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My understanding of gas turbines is as follows:

  • When you spin an impeller, you can compress air (this requires work) and when compressed air is expanded, it can be used to spin a turbine (this generates work).
  • Thermodynamically, when you heat air, you increase its enthalpy, which is proportional to the work output you could get from expanding the air.
  • However hotter air requires more work to compress.
  • So, if we compress cool air (which also has the side effect of increasing temperature), heat the air (combustion chamber), then expand hot air, the power generated by the expansion (turbine) is greater than the power consumed by the compression (compressor). And this difference can be used elsewhere, e.g. to generate electricity.

However, I am struggling to reconcile this understanding with the Carnot cycle, which is the ideal heat engine.

With a Carnot cycle, the heat addition is isothermal, so the enthalpy of the air is not actually increased and the air is not actually "hotter" after heat addition - so my above explanation sort of falls apart.

Is all the heating done during the compression? So you start with cool air, compress it, and then you have high-pressure, hot air?

Could someone pin-point what's wrong with my current understanding?

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  • $\begingroup$ This is a googleable question: thermodynamics-engineer.com/gas-turbine $\endgroup$
    – Gert
    Commented Jan 4, 2022 at 21:56
  • $\begingroup$ @Gert I know that the equations give the right answer, but I want to clarify my physical understanding of how gas turbines work $\endgroup$
    – V. Jain
    Commented Jan 4, 2022 at 22:00
  • $\begingroup$ The 'physical understanding' (whatever you understand by that) comes from the understanding of the equations. Objects like that can only be described by the language of mathematics. $\endgroup$
    – Gert
    Commented Jan 4, 2022 at 22:06
  • $\begingroup$ A search for "Joule cycle" "Carnot cycle" on Google Scholar lists dozens of peer-reviewed articles discussing the two. $\endgroup$
    – Chemomechanics
    Commented Jan 5, 2022 at 1:28

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For a Joule or Brayton cycle (gas turbine), air is compressed, and heat is then added at constant pressure. the heat causes the air to expand, and to maintain constant pressure and maintain mass flow continuity through the engine then requires the hot air to accelerate on its way out of the engine. This means there is a reaction force generated in the engine which is conducted to the engine mounts (via the compressor turbine blades and the turbine disc thrust bearings) which is the thrust output of the engine.

You will find a thermodynamic cycle diagram for the brayton cycle on wikipedia, which details each step in the process including the power extraction that is used to spin the compressor.

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