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Encyclopedia > Axial compressor
An animation of an axial compressor. The darker colored blades are the stators.
An animation of an axial compressor. The darker colored blades are the stators.

Axial compressors are rotating, aerofoil based compressors in which the working fluid principally flows parallel to the axis of rotation. This is in contrast with centrifugal, axi-centrifugal and mixed-flow compressors where the air may enter axially but will have a significant radial component on exit. Image File history File links Axial_compressor. ... Image File history File links Axial_compressor. ... A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume. ...


Axial flow compressors produce a continuous flow of compressed air, and have the benefits of high efficiencies and large mass flow capacity, particularly in relation to their cross-section. They do, however, require several rows of aerofoils to achieve large pressure rises making them complex and expensive relative to other designs (e.g. centrifugal compressor). A centrifugal compressor, also called a radial blower, squirrel cage, or squirrel wheel compressor, consists of an axle to which is mounted a cylindrical assembly of compressor blades. ...


Axial compressors, are widely used in gas turbines, such as jet engines, high speed ship engines, small scale power stations. They are also used in industrial applications such as; large volume air separation plants, blast furnace air, fluid catalytic cracking air, and propane dehydrogenation. The worlds first commercial, oil-free gas turbine is manufactured by Capstone. ... A Pratt and Whitney turbofan engine for the F-15 Eagle is tested at Robins Air Force Base, Georgia, USA. The tunnel behind the engine muffles noise and allows exhaust to escape. ...

Contents

Description

Axial compressors consist of rotating and stationary components. A shaft drives a central drum, retained by bearings, which has a number of annular aerofoil rows attached. These rotate between a similar number of stationary aerofoil rows attached to a stationary tubular casing. The rows alternate between the rotating aerofoils (rotors) and stationary aerofoils (stators), with the rotors imparting energy into the fluid, and the stators converting the increased rotational kinetic energy into static pressure through diffusion. A pair of rotating and stationary aerofoils is called a stage. The cross-sectional area between rotor drum and casing is reduced in the flow direction to maintain axial velocity as the fluid is compressed.

Diagram of an axial flow compressor
Diagram of an axial flow compressor

Image File history File links Axial-flow-compressor. ... Image File history File links Axial-flow-compressor. ...

Design

The increase in pressure for a single stage is limited by the relative velocity between the rotor and the fluid, and the turning and diffusion capabilities of the aerofoils. A typical stage in a commercial compressor will produce a pressure increase of be-tween 15% and 60% (pressure ratios of 1.15-1.6) at design conditions with a polytropic efficiency in the region of 90-95%. To achieve different pressure ratios, axial compressors are designed with different numbers of stages and rotational speeds.


Higher stage pressure ratios are also possible, if the relative velocity between fluid and rotors is supersonic, however this is achieved at the expense of efficiency and operability. Such compressors, with stage pressure ratios of over 2, are only used where minimising the compressor size, weight or complexity is critical, such as in military jets.


The aerofoil profiles are optimised and matched for specific velocities and turning. Although compressors can be run at other conditions with different flows, speeds, or pressure ratios, this can result in an efficiency penalty or even a partial or complete breakdown in flow (know as stall and surge respectively). Thus, a practical limit on the number of stages, and the overall pressure ratio, comes from the interaction of the different stages when required to work away from the design conditions. These “off-design” conditions can be mitigated to a certain extent by providing some flexibility in the compressor. This is achieved normally through the use of adjustable stators or with valves that can bleed fluid from the main flow between stages (inter-stage bleed).


Modern jet engines use a series of compressors, running at different speeds; to supply air at around 40:1 pressure ratio for combustion with sufficient flexibility for all flight conditions.


Development

Early axial compressors offered poor efficiency, so poor that in the early 1920s a number of papers claimed that a practical jet engine would be impossible to construct. Things changed dramatically after A. A. Griffith published a seminal paper in 1926, noting that the reason for the poor performance was that existing compressors used flat blades and were essentially "flying stalled". He showed that the use of airfoils instead of the flat blades would dramatically increase efficiency, to the point where a practical jet engine was a real possibility. He concluded the paper with a basic diagram of such an engine, which included a second turbine that was used to power a propeller. Alan Arnold Griffith (b 13 June 1893 - 13 Oct 1963) was a British engineer, who, among many other contributions, is best known for his work on stress and fracture in metals that is now known as metal fatigue, as well as being one of the first to develop a strong... This article does not cite its references or sources. ...


Although Griffith was well known due to his earlier work on metal fatigue and stress measurement, little work appears to have started as a direct result of his paper. The only obvious effort was a test-bed compressor built by Griffith's colleague at the RAE, Haine Constant. Other early jet efforts, notably those of Frank Whittle and Hans von Ohain, were based on the more robust and better understood centrifugal compressor which was widely used in superchargers. Griffith had seen Whittle's work in 1929 and dismissed it, noting an error in the math and going on to claim that the frontal size of the engine would make it useless on a high-speed aircraft. This article is about a computer game. ... Stress is the internal distribution of force per unit area that balances and reacts to external loads applied to a body. ... Rae is another way of saying LONG LIVE THE LOCUST!!! YAY!!!! // Rachel Raven Rachelle Alex Rae (born 1969), Scottish professional football (soccer) midfielder Arthur Rae, Australian politician Bob Rae (born 1948), Canadian politician Charlotte Rae, (born Charlotte Rae Lubotsky in 1926), American actress and singer Douglas Rae, Scottish businessman John... Air Commodore Sir Frank Whittle, OM, KBE FRS (1 June 1907–9 August 1996) was a Royal Air Force officer who invented the jet engine. ... Hans Joachim Pabst von Ohain (December 14, 1911 – March 13, 1998) was one of the inventors of jet propulsion. ... A centrifugal compressor, also called a radial blower, squirrel cage, or squirrel wheel compressor, consists of an axle to which is mounted a cylindrical assembly of compressor blades. ... A supercharger (also known as a blower) is an air compressor used to force more air (and hence more oxygen) into the combustion chamber(s) of an internal combustion engine than can be achieved at ambient atmospheric pressure (natural aspiration). ...


Real work on axial-flow engines started in the late 1930s, in several efforts that all started at about the same time. In England, Haine Constant reached an agreement with the steam turbine company Metropolitan Vickers (Metrovick) in 1937, starting their turboprop effort based on the Griffith design in 1938. In 1940, after the successful run of Whittle's centrifugal-flow design, their effort was re-designed as a pure jet, the Metrovick F.2. In Germany, von Ohain had produced several working centrifugal engines, some of which had flown including the worlds first jet aircraft (He 178), but development efforts had moved on to Junkers (Jumo 004) and BMW (BMW 003), which used axial-flow designs in the worlds first jet fighter (Messerschmitt Me 262) and jet bomber (Arado Ar 234). In the United States, both Lockheed and General Electric were awarded contracts in 1941 to develop axial-flow engines, the former a pure jet, the latter a turboprop. Northrop also started their own project to develop a turboprop, which the US Navy eventually contracted in 1943. Westinghouse also entered the race in 1942, their project proving to be the only successful one of the US efforts, later becoming the J30. Metropolitan-Vickers, Metrovick, or Metrovicks, was a British heavy industrial firm of the early-to-mid 20th century formerly known as British Westinghouse. ... A schematic diagram showing the operation of a turboprop engine. ... The Metrovick F.2 was one of the earliest jet engines, and the first British design to be based on an axial compressor. ... The Heinkel He 178 was the worlds first aircraft to fly under turbojet power, and the first practical jet plane. ... Junkers & Co was a major German aircraft manufacturer. ... The Jumo 004 was the worlds first turbojet engine in production and operational use. ... BMW, or Bavarian Motor Works, is an independent German company and manufacturer of automobiles and motorcycles. ... The BMW 003 was an early turbojet engine produced in Germany during World War II. Work on its design began earlier than the contemporary Junkers Jumo 004 engine, but prolonged developmental problems meant that the BMW 003 entered production much later, and the aircraft projects that had been designed with... The Messerschmitt Me 262 Schwalbe (German: Swallow) was the worlds first operational jet-powered fighter aircraft. ... The Arado Ar 234 Blitz (Lightning) was the worlds first operational jet powered bomber, built by the Arado company in the closing stages of World War II. In the field it was used almost entirely in the reconnaissance role, but in its few uses as a bomber it proved... The Lockheed SR-71 was remarkably advanced for its time and remains unsurpassed in many areas of performance. ... This article is about the American company. ... The Northrop Corporation was a leading aircraft manufacturer of the United States. ... The United States Navy (USN) is the branch of the United States armed forces responsible for naval operations. ... Westinghouse logo (designed by Paul Rand) The Westinghouse Electric Company, headquartered in Monroeville, Pennsylvania, is an organization founded by George Westinghouse in 1886. ...


By the 1950s every major engine development had moved on to the axial-flow type. As Griffith had originally noted in 1929, the large frontal size of the centrifugal compressor caused it to have higher drag than the narrower axial-flow type. Additionally the axial-flow design could improve its compression ratio simply by adding additional stages and making the engine slightly longer. In the centrifugal-flow design the compressor itself had to be larger in diameter, which was much more difficult to "fit" properly on the aircraft. On the other hand, centrifugal-flow designs remained much less complex (the major reason they "won" in the race to flying examples) and therefore have a role in places where size and streamlining are not so important. For this reason they remain a major solution for helicopter engines, where the compressor lies flat and can be built to any needed size without upsetting the streamlining to any great degree. Bold text The compression ratio is a single number that can be used to predict the performance of any engine (such as an internal-combustion engine or a Stirling Engine). ... In fluid dynamics, a streamline is the path that an imaginary massless particle would make if it followed the flow of a fluid in which it was embedded. ...


Axial-flow jet engines

Low pressure axial compressor scheme of the Olympus BOl.1 turbojet.
Low pressure axial compressor scheme of the Olympus BOl.1 turbojet.

In the jet engine application, the compressor faces a wide variety of operating conditions. On the ground at takeoff the inlet pressure is high, inlet speed zero, and the compressor spun at a variety of speeds as the power is applied. Once in flight the inlet pressure drops, but the inlet speed increases (due to the forward motion of the aircraft) to recover some of this pressure, and the compressor tends to run at a single speed for long periods of time. Image File history File links Metadata Size of this preview: 800 × 529 pixelsFull resolution (1288 × 851 pixel, file size: 267 KB, MIME type: image/jpeg) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Metadata Size of this preview: 800 × 529 pixelsFull resolution (1288 × 851 pixel, file size: 267 KB, MIME type: image/jpeg) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...


There is simply no "perfect" compressor for this wide range of operating conditions. Fixed geometry compressors, like those used on early jet engines, are limited to a design pressure ratio of about 4 or 5:1. As with any heat engine, fuel efficiency is strongly related to the compression ratio, so there is very strong financial need to improve the compressor stages beyond these sorts of ratios. A heat engine is a physical or theoretical device that converts thermal energy to mechanical output. ... Fuel efficiency sometimes means the same as thermal efficiency, that is, the efficiency of converting energy contained in a carrier fuel to kinetic energy or work. ... Bold text The compression ratio is a single number that can be used to predict the performance of any engine (such as an internal-combustion engine or a Stirling Engine). ...


Additionally the compressor may stall if the inlet conditions change abruptly, a common problem on early engines. In some cases, if the stall occurs near the front of the engine, all of the stages from that point on will stop compressing the air. In this situation the energy required to run the compressor drops suddenly, and the remaining hot air in the rear of the engine allows the turbine to speed up whole engine dramatically. This condition, known as surging, was a major problem on early engines and often led to the turbine or compressor breaking and shedding blades. A stall is the slowing or stopping of a process. ...


For all of these reasons, axial compressors on modern jet engines are considerably more complex than those on earlier designs.


Spools

All compressors have a sweet spot relating rotational speed and pressure, with higher compressions requiring higher speeds. Early engines were designed for simplicity, and used a single large compressor spinning at a single speed. Later designs added a second turbine and divided the compressor into "low pressure" and "high pressure" sections, the latter spinning faster. This two-spool design resulted in increased efficiency. Even more can be squeezed out by adding a third spool, but in practice this has proven to be too complex to make it generally worthwhile. That said, there are several three-spool engines in use, perhaps the most famous being the Rolls-Royce RB.211, used on a wide variety of commercial aircraft. A sweet spot is a place, often numerical as opposed to physical, where a combination of factors suggest a particularly suitable solution. ... Rolls-Royce RB211 engine The Rolls Royce RB.211 family is a family of high-bypass turbofan aircraft engines made by Rolls-Royce capable of generating 37,400 to 60,600 pound (166 to 270 kN) thrust. ...


Bleed air, variable stators

As an aircraft changes speed or altitude, the pressure of the air at the inlet to the compressor will vary. In order to "tune" the compressor for these changing conditions, designs starting in the 1950s would "bleed" air out of the middle of the compressor in order to avoid trying to compress too much air in the final stages. This was also used to help start the engine, allowing it to be spun up without compressing much air by bleeding off as much as possible. Bleed systems were already commonly used anyway, to provide airflow into the turbine stage where it was used to cool the turbine blades, as well as provide pressurized air for the air conditioning systems inside the aircraft. A Siemens steam turbine with the case opened. ... Note: in the broadest sense, air conditioning can refer to any form of heating, ventilation, and air-conditioning. ...


A more advanced design, the variable stator, used blades that can be individually rotated around their axis, as opposed to the power axis of the engine. For startup they are rotated to "open", reducing compression, and then are rotated back into the airflow as the external conditions require. The General Electric J79 was the first major example of a variable stator design, and today it is a common feature of most military engines. General Electric J79 General Electric J79 The General Electric J79 is an axial-flow turbojet engine built for use in a variety of fighter aircraft and bomber aircraft. ...


Closing the variable stators progressively, as compressor speed falls, reduces the slope of the surge (or stall) line on the operating characteristic (or map), improving the surge margin of the installed unit. By incorporating variable stators in the first five stages, General Electric Aircraft Engines has developed a ten-stage axial compressor capable of operating at a 23:1 design pressure ratio. General Electric Aircraft Engines (GEAE) is the top supplier of aircraft engines in the world and offers engines for the majority of commercial aircraft. ...


Bypass

For jet engine applications, the "whole idea" of the engine is to move air to provide thrust. In most cases, the engine produces more power to move air than its mechanical design actually allows. Namely, the inlet into the compressor is simply too small to move the amount of air that the engine could, in theory, heat and use. THESE ARE COMPLICATED. A number of engine designs had experimented with using some of the turbine power to drive a secondary "fan" for added air flow, starting with the Metrovick F.3, which placed a fan at the rear of a late-model F.2 engine. A much more practical solution was created by Rolls-Royce in their early 1950's Conway engine, which enlarged the first compressor stage to be larger than the engine itself. This allowed the compressor to blow cold air past the interior of the engine, somewhat similar to a propeller. This technique allows the engine to be designed to produce the amount of energy needed, and any air that cannot be blown through the engine due to its size is simply blown around it. Since that air is not compressed to any large degree, it is being moved without using up much energy from the turbine, allowing a smaller core to provide the same mass flow, and thrust, as a much larger "pure jet" engine. This engine is called a "turbofan." In physics, power (symbol: P) is the rate at which work is performed or energy is transferred. ... The Rolls-Royce Conway was the first by-pass engine to go into service in the world. ... Schematic diagram of high-bypass turbofan engine CFM56-3 turbofan, lower half, side view. ...


This technique also has the added benefit of mixing the cold bypass air with the hot engine exhaust, greatly lowering the exhaust temperature. Since the sound of a jet engine is strongly related to the exhaust temperature, bypass also dramatically reduces the sound of the engine. Early jetliners from the 1960s were famous for their "screaming" sound, whereas modern engines of greatly higher power generally give off a much less annoying "whoosh" or even buzzing.


Mitigating this savings is the fact that drag increases exponentially at high speeds, so while the engine is able to operate far more efficiently, this typically translates into a smaller real-world effect. For instance, the latest Boeing 737's with high-bypass CFM56 engines operates at an overall efficiency about 30% better than the earlier models. Military turbofans, on the other hand, especially those used on combat aircraft, tend to have so low a bypass-ratio that they are sometimes referred to as "leaky turbojets." An object falling through a gas or liquid experiences a force in direction opposite to its motion. ... The Boeing 737 is the worlds most popular short to medium range, single aisle, narrow body airliner. ... CFM International CFM-56 series engines is a family of high-bypass turbofan engines made by CFM International and has a thrust range from 18,500 to 34,000 lbf (82 kN to 151 kN). ...


Turbine cooling

The limiting factor in jet engine design is not the compressor, but the temperature at the turbine. It is fairly easy to build an engine that can provide enough compressed air that when burnt will melt the turbine; this was a major cause of failure in early German engines which were hampered by the availabilty of high temperature metals. Improvements in air cooling and materials have dramatically improved the temperature performance of turbines, allowing the compression ratio of jet engines to increase dramatically. Early test engines offered perhaps 3:1 and production engines like the Jumo 004 were about 4:1, about the same as contemporary piston engines. Improvements started immediately and have not stopped; the latest Rolls-Royce Trent operates at about 40:1, far in excess of any piston engine. Rolls-Royce Trent 900 on A380 prototype Rolls Royce Trent is a family of high-bypass turbofan engines manufactured by Rolls-Royce. ...


Since compression ratio is strongly related to fuel economy, this eightfold increase in compression ratio results in an increase in fuel economy for any given amount of power, which is the reason there is strong pressure in the airline industry to use only the latest designs.


Design notes

Energy Exchange between rotor and fluid

The relative motion of the blades relative to the fluid adds velocity or pressure or both to the fluid as it passes through the rotor. The fluid velocity is increased through the rotor, and the stator converts kinetic energy to pressure energy. Some diffusion also occurs in the rotor in most practical designs.


The increase in velocity of the fluid is primarily in the tangential direction (swirl) and the stator removes this angular momentum. Swirl were a Sydney, Australia-based indie rock band. ...


The pressure rise results in a stagnation temperature rise. For a given geometry the temperature rise depends on the square of the tangential Mach number of the rotor row. Current turbofan engines have fans that operate at Mach 1.7 or more, and require significant containment and noise suppression structures to reduce blade loss damage and noise. Stagnation Temperature is the temperature at a stagnation point in a fluid flow, where all of the kinetic energy in converted into heat energy and is added to the local Static Enthalpy. ... An F/A-18 Hornet breaking the sound barrier. ... Schematic diagram of high-bypass turbofan engine CFM56-3 turbofan, lower half, side view. ...


Velocity diagrams

The blade rows are designed at the first level using velocity diagrams. The velocity diagram shows the relative velocities of the blade rows and the fluid.


The axial flow through the compressor is kept as close as possible to Mach 1 to maximize the thrust for a given compressor size. The tangential Mach number determines the attainable pressure rise.


The blade rows turn the flow through and angle ß and larger turning allows a higher temperature ratio, but requires higher solidity. Look up solid in Wiktionary, the free dictionary. ...


Modern blades rows have lower aspect ratios and higher solidity.


Compressor maps

A map shows the performance of a compressor and allows determination of optimal operating conditions. It shows the mass flow along the horizontal axis, typically as a percentage of the design mass flow rate, or in actual units. The pressure rise is indicated on the vertical axis as a ratio between inlet and exit stagnation pressures.


A surge or stall line identifies the boundary to the left of which the compressor performance rapidly degrades and identifies the maximum pressure ratio that can be achieved for a given mass flow. Contours of efficiency are drawn as well as performance lines for operation at particular rotational speeds.


Compression stability

Operating efficiency is highest close to the stall line. If the downstream pressure is increased beyond the maximum possible the compressor will stall and become unstable.


Typically the instability will be at the Helmholtz frequency of the system, taking the downstream plenum into account. A brass, spherical Helmholtz resonator based on his original design, from around 1890-1900. ...


References


  Results from FactBites:
 
axial compressor: Information from Answers.com (2558 words)
Axial compressors are compressors in which the fluid flows mainly parallel to the rotation axis.
Axial flow compressors have large mass flow capacity and high efficiencies, but have a smaller pressure rise per stage than centrifugal compressors.
Axial compressors are essentially a steam turbine reversed; instead of high-pressure gas flowing into the turbine and forcing it to rotate to provide power, in the compressor role, power is provided from an external source in order to spin the system and compress the gas.
Compressor Section - Generic Information (2327 words)
The air in an axial compressor flows in an axial direction through a series of rotating (rotor) blades and stationary (stator) vanes that are concentric with the axis of rotation.
The dual compressor is a combination either of two axial compressors or of an axial and a centrifugal compressor (Figure 4).
The axial compressor and centrifugal compressor combination is mounted on the same shaft; the compressors turn in the same direction and at the same speed.
  More results at FactBites »

 
 

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