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Detached-Eddy Simulation Applied to Aeroelastic stability analysis in a Last-stage Steam Turbine Blade
KTH, School of Industrial Engineering and Management (ITM), Energy Technology. (Turbomachinery Group)
KTH, School of Industrial Engineering and Management (ITM), Energy Technology. (Turbomachinery Group)
(Beihang University)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Avoiding flutter in the design phase of last stage steam turbines is a practical and known problem for the manufacturers. Therefore, a high accuracy prediction approach of blade flutter characteristics in the design phase is critical for the whole design system. Since the majority of aerodynamic work contributing to flutter is done near the blade tip, resolving the tip leakage flow can significantly increase the accuracy of flutter prediction. Previous research showed that besides the tip leakage vortex, the induced vortices in the tip region could also have an influence on the flow field and time-averaged blade loading near the tip. The structure of induced vortices cannot be resolved by URANS simulations due to the high dissipation in turbulence models. The URANS is the highest fidelity numerical method previously applied in flutter analysis as far as we are aware, and as a result the influence of induced vortices on flutter characteristics has not been investigated. The impact of induced vortices on the aeroelastic stability of a realistic-scale last stage steam turbine is analyzed in this paper. Flutter analysis based on DES approach shows that the induced vortices can influence the predicted aerodynamic work coefficient distribution on the blade surface, especially on the rear half of the blade suction side near the tip. At the least stable IBPA, the induced vortices show a destabilization effect on blade aeroelastic stabilities. Due to the periodic motion of induced vortices in streamwise during blade oscillation, a monotonous increasing trend of predicted aerodynamic damping with the increase of maximum blade vibration amplitude is observed at IBPA equals 0 degrees. In conclusion, resolving the induced vortices can influence the predicted flutter characteristics of the steam turbine test case, while further analysis of this phenomenon is required.

Keywords [en]
Steam turbine, Flutter, Aeroelastic stability, Tip clearance flow, Detached-Eddy Simulation
National Category
Engineering and Technology
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-233766OAI: oai:DiVA.org:kth-233766DiVA, id: diva2:1242504
Note

QC 20180829

Available from: 2018-08-28 Created: 2018-08-28 Last updated: 2018-08-29Bibliographically approved
In thesis
1. Improved Flutter Prediction for Turbomachinery Blades with Tip Clearance Flows
Open this publication in new window or tab >>Improved Flutter Prediction for Turbomachinery Blades with Tip Clearance Flows
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Recent design trends in steam turbines strive for high aerodynamic loading and high aspect ratio to meet the demand of higher efficiency. These design trends together with the low structural frequency in last stage steam turbines increase the susceptibility of the turbine blades to flutter. Flutter is the self-excited and self-sustained aeroelastic instability phenomenon, which can result in rapid growth of blade vibration amplitude and eventually blade failure in a short period of time unless adequately damped. To prevent the occurrences of flutter before the operation of new steam turbines, a compromise between aeroelastic stability and stage efficiency has to be made in the steam turbine design process. Due to the high uncertainty in present flutter prediction methods, engineers use large safety margins in predicting flutter which can rule out designs with higher efficiency. The ability to predict flutter more accurately will allow engineers to push the design envelope with greater confidence and possibly create more efficient steam turbines.

The present work aims to investigate the influence of tip clearance flow on the prediction of steam turbine flutter characteristics. Tip clearance flow effect is one of the critical factors in flutter analysis for the majority of aerodynamic work is done near the blade tip. Analysis of the impact of tip clearance flow on steam turbine flutter characteristics is therefore needed to formulate a more accurate aeroelastic stability prediction method in the design phase.Besides the tip leakage vortex, the induced vortices in the tip clearance flow can also influence blade flutter characteristics. However, the spatial distribution of the induced vortices cannot be resolved by URANS method for the limitation of turbulence models. The Detached-Eddy Simulation (DES) calculation is thus applied on a realistic-scale last stage steam turbine model to analyze the structure of induced vortices in the tip region. The influence of the tip leakage vortex and the induced vortices on flutter prediction are analyzed separately.

The KTH Steam Turbine Flutter Test Case is used in the flutter analysis as a typical realistic-scale last stage steam turbine model. The energy method based on 3D unsteady CFD calculation is applied in the flutter analysis. Two CFD solvers, an in-house code LUFT and a commercial software ANSYS CFX, are used in the flutter analysis as verification of each other. The influence of tip leakage vortex on the steam turbine flutter prediction is analyzed by comparing the aeroelastic stability of two models: one with the tip gap and the other without the tip gap. Comparison between the flutter characteristics predicted by URANS and DES approaches is analyzed to investigate the influence of the induced vortices on blade flutter characteristics.

The multiple induced vortices and their relative rotation around the tip leakage vortex in the KTH Steam Turbine Flutter Test Case are resolved by DES but not by URANS simulations. Both tip leakage vortex and induced vortices have an influence on blade loading on the rear half of the suction side near the blade tip. The flutter analysis results suggest that the tip clearance flow has a significant influence on blade aerodynamic damping at the least stable interblade phase angle (IBPA), while its influence on the overall shape of the damping curve is minor. At the least stable IBPA, the tip leakage vortex shows a stabilization effect on rotor aeroelastic stabilities while the induced vortices show a destabilization effect on it. Meanwhile, a non-linear unsteady flow behavior is observed due to the streamwise motion of induced vortices during blade oscillation, which phenomenon is only resolved in DES results.

Place, publisher, year, edition, pages
Stockholm, Sweden: KTH Royal Institute of Technology, 2018. p. 88
Series
TRITA-ITM-AVL ; 2018:40
Keywords
Steam turbine, Flutter, Aeroelastic stability, Tip clearance flow, Detached-Eddy Simulation
National Category
Engineering and Technology
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-233770 (URN)978-91-7729-909-7 (ISBN)
Presentation
2018-09-18, HPT Learning Theater, M235, Brinellvägen 68, KTH, Stockholm, 14:00 (English)
Opponent
Supervisors
Available from: 2018-08-28 Created: 2018-08-28 Last updated: 2018-08-28Bibliographically approved

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