Why does increased turbine temperature causes increased efficiency?

[MaxHelixAngle]

Hi TheBeak,

Not an expert, however have just asking myself the same question before you asked it and have pulled out the old BGT and Thermodynamics text books.

The Theoretical Thermal Efficency of the engine (theoretical because it does not take into account any practical losses which must occur in running an engine) can be defined as: [work done / heat supplied] or [(heat supplied - heat rejected) / heat supplied]. The easiest way of considering the above is forgetting the calorific effect's of fuel and assuming that the working substance of the engine is just air that is heated by some source, the only addition of energy to the air is heat. A certain amount of heat is supplied to the air and a certain amount is expended, the missing heat must have performed the work. The air is only usable through the turbines and as a propulsive jet, down to a certain temperature, any energy (heat) in the air below that is wasted. Thefore the higher the heat is above that minumum, with the same amount of heat wasted, the greater the efficency.

This can be seen on a PV diagram, where it can be broken down into the work achieved by different parts of the cycle. I don't understand why the Carnot cycle has been mentioned as this is a cycle based of isothermal change (heating the air at a constant temp). Check out the Braynton cycle, this is the theoretical cycle of a turbine engine (heating at constant pressure).

Hope this is of some help, will watch on this thread with much interest to see if some of the more learned contributors can provide a more detailed explination.

Cheers,
MHA