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The Influence of Flow Leakage Modelling on Turbomachinery Blade Forcing Predictions
KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Vibrations in turbomachinery engine components are undesirable as they put the structural integrity of the components at risk and can lead to failure during the lifetime of the turbomachinery engine. Vibrations arising from aerodynamic forces and stability of turbomachinery blades is assessed in the discipline of aeromechanics. Ultimately, aeromechanical considerations limit turbomachinery designs and impose constraints on innovative aerodynamic designs with highly loaded light-weight components. Besides, aeromechanical assessment of blade vibration is done at a late stage of the design process and the number of iterations in the design loop is limited. Aeromechanical calculations can have large uncertainties in the prediction accuracy, especially when made a-priori without test data or without comparable design experience to tune the analysis methods. Therefore, large safety margins are required in the design, given that only a small set of prototype engines of a chosen design can be manufactured for testing. This may result in unnecessarily conservative engines and inhibit efficient or cost-effective design.

Accurate prediction methods together with a reliable estimate of the accuracy and sensitivity of the calculations will allow designers to push the limits and to design machines with highly efficient components. Efficiency directly translates into savings in terms of operational cost, capital cost as well as reductions in emissions when fuels are used.

In the presented work the sensitivity of aerodynamic forcing to the geometry features of a tip gap, hub cavity, tip-shroud cavity and inlet guide vane partial gaps has been investigated by the means of URANS CFD computations. The results indicate that sensitivity is both feature and case dependent, and that the detailing features can significantly alter the aerodynamic forcing function. The work shows, that the features should be included in high-fidelity aerodynamic models used for aeromechanics and highlights the mechanisms in which the features affect the aeromechanic forcing.

Investigations were performed for a subsonic model steam turbine configuration in 1.5 stage simulations, for a transonic turbine stage and for a 1.5 stage transonic research compressor in a 5 row investigation. Computations were performed using time domain simulations on scaled sectors of the blade rows. Results are analysed in terms of generalised modal force, and differences in the flow-field between the investigated detailing configurations are highlighted, marking the influence of the detailing features.

Abstract [sv]

Vibrationer i turbomaskinmotorer är oönskade därför att de riskerar komponenternasstrukturella integritet och kan leda till brott under motorns livslängd.Vibrationer som har sitt ursprung i aerodynamiska krafter, såväl somaerodynamiskt kopplad stabilitet av blad i turbomaskinen studeras inom aeromekanik. I slutändan verkar aeromekaniska överväganden inskränkande i utvecklingav turbomaskiner och utgör begränsningar till innovativa aerodynamiskakonstruktioner med högbelastade lättviktkomponenter. För övrigt görsen aeromekanisk bedömning av bladvibration på en sen fas av konstruktionsprocessenoch antalet iterationer i designslingan är begränsad. Aeromekaniskaberäkningar kan komma med stor osäkerhet angående exakthet av prognoser,speciellt när de görs i förväg utan testdata eller utan jämförbar erfarenhetsom kan hjälpa till att avstämma analysmetoderna, och bara ett begränsatantal testmotorer kan tillverkas för provning.

Därför krävs stora säkerhetsmarginaler i konstruktionen. Detta kan ledatill onödigt konservativa konstruktioner och hindra effektiv eller kostnadseffektivdesign.

Noggranna prediktiva metoder tillsammans med en bra uppskattning avexaktheten och känsligheten av beräkningarna gör det möjligt för konstruktöreratt driva gränserna och konstruera maskiner med högeffektiva komponenter. Effektivitet omvandlas dessutom direkt till besparingar när det gällerdriftskostnad, kapitalkostnad och utsläppsminskningar ifall bränsle användsför att driva motorn.

I det här arbetet har aerodynamikens inflytande av att modellera geometriskadetaljeringar studerats. Rotorspetsmellanrum, navkavitet, spetskavitetoch mellanrum på styrskovlar undersöktes med hjälp av URANS CFDberäkningar.Resultaten visar att påverkan är både detaljberoende och fallberoende,och att detaljeringsfunktionerna kan väsentligt ändra den aerodynamiskainstationära kraften som utför arbete på strukturen. Denna avhandlingvisar att undersökning av geometrisk detaljer bör ingå i noggranna aerodynamiskamodeller som används för aeromekanik och markerar på vilket visdetaljer påverkar den aeromekaniska kraften.

Simuleringsundersökningar utfördes på en subsonisk ångprovturbinkonfigurationi 1,5-steg, ett transoniskt turbinsteg, och på en 1,5-steg transoniskforskningskompressor med 5 skovelrad. Beräkningar utfördes i tidsdomän påskalade sektorer av bladraderna. Resultaten analyserades med fokus på generaliseradmodalkraft. Förändringar i flödesfältet mellan de undersökta detaljeringskonfigurationernamarkeras i samband med aerodynamisk skillnad ibladkraft.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. , p. 109
Series
TRITA-ITM-AVL ; 2018:58
National Category
Engineering and Technology
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-241125ISBN: 978-91-7873-060-5 (print)OAI: oai:DiVA.org:kth-241125DiVA, id: diva2:1278085
Public defence
2019-02-06, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, T7328Swedish Energy Agency, T6527Available from: 2019-01-14 Created: 2019-01-11 Last updated: 2019-05-10Bibliographically approved
List of papers
1. Influence of tip shroud cavity detailing on turbine blade forcing calculations
Open this publication in new window or tab >>Influence of tip shroud cavity detailing on turbine blade forcing calculations
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2014 (English)In: ASME Turbo Expo 2014: Turbine Technical Conference and Exposition, ASME Press, 2014, Vol. 7B, p. V07BT35A021-Conference paper, Published paper (Refereed)
Abstract [en]

Forced response in turbomachinery refers to the vibration of a component due to an excitation originating from another component. Obstacles, such as struts and blade rows in the upstream and downstream flow path of a turbomachine engine lead to engine order (EO) excitations. To be able to predict the severity of these excitations, both aerodynamic and structural calculations are performed. There is a risk of critical high cycle fatigue (HCF) failure when the force acts at a resonance frequency. Customarily, forcing calculations exclude detailing features, such as leakage flows. The current investigation uses a two stage subsonic model steam turbine configuration with shrouded rotor blades to demonstrate the influence of neglecting flow through seal cavities for blade forcing predictions. Upstream and downstream vanes are the excitation sources on the rotor blade. Calculation results are compared for a configuration including and excluding the tip shroud cavity. Computed data is compared to available pressure data from tests in the model turbine. The investigation shows for the first blade passing excitation at design point that the axial and circumferential rotor forcing change by +22%, respectively +4% when including the tip shroud cavity for the investigated configuration. The change in forcing arises from the interaction of the leakage flow with the main stream flow. For highly loaded designs this can be of importance if there is a critical excitation.

Place, publisher, year, edition, pages
ASME Press, 2014
Keywords
Engines, Excited states, Fatigue of materials, Leakage (fluid), Steam turbines, Stream flow, Turbomachine blades, Calculation results, Critical excitation, Downstream flow, Excitation sources, Forced response, High cycle fatigue, Resonance frequencies, Structural calculations
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-160680 (URN)10.1115/GT2014-26724 (DOI)000362240200079 ()2-s2.0-84922237083 (Scopus ID)9780791845776 (ISBN)
Conference
ASME Turbo Expo 2014: Turbine Technical Conference and Exposition, GT 2014, Dusseldorf, Germany, 16 June 2014 through 20 June 2014
Funder
Swedish Energy Agency
Note

QC 20150226

Available from: 2015-02-26 Created: 2015-02-26 Last updated: 2019-01-14Bibliographically approved
2. Unsteady blade force computation sensitivity in a transonic turbine to rotor tip gap, hub and shroud cavity model detail
Open this publication in new window or tab >>Unsteady blade force computation sensitivity in a transonic turbine to rotor tip gap, hub and shroud cavity model detail
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2015 (English)In: Proceedings of the 14th International Symposium on Unsteady Aerodynamics, Aeroacoustics & Aeroelasticity of Turbomachines, 2015Conference paper, Published paper (Refereed)
Abstract [en]

The influence of including geometric detailing features on blade forcing predicted by CFD calculations is investigated. Various features such as rotor tip gap, rotor tip shroud cavity with substantial leakage flow and large rotor upstream hub rim cavity are investigated. The test case is based on a single stage transonic test turbine rig in which the unsteady aerodynamics and blade forcing have previously been investigated numerically.

Including the tip gap and the tip shroud leakage in the computational model resulted in an overall increase in unsteady forcing. The change in forcing is mainly due to a change in secondary flow structure, but also due to a change of the stage flow condition. Conversely, it is shown that including the hub cavity in the computational model, even without the presence of purge-flow, reduces the unsteady force. Lastly, even though there are changes in unsteady blade loading, the blade response amplitude is only changed marginally (maximum 7%) due to forcing variations alone.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-178410 (URN)
Conference
The 14th International Symposium on Unsteady Aerodynamics, Aeroacoustics & Aeroelasticity of Turbomachines, KTH, Stockholm, 2015
Note

Qc 20160318

Available from: 2015-12-07 Created: 2015-12-07 Last updated: 2019-01-14Bibliographically approved
3. Influence of gap detailing on calculated unsteady non-adjacent blade row aero-forcing in a transonic compressor stage
Open this publication in new window or tab >>Influence of gap detailing on calculated unsteady non-adjacent blade row aero-forcing in a transonic compressor stage
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Resonant or close to resonant forced response excitation of compressor blades limits component life time, and can potentially lead to high cycle fatigue failure if the excitingforces are large and damping is insufficient. When numerically quantifying the forcing function by means of simulations,simplifications are typically made in the analysis to reducecomplexity and computational cost. In this paper we numerically investigate how the blade forcing function is influencedby the rotor tip gap flow and by flow across gaps in the upstream VIGV row. Unsteady simulations are made using a testrig geometry where a forcing crossing with an excitation froma non-adjacent blade row had previously been measured. Theeffects of the gaps on the forcing function for the first torsionmode are presented for both the non-adjacent blade row excitation (changes compared with a case without gaps indicating a 20% reduction) and an adjacent excitation (changes indicating an80% increase in terms of forcing function amplitude comparing with a case without gaps).

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-241111 (URN)
Note

QC 20190114

Available from: 2019-01-11 Created: 2019-01-11 Last updated: 2019-01-14Bibliographically approved
4. A NEW VISUALIZATION METHOD FOR HARMONIC UNSTEADY FLOWS IN TURBOMACHINERY
Open this publication in new window or tab >>A NEW VISUALIZATION METHOD FOR HARMONIC UNSTEADY FLOWS IN TURBOMACHINERY
2016 (English)In: PROCEEDINGS OF THE ASME TURBO EXPO: TURBINE TECHNICAL CONFERENCE AND EXPOSITION, 2016, VOL 7B, AMER SOC MECHANICAL ENGINEERS , 2016Conference paper, Published paper (Refereed)
Abstract [en]

Understanding unsteady flow processes is key in the analysis of challenging problems in turbomachinery design such as flutter and forced response. In this paper a new visualization method for harmonic unsteady flow is presented. The method illustrates the direction in which unsteady waves are traveling and transporting energy by the direct visualization of the propagating pressure waves in terms of field lines constructed from the wave group velocity. The group velocity is calculated from the unsteady flow solution by assuming that the local unsteady pressure perturbation of interest can be represented by a single harmonic unsteady wave. The applicability of the method is demonstrated for three test cases including a linear cascade of two-dimensional flat plates and a linear cascade of two-dimensional compressor blade profiles.

Place, publisher, year, edition, pages
AMER SOC MECHANICAL ENGINEERS, 2016
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-196481 (URN)000385461300066 ()2-s2.0-84991735542 (Scopus ID)
Conference
ASME Turbo Expo: Turbine Technical Conference and Exposition
Note

QC 20161124

Available from: 2016-11-24 Created: 2016-11-14 Last updated: 2019-01-14Bibliographically approved

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