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GATE 2024 AE – Question 50

Propulsion · Turbomachinery: Axial compressors, work and compression, degree of reaction, multi-staging · 2 marks · Multiple select

Which of the following statements is/are TRUE for a single stage axial compressor?

  1. Starting from design condition and keeping the mass flow rate constant, if the blade RPM is increased, the compressor rotor may experience positive incidence flow separation (actual relative flow angle greater than the design blade angle)
  2. Starting from design condition at the same blade RPM, if the mass flow rate is increased, the compressor rotor may experience positive incidence flow separation (actual relative flow angle greater than the design blade angle)
  3. Keeping the mass flow rate constant, if the blade RPM is increased, the compressor may experience surge
  4. At the same blade RPM, if the mass flow rate is increased, the compressor may experience surge

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Show answer and explanation

Correct answer: (A) Starting from design condition and keeping the mass flow rate constant, if the blade RPM is increased, the compressor rotor may experience positive incidence flow separation (actual relative flow angle greater than the design blade angle); (C) Keeping the mass flow rate constant, if the blade RPM is increased, the compressor may experience surge

Explanation

The relative flow angle from the axial direction satisfies $\tan\beta_1 = \frac{U}{C_a}$. Raising the speed at a constant mass flow raises $U$ but not $C_a$, so $\beta_1$ increases and the incidence becomes positive, and the blades stall, which can lead to surge (A and C). Raising the mass flow at the same speed raises $C_a$, which reduces $\beta_1$ and gives negative incidence, which leads towards choking, not surge (B and D false).