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Most modern passenger and military aircraft are powered bygas turbine engines, also called jetengines. There are several differenttypes of gas turbine engines,but all turbine engines have some parts in common.All turbineengines have a compressor to increase the pressure of theincoming air before it enters the combustor.Compressor performancehas a large influence on total engineperformance.
As shown in the above figure, there are two main types ofcompressors: axial and centrifugal.In the picture, the compressor on the left iscalled an axial compressor because the flowthrough the compressor travels parallel to the axis of rotation. Thecompressor on the right is called a centrifugalcompressor because the flow through this compressor is turnedperpendicular to the axis of rotation. Centrifugal compressors, whichwere used in the first jet engines, are still used on small turbojetsandturboshaftengines and as pumps onrocketengines. Modern largeturbojet andturbofanengines 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. Asimilar average, single-stage axial compressor increases the pressure by onlya factor of 1.2. But it is relatively easy to link together severalstages and produce a multistage axial compressor. In themultistage compressor, the pressure is multiplied from row to row (8stages at 1.2 per stage gives a factor of 4.3). It is much moredifficult to produce an efficient multistage centrifugal compressorbecause the flow has to be ducted back to the axis at each stage.Because the flow is turned perpendicular to the axis, an engine witha centrifugal compressor tends to be wider, having a greater cross-sectionalarea than a corresponding axial. This creates additional undesirableaircraft drag.For these reasons, most high performance, high compressionturbine engines use multi staged axial compressors. But, if only amoderate amount of compression is required, a centrifugal compressoris much simpler to use.
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