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How is energy wasted in a jet exhaust leaving the jet engine?
More is the momentum imparted to the jet exhaust leaving out back, more is the thrust that can be generated. How is the energy wasted?
Answer:
If you measure the temperature at each turbine stage, you will find that the gas temperature decreases as energy is taken out to turn each rotor group. For example, in a typical three stage turbine engine, the first turbine may be subjected to a gas temperature (turbine inlet temperature) exceeding 1900 deg F. As the energy from the flow is used to turn the first turbine, that temperature is reduced. The second turbine may only see 1600 deg F, and the third turbine perhaps 1400 deg F. The exhaust temperature out the back (after the third stage) may be "down" in the 1200 deg F range. But ideally, if all the energy were efficiently removed from the gas path, the exhaust temperature would be the same as the surrounding air. That cannot occur because of issues like entropy and irreversible processes, but what's left over is wasted energy that cannot be recovered - like the heat from your car engine.
ADDED: As mentioned above, there are other areas where waste occurs; noise, vibration, etc. but the largest by far is heat.
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