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Investigation of tip clearance flow effects on an open 3D steam turbine flutter test case
KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
2017 (English)In: Proceedings of the ASME Turbo Expo, American Society of Mechanical Engineers (ASME) , 2017Conference paper, Published paper (Refereed)
Abstract [en]

Blade failure caused by flutter is a major problem in the last stage of modern steam turbines. It is because rotor at this stage always has a large scale in spanwise, which provides low structural frequency as well as supersonic tip speeds. Since most of the unsteady aerodynamic work is done in the tip region, transonic tip-leakage flow that influences the tip region flow could have a remarkable effect on the aerodynamic stability of rotor blades. However, few research had been done on the tipleakage flow influence on flutter characteristic based on fullscale steam turbine numerical models. In this paper, an open 3D steam turbine stage model designed by Durham University was applied, which was widely analyzed and representative for the last stage of modern industrial steam turbines. The average Mach number at the rotor outlet is 1.1. URANS simulation carried by both numerical software CFX and LUFT code is applied, and the two solvers show an agreement on steady and unsteady results. The numerical results indicate that the influence of tip leakage flow on blade stability is based on two types of flow mechanisms. Both mechanisms act on the suction side of near tip region. The first type of mechanism is produced by the reduction of passage shock near the leading edge, and the other type of mechanism at the rear of blade is caused by the interaction between tip leakage vortex and trailing edge shock of the neighbor blade. In conclusion, tip leakage flow has a significant influence on steam turbine flutter boundary prediction and requires further analysis in the future.

Place, publisher, year, edition, pages
American Society of Mechanical Engineers (ASME) , 2017.
Keywords [en]
Aerodynamic stability, Flutter, Steam turbine, Tip clearance, Aerodynamics, Computer software, Flutter (aerodynamics), Gas turbines, Leakage (fluid), Steam, Turbines, Turbomachine blades, Turbomachinery, Unsteady flow, Flutter boundary predictions, Numerical results, Numerical software, Structural frequencies, Tip clearance flow, Tip leakage vortex, Unsteady aerodynamics, Steam turbines
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-216582DOI: 10.1115/GT2017-64021ISI: 000412862600056Scopus ID: 2-s2.0-85028966551ISBN: 9780791850954 OAI: oai:DiVA.org:kth-216582DiVA, id: diva2:1154031
Conference
ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition, GT 2017, 26 June 2017 through 30 June 2017
Note

QC 20171101

Available from: 2017-11-01 Created: 2017-11-01 Last updated: 2018-08-28Bibliographically 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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