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Design of Internal Cooling Passages: Investigation of Thermal Performance of Serpentine Passages
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Gas turbines are used to convert thermal energy into mechanical energy. The thermal efficiency of the gas turbine is directly related to the turbine inlet temperature. The combustion and turbine technology has improved to such an extent that the operating temperature in the turbine inlet is higher than the melting temperature of the turbine material. Different techniques are used to cope with this problem. One of the most commonly used methods is internal cooling of the turbine blades. Conventionally air from the compressor is used for this purpose but due to higher heat capacity, steam can be used as coolant. This opens up the possibility to increase the gas temperature. In the case of a combined cycle power plant, its availability provides a good opportunity to be used as a coolant.

The trailing edge of the gas turbine blades is an important region as it affects the aerodynamics of the flow. The aerodynamics demands a sharp and thin trailing edge to reduce profile losses. The conventional method is the release of a lot of cooling air though a slot along the airfoil trailing edge. However in the case of internal only cooling designs, the coolant is not allowed to leave the channel except from the root section to avoid mixing of the gas in the main flow path with the coolant and loss of cooling medium.

The challenge is to design an inner cooling channel, with the cooling medium entering and leaving the blade at the root section, which reduces the metal temperatures to the required values without an increase of the profile losses and at acceptable cooling flow rate and pressure drop.

This thesis presents Computational Fluid Dynamic (CFD) based numerical work concentrated firstly on the flow and heat transfer in two-pass rectangular channels with and without turbulator ribs. The aspect ratio of the inlet pass was reduced to accommodate more channels in the blade profile in chord-wise direction. Additionally, the divider-to-tip wall distance was varied for these channels. Their effect on heat transfer and pressure drop was studied for smooth as well as ribbed channels.  It was followed by a numerical heat transfer study in the trapezoidal channel. Different RANS based turbulence models were used to compare the numerical results with the experimental results. Further, new designs to enhance heat transfer in the channel’s side walls (named as trailing edge wall) were studied. These include the provision of ribs at the trailing edge wall only, inline arrangement of ribs at the bottom as well as at the trailing edge wall and a staggered arrangement of these ribs. The final study was a conjugate heat transfer problem with an aim to propose the best internal cooling channel design to reduce the metal temperature of the trailing edge surface for the given thermal and flow conditions. A number of different options were studied and changes were made to get the best possible channel design.

The results show that for a two-pass rectangular channel (both smooth and ribbed), the reduction in inlet channel aspect ratio reduces the pressure drop. For a smooth channel the reduction in the width of the inlet pass does not affect the heat transfer enhancement at the inlet pass and outlet pass regions. In case of ribbed channels, heat transfer decreases at the tip and bend bottom with decrease in the width of the inlet pass. Among different turbulence models used to validate numerical results against experimental results for case of trapezoidal channel, the low-Re k-epsilon model is found to be the most appropriate. Using the turbulence model that yields results that are closest to the experimental data, the staggered arrangement of ribs at the trailing edge wall is found to have maximum thermal performance. The results from the conjugate heat transfer problem suggest using steam as coolant if it is available as it requires less mass flow rate to get similar wall temperature values as compared to air at similar thermal and flow conditions. It is also found that staggered arrangement of ribs is the best option compared to others to enhance heat transfer in trailing edge of the gas turbine blade with the pressure drop in the cooling duct in the acceptable range.

Abstract [sv]

Gasturbiner används för att omvandla värmeenergi till mekanisk energi. Den termiska verkningsgraden för en gasturbin är direkt relaterad till turbinen inloppstemperatur. Förbrännings- och turbintekniken har förbättrats så mycket att gastemperaturen i turbininloppet är högre än smälttemperaturen för turbinmaterialet. Olika tekniker används för att hantera detta problem. En av de vanligaste metoderna är intern kylningen av turbinbladen. Konventionellt luft från kompressorn används för detta ändamål, men på grund av högre värmekapacitet kan ånga användas som kylmedel. Detta öppnar för möjligheten att höja gasens temperatur. Vid ett kombikraftverk, ger dess tillgänglighet ett bra tillfälle att användas som kylmedel.

 

Den bakre delen av turbinbladen är ett viktigt område eftersom geometrin påverkar strömningen. Aerodynamiken kräver en skarp och tunn bakkant för att minska profilförlusterna. Den konventionella metoden för kylning av denna är att släppa ut en stor mängd kylluft genom en spalt längs bakkanten. Men i fallet med enbart inre kylning får kylmediet inte lämna skovelprofilen i strömningskanalen utan endast genom rotsektionen för att undvika blandning av förbränningsluften i turbinens strömningskanal med kylmediet och förlust av kylmedium.

 

Utmaningen är att utforma en inre kylkanal, i vilken kylmediet kommer in och lämnar bladet i rotsnittet som är tillräckligt bra för att hålla metalltemperaturen på normala värden utan att öka profilförlusterna och med acceptabla kylluftflöden och tryckfall.

 

Denna avhandling består av ett Computational Fluid Dynamics (CFD) baserat numeriskt arbetet koncentrerat på strömning och värmeöverföring först i två-pass rektangulära kanaler med och utan turbulensalstrande ribbor. Geometrin för inloppspassagen reducerades för att ge utrymme för fler kylkanaler inom bladets profil i kordans riktning. Dessutom varierades mellanväggens avstånd till toppväggen. Effekten på värmeöverföring och tryckfall studerades för båda kanalerna. Därefter följde en numerisk studie av värmeöverföringen i liknande men trapetsformade kanaler. Olika RANS baserade turbulensmodeller användes för att jämföra numeriska och experimentella resultat. Vidare har nya konstruktioner för att förbättra värmeöverföringen i kanalens sidoväggar och bakkant studeras. Dessa inkluderar turbulensribbor på enbart bakkantsväggen samt ribbor på såväl sidoväggar som på bakkantsväggen i linje med och förskjutna mot varandra. Den slutliga studien var ett sammansatt värmeöverföringsproblem bakkantens yta för ett visst angivet tillstånd i form av värmebelastning, tryck, temperatur och flöden. Ett antal olika alternativ har studerats och modifierats för att bästa möjliga kanalutformningen.

 

Resultaten visar att för en två-pass rektangulär kanal (både släta och ribbade), minskar tryckfallet när inloppskanalens geometri reducerades. För en slät kanal påverkar inte den minskade bredden på inloppskanalen värmeöverförning i inlopps- och utloppskanalerna. Vid ribbade kanaler minskar värmeöverföring vid toppen och på toppväggen med minskad bredd på inloppskanalen. Av de olika turbulensmodeller som används för att validera numeriska resultat mot experimentella för fallet med trapetsformad kanal visade sig låg-Re k-epsilon modellen den mest lämpliga. Genom att använda den turbulensmodell som är närmast experimentella data visar det att geometrin med förskjutna ribbor på bakkantsväggen har maximal termiska prestanda. Resultaten från det sammansatta värmeöverföringsproblemet framhåller användning av ånga som kylmedium om den finns tillgänglig eftersom den kräver mindre massflöde för att få samma värden på väggtemperaturerna jämfört med luft vid samma termiska tillstånd. Det kunde också visas att förskjutna turbulensribbor är det bästa alternativet jämfört med andra för att öka värmeöverföringen i bakkanten av ett gasturbinblad med acceptabelt tryckfall i kylkanalen.  

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , xviii, 110 p.
Series
Trita-KRV, ISSN 1100-7990 ; 11:09
Keyword [en]
Gas turbine, heat transfer, CFD, Serpentine passages
National Category
Energy Engineering
Research subject
SRA - Energy
Identifiers
URN: urn:nbn:se:kth:diva-47161ISBN: 978-91-7501-147-9 (print)OAI: oai:DiVA.org:kth-47161DiVA: diva2:454498
Public defence
2011-11-30, Sal M2, Brinellvägen 64, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
StandUp
Note
QC 20111108Available from: 2011-11-08 Created: 2011-11-07 Last updated: 2011-11-08Bibliographically approved
List of papers
1. Flow structure, heat transfer and pressure drop in varying aspect ratio two-pass rectangular smooth channels
Open this publication in new window or tab >>Flow structure, heat transfer and pressure drop in varying aspect ratio two-pass rectangular smooth channels
Show others...
2012 (English)In: Heat and Mass Transfer, ISSN 0947-7411, E-ISSN 1432-1181, Vol. 48, no 5, 735-748 p.Article in journal (Refereed) Published
Abstract [en]

Two-pass channels are used for internal cooling in a number of engineering systems e.g., gas turbines. Fluid travelling through the curved path, experiences pressure and centrifugal forces, that result in pressure driven secondary motion. This motion helps in moving the cold high momentum fluid from the channel core to the side walls and plays a significant role in the heat transfer in the channel bend and outlet pass. The present study investigates using Computational Fluid Dynamics (CFD), the flow structure, heat transfer enhancement and pressure drop in a smooth channel with varying aspect ratio channel at different divider-to-tip wall distances. Numerical simulations are performed in two-pass smooth channel with aspect ratio W-in/H = 1:3 at inlet pass and W-out/H = 1:1 at outlet pass for a variety of divider-to-tip wall distances. The results show that with a decrease in aspect ratio of inlet pass of the channel, pressure loss decreases. The divider-to-tip wall distance (W-el) not only influences the pressure drop, but also the heat transfer enhancement at the bend and outlet pass. With an increase in the divider-to-tip wall distance, the areas of enhanced heat transfer shifts from side walls of outlet pass towards the inlet pass. To compromise between heat transfer and pressure drop in the channel, W-el/H = 0.88 is found to be optimum for the channel under study.

Keyword
Gas turbine, Two-pass channel, Heat Transfer
National Category
Energy Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-47339 (URN)10.1007/s00231-011-0926-1 (DOI)000303399500002 ()2-s2.0-84861528237 (Scopus ID)
Funder
StandUp
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QC 20120628

Available from: 2011-11-08 Created: 2011-11-08 Last updated: 2017-12-08Bibliographically approved
2. On flow structure, heat transfer and pressure drop in varying aspect ratio two-pass rectangular channel with ribs at 45 degrees
Open this publication in new window or tab >>On flow structure, heat transfer and pressure drop in varying aspect ratio two-pass rectangular channel with ribs at 45 degrees
Show others...
2013 (English)In: Heat and Mass Transfer, ISSN 0947-7411, E-ISSN 1432-1181, Vol. 49, no 5, 679-694 p.Article in journal (Refereed) Published
Abstract [en]

To increase the thermal efficiency of gas turbines, inlet temperature of gas is increased. This results in the requirement of cooling of gas turbine blades and vanes. Internal cooling of gas turbine blades and vanes is one of several options. Two-pass channels are provided with ribs to enhance heat transfer at the expense of an increased pressure drop. The space in the blade is limited and requires channels with small aspect ratios. Numerical simulations have been performed to investigate heat transfer, flow field and pressure loss in a two-pass channel equipped with 45A degrees ribs with aspect ratio (W-in/H) equal to 1:3 in the inlet pass and 1:1 in the outlet pass with both connected together with a 180A degrees bend. The results are compared with a higher aspect ratio channel (W-in/H = 1:2, inlet pass). In the ribbed channel, a decrease in pressure drop was observed with a decrease in the aspect ratio of the channel. The smaller aspect ratio channel not only allows using more cooling channels in the blade, but also results in more heat transfer enhancement. The divider-to-tip wall distance (W-el) has influence on the pressure drop, as well as on the heat transfer enhancement at the bend and outlet pass. Heat transfer decreases with decrease in aspect ratio of the inlet pass of the two-pass channel. With increase in divider-to-tip wall distance, heat transfer tries to attain a constant value.

Keyword
Gas Turbine, Heat Transfer, Numerical Study, Two-Pass Ribbed Channel.
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-47342 (URN)10.1007/s00231-013-1111-5 (DOI)000317854500006 ()2-s2.0-84878323147 (Scopus ID)
Note

QC 20130523. Updated from submitted to published.

Available from: 2011-11-08 Created: 2011-11-08 Last updated: 2017-12-08Bibliographically approved
3. Validation and Analysis of Numerical Results for a Two-Pass Trapezoidal Channel With Different Cooling Configurations of Trailing Edge
Open this publication in new window or tab >>Validation and Analysis of Numerical Results for a Two-Pass Trapezoidal Channel With Different Cooling Configurations of Trailing Edge
2012 (English)In: Journal of turbomachinery, ISSN 0889-504X, E-ISSN 1528-8900, Vol. 135, no 1, 011027- p.Article in journal (Refereed) Published
Abstract [en]

High inlet temperatures in a gas turbine lead to an increase in the thermal efficiency of the gas turbine. This results in the requirement of cooling of gas turbine blades/vanes. Internal cooling of the gas turbine blade/vanes with the help of two-pass channels is one of the effective methods to reduce the metal temperatures. In particular, the trailing edge of a turbine vane is a critical area, where effective cooling is required. The trailing edge can be modeled as a trapezoidal channel. This paper describes the numerical validation of the heat transfer and pressure drop in a trapezoidal channel with and without orthogonal ribs at the bottom surface. A new concept of ribbed trailing edge has been introduced in this paper which presents a numerical study of several trailing edge cooling configurations based on the placement of ribs at different walls. The baseline geometries are two-pass trapezoidal channels with and without orthogonal ribs at the bottom surface of the channel. Ribs induce secondary flow which results in enhancement of heat transfer; therefore, for enhancement of heat transfer at the trailing edge, ribs are placed at the trailing edge surface in three different configurations: first without ribs at the bottom surface, then ribs at the trailing edge surface in-line with the ribs at the bottom surface, and finally staggered ribs. Heat transfer and pressure drop is calculated at Reynolds number equal to 9400 for all configurations. Different turbulent models are used for the validation of the numerical results. For the smooth channel low-Re k-e model, realizable k-e model, the RNG k-ω model, low-Re k-ω model, and SST k-ω models are compared, whereas for ribbed channel, low-Re k-e model and SST k-ω models are compared. The results show that the low-Re k-e model, which predicts the heat transfer in outlet pass of the smooth channels with difference of +7%, underpredicts the heat transfer by -17% in case of ribbed channel compared to experimental data. Using the same turbulence model shows that the height of ribs used in the study is not suitable for inducing secondary flow. Also, the orthogonal rib does not strengthen the secondary flow rotational momentum. The comparison between the new designs for trailing edge shows that if pressure drop is acceptable, staggered arrangement is suitable for the outlet pass heat transfer. For the trailing edge wall, the thermal performance for the ribbed trailing edge only was found about 8% better than other configurations.

Keyword
Gas turbines, Pressure drop, Reynolds number, Rhenium, Secondary flow, Turbulence models
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-47348 (URN)10.1115/1.4006534 (DOI)000312989900027 ()2-s2.0-84868158525 (Scopus ID)
Funder
StandUp
Note

QC 20121210

Available from: 2011-11-08 Created: 2011-11-08 Last updated: 2017-12-08Bibliographically approved
4. Design of internally cooled trailing edge at engine similar conditions- A conjugate heat transfer problem
Open this publication in new window or tab >>Design of internally cooled trailing edge at engine similar conditions- A conjugate heat transfer problem
2012 (English)In: ASME Turbo Expo 2012: Turbine Technical Conference and Exposition, ASME Press, 2012, 1357-1372 p.Conference paper, Published paper (Refereed)
Abstract [en]

Gas turbines are operated at elevated temperatures as the thermal efficiency of the gas turbine is directly linked to the turbine inlet gas temperature. The different regions of the turbine blade require different means of cooling. This paper presents different designs of the two-pass trapezoidal channel which represents the trailing edge of a real engine. Engine similar boundary conditions are applied and conjugate heat transfer method is used to predict the wall temperatures. The aim is to design a cooling channel that through use of steam can reduce wall temperatures to below a target value while maintaining minimal pressure drop. The variations in design of a smooth two-pass channel were made to achieve the design target. These variations included installation of ribs at the walls, tapered divider wall, tilted divider wall and L-shaped divider wall to promote fluid impingement on the trailing wall. The results suggest that a channel with staggered ribs at the outlet pass, a tilted divider wall and impingement at the corner is the best arrangement for reducing wall temperatures below the target value; however, it has low overall aerothermal performance due to high pressure losses. A similar channel without impingement can yield acceptable results if a thermal barrier coating is applied or if a small corner of the tip-trailing edge is truncated to reduce material volume. This modification though can improve the thermal performance of the channel, is to result in higher profile and aerodynamics losses.

Place, publisher, year, edition, pages
ASME Press, 2012
Keyword
Conjugate heat transfer, Conjugate heat transfer problem, Elevated temperature, Thermal efficiency, Thermal Performance, Trapezoidal channels, Turbine inlet gas, Wall temperatures
National Category
Energy Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-47349 (URN)10.1115/GT2012-68557 (DOI)2-s2.0-84881134587 (Scopus ID)978-079184470-0 (ISBN)
Conference
ASME Turbo Expo 2012: Turbine Technical Conference and Exposition, GT 2012; Copenhagen; Denmark; 11 June 2012 through 15 June 2012
Funder
StandUp
Note

QC 20111108

Available from: 2011-11-08 Created: 2011-11-08 Last updated: 2014-04-29Bibliographically approved

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