One of the purposes of compressor bleed valves in a gas turbine engine is to…
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A
decrease the airflow across the rear compressor stages, whilst that across the early stages is increased.
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B
decrease the load on the compressor when the engine is approaching maximum rpm.
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C
limit the acceleration period of the engine.
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D
increase the airflow across the rear compressor stages, whilst that across the early stages is decreased.
Refer to figure.
A compressor stall occurs when there is an imbalance between the airflow supply and the airflow demand. Due to a low rotor speed, airflow velocity becomes very small and air stagnates in the REAR stages (increase of back pressure), which results in a lack of air in the FRONT stages. This further induces a subsequent low axial velocity in front stages and will lead to a high angle of attack of the rotor blades determining a STALL (at the front stages first). Usually, this is followed by a complete compressor stall, and possibly a surge depending on the severity of the stall.
That’s why the tapped excessive pressure is relieved by means of a bleed valve that will open at the front stages and recirculate the bleed air towards the air conditioning unit and other systems. This is determined by the engine operating regime so that any unwanted rise in air pressure can be relieved and prevent the conditions which will cause a surge.
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