AE-Seals

Gas turbines work by manipulatingthe flow of gas which is central to theefficiency of the system. Gas pathsealing is therefore a fundamental areaof interest when seeking improvementsin the efficiency and performance ofaircraft. By reducing the level of leakagefrom the gasflow, efficient sealing helps

retain the energy introduced to the gasstream. Leakage occurs in a number ofareas and is both accidental, such asbetween pressure stages, and intended,for example secondary air systems.These secondary airflow systemsinclude internal engine cooling flows,external bleed air for accessories and

Sealing technologies –signed, sealed and deliveringemissions savings

A significant level of development in the area of aviation sealing technology has occurred in recent years.Professor Sir David King, Dr Oliver Inderwildi and Dr Chris Carey of the Smith School of Enterprise and theEnvironment look at the advantages that new sealing technology can offer in order to reduce aviation’senvironmental impact.

cabin feed as well as unwanted leakage.Reducing secondary flow by precisecontrol has the potential to result in afour to six per cent increase in powerand a reduction in specific fuel con-sumption (SFC) by between three andfive per cent. There are a numberof key sealing locations in gas turbines:the fan and compressor shroud, thecompressor interstage and dischargeseals, the turbine interstage and bearingseal locations.

Sealing in the energy As gas turbines are the main source of propulsion in aviation, developmentswhich improve them can produce a significant reduction in fuelconsumption and consequently in CO2 emissions. Hence, aviation’s impacton the environment can be mitigated by advances in turbine technology.

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Figure 1: schematic of approximate seal locations and example sealing technologies in turbine engines.

AVE08_Sealing smith school:Layout 1 10/8/09 09:26 Page 44

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The Smith School of Enterprise and the Environment is a uniqueinterdisciplinary hub where academics from around the worldwork with the private sector and government to pioneer solutionsto the major environmental challenges of the 21st century.

Sir David King

Dr Oliver Inderwildi

Dr Chris Carey

Keepin’ it tight!Applicable seal technology depends onthe location which requires sealing.Figure 1 indicates examples of seallocations and the types of sealscurrently used. The following sectionidentifies current technology used aswell as future techniques which couldreduce leakage rates for outer air sealsand inter stage locations.

Outer air sealsOuter air seals are located between thetip of the blades and the casing orshroud and are used to seal the cavityover the rotating blade. The environ-mental conditions at the tip seallocation are harsh, reaching temper-atures in excess of 1,400°C, pressuresover 4,000 kPa, and high surfacespeeds over 600 ms-1. The presence ofunburnt fuel and ingested debris,such as sand, complicates the sealingprocess, as do changes in partdimensions due to mechanical andthermal loading. This causes a variationin the clearance or gap between theblade tip and shroud.

However, as engines age andturbine blades become worn, theclearance between blade and shroudcan increase up to 50mm, leading to aloss of thrust. To overcome this, anincreased throttle setting is requiredwhich increases fuel use. For each 1mmof additional blade tip clearance, fueluse increases by one per cent. Theincreased distance between the bladetip and the shroud accounts for bet-ween 80 and 90 per cent of engineperformance degradation.

Abradable sealsThe current method of sealing used atthe outer air seal location is abradablecoatings on the shroud. Materials usedfor this are dependent on the tem-perature of the location. Low tem-perature locations, such as fan tipsealing, use epoxy systems. Hightemperature locations, such as turbineblade tip sealing, use porous ceramics.The coating allows for rubbing of theshroud by the blades during service,whilst providing the minimumclearance. Without abradable shroudsthe clearances when the engine is cold

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must be large enough to ensure bladerub does not occur. The surface mustalso remain smooth to reduce aero-dynamic losses. Seals come in threemain types, abradable (sintered orsprayed porous material), compliant (aporous material that compacts on bladerub), or low shear strength (sprayedaluminium).

Advanced sealingtechniquesA number of methods are beinginvestigated to reduce blade tipclearance. The two main ways of alter-ing this are the use of actuation andregeneration of the abradable seal.Both techniques can be identified bythe control architecture used and themethod of actuation. The method ofcontrol can be either active, where anexternal control method is used, suchas sensors, or passive where theenvironment within the turbine, such asthe temperature, induces the necessaryresponse from the system. The methodof actuation can be mechanical,thermal or pneumatic.

Thermal actuationActive thermal control is the use ofselective cooling of the turbine caseduring operation, resulting in areduction in blade tip clearance. Anumber of commercial engines, such asPW4000 and GEnx (see figure 2), usethis method. Thermal systems arerelatively slow and cannot be usedduring transient events, such as take-


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off and reacceleration and are thereforeonly usually used at cruise conditions.The main area of development in thesesystems is in producing thermally effici-ent materials which could provide amore responsive system. Faster res-ponse of the system would allow theuse of tip clearance control throughoutthe mission, rather than just at cruise.Passive thermal control uses the materialproperties and engine operatingtemperature to alter the blade tipclearance. The stators (fixed turbineblades which re-align the airflow after arotor stage) consist of a combination ofmaterials with varying co-efficients ofthermal expansion. This means thatdifferent parts of the engine contractand expand at differing rates withoutexternal control inputs. While accurateand reliable, active thermal systems onlyprovide optimum clearance for theminimum clearance conditions, such astake off and manoeuvring. They do notcapitalise on the longer stabilised cruise conditions of the flight profilemeaning the overall benefits for a givenflight plan are minimal. Passive thermalsystems are not currently in use in anysystem.

Mechanical actuationAs mechanical actuation relies on amechanical movement to vary the tipclearance it doesn’t lend itself to passivecontrol. Motive power required to closethe gap is supplied by hydraulic, electro-magnetic and magnetic systems. Thesignificant challenge specific to thismethodology is the development ofactuation systems which can withstandthe harsh environment of a gas turbine.In addition, secondary sealing systemsfor the moveable shroud and theweight and the complexity of thenecessary mechanisms involved are ofconcern. One way of bypassing some ofthese problems is the use of shapememory alloy (SMA) actuators. Initialstudies by NASA into SMA compressorcase compensator rings show thatcopper-aluminium-nickel actuator ringscan reduce the blade tip clearancefrom 5mm to 1mm. Calculationssuggest that the full application of thistechnique through the axial compressorcould increase engine efficiency by 0.7per cent but would require a high tem-perature SMA. Current indications arethat SMA actuators will find their wayinto the next generation of turbineengine providing the reliability criteriaare met.

Pneumatic actuationActive pneumatic systems use thepressure generated by the compressorstage to move sealed shroud sections toreduce the blade tip clearance. Thesesystems are very sensitive to pressurebalancing and suffer from high cyclefatigue. They also require a great dealof system pressure which reduces theoverall efficiency of the turbine,reducing the efficiency increaseprovided by the gap reduction.

Passive pneumatic systems are drivenby engine-generated gas pressures orhydrodynamic effects. They coverconcepts such as floating shroudsegments and blade tip cooling airdischarge. They rely on a very limitedblade tip area, extremely accuratepositioning, tight alignment tolerances,friction levels and secondary seal hang-up that make these systems veryunattractive to engine manufacturers.The flexible bellows which deflect theshroud are also subject to high cyclefatigue. Pneumatic actuation will haveto overcome several problems before itcan be considered for in-flight use.

RegenerationOther systems use a rub-avoidancemethodology, but regeneration systemsaim to regenerate or restore the tip seal.They can be either passive or active incontrol and produce regeneration of theseal where it has been abraded by bladecontact. An example of a restorationconcept is mechanical restoration whereground crew can alter the clearancesthrough the use of linkages. Thedisadvantage of this is the additionalweight and volume due to themechanical systems as well as theaccessibility of the mechanism forroutine adjustment by ground crew.

Regeneration of the seal usesspecially engineered material systemsthat undergo growth because ofthermal, chemical or electrical inter-actions. They remove the requirementfor secondary sealing and can use theenvironment for passive control, re-ducing the complexity of the systemand therefore the overall weight. Theycan be actively controlled by usingchemical reactions that are progressedor retarded by electrical stimulus.

Figure 2: GEnx engine illustrating thermal tip clearance control mechanism on turbine stage.


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reducing the thermal expansion effectson the clearance of the seal. Thesesavings relate to a 1.8 per cent increasein power and a 1.3 per cent increase inefficiency.

Problems with these systems includeuniformity of growth, strength ofgrown material and growth rate, aswell as excessive blade-tip wear causedby the constant abrasion.

Interstage locationsThe current technology of theselocations is mainly labyrinth type seals.Developments in this area aresignificant and varied due to thenumber of locations within the enginewith individual demands. Advancedseals will generally be expected tooperate in higher temperatureenvironments, with greater pressuresacross the seal, in order toaccommodate higher pressure ratios aswell as operating at higher speeds.

Labyrinth sealsLabyrinth seals are the most commonflow path seals applied to turbineengines. They consist of several knifeedges, typically five, in close clearance(0.25 to 0.50mm) in a number ofconfigurations (figure 3). Labyrinthseals rely on controlled leakage acrossthe seal. This is driven by the pressuredifferential between the seal ends. Thedesign of the seal forces the flow toseparate at the knife edge causing aloss of kinetic energy and pressure fromthe gas flow. This is repeated in thenext cell and so on until the gas leak-age reaches the seal exit. Currentlabyrinth seal applications can with-stand temperatures as high as 700°Cand pressure differences of up to3,000 KPa.

However, vibration of the shaft cancause the blades to bite or rub into theshroud, increasing the flow betweencells and reducing the sealing efficiency.Current advances in labyrinth sealsfocus on the use of abrasive knives andsacrificial ceramic shrouds to reduceleakage rates while allowing forrubbing to occur. The use of sacrificialabrasive layers results in a reduction ofthe running clearance by 90 per cent,allowing for an overall improvement inengine efficiency by one per cent. Theceramic coating also reduces theunderlying metals’ temperature by 20per cent and thermal displacement by34 per cent. This has the effect of

Brush sealsBrush seals consist of a dense pack ofbristles sandwiched between a face andbacking plate (figure 4). A significantbenefit of brush seals over labyrinthseals is the ability to accommodatemovement of the shaft and return tosmall running clearances. It has beenreported that brush seals have 5-10 percent the leakage rates of similarlabyrinth seals with a 34 per centreduction in footprint.

However, they can still be subject tocatastrophic wear at the brush-shaftinterface due to excessive thermalloading. The clearance, and thereforeleakage, rates of brush seals are relatedto the rotational velocity of the shaft.As at high velocities, hydrodynamicforces can push the bristles away fromthe shaft. Bristles material currently

Figure 3: examples of labyrinth seal configurationa) straight, b) angled, and c) stepped.

includes cobalt based super-alloys whichcan withstand temperatures of up to700°C and rotational speeds of 300 ms-1. In order to meet the demands ofadvance engine designs, nickel-based

super alloy will be used to allowmaximum working parameters of800°C and 600 ms-1.

Developments of brush seals includeshoed brush seal, where pads are spotwelded on to the end of the brushes,giving a closer and more consistent faceto reduce leakage rate further. Otheradvancements in bush seals includehybrid brush seals which are an effort toreduce brush wear, and the use ofceramic fibres, such as silicon carbideand aluminium oxide, to reduce fibrewear.

The stiffness of the wire limits thepressure differential across brush seals,which is currently 670 kPa per stage.New stiffer bristle materials such asceramic and super alloys will increasethis value to approximately 950 kPa perstage. Leaf seals overcome this byreplacing the bristle with plates, withgreater axial strength. This allows alarger pressure differential across theseal. Laminar leakage between platesis low and hydrodynamic lift preventscontact on operation thus reducingwear. A 66 per cent reduction inleakage rates has been reportedcompared to labyrinth seals. Anothervariation of this is the wafer seal whichdiffers in the root attachment methodand moment of inertia but operates onthe same principles.Figure 4: detail of brush seal.


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Film riding or foilsealsFilm riding seals consist of thin foilmetal fingers, sometimes spring loaded,which are designed to operate withoutcontacting the shaft by the use ofhydrodynamic forces (figure 5). Thisgreatly increases seal life, keeping theengine within performance character-istics for longer resulting in better fueleconomy. These seals can withstandhigher pressures (5,500 kPa) andtemperatures (800°C) than other seal-ing methods. When compared withlabyrinth seals, a 66 per cent reductionin leakage has been reported for filmriding seals. While most designs areface seals, some turbine rim sealapplications have also been suggestedas possible. Problems with this type ofseal include dust ingestion and aircraftmanoeuvring loads which can causecontact between seal faces.

Finger sealsFinger seals consist of multiple fingersor flexural elements aligned around thecircumference of the shaft (figure 6).The finger acts as a cantilever allowingradial movement of the seal face andmovement of the rotor. The seals areapplied in offset groups, reducing theleakage through the fingers. Fingerseals are designed to move radiallyinward, towards the rotor whenpressure differentials develop. Initialstudies show that these seals have alonger average life than brush seals,with 60-70 per cent of the flow factor.

ConclusionOuter air sealsThe degradation of outer air seals is asignificant factor in the reduction ofengine performance. While abradableshrouds and thermal contraction ofturbine casing have increased efficiency,improvements so far are limited and arehindered by slow reaction times and alack of suitable measurement systems.Using any of the systems discussed hereto reduce the blade tip clearance by1mm results in a one per cent saving inSFC. With U.S. aviation expected to useapproximately 19 billion gallons ofaviation fuel in 2009, this equates to asaving of 200 million gallons of fuel,and subsequent savings in emissions, ata cost saving of nearly $400m in oneyear in the U.S. alone at current prices.Another issue is the development ofmeasuring techniques which can workreliably in the extreme environmentswithin jet turbines.

There are a number of significantdevelopments in the pipeline in thearea of gas flow sealing and whenapplied, either in full or in part in thenext generation of turbine engines,they will help manufacturers reach thechallenging targets for fuel-burnreduction, thrust increase and increasedservice life.

Interstage sealsThe benefits of advanced sealtechnology are significant. Studies onan AE3007, a modern 40kN thrustregional engine, showed a reduction in

Figure 5: foil seal schematic.

SFC (1.96 per cent) and increase inthrust (4.93 per cent) by implementingbrush seals at two turbine interstagelocations. The use of more advancedfilm riding seals in place of the brushseals resulted in a 2.62 per centreduction in SFC and a 6.95 per centincrease in thrust.

The applications of low leakage film-riding seals can reduce the cycle airused to purge high pressure turbinecavities by 50 per cent. This saving involume of airflow can be used to holdthe rotor inlet temperature constantallowing for a SFC drop of 0.9 per cent,a thrust increase of 2.5 per cent or amission fuel burn drop of 1.3 per cent.This 0.9 per cent drop in SFC couldresult in a saving of more than 500kgof fuel per hour on a CF6 80C2-B1F-powered 747, equating to nearly 40tonnes over a 75,000 hour life of anaircraft.

Another benefit of advanced sealingtechnologies is the ability to increasethe pressure ratio of turbines.Increasing the pressure ratio results inmore of the thermal energy beingconverted to jet speed, thus increasingthe engines’ efficiency and lowering theSFC. However, increasing the pressureratio causes a greater strain on thecomponents involved, with seals beingthe weaker link in the chain. Increasingengine pressure ratios has been shownto cause a decrease in overall engineweight. Advancements in sealtechnology can be made with muchsmaller investments; 20-25 per cent ofdevelopment costs for compressor orturbine developments.

Figure 6: detail of finger sealsshowing offset.


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