A turbine compressor stages regulate the stream of air that flows into the machine through the stator assembly. The compressor contains between 8 and 16 stages, depending on the type of turbine or jet engine. Each one of them consists of a rotor disc, or wheel, containing rotating blades. These are followed by a stator assembly having stationary blades, which are sometimes also called “vanes”.
A stator assembly is usually employed to supply the right flow direction for the compressor first rotor wheel, with vane incidence made adjustable to allow for different thrust settings. The operation of a stage can be visualized in the middle section of the compressor, which is an array of aerofoils of rotor and stator blades. Located in the middle between hub and tip, this array of vanes in the middle section is called “cascade”. When studying it, flow in a cascade is considered to be two-dimensional, but in an actual compressor, three dimensional effects occur.
Depending on the position of the observer, the flow will have three velocities when approaching a rotating blade; 1) absolute velocity V as “seen” by an external observer standing next to the engine; 2) circumferential velocity U, which depends on rotating speed (rpm) and radial position; 3) relative velocity V-rel as seen by an observer “sitting” on the rotating blade and moving with it. All three velocities may be combined into a velocity triangle. Since velocities not only have a magnitude but also a direction, they may be expressed as vectors, which are arrows of a length corresponding to their respective velocity and pointing in the direction of the flow. Then the energy content of the flow is determined by the static pressure P1, static temperature t1, and kinetic energy.
Because there is an increase in flowpath cross-sectional area between adjacent rotor blades downstream, a diffusing action arises, causing relative velocity to decrease and pressure to increase. The flow exits the rotor at relative velocity V2-rel, which is smaller than V1-rel. at rotor entrance. Direction of exit velocity V2-rel. corresponds to the mean line slope of the blade trailing edge. Together with circumferential velocity U, the velocity triangle at rotor exit can be drawn to yield absolute velocity V2.
