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GATE 2024 AE – Question 49

Propulsion · Turbomachinery: Centrifugal compressors and axial turbines · 2 marks · Multiple select

Which of the following statements is/are TRUE for an axial turbine?

  1. For a fixed rotational speed, the mass flow rate increases with increase in the flow coefficient
  2. The absolute stagnation enthalpy of the flow decreases across the nozzle row
  3. The relative stagnation enthalpy remains unchanged through the rotor
  4. For a fixed rotational speed, the mass flow rate remains unchanged with a change in the flow coefficient

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Correct answer: (A) For a fixed rotational speed, the mass flow rate increases with increase in the flow coefficient; (C) The relative stagnation enthalpy remains unchanged through the rotor

Explanation

The flow coefficient is $\phi = \frac{C_a}{U}$. At a fixed speed $U$, a larger $\phi$ means a larger axial velocity and so a larger mass flow rate (A true, D false). The stationary nozzle row does no work, so the absolute stagnation enthalpy is unchanged across it (B false). Across the rotor, at constant radius, the relative stagnation enthalpy (the rothalpy) stays constant (C true).