Figure 2-5: T-s diagram for an ideal jet propulsion cycle along with a turbojet engine schematic (from [6]).
2.3
How current APUs work
Figure 2-6: APU centrifugal compressor rotor with inducer vanes (from [3]).
Air drawn into the engine first goes through a centrifugal compressor rotor. Curved vanes at the compressor intake area, called inducers, guide the air into the compressor. Rotors without inducers are usually very noisy due to flow separation [5]. As the air passes through the compressor, it accelerates outward at high speed and slows down in a ring of stationary vanes called the diffuser. This causes the air pressure to rise. Immediately after the compressor section, an air bleed system is usually present. This releases a portion of the airflow in the engine. Since this bleed air is very energetic, it can pressurize aircraft cabins or drive small cold turbines to develop shaft horsepower. Valves or venturis control this air bleed to within pre-determined limits [3].
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