Most modern passenger and military aircraft are powered by gas turbine engines, which are also called jet engines. There are several different types of jet engines, but all jet engines have some parts in common. All jet engines have a compressor to increase the pressure of the incoming air. Mathematical details of this process is given on a separate slide. There is also a picture slide that shows several photographs of actual compressors.
There are two main types of compressors used in modern jet engines; axial compressors are discussed on this slide, and centrifugal compressors are discussed on another slide. In the axial compressor, the air flows parallel to the axis of rotation. The compressor is composed of several rows of airfoil cascades. Some of the rows, called rotors, are connected to the central shaft and rotate at high speed. Other rows, called stators, are fixed and do not rotate. The job of the stators is to increase pressure and keep the flow from spiraling around the axis by bringing the flow back parallel to the axis. In this figure we show a computer animation of an axial compressor. The stators of this compressor are connected to the outer casing, which has been cut away. The rotors are connected to the central blue shaft, which would be connected to the power turbine of the engine.
How does an axial compressor work? The details are quite complex because the blade geometries and the resulting flows are three dimensional, unsteady, and can have important viscous and compressibility effects. Each blade on a rotor or stator produces a pressure variation much like the airfoil of a spinning propeller. But unlike a propeller blade, the blades of an axial compressor are close to one another, which seriously alters the flow around each blade. Compressor blades continuously pass through the wakes of upstream blades that introduce unsteady flow variations. Compressor designers must rely on wind tunnel testing and sophisticated computational models (http://hpcc.lerc.nasa.gov/) to determine the performance of an axial compressor. The performance is characterized by the pressure ratio across the compressor (CPR), the rotational speed of the shaft necessary to produce the pressure increase, and an efficiency factor that indicates how much additional work is required relative to an ideal compressor. There are some additional important topics, like compressor stall and surge, that will be added to these pages in the future.
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