backstreet: Axial compressor

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An animation of an axial compressor. The static 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 other rotating compresors such as centrifugal, axi-centrifugal and mixed-flow compressors where the air may enter axially but will have a significant radial component on exit.
Axial flow compressors produce a continuous flow of compressed gas, 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).
Axial compressors are widely used in gas turbines, such as jet engines, high speed ship engines, and 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. Axial compressors, known as superchargers, have also been used to boost the power of automotive reciprocating engines by compressing the intake air, though these are very rare. A good example of an axial supercharger is the aftermarket Latham type built between 1955-65 which were used on hot rods and aircooled Volkwagens at that time, but these didn't catch on.
Contents
1 Description
2 Design
3 Development
4 Axial-flow jet engines
4.1 Spools
4.2 Bleed air, variable stators
4.3 Bypass
4.4 Turbine cooling
5 Design notes
5.1 Energy exchange between rotor and fluid
5.2 Velocity diagrams
5.3 Compressor maps
5.4 Compression stability
6 References
7 External links
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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
Design
The increase in pressure produced by 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 between 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 (known as compressor stall and pressure 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 鎼奻f-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...(and so on)