Photographs of an axial and a centrifugal compressor.
 Computer drawing of engine showing location of compressor.

Most modern passenger and military aircraft are powered by gas turbine engines, also called jet engines. All types of gas turbine engines have some parts in common. All turbine engines have a compressor to increase the pressure of the incoming air before it enters the combustor. The mathematical details of the work process performed by the compressor to increase air pressure are given on a separate slide.

As shown in the above figure, there are two main types of compressors: axial and centrifugal. The compressor on the left is called an axial compressor because the flow through the compressor travels parallel to the axis of rotation. The compressor on the right is called a centrifugal compressor because the flow through this compressor is turned perpendicular to the axis of rotation. Centrifugal compressors, which were used in the first jet engines, are still used on small turbojets and turboshaft engines and as pumps on rocket engines. Modern large turbojet and turbofan engines usually use axial compressors.

Why the change to axial compressors? An average, single-stage, centrifugal compressor can increase the pressure by a factor of 4. A similar single-stage axial compressor increases the pressure by only a factor of 1.2. But it is relatively easy to link together several stages and produce a multistage axial compressor. In the multistage compressor, the pressure is multiplied from row to row (8 stages at 1.2 per stage gives a factor of 4.3). It is much more difficult to produce an efficient multistage centrifugal compressor because the flow has to be ducted back to the axis at each stage. Because the flow is turned perpendicular to the axis, an engine with a centrifugal compressor tends to be wider (greater cross-sectional area) than a corresponding axial. This creates additional undesirable aircraft drag. Centrifugal compressors are also less efficient than axial compressors. For all of these reasons, most high compression jet engines use multi staged axial compressors. But, if only a moderate amount of compression is required, a centrifugal compressor is much simpler to use.

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byTom Benson
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