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Cavity Purge Flows in High Pressure Turbines
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.ORCID iD: 0000-0003-2784-2534
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Turbomachinery forms the principal prime mover in the energy and aviation industries. Due to its size, improvements to this fleet of machines have the potential of significant impact on global emissions. Due to high gas temperatures in stationary gas turbines and jet engines, areas of flow mixing and cooling are identified to benefit from continued research. Here, sensitive areas are cooled through cold air injection, but with the cost of power to compress the coolant to appropriate pressure. Further, the injection itself reduces output due to mixing losses.A turbine testing facility is center to the study, allowing measurement of cooling impact on a rotating low degree of reaction high pressure axial turbine. General performance, flow details, and cooling performance is quantified by output torque, pneumatic probes, and gas concentration measurement respectively. The methodology of simultaneously investigating the beneficial cooling and the detrimental mixing is aimed at the cavity purge flow, used to purge the wheelspace upstream of the rotor from hot main flow gas.Results show the tradeoff between turbine efficiency and cooling performance, with an efficiency penalty of 1.2 %-points for each percentage point of massflow ratio of purge. The simultaneous cooling effectiveness increase is about 40 %-points, and local impact on flow parameters downstream of the rotor is of the order of 2° altered turning and a Mach number delta of 0.01. It has also been showed that flow bypassing the rotor blading may be beneficial for cooling downstream.The results may be used to design turbines with less cooling. Detrimental effects of the remaining cooling may be minimized with the flow field knowledge. Stage performance is then optimized aerodynamically, mixing losses are reduced, and the cycle output is maximized due to the reduced compression work. The combination may be used to provide a significant benefit to the turbomachinery industry and reduced associated emissions.

Abstract [sv]

Strömningsmaskinen i dess olika variationer bildar den främsta drivmotorn inom kraftproduktion och flygindustrin. En förbättring av denna väldiga maskinpark har potentialen till betydande inverkan på globala utsläpp. Områden som identifierats kunna dra nytta av vidare forskning är ombandningsprocesser och kylning. Dessa områden är inneboende i stationära gasturbiner och jetmotorer på grund av de heta gaser som används. Kylning uppnås genom injektion av kall luft i kritiska områden och försäkrar därmed säker drift. Kylningen kommer dock till en kostnad. På cykelnivå krävs arbete för att komprimera flödet till korrekt tryck. Dessutom medför injektionen i sig förluster som kan härledas till omblandningsprocessen.

Syftet med detta arbete är att samtidigt undersöka de fördelaktiga kylegenskaperna som nackdelarna med inblandning för att på så sätt bestämma den uppoffring som måste göras för en viss kylning. Alla förbättringar tros dock inte behöva föregås av en uppoffring. Om påverkan av kylningen på huvudflödet är välförstådd kan designen justeras för att ta hänsyn till denna förändring och minimera inverkan. Denna metodologi riktar sig mot ett särskilt kylflöde, kavitetsrensningsflödet, som har till uppgift att avlägsna het luft från den kavitet som uppkommer uppströms rotorskivan i ett högtrycksturbinsteg.

Studien kretsar kring en turbinprovanläggning som möjliggör detaljerade strömningsmätningar i ett roterande turbinsteg under inverkan av kavitetsrensningsflödet. Högtrycksturbinsteget som används för undersökningen är av låg reaktionsgrad. Här kvantifieras generell prestanda genom mätning av vridmomentet på utgående axel. Flödesfältet kvantifieras med pneumatiska sonder, och kylningsprestandan predikteras genom gaskoncentrationsmätningar.

Resultaten visar avvägningen och sambandet mellan turbinverkningsgrad och kylning i kavitet samt huvudkanal. Flödet mäts i detalj, och de effekter som kan förväntas uppkomma då ett turbinsteg utsätts för en viss mängd av kylflödet kvantifieras. De kvantitativa resultaten för det undersökta steget visar på en förlust i verkningsgrad på 1.2 procentenheter för varje procentenhet av kavitetsrensningsflödet i termer om massflödesförhållande. Samtidigt ses kyleffektiviteten öka med 40 procentenheter. Den lokala inverkan på flödesfältet nedströms rotorn för det undersökta steget är 2° i flödesvinken och en ändring på 0.01 i Machnummer för varje procentenhet av kylflödet. Dessa ändringar ses i form av ökad omlänkning och reducerad hastighet nära hubben, och vice versa omkring halva spännvidden. Inverkan av aktuell driftpunkt understryks genom arbetet. Det har också visats att ett läckage som kringgår rotorbladen i vissa kan fall ge fördelaktig kylning i områden nedströms.

Denna kombinerade kunskap kan användas för design av turbiner med så låg mängd kylning som möjligt samtidigt som säker drift bibehålls. Den negativa inverkan av den återstående kylningen kan minimeras genom kunskapen om hur flödesfältet påverkas. Genom detta optimeras stegverkningsgraden aerodynamiskt, omblandningsförluster minimeras, och cykeleffekten maximeras genom det minskade kompressionsarbetet till följd av de reducerade kylmängderna. Kombinationen kan ge en betydande förbättring för turbinindustrin och minskade utsläpp.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , p. 62
Series
TRITA-KRV ; Report 17/07
Keyword [en]
turbomachinery; axial turbine; cavity purge; purge flow; wheelspace; rim seal; spanwise transport; radial transport; effectiveness; cooling; efficiency
Keyword [sv]
strömningsmaskiner; axialturbin; kavitetsrensningsflöde; kavitetsflöde; tätkant; spännviddsvis transport; radiell transport; effektivitet; kylning; verkningsgrad
National Category
Aerospace Engineering Energy Engineering Fluid Mechanics and Acoustics
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-218468ISBN: 978-91-7729-626-3 (print)OAI: oai:DiVA.org:kth-218468DiVA, id: diva2:1160991
Public defence
2018-02-15, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, P30419-2
Note

QC 20171129

Available from: 2017-11-29 Created: 2017-11-28 Last updated: 2018-02-21Bibliographically approved
List of papers
1. TEST TURBINE INSTRUMENTATION FOR CAVITY PURGE INVESTIGATIONS
Open this publication in new window or tab >>TEST TURBINE INSTRUMENTATION FOR CAVITY PURGE INVESTIGATIONS
2014 (English)In: The XXII Symposium on Measuring Techniques in Turbomachinery, Lyon, 4-5 September 2014, 2014Conference paper, Published paper (Other academic)
Abstract [en]

The upstream wheelspace of the KTH Test Turbine has been instrumented with the aim of investigating cavity flow phenomena, as well as cavity-main annulus interaction. Measurements include static pressure, unsteady pressure and temperature.The stage used is of high pressure steam turbine design. The trials include investigating the design point and also a high pressure, high speed operating point, assimilating gas turbine operation. At each point, varying amounts of purge flow are superposed and the influences on the measurements studied.Initial results show considerable dependence of both operating

Keyword
Turbomachinery, Turbine, Intrumentation, Pressure, Temperature, Unsteady pressure, Measurement, Cavity flows, Cavity purge
National Category
Energy Engineering Fluid Mechanics and Acoustics Aerospace Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-181479 (URN)
Conference
The XXII Symposium on Measuring Techniques in Turbomachinery,Lyon, 4-5 September 2014
Funder
Swedish Energy Agency
Note

QC 20160308

Available from: 2016-02-02 Created: 2016-02-02 Last updated: 2017-11-29Bibliographically approved
2. Experimental flow and performance investigations of cavity purge flows in a high pressure turbine stage
Open this publication in new window or tab >>Experimental flow and performance investigations of cavity purge flows in a high pressure turbine stage
2015 (English)In: 11th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2015, European Conference on Turbomachinery (ETC) , 2015Conference paper, Published paper (Refereed)
Abstract [en]

A high pressure turbine stage has been investigated from the aspect of flow and performance impact associated with cavity purge. Performance is referred to as the operating parameters of the turbine, mainly based on the continuous output torque monitoring. The flow parameters were studied through measurements featuring temperature and pressure throughout the flow path, as well as in the cavity. Purge and main flow velocities were quantified in the vane exit section, and degree of sealing based on purge-amount correlations and pressure readings. Results were related to turbine efficiency based on a simple correlation, and also entropy generation. Change of operating point was found to have a significant effect on degree of sealing, while the change of efficiency was found to be linear with respect to relative purge rate and independent of operating point.

Place, publisher, year, edition, pages
European Conference on Turbomachinery (ETC), 2015
Keyword
Entropy, Fluid dynamics, Gas turbines, Thermodynamics, Turbomachinery, Entropy generation, High pressure turbine stage, Operating parameters, Operating points, Performance impact, Pressure reading, Temperature and pressures, Turbine efficiency, Turbines
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-174784 (URN)000380606100084 ()2-s2.0-85043422715 (Scopus ID)9780000000002 (ISBN)
Conference
11th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2015, 23 March 2015 through 27 March 2015
Funder
Swedish Energy Agency
Note

QC 20151208

Available from: 2015-12-08 Created: 2015-10-07 Last updated: 2018-05-16Bibliographically approved
3. Experimental Investigation of Turbine Stage Flow Field and Performance at Varying Cavity Purge Rates and Operating Speeds
Open this publication in new window or tab >>Experimental Investigation of Turbine Stage Flow Field and Performance at Varying Cavity Purge Rates and Operating Speeds
2018 (English)In: Journal of turbomachinery, ISSN 0889-504X, E-ISSN 1528-8900, Vol. 140, no 3, article id 031001Article in journal (Refereed) Published
Abstract [en]

The aspect of hub cavity purge has been investigated in a high-pressure axial lowreaction turbine stage. The cavity purge is an important part of the secondary air system, used to isolate the cavities below the hub level from the hot main annulus flow. A fullscale cold-flow experimental rig featuring a rotating stage was used in the investigation, quantifying main annulus flow field impact with respect to purge flow rate as it was injected upstream of the rotor. Five operating speeds were investigated of which three with respect to purge flow, namely, a high loading design case, and two high-speed points encompassing the peak efficiency. At each of these operating speeds, the amount of purge flow was varied from 0% to 2%. Observing the effect of the purge rate on measurement plane averaged parameters, a minor flow angle decrease and Mach number increase is seen for the low speed case, while maintaining near constant values for the higher operating speeds. The prominent effect due to purge is seen in the efficiency, showing a linear sensitivity to purge of 1.3%-points for every 1% of added purge flow for the investigated speeds. While spatial average values of flow angle and Mach number are essentially unaffected by purge injection, important spanwise variations are observed and highlighted. The secondary flow structure is strengthened in the hub region, leading to a generally increased over-turning and lowered flow velocity. Meanwhile, the added volume flow through the rotor leads to higher outlet flow velocities visible at higher span, with associated decreased turning. A radial efficiency distribution is utilized, showing negative impact through span heights from 15% to 70%. Pitchwise variation of investigated flow parameters is significantly influenced by purge flow, making this a parameter to include for instance when evaluating benefits of stator clocking positions.

Place, publisher, year, edition, pages
ASME Press, 2018
National Category
Aerospace Engineering Energy Engineering
Identifiers
urn:nbn:se:kth:diva-218464 (URN)10.1115/1.4038468 (DOI)000424289100001 ()2-s2.0-85039768466 (Scopus ID)
Funder
Swedish Energy Agency, P30419-2
Note

QC 20171206

Available from: 2017-11-28 Created: 2017-11-28 Last updated: 2018-02-22Bibliographically approved
4. SEEDGAS INVESTIGATION OF TURBINE STAGE AND SEAL PERFORMANCE AT VARYING CAVITY PURGE RATES AND OPERATING SPEEDS
Open this publication in new window or tab >>SEEDGAS INVESTIGATION OF TURBINE STAGE AND SEAL PERFORMANCE AT VARYING CAVITY PURGE RATES AND OPERATING SPEEDS
2017 (English)In: PROCEEDINGS OF THE ASME TURBO EXPO: TURBINE TECHNICAL CONFERENCE AND EXPOSITION, 2017, VOL 2A, AMER SOC MECHANICAL ENGINEERS , 2017Conference paper, Published paper (Refereed)
Abstract [en]

The topic of hub cavity purge is investigated in a high-pressure axial low-reaction turbine stage. Both the sealing ability of the purge flow and the performance impact associated with its injection into the main flow are studied. Three operating speeds are investigated, namely a high loading case, the peak efficiency, and a high speed case, and purge flow rates across a wide range. The operating points coincide with investigations previously reported, where the flow field and stage efficiency was quantified using pneumatic probes. Comparative measurements are also performed, varying a leakage flow through the rotor below the hub platform. The purge flow is now seeded with CO2 in order to measure its distribution throughout the stage, as it is injected into the wheelspace upstream of the rotor, allowing for quantification of the sealing effectiveness. This is done at a number of defined locations along the stator-side wall in the wheel space, resolving the radial variation through the cavity. Important radial variations of effectiveness are observed, confirming that the flow is in the regime of merged boundary layers, due to the narrow cavity, as compared to typical gas turbine operation with separated. boundary layers. The trends are found to be related to operating speed and platform leakage. With known sealing effectiveness, industry correlations may be adapted to make use of the variation of necessary purge rate to obtain a certain degree of sealing at a given operating point, and thereby optimize the efficiency. In addition to quantification of potential hot-gas ingestion, the paper initiates an investigation of the transport of the purge flow in the main annulus, through sampling on the hub, as well as area traverse downstream of the rotor. The amount of sealing gas leads to opportunity to quantify the cooling performance of the purge flow in the main annulus. Both the cooling performance in the main annulus and cavity are shown to be significantly influenced by the rotor leakage, while its effect on efficiency is minor.

Place, publisher, year, edition, pages
AMER SOC MECHANICAL ENGINEERS, 2017
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-217086 (URN)10.1115/GT2017-64295 (DOI)000412624400068 ()2-s2.0-85028981739 (Scopus ID)
Conference
PROCEEDINGS OF THE ASME TURBO EXPO 2017
Note

QC 20171106

Available from: 2017-11-06 Created: 2017-11-06 Last updated: 2018-02-26Bibliographically approved
5. Purge Flow Impact on Turbine Stage and Seal Performance at Varying Cavity Purge Rates and Operating Speed
Open this publication in new window or tab >>Purge Flow Impact on Turbine Stage and Seal Performance at Varying Cavity Purge Rates and Operating Speed
(English)In: International Journal of Turbomachinery, Propulsion and Power, ISSN 2504-186XArticle in journal (Refereed) Submitted
Abstract [en]

The impact of the wheelspace cavity purge flow on a high-pressure axial low-reaction turbine stage is investigated. Both the flow's sealing ability and the performance impact associated with its injection are studied. Two operating speeds are tested, namely a high loading case and the peak efficiency, with purge flow rates covering a wide range. As the purge flow is injected upstream of the rotor, the sealing effectiveness is quantified both radially and tangentially close to the rim seal, where the tangential variation is used to identify the seal mixing region. Having passed the rotor blading, the purge flow distribution in the main annulus is quantified, showing an influence of operating speed. The purge flow core is localized to the trace of the vane wake, however somewhat migrated while passing through the blading. The combination of measurements shows that the impact on flow parameters cannot be used to determine the spanwise transport of the purge flow; hence two techniques are necessary to both judge the spanwise transport and impact on flow. With known sealing effectiveness, industry correlations may be adapted to make use of the variation of necessary purge rate to obtain a certain degree of effectiveness at a given operating point, and thereby optimize the efficiency. Also the distribution of the coolant in the main flow path may be used to optimize film cooling in that area.

National Category
Energy Engineering Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-218465 (URN)
Note

QC 20171129

Available from: 2017-11-28 Created: 2017-11-28 Last updated: 2017-11-29Bibliographically approved

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