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 widely used in gas turbines, notably jet engines. Engines using an axial compressor are known as axial-flow. Almost all modern engines are axial-flow, the notable exception being those used in helicopters, where the smaller size of the centrifugal compressor is useful.

Description

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.

Diagram of an axial flow compressor

A typical axial compressor has a rotor which looks like a fan with contoured blades followed by a stationary set of blades, called a stator. As the diagram illustrates, compressor blades/vanes are relatively flat in section. Turbine blades/vanes, on the other hand, have significant curvature. Each pair of rotors and stators is referred to as a stage, and most axial compressors have a number of such stages placed in a row along a common power shaft in the center. The stator blades are required in order to ensure reasonable efficiency; without them the gas would rotate with the rotor blades resulting in a large drop in efficiency. Improvements can be made by replacing the stators with a second set of fans rotating in the opposite direction, but these designs have generally proven to be too complex to be worthwhile.

Each stage is smaller than the last, as the volume of air is reduced by the compression of the preceding stage. Axial compressors therefore generally have a conical shape, widest at the inlet. Compressors typically have between 9 and 15 stages.

In a jet engine the compressor is powered by a turbine placed in the hot exhaust, using up some of its energy. In such a system axial compressors typically use between 60% and 65% of the engine’s power in order to run. This explains why jet engines are not used in cars; even when the car is standing still on idle, a turbine engine will still have to run close to full power, thus drastically reducing efficiency. In aircraft this is not an issue, since it is never on “idle” and its engine is always running close to full power for an entire trip.