12: COMPRESSORS AND TURBINES
Objective: To increase the pressure of the air before heat addition. We have seen previously that the cycle efficiency is strongly dependent on the ratio of the highest to the lowest pressure in an engine. Unless the pressure rise due to deceleration of the fluid is extremely high (because the fluid velocity is in the high supersonic range), a mechanical compressor is required to achieve the pressure rise.
Compressors usually operate in two steps (though in some designs it may not be possible to distinguish between these steps):
Increase the momentum of the air by doign work on it (using rotor blades, for
2. Decelerate the air to increase static pressure (using stator stages).
Often, the compression takes place through several compressor stages, with each stage increasing the pressure by a factor of, say, 1.8 or 2. Modern jet engines may have as many as 15 compressor stages, rotating on as many as three independent concentric shafts.
Both "Centrifugal" and "Axial" compressors are used.
The tuboprop engine shown above uses two stages of centrifugal compression. Here the air enters the compressor close to the hub, and is then impelled outwards by the blades. It is then passed through an expanding duct at the periphery before being led back towards the hub for the next compression stage. The blades that push the air towards the periphery and increase its momentum are the rotor blades, while the expanding duct is the stator or diffuser passage.
Features of centrifugal compressors
Large pressure ratio per stage: stage pressure ratios can be as high as 3 or 4.
2. Few moving parts.
Such compressor stages are used on the low-pressure shaft of helicopter engines and turboprop engines. One famous application is in the Space Shuttle Main Engine turbopumps.