Best practices for underplatform damper designers

2018 ◽  
Vol 232 (7) ◽  
pp. 1221-1235 ◽  
Author(s):  
Chiara Gastaldi ◽  
Teresa M Berruti ◽  
Muzio M Gola
Keyword(s):  
Turbine Blades ◽  
Operating Regime ◽  
Test Case ◽  
Functional Requirements ◽  
Speed Range ◽  
Early Design Stages ◽  
Computationally Intensive ◽  
Bending Modes ◽  
Underplatform Damper ◽  
Practical Demonstration

The purpose of this paper is to offer a practical demonstration of how essential preoptimization is in the design of underplatform dampers for turbine blades. Preoptimization can be thought of as a “prescreening” which allows excluding, since the early design stages, a high number of poorly performing damper–platform configurations. This concept, previously presented by the authors, is here extended and its generality for all blade bending modes is rigorously demonstrated. The paper addresses a test case where the introduction of curved-flat underplatform dampers is necessary to avoid a dangerous resonance crossing in the operating rotational speed range of a real turbine disk. It is shown how preoptimized dampers are the only ones that manage to satisfy all functional requirements, including those in the nonlinear operating regime. The same set of dampers may have been identified by exploring, through hundreds of computationally intensive nonlinear calculations, the performance of all possible damper configurations. The latter approach, i.e. iterative design, is unpractical and has to be repeated for each new set of blades since it is based on a test case-specific trial-and-error procedure. Preoptimization substitutes iterations with knowledge of the damper behavior and can therefore be considered as “informed design”: viable damper configurations are instantly singled out through simple but insightful considerations on the damper equilibrium of forces and moments.

Assessment of a 3D Linear Euler Flutter Prediction Tool Using Sector Cascade Test Data

2005 ◽  
Author(s):  
Hans Ma˚rtensson ◽  
Damian M. Vogt ◽  
Torsten H. Fransson
Keyword(s):  
Turbine Blades ◽  
Tip Clearance ◽  
Mode Shapes ◽  
Test Case ◽  
Prediction Tool ◽  
Reduced Frequency ◽  
3D Geometry ◽  
Annular Sector ◽  
Lp Turbine ◽  
Bending Modes

An assessment and validation of a numerical prediction tool for flutter are made using new experimental data from experiments on turbine blades in a sector cascade. The 3D geometry is that of a low-pressure (LP) turbine blade with twist and a profile that changes along span in an annular sector cascade. The numerical model is a linear harmonic Euler equation solver. Rig results are obtained for the blade by oscillating 1 blade out of 7 in the annular sector cascade. The blade is oscillated in the rig using a mechanical type of actuator to control the mode. The mode shapes in the rig consist of torsion and bending modes around a pivot mechanism fixed inside the hub end wall. The frequencies obtained in the rig are in the range up to 219 Hz, or reduced frequency based on full chord k = 0.5, which covers the range of useful reduced frequencies typically found in turbine designs. Under reference running conditions the unsteady pressure responses are found qualitatively in line with the experiment. The test case is shown to be challenging to the numerical tool in terms of effects of tip clearance as well as off-design effects. In order to improve results tip clearance modeling and inclusion of viscous terms are identified as key factors.

Guided Tuning of Turbine Blades: A Practical Method to Avoid Operating at Resonance

2013 ◽  
Vol 135 (5) ◽  
Author(s):  
Loc Duong ◽  
Kevin D. Murphy ◽  
Kazem Kazerounian
Keyword(s):  
Natural Frequencies ◽  
Turbine Blades ◽  
Practical Method ◽  
Laser Vibrometer ◽  
Operating Speed ◽  
Design Procedures ◽  
Iterative Design ◽  
At Resonance ◽  
Speed Range ◽  
Computationally Intensive

In gas turbine applications, forced vibrations of turbine blades under resonant—or nearly resonant—conditions are undesirable. Usually in airfoil design procedures, at least the first three blade modes are required to be free of excitation in the operating speed range. However, not uncommonly, a blade may experience resonance at other higher natural frequencies. In an attempt to avoid resonant oscillations, the structural frequencies are tuned away from the excitation frequencies by changing the geometry of the blade. The typical iterative design process—of adding and removing material through restacking the airfoil sections—is laborious and in no way assures an optimal design. In response to the need for an effective and fast methodology, the guided tuning of turbine blades method (GTTB) is developed and presented in this paper. A practical tuning technique, the GTTB method is based on structural perturbations to the mass and stiffness at critical locations, as determined by the methodology described herein. This shifts the excited natural frequency out of the operating speed range, while leaving the other structural frequencies largely undisturbed. The methodology is demonstrated here in the redesign of an actual turbine blade. The numerical results are experimentally validated using a laser vibrometer. The results indicate that the proposed method is not computationally intensive and renders effective results that jibe with experiments.

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

2018 ◽  
Vol 140 (6) ◽  
Author(s):  
Chiara Gastaldi ◽  
Teresa M. Berruti ◽  
Muzio M. Gola
Keyword(s):  
Turbine Blades ◽  
Time Integration ◽  
Optimization Procedure ◽  
Forced Response ◽  
Contact Forces ◽  
Test Case ◽  
Equilibrium Equations ◽  
Bending Modes ◽  
The One ◽  
High Level

The purpose of this paper is to propose an effective strategy for the design of turbine blades with underplatform dampers (UPDs). The strategy involves damper “pre-optimization,” already proposed by the authors, to exclude, before the blades-coupled nonlinear calculation, all those damper configurations leading to low damping performance. This paper continues this effort by applying pre-optimization to determine a damper configuration which will not jam, roll, or detach under any in-plane platform kinematics (i.e., blade bending modes). Once the candidate damper configuration has been found, the damper equilibrium equations are solved by using both the multiharmonic balance method (MHBM) and the direct-time integration (DTI) for the purpose of finding the correct number of Fourier terms to represent displacements and contact forces. It is shown that contrarily to non-preoptimized dampers, which may display an erratic behavior, one harmonic term together with the static term ensures accurate results. These findings are confirmed by a state-of-the-art code for the calculation of the nonlinear forced response of a damper coupled to two blades. Experimental forced response functions (FRF) of the test case with a nominal damper are available for comparison. The comparison of different damper configurations offers a “high-level” validation of the pre-optimization procedure and highlights the strong influence of the blades mode of vibration on the damper effectiveness. It is shown that the pre-optimized damper is not only the most effective but also the one that leads to a faster and more flexible calculation.

A Novel Method of Coupling the Steam Turbine Exhaust Hood and the Last Stage Blades Using the Non-Linear Harmonic Method

2013 ◽  
Author(s):  
Zoe Burton ◽  
Grant Ingram ◽  
Simon Hogg
Keyword(s):  
Steam Turbine ◽  
Turbine Blades ◽  
Computation Time ◽  
Computational Effort ◽  
Alternative Methods ◽  
Test Case ◽  
Exhaust Hood ◽  
Harmonic Method ◽  
Non Linear ◽  
Last Stage

The exhaust hood of a steam turbine is a vital area of turbomachinery research its performance strongly influences the power output of the last stage blades. It is well known that accurate CFD simulations are only achieved when the last stage blades are coupled to the exhaust hood to capture the strong interaction. This however presents challenges as the calculation size grows rapidly when the full annulus is calculated. The size of the simulation means researchers are constantly searching of methods to reduce the computational effort without compromising solution accuracy. This work uses a novel approach, by coupling the last stage blades and exhaust hood by the Non-Linear Harmonic Method, a technique widely used to reduce calculation size in high pressure turbine blades and axial compressors. This has been benchmarked against the widely adopted Mixing Plane method. The test case used is the Generic Geometry, a representative exhaust hood and last stage blade geometry that is free from confidentiality and IP restrictions and for which first calculations were presented at last year’s conference [1]. The results show that the non-uniform exhaust hood inlet flow can be captured using the non-liner harmonic method, an effect not previously achievable with single passage coupled calculations such as the mixing plane approach. This offers a significant computational saving, estimated to be a quarter of the computation time compared with alternative methods of capturing the asymmetry with full annulus frozen rotor calculations.

Towards Multidisciplinary Turbine Blade Tolerance Design Assessment Using Adjoint Methods

2020 ◽  
Author(s):  
Alexander Liefke ◽  
Peter Jaksch ◽  
Sebastian Schmitz ◽  
Vincent Marciniak ◽  
Uwe Janoske ◽  
...  
Keyword(s):  
Assessment Tool ◽  
Low Cycle Fatigue ◽  
Turbine Blades ◽  
Computational Cost ◽  
Life Time ◽  
Test Case ◽  
Evaluation Tool ◽  
Design Assessment ◽  
Tool Chain ◽  
Relative Errors

Abstract This paper shows how to use discrete CFD and FEM adjoint surface sensitivities to derive objective-based tolerances for turbine blades, instead of relying on geometric tolerances. For this purpose a multidisciplinary adjoint evaluation tool chain is introduced to quantify the effect of real manufacturing imperfections on aerodynamic efficiency and probabilistic low cycle fatigue life time. Before the adjoint method is applied, a numerical validation of the CFD and FEM adjoint gradients is performed using 102 heavy duty turbine vane scans. The results show that the relative error for adjoint CFD gradients is below 0.5%, while the FEM life time gradient relative errors are below 5%. The adjoint assessment tool chain further reduces the computational cost by around 85% for the investigated test case compared to non-linear methods. Through the application of the presented tool chain, the definition of specified objective-based tolerances becomes available as a design assessment tool and allows to improve overall turbine efficiency and the accuracy of life time prediction.

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.

Effects of Contact Interface Parameters on Vibration of Turbine Bladed Disks With Underplatform Dampers

2011 ◽  
Author(s):  
C. W. Schwingshackl ◽  
E. P. Petrov ◽  
D. J. Ewins
Keyword(s):  
Structural Integrity ◽  
Turbine Blades ◽  
Contact Interface ◽  
Bladed Disks ◽  
Normal Contact ◽  
Input Parameters ◽  
Software Codes ◽  
Model Setup ◽  
Interface Parameters ◽  
Underplatform Damper

The design of high cycle fatigue resistant bladed disks requires the ability to predict the expected damping of the structure in order to evaluate the dynamic behaviour and ensure structural integrity. Highly sophisticated software codes are available today for this nonlinear analysis but their correct use requires a good understanding of the correct model generation and the input parameters involved to ensure a reliable prediction of the blade behaviour. The aim of the work described in this paper is to determine the suitability of current modelling approaches and to enhance the quality of the nonlinear modelling of turbine blades with underplatform dampers. This includes an investigation of a choice of the required input parameters, an evaluation of their best use in nonlinear friction analysis, and an assessment of the sensitivity of the response to variations in these parameters. Part of the problem is that the input parameters come with varying degrees of uncertainty, since some are experimentally determined, others are derived from analysis and a final set are often based on estimates from previous experience. In this investigation the model of a commercial turbine bladed disc with an underplatform damper is studied and its first flap, first torsion and first edgewise modes are considered for 6EO and 36EO excitation. The influence of different contact interface meshes on the results is investigated, together with several distributions of the static normal contact loads to enhance the model setup and hence increase accuracy in the response predictions of the blade with an underplatform damper. A parametric analysis is carried out on the friction contact parameters and the correct setup of the nonlinear contact model to determine their influence on the dynamic response and to define the required accuracy of the input parameters.

An Optimum Aero-Design Process of Turbine Blades

2002 ◽  
Author(s):  
C. Xu ◽  
R. S. Amano
Keyword(s):  
Design Process ◽  
High Efficiency ◽  
Pressure Ratio ◽  
Turbine Blades ◽  
Three Dimensional ◽  
Optimization Procedure ◽  
Compressor Blade ◽  
Advanced Technology ◽  
Test Case ◽  
Blade Design

With the development of the advanced technology, the combustion temperature is raised for increased efficiencies. At the same time, the turbine and compressor pressure ratio and the mass flow rate rise; thus causing turbine and compressor blades turning and blade lengths increase. Moreover, the high efficiency requirements had made the turbine and compressor blade design difficult. A turbine airfoil has been custom designed for many years, but an optimization for the section design in a three-dimensional consideration is still a challenge. For a compressor blade design, standard section cannot meet the modern compressor requirements. Modern compressor design has not only needs a custom designed section according to flow situation, but also needs three-dimensional optimizations. Therefore, a good blade design process is critical to the turbines and compressors. A blade design of the turbomachines is one of the important steps for a good turbomachine design. A blade design process not only directly influences the overall machine efficiency but also dramatically impact the design time and cost. In this study, a blade design and optimization procedure was proposed for both turbine and compressor blade design. A compressor blade design was used as a test case. It was shown that the current design process had more advantages than conventional design methodology.

Performance of Two-Equation Turbulence Models for Flat Plate Flows With Leading Edge Bubbles

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
S. Collie ◽  
M. Gerritsen ◽  
P. Jackson
Keyword(s):  
Flat Plate ◽  
Turbine Blades ◽  
Turbulence Models ◽  
Leading Edge ◽  
Test Case ◽  
Sst Model ◽  
Two Dimensional Flow ◽  
Experimental Values ◽  
University Of Bristol ◽  
Sharp Leading Edge

This paper investigates the performance of the popular k-ω and SST turbulence models for the two-dimensional flow past the flat plate at shallow angles of incidence. Particular interest is paid to the leading edge bubble that forms as the flow separates from the sharp leading edge. This type of leading edge bubble is most commonly found in flows past thin airfoils, such as turbine blades, membrane wings, and yacht sails. Validation is carried out through a comparison to wind tunnel results compiled by Crompton (2001, “The Thin Aerofoil Leading Edge Bubble,” Ph.D. thesis, University of Bristol). This flow problem presents a new and demanding test case for turbulence models. The models were found to capture the leading edge bubble well with the Shear-Stress Transport (SST) model predicting the reattachment length within 7% of the experimental values. Downstream of reattachment both models predicted a slower boundary layer recovery than the experimental results. Overall, despite their simplicity, these two-equation models do a surprisingly good job for this demanding test case.

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