Compression path in a closed-cycle gas turbine (theory vs. reality) In a closed Brayton-cycle gas turbine, how is the compression process most accurately described in practice?

Difficulty: Easy

Correct Answer: polytropically

Explanation:


Introduction / Context:
Thermodynamic textbooks idealize compression as isentropic for efficiency calculations. Real compressors, however, exhibit losses and follow a polytropic path, which more accurately captures the small-step efficiency across many blade rows.



Given Data / Assumptions:

  • Closed Brayton cycle using a working gas (e.g., helium, nitrogen, or air in sealed loop).
  • Finite mechanical and aerodynamic losses.
  • Steady-state operation.


Concept / Approach:
Polytropic compression assumes p * v^n = constant with an exponent n between 1 (isothermal) and gamma (isentropic for ideal gas). This provides an incremental efficiency representation and aligns with measured compressor maps.



Step-by-Step Solution:
Ideal (reference) path: isentropic.Real path: polytropic with n slightly greater than isothermal and less than isentropic exponent-related equivalent.Therefore, “polytropic” best describes the practical compression process.



Verification / Alternative check:
Manufacturers quote polytropic efficiency to characterise multi-row compression more robustly than a single isentropic efficiency figure.



Why Other Options Are Wrong:

  • Isothermal: unattainable without ideal heat removal during compression.
  • Isentropic: a useful ideal limit, not the real path.
  • Isochoric: volume does not remain constant during compression.


Common Pitfalls:
Confusing “closed cycle” with “isothermal”; the cycle closure does not imply isothermal compression.



Final Answer:
polytropically

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