19 th Blade Mechanics Seminar: Abstracts

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1 19 th Blade Mechanics Seminar: Abstracts ZHAW Zurich University of Applied Sciences IMES Institute of Mechanical Systems in Winterthur, Switzerland Thursday, September 11 th, 2014 Eulachpassage, TN E0.58, Technikumstrasse 71, 8401 Winterthur Jaroslaw Szwedowicz Alstom (Switzerland) Ltd, Switzerland Title key note presentation: Blade Design for Damping Past, Current and Future Industrial Practice HCF problems in turbine blades probably date back to the first design of a Parsons steam turbine in Campbell s (1924) 5-year investigation of 227 steam turbines with ratings above 5 MW resulted in the first resonance design guideline, which neglects damping. Significant advances were made during the period starting with the first fundamental damping study of den Hartog (1931) and continuing through the late 70s, with the development of many closed-form equations supplemented by a great deal of empirical knowledge. Computerization in the 80s allowed for further expansion of the science of damping, offering many specific solutions for friction underplatform or insert dampers; bolt, shroud or winglet contact dissipation; or coating damping. Aerodamping and impact-damping devices and the recently demonstrated piezoelectric-materials and magnetic dissipation widely extend the damping technology portfolio. These newer models, however, are not readily incorporated into common approaches such as the Campbell Diagram and the Mean Stress Diagram (1963), and blade damping is still treated too frequently as an overall empirical factor. Regarding the principles of forced vibrations, 4 standard resonance-mitigation strategies (Mass, Stiffness, Mistuning and Damping) are discussed and damping approaches of material and clamping dissipation are reviewed. Then, friction damping technology, such as use of an external or internal damper and shroud or bolt contact dissipation, is summarized in the form of engineering guidelines. With respect to the physical uncertainties in design, evaluation of damping performance and frequency sensitivity are discussed as necessary analyses for integrated bladedamper solutions. Scaling effects in friction damping are demonstrated by considering performance in different sizes of the turbine blade. Coating damping and aero-damping are also briefly discussed.

2 The market demand for larger AN 2 turbine stages will require longer freestanding blades with reliable damping technologies. Therefore, the presentation explores alternative dissipation technologies based on piezoelectric-materials and magnetic damping. The presentation strengthens the overall engineering understanding of damping blade technologies. Jacob Laborenz Leibniz University Hannover, Germany Eddy Current Damping Concept for Last Stage Steel Blading Within this talk, an eddy current based damping element for the vibration reduction of a last stage steel blading will be presented. In this approach, permanent magnets and conductive plates are embedded in the blading such that relative motion will induce eddy currents and, therefore, vibration energy is dissipated into heat. A simulation model will be presented consisting of a two-stage process utilizing the finite element method in time and frequency domain. The former are used for the evaluation of magnetic fields including motion induced eddy currents such that equivalent viscous damping constants can be identified. Based on those parameters, the forced response prediction is carried out in the frequency domain. Malte Krack Leibniz University Hannover, Germany Efficient Vibration Analysis of Friction-Damped Bladed Disks Near Isolated Resonances For the special but particularly relevant situation of an isolated resonance, the essential vibration behavior can be represented well by a single equivalent (modal) oscillator. Its amplitudedependent modal properties are computed using nonlinear modal analysis. A parametric reduced order model is then constructed that permits the vibration prediction under various operation conditions. The overall approach is particularly suited for the computational cost reduction of extensive parameter studies. Hence, more accurate models can be afforded and parameter 2

3 uncertainties can be addressed more properly. The proposed approach thus facilitates the design of more effective and more reliable friction-damped bladed disks. Florian Schönleitner Graz University of Technology, Austria Evaluation of the Modal Characteristics of a Low Pressure Turbine Rotor A Comparison between Different Numerical and Experimental Methods For a serious prediction of vibration characteristics under operating conditions of any structure a detailed knowledge of the modal characteristics is essential. The quality of numerical results is strongly dependent on boundary conditions and coupling conditions. Based on the model of a low pressure turbine rotor of a test rig at Graz University of Technology the presentation shows different numerical models for modal characterization. A big challenge in the FE modeling process is the coupling between rotor disk and rotor blades. In general, software packages for finite element analysis provide a number of contact models and formulations. In a detailed study different contact models and formulations were applied and investigated. The combination of contact type as well as formulation type is of particular importance. Eigenvalues and mode shapes of the blades are presented and illustrated as the results of the numerical analysis and can be compared with experimental data which were generated in a separate study. Hence also the experimental characterization was carried out with different methods, such as impact hammer tests, shaker tests or measurements with strain gages in combination with an electromagnetic excitation. The modal characteristics of the blades are of particular interest especially for further aero elastic investigations. All investigations were performed on different simplified models as well as on the assembling model of the turbine rotor. Evaluated modal characteristics are prepared and shown in a way to provide a better understanding for the importance of using correct contact models for an efficient numerical analysis. Finally, a simplified numerical model with sufficient quality can be recommended which shows a minimum deviation of the numerical results compared to experimentally evaluated data. 3

4 Simone Bistolfi Franco Tosi Meccanica, Italy Mutual Influence of Shaft and L-0 Blade Row Dynamics in a Steam Turbine Two modal models (for the 0 ND tangential mode and for the 1ND axial mode) of a L-0 blade row a single flow axial exhaust 67 MW condenser steam turbine driving a generator have been developed and introduced in the 1D beam model of the rotor, for analyzing its behavior, separately for lateral and torsion vibrations. The analysis shows that besides the well-known influence of shaft torsion vibrations on blade row vibrations, also lateral shaft vibrations can excite easily the blade row vibrations and conversely that blade row vibrations excited by the steam flow can be detected by lateral vibration measurements in the bearings of the steam turbine. Andreas Hartung MTU Aero Engines AG, Germany Blade Vibrations: Steady State Forced Response and Transient Measurements An analytical way for: prediction of optimal transient testing based on forced response; analytical evaluation of transient measurements backward to the steady state forced response; dealing with measured mistuning of structure damping; evaluation of measured strong non-linear transient vibrations is presented. 4

5 Ulrich Retze MTU Aero Engines AG, Germany Excitation Rig A rotating test facility for: vibration tests of blade stages with adjustable and analytically predictable excitations accordingly to excitation orders and magnitudes of the engine excitations; endurance HCF- and LCF-Tests; fretting HCF- and LCF-Tests is presented. Marius Bonhage Leibniz University Hannover, Germany Transient Amplitude Amplification of Mistuned Structures This presentation discusses the problem of exceeding amplitudes of mistuned structures caused by travelling waves-type excitation with time-variant frequency. These waves cause resonance passages of rotating structures. A wide-spread opinion based on the work of e. g. [Goloskokow1971] is that the maximum amplitude of the transient vibration response during resonance passage is smaller than the maximum amplitude of the frequency response function. Processing extensive parametric studies of a bladed disk it can be pointed out under which circumstances the probability rises that higher maximum amplitudes occur during resonance passage compared to steady state conditions. [Goloskokow1971]: Goloskokov, E. G., and Filippov, A., Instationäre Schwingungen mechanischer Systeme. Akademie-Verlag. 5

6 Tomokazu Miyakozawa Rolls Royce, United Kingdom Numerical Flutter Analyses on Welded-Pair Low Pressure Turbine Rotor Blades Low pressure turbine rotor blades in large civil turbofans can exhibit limit cycle flutter during engine testing often minimised by pairing blades at the shroud. One such example has been explored by performing simulations investigating the aerodynamic damping levels using both 3D URANS single passage and Fast influence coefficient techniques. The influence of different exhaust hot-nozzle configurations are also explored emulating the actual test set up conditions. Comparisons of the steady and unsteady pressures will be discussed providing insight into the flutter stability and finally the predicted damping levels compared with engine results. Harald Schoenenborn MTU Aero Engines AG, Germany Comparison of Non-Linear and Linearized CFD Analysis of the Stator-Rotor Interaction of a Compressor Stage The prediction of resonance amplitudes due to stator-rotor interactions is still an important task within the design process of turbo machinery bladings. In this presentation the stator-rotor interaction of a compressor stage which consists of an inlet guide vane and a rotor blade is studied with a non-linear and a linearized CFD code. First, a quasi-3d-study of a section close to the tip region is considered. The passing of the wake of the inlet guide vane over the rotor is studied for six different vibration mode shapes with an increasing complexity (first bending mode up to 4th chordwise bending mode). Whereas for low rotor speeds the comparison between linearized and non-linear calculations is quite good, large differences are found for high rotor speeds. It is shown that an acoustic interaction between the two stages with a cut-on mode is the cause for the large differences, leading to much higher unsteady pressure amplitudes on the rotor blade. This in turn leads to different aerodynamic work on the rotor blade for the different mode shapes. The extension of the investigations to 3D shows essentially the same effects. 6