Regeneration-Induced Forced Response in Axial Turbines

2013 ◽  
Author(s):  
Jens Aschenbruck ◽  
Christopher E. Meinzer ◽  
Linus Pohle ◽  
Lars Panning-von Scheidt ◽  
Joerg R. Seume
Keyword(s):  
Turbine Blades ◽  
Forced Response ◽  
Response Analysis ◽  
Geometrical Parameters ◽  
Turbine Stage ◽  
Structural Dynamic ◽  
Air Turbine ◽  
Axial Turbines ◽  
Interaction Approach ◽  
Stagger Angle

The regeneration of highly loaded turbine blades causes small variations of their geometrical parameters. To determine the influence of such regeneration-induced variances of turbine blades on the nozzle excitation, an existing air turbine is extended by a newly designed stage. The aerodynamic and the structural dynamic behavior of the new turbine stage are analyzed. The calculated eigenfrequencies are verified by an experimental modal analysis and are found to be in good agreement. Typical geometric variances of overhauled turbine blades are then applied to stator vanes of the newly designed turbine stage. A forced response analysis of these vanes is conducted using a uni-directional fluid-structure interaction approach. The effects of geometric variances on the forced response of the rotor blade are evaluated. It is shown that the vibration amplitudes of the response are significantly higher for some modes due to the additional wake excitation that is introduced by the geometrical variances e.g. 56 times higher for typical MRO-induced variations in stagger-angle.

Experimentally Verified Study of Regeneration-Induced Forced Response in Axial Turbines

2014 ◽  
Vol 137 (3) ◽  
Author(s):  
Jens Aschenbruck ◽  
Joerg R. Seume
Keyword(s):  
Vibration Amplitude ◽  
Turbine Blades ◽  
Forced Response ◽  
Rotor Blades ◽  
Response Analysis ◽  
Reference Case ◽  
Angle Variation ◽  
Air Turbine ◽  
Axial Turbines ◽  
Stagger Angle

Geometrical variations occur in highly loaded turbine blades due to operation and regeneration. To determine the influence of such regeneration-induced variances of turbine blades on the aerodynamic excitation, a typical stagger angle variation of overhauled turbine blades is applied to stator vanes of an air turbine. This varied turbine stage is numerically and experimentally investigated. For the aerodynamic investigation of the vane wake, computational fluid dynamics (CFD) simulations are conducted. It is shown that the wake is changed due to the stagger angle variation. These results are confirmed by aerodynamic probe measurements in the air turbine. The vibration amplitude of the downstream rotor blades has been determined by a computational forced response analysis using a unidirectional fluid–structure interaction (FSI) approach and is experimentally verified here by tip-timing measurements. The results of the simulations and the measurements both show significantly higher amplitudes at certain operating points (OPs) due to the additional wake excitation. For typical regeneration-induced variations in stagger angle, the vibration amplitude is up to five times higher than in the reference case of uniform upstream stators. Based upon the present results, the influence of these variations and of the vane patterns on the vibration amplitude of the downstream rotor blade can and should be estimated in the regeneration process to minimize the dynamic stresses of the blades.

Experimentally Verified Study of Regeneration-Induced Forced Response in Axial Turbines

2014 ◽  
Author(s):  
Jens Aschenbruck ◽  
Joerg R. Seume
Keyword(s):  
Vibration Amplitude ◽  
Turbine Blades ◽  
Forced Response ◽  
Rotor Blades ◽  
Response Analysis ◽  
Reference Case ◽  
Angle Variation ◽  
Air Turbine ◽  
Axial Turbines ◽  
Stagger Angle

Geometrical variations occur in highly loaded turbine blades due to operation and regeneration. To determine the influence of such regeneration-induced variances of turbine blades on the aerodynamic excitation, a typical stagger angle variation of overhauled turbine blades is applied to stator vanes of an air turbine. This varied turbine stage is numerically and experimentally investigated. For the aerodynamic investigation of the vane wake, CFD simulations are conducted. It is shown that the wake is changed due to the stagger angle variation. These results are confirmed by aerodynamic probe measurements in the air turbine. The vibration amplitude of the downstream rotor blades has been determined by a computational forced response analysis using a uni-directional fluid-structure interaction approach and is experimentally verified here by tip-timing measurements. The results of the simulations and the measurements both show significantly higher amplitudes at certain operating points due to the additional wake excitation. For typical regeneration-induced variations in stagger angle, the vibration amplitude is up to five times higher than in the reference case of uniform upstream stators. Based upon the present results, the influence of these variations and of the vane patterns on the vibration amplitude of the downstream rotor blade can and should be estimated in the regeneration process to minimize the dynamic stresses of the blades.

Influence of Detailing on Aerodynamic Forcing of a Transonic Axial Turbine Stage and Forced-Response Prediction for Low-Engine-Order (LEO) Excitation

2017 ◽  
Author(s):  
Tobias R. Müller ◽  
Damian M. Vogt ◽  
Klemens Vogel ◽  
Bent A. Phillipsen ◽  
Peter Hönisch
Keyword(s):  
Numerical Study ◽  
Response Prediction ◽  
Forced Response ◽  
Resonance Excitation ◽  
Response Analysis ◽  
Turbine Stage ◽  
Flow Induced Vibration ◽  
Axial Turbine ◽  
Timing Data ◽  
Generalized Force

The effects of detailing on the prediction of forced-response in a transonic axial turbine stage, featuring a parted stator design, asymmetric inlet and outlet casings as well as rotor cavities, is investigated. Ensuring the mechanical integrity of components is of paramount importance for the safe and reliable operation of turbomachines. Among others, flow induced resonance excitation can lead to high-cycle fatigue (HCF) and potentially to damage of components unless properly damped. This numerical study is assessing the necessary degree of detailing in terms of spatial and temporal discretization, boundary conditions of the pre-stressed rotor geometry as well as geometrical detailing for the reliable prediction of the aerodynamic excitation of the structure. In this context, the sensitivity of the aerodynamic forcing is analyzed by means of the generalized force criterion, showing a significant influence for some of the investigated variations of the numerical model. Moreover, the origin and further progression of several low-engine-orders (LEO) within the flow field, as well as their interaction with different geometric details has been analyzed based on the numerical results obtained from a full 360° CFD-calculation of the investigated turbine stage. The predicted flow induced vibration of the structure has been validated by means of a full forced-response analysis, where a good agreement with tip-timing data has been found.

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.

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.

The Relevance of Damper Pre-Optimization and its Effectiveness on the Forced Response of Blades

2017 ◽  
Author(s):  
Chiara Gastaldi ◽  
Teresa M. Berruti ◽  
Muzio M. Gola
Keyword(s):  
Turbine Blades ◽  
Time Integration ◽  
Optimization Procedure ◽  
Forced Response ◽  
Contact Forces ◽  
Design Stage ◽  
Geometrical Parameters ◽  
Test Case ◽  
Equilibrium Equations ◽  
Contact Points

The paper presents a calculation procedure for the design of turbine blades with underplatform dampers. The procedure involves damper “pre-optimization” before the coupled calculation with the blades. The pre-optimization procedure excludes, since the early design stage, all those damper configurations leading to low damping performance. Pre-optimization involves plotting a design “damper map” with forbidden areas, corresponding to poorly performing damper geometries and admissible design areas, where effective solutions for the damper shape can be explored. Once the candidate damper configurations have been selected, the damper equilibrium equations are solved by using both the multi-harmonic balance (MHB) method, and the direct time integration method (DTI). Direct time integration of the damper dynamic equations is implemented in order to compute the trend of the contact forces in time and the shape of the hysteresis cycles at the different contact points. Based on these trends, the correct number of Fourier terms to represent the contact forces on the damper is chosen. It is shown that one harmonic term together with the static term, are enough in the MHB calculation of a pre-optimized damper. The proposed method is applied to a test case of a damper coupled with two blades. Experimental forced response functions of the test case with a nominal damper are available for comparison. The purpose of this paper is to show the effectiveness of the “damper maps” in excluding all those damper configurations, leading to undesirable damper behavior and to highlight the strong influence of the blades mode of vibration on the damper effectiveness. From the comparison of dampers with different geometrical parameters, the pre-optimized damper proved to be not only the most effective, in terms of damping capability, but also the one that leads to a faster and more flexible calculation of the damper, coupled with the blades.

scholarly journals Implementation of Speed Variation in the Structural Dynamic Assessment of Turbomachinery Flow Path Components

2013 ◽  
Author(s):  
Andrew M. Brown ◽  
R. Benjamin Davis ◽  
Michael K. DeHaye
Keyword(s):  
Resonant Frequency ◽  
Dynamic Assessment ◽  
Excitation Frequency ◽  
Flow Path ◽  
Forced Response ◽  
Response Analysis ◽  
Structural Dynamic ◽  
Speed Variation ◽  
Time Histories ◽  
Life Analysis

During the design of turbomachinery flow path components, the assessment of possible structural resonant conditions is critical. Higher frequency modes of these structures are frequently found to be subject to resonance, and in these cases, design criteria require a forced response analysis of the structure with the assumption that the excitation speed exactly equals the resonant frequency. The design becomes problematic if the response analysis shows a violation of the HCF criteria. One possible solution is to perform “finite-life” analysis, where Miner’s rule is used to calculate the actual life in seconds in comparison to the required life. In this situation, it is beneficial to incorporate the fact that, for a variety of turbomachinery control reasons, the speed of the rotor does not actually dwell at a single value but instead dithers about a nominal mean speed and during the time that the excitation frequency is not equal to the resonant frequency, the damage accumulated by the structure is diminished significantly. Building on previous investigations into this process, we show that a steady-state assumption of the response is extremely accurate for this typical case, resulting in the ability to quickly account for speed variation in the finite-life analysis of a component which has previously had its peak dynamic stress at resonance calculated. A technique using Monte Carlo simulation is also presented which can be used when specific speed time histories are not available. The implementation of these techniques can prove critical for successful turbopump design, as the improvement in life when speed variation is considered is shown to be greater than a factor of two.

On the Effect of Axial Spacing Between Rotor and Stator Onto the Blade Vibrations of a Low Pressure Turbine Stage at Engine Relevant Operating Conditions

2016 ◽  
Author(s):  
Andreas Marn ◽  
Florian Schönleitner ◽  
Mathias Mayr ◽  
Thorsten Selic ◽  
Franz Heitmeir
Keyword(s):  
Rotor Blade ◽  
Low Pressure ◽  
Forced Response ◽  
Operating Conditions ◽  
Turbine Stage ◽  
Blade Vibration ◽  
Structural Dynamic ◽  
Flow Measurements ◽  
Vibration Measurements ◽  
Excitation Mechanisms

In order to achieve the ACARE targets regarding reduction of emissions, it is essential to reduce fuel consumption drastically. Reducing engine weight is supporting this target and one option to reduce weight is to reduce the overall engine length (shorter shafts, nacelle). However, to achieve a reduction in engine length, the spacing between stator and rotor can be minimised, thus changing the rotor blade excitation. Related to the axial spacing, a number of excitation mechanisms with respect to the rotor blading must already be considered during the design process. Based on these facts several setups have been investigated at different engine relevant operating points and axial spacing between the stator and rotor in the subsonic test turbine facility (STTF-AAAI) at the Institute for Thermal Turbomachinery and Machine Dynamics at Graz University of Technology. In order to avoid upstream effects of supporting struts, these struts are located far downstream of the stage which is under investigation. For rotor blade vibration measurements, a novel telemetry system in combination with strain gauges is applied. To the best of the author’s knowledge, the present paper is the first report of blade vibration measurements within a rotating system in the area of low pressure turbines under engine relevant operating conditions. In addition, aerodynamic measurements including unsteady flow measurements have been conducted, but will not be presented in this paper. By analysing the flow field, aerodynamic excitation mechanisms can be identified and assigned to the blade vibration. However, this is not presented in this paper. Within this paper, the flow fields are analysed in both upstream and downstream of the turbine stage, visualised for two axial gaps and then compared to the forced response of the blading. Detailed structural dynamic investigations show critical modes during the operation which are identified by the telemetry measurements as well. Finally the influence of the axial spacing regarding the rotor blade excitation and vibration can be elaborated and is prepared to get a better understanding of basic mechanisms. The paper shows that reducing axial spacing is a promising option for reducing engine weight, but aeroelasticity must be carefully taken into account.

Advanced Modelling of Underplatform Friction Dampers for Analysis of Bladed Disc Vibration

2006 ◽  
Author(s):  
E. P. Petrov ◽  
D. J. Ewins
Keyword(s):  
Large Scale ◽  
Dynamic Models ◽  
Normal Load ◽  
Forced Response ◽  
Response Analysis ◽  
Finite Element Models ◽  
Structural Dynamic ◽  
Stick Slip ◽  
Friction Dampers ◽  
Inertia Forces

Advanced structural dynamic models for both wedge and split underplatform dampers have been developed. The new damper models take into account inertia forces and the effects of normal load variation on stick-slip transitions at the contact interfaces. The damper models are formulated for the general case of multiharmonic forced response analysis. An approach for using the new damper models in the dynamic analysis of large-scale finite element models of bladed discs is proposed and realised. Numerical investigations of bladed discs are performed to demonstrate the capabilities of the new models and an analysis of the influence of the damper parameters on the forced response of bladed discs is made.

Vibratory Stress Suppression in Turbine Blades Subjected to Aerodynamic Loading

2013 ◽  
Author(s):  
Imran Aziz ◽  
Wasim Tarar ◽  
Imran Akhtar ◽  
M. Nadeem Azam
Keyword(s):  
Gas Turbine ◽  
Solid Mechanics ◽  
Turbine Blades ◽  
Free Vibration Analysis ◽  
Forced Response ◽  
Mode Shapes ◽  
Response Analysis ◽  
Published Data ◽  
Element Analysis ◽  
Aerodynamic Loading

Vibratory stresses are the main cause of failure in gas turbine engines and other rotating machinery components. These stresses must be attenuated to an acceptable level through an efficient process in order to prevent failures in turbine blades. Research [8] has shown that a thin magneto mechanical coating layer can make a significant contribution to the damping and reduction of these vibratory stresses. Previous studies on analyzing the damping characteristics of these coatings for various applications, such as beams and turbine blades, employed general solid mechanics loads. In this study, we numerically compute aerodynamic loads on one and a half stage axial turbine in order to bring more reality to the problem. We employ a three-dimensional finite-volume based solver to simulate the flow in the turbine using SST model to account for turbulence effects. Sliding mesh technique is used to allow the transfer of flow parameters across the sliding rotor/stator interfaces. In order to model a single passage configuration, profile transformation method is used. A free vibration analysis has been performed to obtain natural frequencies and corresponding mode shapes to analyze resonance conditions. The computed CFD loads are then applied to an uncoated and coated turbine blade through a finite-element analysis (FEA) package. A forced response analysis is performed at the critical frequencies to obtain vibratory stresses. Numerical results show suppression of vibratory stresses at various low and high frequency vibration modes. The results are benchmarked against published data and closely match the expected outcome. The research presents an effective procedure for suppression of vibratory stresses in gas turbine engine component subjected to real world aerodynamic loading. The new procedure is a significant improvement towards more realistic simulation based solutions for vibration suppression problems.

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