Axial compressor

An animation of an axial compressor. Thestatic blades are the stators.

Axial compressors are rotating, aerofoilbased compressors in which the working flu-id principally flows parallel to the axis of ro-tation. This is in contrast with other rotatingcompresors such as centrifugal, axi-centrifu-gal and mixed-flow compressors where theair may enter axially but will have a signific-ant radial component on exit.

Axial flow compressors produce a continu-ous flow of compressed gas, and have the be-nefits of high efficiencies and large mass flowcapacity, particularly in relation to theircross-section. They do, however, require sev-eral rows of aerofoils to achieve large pres-sure rises making them complex and expens-ive relative to other designs (e.g. centrifugalcompressor).

Axial compressors are widely used in gasturbines, such as jet engines, high speed shipengines, and small scale power stations. Theyare also used in industrial applications suchas large volume air separation plants, blastfurnace air, fluid catalytic cracking air, andpropane dehydrogenation. Axial compressors,known as superchargers, have also been usedto boost the power of automotive reciprocat-ing engines by compressing the intake air,though these are very rare. A good exampleof an axial supercharger is the aftermarketLatham type built between 1955-65 whichwere used on hot rods and aircooled

Volkswagens at that time, but these didn’tcatch on.

DescriptionAxial compressors consist of rotating and sta-tionary components. A shaft drives a centraldrum, retained by bearings, which has anumber of annular aerofoil rows attached.These rotate between a similar number ofstationary aerofoil rows attached to a station-ary tubular casing. The rows alternatebetween the rotating aerofoils (rotors) andstationary aerofoils (stators), with the rotorsimparting energy into the fluid, and thestators converting the increased rotationalkinetic energy into static pressure throughdiffusion. A pair of rotating and stationaryaerofoils is called a stage. The cross-sectionalarea between rotor drum and casing is re-duced in the flow direction to maintain axialvelocity as the fluid is compressed.

Diagram of an axial flow compressor

DesignThe increase in pressure produced by asingle stage is limited by the relative velocitybetween the rotor and the fluid, and the turn-ing and diffusion capabilities of the aerofoils.A typical stage in a commercial compressorwill produce a pressure increase of between15% and 60% (pressure ratios of 1.15-1.6) atdesign conditions with a polytropic efficiencyin the region of 90-95%. To achieve differentpressure ratios, axial compressors are

From Wikipedia, the free encyclopedia Axial compressor

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designed with different numbers of stagesand rotational speeds.

Higher stage pressure ratios are also pos-sible if the relative velocity between fluid androtors is supersonic, however this is achievedat the expense of efficiency and operability.Such compressors, with stage pressure ratiosof over 2, are only used where minimising thecompressor size, weight or complexity is crit-ical, such as in military jets.

The aerofoil profiles are optimised andmatched for specific velocities and turning.Although compressors can be run at otherconditions with different flows, speeds, orpressure ratios, this can result in an effi-ciency penalty or even a partial or completebreakdown in flow (known as compressorstall and pressure surge respectively). Thus,a practical limit on the number of stages, andthe overall pressure ratio, comes from the in-teraction of the different stages when re-quired to work away from the design condi-tions. These “off-design” conditions can bemitigated to a certain extent by providingsome flexibility in the compressor. This isachieved normally through the use of ad-justable stators or with valves that can bleedfluid from the main flow between stages(inter-stage bleed).

Modern jet engines use a series of com-pressors, running at different speeds; to sup-ply air at around 40:1 pressure ratio for com-bustion with sufficient flexibility for all flightconditions.

DevelopmentEarly axial compressors offered poor effi-ciency, so poor that in the early 1920s a num-ber of papers claimed that a practical jet en-gine would be impossible to construct. Th-ings changed dramatically after A. A. Griffithpublished a seminal paper in 1926, notingthat the reason for the poor performance wasthat existing compressors used flat bladesand were essentially "flying stalled". Heshowed that the use of airfoils instead of theflat blades would dramatically increase effi-ciency, to the point where a practical jet en-gine was a real possibility. He concluded thepaper with a basic diagram of such an en-gine, which included a second turbine thatwas used to power a propeller.

Although Griffith was well known due tohis earlier work on metal fatigue and stressmeasurement, little work appears to have

started as a direct result of his paper. Theonly obvious effort was a test-bed compressorbuilt by Griffith’s colleague at the Royal Air-craft Establishment, Haine Constant. Otherearly jet efforts, notably those of FrankWhittle and Hans von Ohain, were based onthe more robust and better understood cent-rifugal compressor which was widely used insuperchargers. Griffith had seen Whittle’swork in 1929 and dismissed it, noting an er-ror in the math and going on to claim that thefrontal size of the engine would make it use-less on a high-speed aircraft.

Real work on axial-flow engines started inthe late 1930s, in several efforts that all star-ted at about the same time. In England,Haine Constant reached an agreement withthe steam turbine company MetropolitanVickers (Metrovick) in 1937, starting theirturboprop effort based on the Griffith designin 1938. In 1940, after the successful run ofWhittle’s centrifugal-flow design, their effortwas re-designed as a pure jet, the MetrovickF.2. In Germany, von Ohain had producedseveral working centrifugal engines, some ofwhich had flown including the world’s firstjet aircraft (He 178), but development effortshad moved on to Junkers (Jumo 004) andBMW (BMW 003), which used axial-flowdesigns in the world’s first jet fighter (Mess-erschmitt Me 262) and jet bomber (Arado Ar234). In the United States, both Lockheedand General Electric were awarded contractsin 1941 to develop axial-flow engines, theformer a pure jet, the latter a turboprop.Northrop also started their own project to de-velop a turboprop, which the US Navy even-tually contracted in 1943. Westinghouse alsoentered the race in 1942, their project prov-ing to be the only successful one of the US ef-forts, later becoming the J30.

By the 1950s every major engine develop-ment had moved on to the axial-flow type. AsGriffith had originally noted in 1929, thelarge frontal size of the centrifugal com-pressor caused it to have higher drag thanthe narrower axial-flow type. Additionally theaxial-flow design could improve its compres-sion ratio simply by adding additional stagesand making the engine slightly longer. In thecentrifugal-flow design the compressor itselfhad to be larger in diameter, which wasmuch more difficult to "fit" properly on theaircraft. On the other hand, centrifugal-flowdesigns remained much less complex (themajor reason they "won" in the race to flying

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examples) and therefore have a role in placeswhere size and streamlining are not so im-portant. For this reason they remain a majorsolution for helicopter engines, where thecompressor lies flat and can be built to anyneeded size without upsetting the streamlin-ing to any great degree.

Axial-flow jet engines

Low pressure axial compressor scheme of theOlympus BOl.1 turbojet.

In the jet engine application, the compressorfaces a wide variety of operating conditions.On the ground at takeoff the inlet pressure ishigh, inlet speed zero, and the compressorspun at a variety of speeds as the power isapplied. Once in flight the inlet pressuredrops, but the inlet speed increases (due tothe forward motion of the aircraft) to recoversome of this pressure, and the compressortends to run at a single speed for long peri-ods of time.

There is simply no "perfect" compressorfor this wide range of operating conditions.Fixed geometry compressors, like those usedon early jet engines, are limited to a designpressure ratio of about 4 or 5:1. As with anyheat engine, fuel efficiency is strongly relatedto the compression ratio, so there is verystrong financial need to improve the com-pressor stages beyond these sorts of ratios.

Additionally the compressor may stall ifthe inlet conditions change abruptly, a com-mon problem on early engines. In somecases, if the stall occurs near the front of theengine, all of the stages from that point onwill stop compressing the air. In this situ-ation the energy required to run the com-pressor drops suddenly, and the remaininghot air in the rear of the engine allows theturbine to speed up the whole engine dramat-ically. This condition, known as surging, was

a major problem on early engines and oftenled to the turbine or compressor breakingand shedding blades.

For all of these reasons, axial compressorson modern jet engines are considerably morecomplex than those on earlier designs.

SpoolsAll compressors have a sweet spot relatingrotational speed and pressure, with highercompressions requiring higher speeds. Earlyengines were designed for simplicity, andused a single large compressor spinning at asingle speed. Later designs added a secondturbine and divided the compressor into "lowpressure" and "high pressure" sections, thelatter spinning faster. This two-spool designresulted in increased efficiency. Even morecan be squeezed out by adding a third spool,but in practice this has proven to be too com-plex to make it generally worthwhile as thereis a trade off between higher fuel efficiencyand the higher maintenance involved pushingup total cost of ownership compared to a twospool design. That said, there are severalthree-spool engines in use, perhaps the mostfamous being the Rolls-Royce RB.211, usedon a wide variety of commercial aircraft.

Bleed air, variable statorsSee also: Bleed airAs an aircraft changes speed or altitude, thepressure of the air at the inlet to the com-pressor will vary. In order to "tune" the com-pressor for these changing conditions,designs starting in the 1950s would "bleed"air out of the middle of the compressor in or-der to avoid trying to compress too much airin the final stages. This was also used to helpstart the engine, allowing it to be spun upwithout compressing much air by bleedingoff as much as possible. Bleed systems werealready commonly used anyway, to provideairflow into the turbine stage where it wasused to cool the turbine blades, as well asprovide pressurized air for the air condition-ing systems inside the aircraft.

A more advanced design, the variablestator, used blades that can be individuallyrotated around their axis, as opposed to thepower axis of the engine. For startup theyare rotated to "open", reducing compression,and then are rotated back into the airflow asthe external conditions require. The GeneralElectric J79 was the first major example of a

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variable stator design, and today it is a com-mon feature of most military engines.

Closing the variable stators progressively,as compressor speed falls, reduces the slopeof the surge (or stall) line on the operatingcharacteristic (or map), improving the surgemargin of the installed unit. By incorporatingvariable stators in the first five stages, Gen-eral Electric Aircraft Engines has developeda ten-stage axial compressor capable of oper-ating at a 23:1 design pressure ratio.

BypassFor jet engine applications, the "whole idea"of the engine is to move air to provide thrust.In most cases, the engine produces morepower to move air than its mechanical designactually allows. Namely, the inlet into thecompressor is simply too small to move theamount of air that the engine could, in the-ory, heat and use. A number of enginedesigns had experimented with using some ofthe turbine power to drive a secondary "fan"for added air flow, starting with the Metro-vick F.3, which placed a fan at the rear of alate-model F.2 engine. A much more practicalsolution was created by Rolls-Royce in theirearly 1950s Conway engine, which enlargedthe first compressor stage to be larger thanthe engine itself. This allowed the com-pressor to blow cold air past the interior ofthe engine, somewhat similar to a propeller.This technique allows the engine to be de-signed to produce the amount of energyneeded, and any air that cannot be blownthrough the engine due to its size is simplyblown around it. Since that air is not com-pressed to any large degree, it is beingmoved without using up much energy fromthe turbine, allowing a smaller core toprovide the same mass flow, and thrust, as amuch larger "pure jet" engine. This engine iscalled a "turbofan."

This technique also has the added benefitof mixing the cold bypass air with the hot en-gine exhaust, greatly lowering the exhausttemperature. Since the sound of a jet engineis strongly related to the exhaust temperat-ure, bypass also dramatically reduces thesound of the engine. Early jetliners from the1960s were famous for their "screaming"sound, whereas modern engines of greatlyhigher power generally give off a much lessannoying "whoosh" or even buzzing.

Mitigating this savings is the fact thatdrag increases exponentially at high speeds,

so while the engine is able to operate farmore efficiently, this typically translates intoa smaller real-world effect. For instance, thelatest Boeing 737’s with high-bypass CFM56engines operates at an overall efficiencyabout 30% better than the earlier models.Military turbofans, on the other hand, espe-cially those used on combat aircraft, tend tohave so low a bypass-ratio that they aresometimes referred to as "leaky turbojets."

Turbine coolingThe limiting factor in jet engine design is notthe compressor, but the temperature at theturbine. It is fairly easy to build an enginethat can provide enough compressed air thatwhen burnt will melt the turbine; this was amajor cause of failure in early German en-gines which were hampered by the availabil-ity of high temperature metals. Improve-ments in air cooling and materials have dra-matically improved the temperature perform-ance of turbines, allowing the compressionratio of jet engines to increase dramatically.Early test engines offered perhaps 3:1 andproduction engines like the Jumo 004 wereabout 4:1, about the same as contemporarypiston engines. Improvements started imme-diately and have not stopped; the latest Rolls-Royce Trent operates at about 40:1, far in ex-cess of any piston engine.

Since compression ratio is strongly relatedto fuel economy, this eightfold increase incompression ratio results in an increase infuel economy for any given amount of power,which is the reason there is strong pressurein the airline industry to use only the latestdesigns.

Design notesEnergy exchange between rotor andfluidThe relative motion of the blades relative tothe fluid adds velocity or pressure or both tothe fluid as it passes through the rotor. Thefluid velocity is increased through the rotor,and the stator converts kinetic energy topressure energy. Some diffusion also occursin the rotor in most practical designs.

The increase in velocity of the fluid isprimarily in the tangential direction (swirl)and the stator removes this angularmomentum.

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The pressure rise results in a stagnationtemperature rise. For a given geometry thetemperature rise depends on the square ofthe tangential Mach number of the rotor row.Current turbofan engines have fans that op-erate at Mach 1.7 or more, and require signi-ficant containment and noise suppressionstructures to reduce blade loss damage andnoise.

Velocity diagramsThe blade rows are designed at the first levelusing velocity diagrams. A velocity diagramshows the relative velocities of the bladerows and the fluid.

The axial flow through the compressor iskept as close as possible to Mach 1 to maxim-ize the thrust for a given compressor size.The tangential Mach number determines theattainable pressure rise.

The blade rows turn the flow through anangle β; larger turning allows a higher tem-perature ratio, but requires higher solidity.

Modern blades rows have low aspect ra-tios and high solidity.

Compressor mapsA map shows the performance of a com-pressor and allows determination of optimaloperating conditions. It shows the mass flowalong the horizontal axis, typically as a per-centage of the design mass flow rate, or inactual units. The pressure rise is indicated onthe vertical axis as a ratio between inlet andexit stagnation pressures.

A surge or stall line identifies the bound-ary to the left of which the compressor per-formance rapidly degrades and identifies the

maximum pressure ratio that can be achievedfor a given mass flow. Contours of efficiencyare drawn as well as performance lines foroperation at particular rotational speeds.

Compression stabilityOperating efficiency is highest close to thestall line. If the downstream pressure is in-creased beyond the maximum possible thecompressor will stall and become unstable.

Typically the instability will be at theHelmholtz frequency of the system, takingthe downstream plenum into account. ..

References• Hill, Philip and Carl Peterson. ’Mechanics

and Thermodynamics of Propulsion,’ 2ndedn, Prentice Hall, 1991. ISBN0201146592.

• Kerrebrock, Jack L. ’Aircraft Engines andGas Turbines,’ 2nd edn, Cambridge,Massachusetts: The MIT Press, 1992.ISBN 0-262-11162-4.

• Rangwalla, Abdulla. S. ’Turbo-MachineryDynamics: Design and Operation,’ NewYork: McGrawl-Hill: 2005. ISBN0-07-145369-5.

• Wilson, David Gordon and TheodosiosKorakianitis. ’The Design of High-Efficiency Turbomachinery and Turbines,’2nd edn, Prentice Hall, 1998. ISBN0133120007.

External links• AxSTREAM turbomachinery design

software

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Categories: Compressors, Chemical engineering, Mechanical engineering

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