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Which of these statements about compressor stall in a gas turbine engine are correct or incorrect?

I. A compressor stall is most likely to occur in a compressor designed for a high pressure ratio operating at low RPM.
II. A compressor stall is most likely to occur when the air flow stagnates in the rear stages of the compressor.

  • A
    I is incorrect, II is correct.
  • B
    I is correct, II is incorrect.
  • C
    I is correct, II is correct.
  • D
    I is incorrect, II is incorrect.
"I. A compressor stall is most likely to occur in a compressor designed for a high pressure ratio operating at low RPM." CORRECT
  • A compressor stall occurs when there is an imbalance between the air flow supply and the airflow demand - in other words, a pressure ratio that is incompatible with the engine RPM. As a result, smooth airflow is interrupted and pressure fluctuations are created within the turbine. It occurs when the angle of attack of the blades exceeds the critical angle of attack (just like a wing). Stalls at low speed usually occur because the acceleration of the engine is exceeding the acceleration at which air can move towards the intake of the engine, causing a stagnation of air at the rear of the compressor stages. Therefore, there is a lack of air ahead of the front stage of the compressor, making it stall first. Usually this is followed by a complete compressor stall, and possibly a surge depending on the severity of the stall.
"II. A compressor stall is most likely to occur when the air flow stagnates in the rear stages of the compressor." CORRECT
  • As mentioned above, air stagnates in the REAR stages, which results in a lack of air in the FRONT stages with subsequent low axial velocity in front stages. This will lead to a high angle of attack of the rotor blades determining a STALL.
    To avoid such situation, turbine engines are fitted with VIGV (variable inlet guide vanes) and VSV (variable stator vanes) that ensure correct airflow through the stages of the engine.

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