“ACTIVE CORE” – A KEY TECHNOLOGY FO R MORE …

26TH INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES

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Abstract

The continuous growth of air traffic and raisedenvironmental awareness put increasingpressure on aero engine manufacturers toreduce fuel burn and emissions. NEWAC is anew integrated program of the European Unionwith focus on innovative core engine concepts toachieve this task. Within NEWAC, four differentcore engine configurations will be investigated:

• Intercooled recuperative aero engine• Intercooled core• Active core• Flow controlled core

These configurations are complemented by thedevelopment of innovative, low emission com-bustor technologies.

This paper presents the overall structure ofNEWAC and focuses on the sub-project “ActiveCore”. In this sub-project, the emphasis lies onactive systems in the core engine which alterand improve current thermodynamic cycles.Two distinct active technologies are presented,one using active cooling air control for the highpressure turbine and the other using activeelements in the high pressure compressor.Both technologies are presented and a snapshotof the ongoing research and current results onboth topics is given.

Abbreviations

AEROHEX Advanced Exhaust Gas RecuperatorTechnology for Aero-EngineApplications

ACAC Active Cooling Air Cooling

ACARE Advisory Council of AeronauticResearch in Europe

ACC Active Clearance Control

ASC Active Surge Control

BPR Bypass Ratio

CFD Computational Fluid Dynamics

CLEAN Component Validator forEnvironmentally Friendly Aero-Engine

CO2 Carbon Dioxide

EEFAE Efficient and Environmentally FriendlyAero- Engine

EU European Union

HP High Pressure

HPC High Pressure Compressor

LDI Lean Direct Injection

LPP Lean Premixed Prevaporized

NEWAC New Aero Engine Core Concepts

NOx Nitrogen Oxides

OPR Overall Pressure Ratio

PERM Partial Evaporation and Rapid Mixing

SFC Specific Fuel Consumption

SP Sub-Project

SRA Strategic Research Agenda

TERA Technoeconomic and EnvironmentalRisk Analysis

T/O Take-off

TiAl Titanium-Aluminum Alloys

VITAL Environmentally Friendly Engine

“ACTIVE CORE” – A KEY TECHNOLOGY FOR MOREENVIRONMENTALLY FRIENDLY AERO ENGINES

BEING INVESTIGATED UNDER THE NEWACPROGRAM

Stephan Bock*, Wolfgang Horn*, Jörg Sieber**MTU Aero Engines

Keywords: Active HPC Control NEWAC

Bock S, Horn W, Sieber J.

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1 Introduction

With the increasing amount of air traffic, thereduction of its environmental impacts isbecoming ever more important. Over the last 40years, aero engines have already improvedtremendously in fuel consumption, noise and airpollutant emissions while, at the same time,becoming more reliable. However, with theincreasing evidence for global warming, theconstant rise in air traffic volume and theeconomic pressures of high fuel prices, evenmore improvement is needed.

In 2001, research for cleaner, moreefficient aero engines was intensified after the“Vision for 2020” report was published byACARE which defined ambitious goals in theareas of NOX, noise and CO2 reduction. In orderto reach these goals, several research projectshave been launched.

One major project, the EU integratedproject VITAL [1], is focusing on technologiesfor low pressure system improvements to reduceCO2 and noise. To further reduce theseemissions and also reduce the NOX production,however, complementary research is needed onthe technology for high pressure componentsand combustors. This research is beingperformed in the EU integrated program forNEW Aero engine Core concepts (NEWAC).

2 Overview of the Activities Within NEWAC

NEWAC has set forth its goals of developingtechnologies for a reduction of 6% in CO2

emissions and 16% in NOX emissions.

Fig. 1. CO2 reduction target for NEWAC

Fig. 1 illustrates the reduction of fuelconsumption achieved over the last 12 years andthe 2010 targets for NEWAC as furtherimprovements on EEFAE results. In order to

reach these goals, four major core concepts arebeing evaluated and additional research iscarried out in the development of improvedcombustors. The four core concepts as depictedin Fig. 2 are:

Fig. 2. Engine configurations investigated under NEWAC

• Intercooled Recuperated CoreThis concept exploits the heat of the engineexhaust gas and maximizes the heat pick upcapacity of the combustor inlet air byintercooling in front of the HP compressor.

• Intercooled CoreThe introduction of an intercooler to a coreconfiguration allows for very high overallpressure ratios. It reduces the compression workfor such cycles and improves fuel burn.

• Active CoreThe active core incorporates actively controlledsystems such as surge control, tip clearancecontrol and cooling air cooling. These activesystems enable the engine to be adjusted to thecurrent flight condition and allow for thereduction of design margin that is needed inconventional systems to cover various engineconditions.

• Flow Controlled CoreHigh BPR direct driven turbofan enginesrequire a compact HP compressor with veryhigh pressure ratio which has to compensate forthe low booster pressure ratio. Therefore, flowcontrol technologies are being investigatedunder NEWAC that strongly increase efficiencyand stall margin which helps in the specific fieldof aerodynamically very highly loaded HPcompressors.

These four core concepts are beinginvestigated in the dedicated NEWAC sub-projects (SPs) SP2 to SP5. To complement thiscore architecture research, additional research isongoing in SP6 to improve upon currentcombustor technology. Here, especially “lean”

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“ACTIVE CORE” - A KEY TECHNOLOGY FOR ENVIRONMENTALLY FRIENDLY AERO ENGINES

combustion, which operates with an excess ofair, is investigated to significantly lower flametemperatures and, therefore, greatly reduce NOX

formation in order to reach the 16% targetshown in Fig. 3.

Fig. 3. NOx reduction targets for NEWAC

The research comprises three major conceptsthat cover a wide range of overall pressureratios in the engine. These concepts are

• Lean Pre-Mixed Pre-vaporized (LPP)• Partial Evaporation & Rapid Mixing

(PERM)• Lean Direct Injection (LDI)

The evaluation of the effects of all thetechnologies investigated in the different coreconcepts mentioned above and the combinationwith combustor technology is carried out by thededicated sub project SP1. Fig. 4 shows theentire sub project structure of NEWAC and theconnecting role of SP1.

Fig. 4. Structure of NEWAC

This engine integration task started in 2006 withthe definition of the requirements and objectivesfor all other tasks by using whole engineperformance cycle data. It will then compare,assess and rank the benefits of the advanced

concepts, ensuring consistency of the results andmonitoring progress towards the ACAREtechnical and economic objectives. The newengine designs will be assessed in typicalaircraft applications and for relevant flightmissions as illustrated in Fig. 5.

Fig. 5. Illustration for the approach to whole engineevaluation with different aircraft and engineconfigurations as well as mission and performanceparameters.

In this context, NEWAC will also adapt andfurther develop the software tool “TERA 2020”(Techno-economic and Environmental RiskAnalysis), previously created under the VITALprogram, in order to compare the environmentaland economic impacts of the new designs.

Fig. 6. Approach for whole engine optimization withinTERA 2020

Parameters that are included in “TERA 2020”include performance, weight, engineinstallation, NOX and CO2 emissions, noise, fuelcosts, maintenance cost and aircraft flight pathand altitude. This techno-economic model willidentify engines with minimum global warmingpotential and lowest cost of ownership as shownin Fig. 6.


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3 Research Performed within SP4 – “ActiveCore”

The sub project SP4, the “Active Core”, is beinglead by Germany’s MTU Aero Engines. Itsconcept follows two major strategies. First, itaims at developing new active systems toimprove stability and efficiency of the highpressure compressor and second, it tries toimprove the overall core efficiency by coolingof high pressure turbine cooling air in order toimprove the cooling efficiency and reduce therequired cooling air mass flow. Both strategiescomplement each other, but can also be usedseparately if desired.

3.1 Research on smart HPC technologies

Today’s compressor designs require highefficiency levels to provide low fuelconsumption and high aerodynamic loading forlightweight and cost effective engine designs.At the same time, safety and reliability demandsrequire the provision of sufficient surge marginfor the compressors. In the high pressurecompressor, the handling of tip leakage flowsand the off-design behavior are the majorchallenges to achieve these ambitious targets.NEWAC is focusing on the improvement ofcurrent designs by the means of active control.Compressor efficiency and stability will beoptimized by a reduction of tip leakage lossesdue to an innovative radial clearancemanagement while the compressor stability willbe enhanced with active surge control (ASC)through air injection. Additional aerodynamicstability offers new improvement potentials,such as a higher overall pressure ratio, reducedblade count or even fewer compressor stages.This opens up opportunities for improvement ofSFC as well as cost and weight reductions.

Fig. 7. Schematic of closed loop control system for ACCand ASC

3.1.1 Active Clearance ControlTip clearance, the radial gap between rotatingblades and the stationary compressor casing,varies significantly during different operatingconditions due to centrifugal forces on the rotorand different thermal expansions of airfoils,disks and the casing as shown in Fig. 8.

Fig. 8. Tip clearance behavior over time during anacceleration for take-off (T/O)

To avoid any contact between rotating andstationary parts, the radial clearance has to bedesigned with enough margin to cover anyworst case scenario. Another approach consistsof the use of abradable liners which allow a rubof the blades into the casing. Both approacheslead to larger clearances than necessary duringmost of the flight and especially during cruise,leading to higher fuel consumption and lowersafety margin. Aircraft engines which have beenin service for a long time period often sufferfrom large clearances due to worn airfoils andliners. Active clearance control systems (ACC)can help overcome this situation. They targetoptimum performance and operability over thefull flight envelope by adjusting the radial

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casing position to a minimal gap as shown bythe bottom curve in Fig. 8. Currently, different“conventional” ACC systems are in use, bothfor turbines and compressors [2,8].

These conventional systems are slowacting and of the “open loop” type without anyfeedback signals, which restricts the benefit to alarge extent. NEWAC is evaluating innovative“closed loop” systems, which promise betterperformance [3]. A thermally and amechanically actuated ACC compressor casinghave been studied to provide the lowestclearance levels in any flight condition. Thetechnology is applied to the rear stages of amodern high pressure compressor as thesestages are more susceptible to tip clearanceproblems. This is in part due to larger thermalexpansion resulting from higher gastemperatures, but also because of the higherimpact on the aerodynamic losses due to smallerblades in the last stages.

In order to control the tip clearance, twoconcepts have been investigated underNEWAC. The first one is a thermal systemusing customer bleed air from front stages tocool the rear stages, the second is a fast acting,mechanical system using hydraulic actuatorsand variable liner segments (Fig. 9).

Fig. 9. Schematic of hydraulically actuated mechanicalACC with adjustable liner segments.

Both systems were compared with respect toefficiency improvement, surge margin gain,weight, cost, safety, reliability and ultimatelyoverall fuel burn benefit. The results showedthat the thermal system, while beingsignificantly simpler and inherently morereliable, showed only a small benefit. One majorreason for this small benefit lies in the slowresponse time of such a system, that makes itimpossible to react to fast transient operating

conditions and, therefore, still requires relativelylarge tip clearances in the HPC.

Therefore, the more promising – albeitcomplex - technology of a mechanical ACCsystem was chosen to be investigated in moredetail. Currently, such a system is in the detaildesign phase and prototype testing will start in2009. The technological challenges for such asystem lie in the hot environment, the need foralmost complete elimination of leakage and play(caused by tolerances, thermal expansion andwear) as well as the weight and size of theoverall system. These are topics that will beaddressed and investigated in a series ofcomponent tests that pave the road for aprototype test of a complete ACC system on arig towards the end of the program.

In order to create a “closed loop” system,sensors have to measure the actual tip clearanceand feed their data into a control loop thatactuates the clearance control system. Onepartner, Vibrometer of Switzerland, has thededicated task to study and develop sensors forsuch active systems, that can withstand therigors of revenue service. These hightemperature gap sensors are currently beingtested and will become a cornerstone of anyactive tip clearance system.

3.1.2 Active Surge ControlAt low power settings, engine operability can becritical due to compressor stage matching andspecial flight or engine conditions. Especiallythe compressor front stages operate close totheir stability limit at low off-design conditions.Here, active flow control offers newpossibilities to enhance compressor stability.NEWAC is focusing on the application ofcontrolled air injection through the compressorcasing into the blade tip region as shown in Fig.10.

Fig. 10. Cross section through HPC tip injection system


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The high speed injection jet alters the three-dimensional flow field at the rotor tip,especially the behavior of the tip clearancevortex. Additionally, the injected jet reduces theblade incidence and thus unloads the rotor tip[4].

Fig. 11. Cross section through a flow field near the surgeline with tip injection (top) and reduced blockage close tothe casing wall as result of the injection (bottom).

Fig. 11 shows the injector flow (top) as well asthe effect of reduced blockage close to the outercasing wall (bottom) in a CFD-simulatedcompressor flow field.

The required mass flow is taken from adownstream compressor stage. Since thisrecirculation is associated with an overallefficiency penalty, only a small portion of thegas flow can be used for flow control [5].Modulated injection flows are generallyregarded as being more effective than steady-state blowing while obtaining a comparablestability enhancement. Their control andapplication, however, are more complex thanthose of steady state flows.

NEWAC SP4 currently plans to investigatethe effects of tip injection in two campaigns.The first campaign focuses on calibration of theapplied CFD tools and results in a first databasefor the use of tip injection in multi-stagecompressors. The second campaign will thenfine tune the injection parameters in order tomaximize the compressor surge margin. Thiscampaign will take into account, that the workbalance of the compressor is affected by tip

injection. Therefore, a completely new stagematching is likely to be needed. This new stagematching will be carried out with the use ofvariable injection parameters as well as the useof variable stator vanes.

Currently, the complex interaction betweeninjection flow, main flow field and downstreamstages is poorly understood. The NEWACprogram will help the consortium partners toincrease their knowledge of these effects.

In parallel to the active tip injectionexperiments, the research program also studiesadvanced multistage casing treatment designs asbenchmark and alternative solution for stabilityenhancement. This work is lead by the RWTHAachen University on a two stage compressorrig setup. Today’s third generation of casingtreatments, as shown in Fig. 12, is capable ofimproving the surge margin of a compressorstage almost without efficiency penalty [6].

Fig. 12. HPC casing treatment

The aerodynamic design and prediction is achallenging task, however, as the related flowmechanisms are highly complex and thecalculation of aerodynamic stability requires theuse of unsteady CFD codes over several stages.Fig. 13 shows the complexity and requiredresolution of such a computational domain.

Fig. 13. Computational domain (grid) for an unsteady,two stage casing treatment calculation.

Without tip injection: With tip injection:

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In addition to the goal of significant stabilityenhancement, the design point efficiency will beimproved as well by lowering the tip clearancesensitivity of the rear stages. Besides the impacton aerodynamics, design issues likeproducibility, weight and cost will be addressed.

The smart compressor technologyeliminates the need to design for worst caserequirements and enables an engineoptimization to individual flight missions.Active control offers the possibility of adaptingthe core engine to each operating condition and,therefore, allows an optimization of componentand cycle behavior. The compressor can bedesigned for better efficiency, higher powerdensity, lower size and weight. Finally,efficiency and surge margin reduction due todeterioration can be compensated, to a certaindegree, by adjusting the core to actualconditions.

All these benefits are being assessed andquantified under NEWAC. The expected SFCimprovement in the order of 2% has to beweighed against the drawbacks of thesetechnologies, including weight, cost orcomplexity. This overall evaluation is currentlybeing performed in various models oncomponent, module and whole engine level andis closely related to the work performed in SP1.

3.2 Research on active cooling air cooling

Modern jet engines with their high turbine entrytemperatures require effective cooling in ordernot to exceed the limits of the used materials. Intoday’s engines, the total cooling airconsumption is in the order of 25% whichrepresents a significant portion of the total airflow. While some of this air participates in thethermodynamic cycle, it still causes adeterioration of the overall engine performance.

A reduction of this amount of cooling aircan, therefore, have a very beneficial effect onengine performance as shown in Fig. 14.NEWAC SP4 is targeting such a reduction incooling air mass flow by introduction of activecooling air cooling (ACAC).

Fig. 14. Possible SFC reduction for differentimprovements in a turbofan engine

3.2.1 Concept of Active Cooling Air CoolingActive cooling air cooling is based on the factthat in current engine designs, the cooling ofcrucial components is not actively controlledand has to be sized for the hottest conditionsduring the take off and climb phase of a flightmission. During the less demanding cruiseportion of the flight, the cooling system deliversmore cooling than would be needed, whichlowers the efficiency of the engine.

With active cooling air cooling, thetemperature of the cooling air is controlled byintroduction of an additional heat exchanger inthe engine bypass to reduce the cooling airtemperature. The use of lower temperature airallows for the reduction of the total amount ofcooling air consumed to achieve the samecooling benefit on hot parts.

The flow through the heat exchanger,however, has to be actively controlled becausethe heat that is dumped in the bypass has anegative impact on the overall engineefficiency. Therefore, NEWAC is investigatingan active system in which the heat exchanger isused only during the relatively short period oftake off and climb and will be bypassed duringcruise, approach and landing.

3.2.2 General Concept StudyThe research that is performed on this systemcan be divided into four portions. The firstencompasses a general concept study in whichthe complete system is laid out and its benefitfrom an overall engine performance point ofview is being evaluated. In addition, keycomponents are being investigated in detail.


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Such key components include the HP turbineairfoils, that see increased thermal stress levelsor the piping and ducting for the cooling air inwhich pressure losses and heat pickup have tobe tightly controlled.

Fig. 15. Air flow scheme for active cooling air cooling

The general layout is depicted in a schematicdrawing in Fig. 15. The compressed air is bledoff the combustor case (1), lead through the heatexchanger matrix (2) and cooled by bypass air(3). A valve (4) allows partial or full bypassingof the heat exchanger in order to control thecooling air temperature. The cooled cooling airis then delivered to the compressor rear cone (5)and the HPT airfoils (6).

As mentioned above, one key question iswhether the HP turbine airfoils can withstandthe increased thermal stresses that result fromcooling with cooled air. In order to evaluatethis, a state-of-the-art turbine airfoil has beenevaluated with a multitude of optimized coolingconfigurations varying mass flows andtemperature levels. The basic approach for thisoptimization is illustrated in Fig. 16.

Fig. 16. Optimization process for a cooled HPT vanecluster

In a first step, the cooling air temperature waslowered which resulted in a cooler part. In asecond step, the cooling air flow was reduced

and redistributed to achieve a similartemperature and stress level distribution as inthe original part.

This strategy gives a first indication onhow much reduction in cooling air mass flow isviable, but it turns out that in order to get anoptimized solution, the cooling concept of theairfoil itself will have to be adjusted. This is stillsubject to further research, but current valuesfor cooling air savings range between 20% and30% which would then lead to an improvementof up to 2% SFC for the investigated enginecycle.

3.2.3 Heat Exchanger StudyBesides this general concept study, the secondportion of the research investigates the size,location, number and type of the heatexchangers needed to realize such a system. Thecurrent proposal for active cooling air coolinguses a heat exchanger with non circular tubesfor the matrix. This design has been proven toprovide low aerodynamic losses whilemaintaining a very small matrix volume whichreduces the integration problems for such a heatexchanger. The use of this type of tube shape,however, has not been investigated prior toNEWAC for the high pressures encountered inmodern jet engine cores.

Preliminary results show that the tubinggeometry used in CLEAN and AEROHEX [7]has to be optimized in order to guaranteesufficient creep life of the heat exchanger. Fig.17 shows a creep strain plot of a cross section ofsuch a noncircular tube. As expected, the highpressure inside the tube creates significantstresses in the “corners” that lead to reducedcreep life and require further optimization.

Fig. 17. Creep strain in cross section of heat exchangermatrix tube and optimization.

In addition to the work on the heat exchangermatrix, NEWAC is investigating the effects ofsuch a complex system on the reliability of the

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aero engine. Scenarios ranging from sensorfailure or the breaking of an air pipe to completeloss of a single heat exchanger due to foreignobject damage are being investigated with thegoal of guaranteeing safe operation and controlof the engine. This, however, typically requiresbuilt in redundancy which negatively impactsthe engine performance due to additionalweight.

As of today, these effects have beeninvestigated and show that a feasible design forACAC can be achieved without excessiveweight or cost penalties. One result of thisanalysis shows that any system with less thanthree independent heat exchangers is notdesirable because it leads to unacceptableeffects in case of a failure. With a systemhaving three or more heat exchangers incombination with an intelligent sensor andactuator layout, the rate of in flight engineshutdowns can be kept at a level comparable toconventional systems without ACAC.

3.2.4 Combustor Case InvestigationThe third portion of the task deals with thedesign of the combustor case for a system withactive cooling air cooling. The combustor case,besides being a highly loaded pressure vessel,has to be optimized for a smooth bleed ofcooling air, allow for efficient transport of thecooled air towards the HPT and reduce theinevitable heat pickup of the cooled air on itspath. These requirements, in addition to the everpresent tasks of cost and weight reduction,present a challenge for the design andmanufacturing of the combustor case.

Under NEWAC, Volvo Aero has theresponsibility of designing a combustor casethat will fulfill these requirements. The goal isto build and demonstrate a combustor case thatis optimized for active cooling air cooling. Thehigh temperature gradients and the number ofadditional bosses for external tubing to and fromthe heat exchanger will be investigated and asolution that still allows for economicmanufacturing is the goal for this task.

3.2.5 Weight offset investigationsEven though current results on cooling aircooling already show promising prospects,keeping the weight of the extra components

under control is one of the most challengingtasks. Therefore, WSK of Poland isinvestigating methods for weight reductionthrough the introduction of new materials,namely Titanium-Aluminum (TiAl) alloys in theHPC. These alloys are just now making theirdebut in commercial jet engines and very littleresearch has been done so far on how they canbe used in the HPC for airfoils or structuralcomponents. The results from this task willopen up more opportunities for the use of thispromising material in jet engines.

3.2.6 Rear Cone Cooling StudyThe fourth group of problems associated withactive cooling air cooling is dedicated tooptimization of the rear cone of the HPC. Thispart is one of the limiting components in today’sjet engines because it is highly loaded bypressure, temperature and centrifugal forces andcannot be cooled effectively in current turbomachines. The availability of cooled cooling airopens a new opportunity for improvement ofthis critical component as it allows for reducedmaterial temperatures. This can be used tochange the material or reduce the wall thicknessof the rear cone. However, machining such thincomponents presents a new challenge that willbe addressed under NEWAC. Manufacturing

Fig. 18. Comparison of measured Almen-probe intensities(top) with simulated values (bottom) for ultrasonic shotpeening of the inside of a rear HPC cone.


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technologies, such as ultrasonic shot peeningand electron multi-beam welding on complex,thin walled structures, are currently developedin order to address these challenges.Fig. 18 shows a first result of these efforts. Forthe cross section of a rear cone, relative Almen-probe intensity values (top) can be correlatedwith values from a simulation (bottom). Suchsimulations can now be used to predict peeningintensities and coverage for complex parts andhelp to reduce the amount of trial pieces toestablish a satisfactory shot peening process.

4 Summary

The research performed under NEWAC willlead to novel technologies that help to reducethe environmental impact of air traffic. Byfocusing the efforts on the core engine,NEWAC complements research performedunder the VITAL program and will contribute toreaching the ACARE “Vision for 2020” goals.

In SP4, the focus lies in the developmentand evaluation of active systems, such as activeclearance control, active surge control andactive cooling air cooling. These technologiesare currently being evaluated and first resultsare already available. The next step will then beto finalize designs and test all technologies.Several large scale rig tests are planned for 2009with results being available in 2010. These testsare expected to verify a 4% reduction in CO2

and will help to pave the way for the nextgeneration of environmentally friendly aeroengines.

5 Acknowledgements

The reported work is performed within the EUprogram NEWAC (FP6-030876). The authorswish to thank the EU for supporting thisprogram. The permission for publication isgratefully acknowledged.

6 References

[1] Korsia J, Spiegeler G. VITAL – European R&DProgramme for Greener Aero Engines. 18th ISABE,Beijing, China, 2007.

[2] Hennecke D.K, Trappmann K. Turbine Tip ClearanceControl in Gas Turbine Engines, AGARD, AGARD-CP-324, 1983.

[3] Neal P.F, Green W.E. Advanced Clearance ControlSystems. 8th ISABE, Cincinnati, USA, 1987.

[4] Deppe A, Saathoff H, Stark U. Spike-type stallinception in Axial Flow Compressors. 6th Conference onTurbomachinery, Fluid Dynamics and Thermodynamics,Lille, France, 2005.

[5] Horn W, Schmidt K.J, Staudacher S. Effects ofCompressor Tip Injection on Aircraft Engine Performanceand Stability. ASME Turbo Expo GT2007-27574,Montreal, Canada, 2007.

[6] Broichhausen K.D, Ziegler K.U. Supersonic andTransonic Compressors: Past, Status and TechnologyTrends. ASME Turbo Expo GT2005-69067, Reno-Tahoe,USA, 2005.

[7] Wilfert G, Kriegl B, Wald L, Johanssen O. CLEAN -Validation of a GTF High Speed Turbine and Integrationof Heat Exchanger Technology in an EnvironmentalFriendly Engine Concept, 17th ISABE, Munich, Germany,2005.

[8] Lattime S, Steinetz B. High-Pressure-TurbineClearance Control Systems: Current Practices and FutureDirections. Journal of Propulsion and Power, Vol. 20,No. 2, pp 302-311, 2004.

Copyright Statement

The authors confirm that they, and/or their company orinstitution, hold copyright on all of the original materialincluded in their paper. They also confirm they haveobtained permission, from the copyright holder of anythird party material included in their paper, to publish it aspart of their paper. The authors grant full permission forthe publication and distribution of their paper as part ofthe ICAS2008 proceedings or as individual off-printsfrom the proceedings.

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“ACTIVE CORE” – A KEY TECHNOLOGY FO R MORE …