agreement

Abstract The dynamics of turbine blades with underplatform dampers (UPDs) is often experimentally explored by using small test rigs like two-blade models for cost and complexity reasons. In this paper, the dynamics of a large-scale academic turbine disk is measured on a special rotation test rig. Such measurements have rarely been published so far. The test rig supports speeds up to 3600 rpm and turbine disks up to a diameter of 1.2 m. The turbine disk is tested linearly as well as with asymmetric and cylindrical UPDs. The excitation forces and the excitation order are varied. The results prove the damper effectiveness by lowering resonance amplitudes. Additionally, the mistuning influence on the result depiction is discussed. The measurements are compared to simulations of the nonlinear frequency response functions (FRFs), showing good agreement.

Academic Blade Geometries for Baseline Comparisons of Industry-Specific Forced Response Predictions

2017 ◽  
Author(s):  
James H. Little ◽  
Jeffrey L. Kauffman ◽  
Matthias Huels
Keyword(s):  
Model Reduction ◽  
Turbine Blades ◽  
Reduction Process ◽  
Forced Response ◽  
Vibration Characteristics ◽  
Contact Dynamics ◽  
Qualitative Behavior ◽  
Friction Damper ◽  
Blade Vibration ◽  
Reduced Order

Predicting the energy dissipation associated with contact of underplatform dampers remains a critical challenge in turbomachinery blade and friction damper design. Typical turbomachinery blade forced vibration response analyses rely on reduced order models and simplified nonlinear codes to predict blade vibration characteristics in a computationally tractable manner. Recent research has focused on both the model reduction process and simulation of the contact dynamics. This paper proposes two academic turbine blade geometries with coupled underplatform dampers as vehicles by which these model reduction and forced response simulation techniques may be compared. The blades correspond to two types of freestanding turbine blades and demonstrate the same qualitative behavior as more complex industry geometries. The blade geometries are fully described here and analyzed using the same procedure as used for an industry-specific blade. Standard results are presented in terms of resonance frequency, amplitude, and damping across a range of aerodynamic excitation. In addition, the predicted blade vibration characteristics are examined under variations in the contact interface: friction coefficient, damper / platform surface roughness, and damper mass, with relative sensitivities to each term generated. Finally, the effect of the number of modes retained in the reduced order model is studied to uncover patterns of convergence as well as to provide additional sets of standard data for comparison with other model reduction and forced response simulation methods.

Non-Linear Dynamics of Steam Turbine Blades With Shroud: Numerical Analysis and Experiments

2012 ◽  
Author(s):  
Stefano Zucca ◽  
Muzio M. Gola ◽  
Francesco Piraccini
Keyword(s):  
Frequency Domain ◽  
Steam Turbine ◽  
Turbine Blades ◽  
Harmonic Balance Method ◽  
Low Pressure ◽  
Forced Response ◽  
Numerical Code ◽  
Accurate Information ◽  
Normal Contact ◽  
Non Linear

The prediction of the aeromechanical behavior of low-pressure blades represents one of the main challenges in the Steam Turbine Industry. The evaluation of forced response and damping is critical for the reliability of new designs and usually requires expensive validation campaigns such as Wheel Box Tests (WBT). A WBT consists of one or more blade rows assembled on a rotor and spun at the desired rotating speed in a vacuum cell, with synchronous excitation provided by various sources. The WBT provides accurate information about the blade modes frequency, the alternating response level, and allows the evaluation of the mechanical damping. Given the large effort in terms of costs and time associated to the experimental activity, the possibility to rely on the output of a numerical code either during the first steps of a new design or to investigate the effect of minor changes to a current design would be extremely beneficial to the development of future products. In order to compute the non-linear forced response of shrouded blades of steam turbines, custom numerical solvers must be developed, since commercial finite element (FE) solvers do not perform this kind of analysis in the frequency domain. In this paper, the forced response of a blade with shrouds of a low pressure steam turbine is computed and numerical results are compared with the experimental Wheel Box Tests performed at GE Oil & Gas. The calculations require a three-step procedure: in the first step, a non-linear static analysis is performed in ANSYS® in order to compute the actual contact area on the shroud surface and the distribution of static normal loads, then a reduced order model of the blade is generated in ANSYS® taking into account the stiffening effect on the blade of the pre-stress due to the centrifugal force, finally the reduced model is imported in a numerical code and the non-linear forced response of the blade is computed. The numerical code solves the balance equations of the system in the frequency domain, by means of the Harmonic Balance Method, imposing cyclic symmetry boundary conditions of the system. An interpolation procedure is implemented in order to manage the non-perfectly matching meshes of the shroud contact surfaces, while the tangential and normal contact stiffness is computed with a numerical model based on the contact mechanics principles. The numerical and the experimental results around some of the critical resonances of the system are compared in order to assess the reliability and accuracy of the numerical tool for its future implementation in the mechanical design practice of the blades.

A Test Rig for Non-Contact Travelling Wave Excitation of a Bladed Disk With Underplatform Dampers

2010 ◽  
Author(s):  
Teresa Berruti ◽  
Christian M. Firrone ◽  
Muzio M. Gola
Keyword(s):  
Numerical Models ◽  
Experimental Tests ◽  
Calibration Procedure ◽  
Travelling Wave ◽  
Forced Response ◽  
Wave Excitation ◽  
Test Rig ◽  
Response Measurement ◽  
Static Test ◽  
Bladed Disk

The paper presents a static test rig called “Octopus” designed for the validation of numerical models aimed at calculating the nonlinear dynamic response of a bladed disk with underplatform dampers (UPDs). The test rig supports a bladed disk on a fixture and each UPD is pressed against the blade platforms by wires pulled by dead weights. Both excitation system and response measurement system are noncontacting. The paper features the design and the set-up of the noncontacting excitation generated by electromagnets placed under each blade. A travelling wave excitation is generated according to a desired engine order by shifting the phase of the harmonic force of one electromagnet with respect to the contiguous exciters. Since the friction phenomenon generated by UPDs introduces nonlinearities on the forced response, the amplitude of the exciting force must be kept constant at a known value on every blade during step-sine test to calculate Frequency Response Functions. The issue of the force control is therefore addressed since the performance of the electromagnet changes with frequency. The system calibration procedure and the estimated errors on the generated force are also presented. Examples of experimental tests that can be performed on a dummy integral bladed disk (blisk) mounted on the rig are described in the end.

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