What is the purpose of combustor in a gas turbine

Question

I have read online that a gas turbine converts the chemical energy in the fuel gas to mechanical energy of the turbine blades. The way I see it, because of this hot air one would have to go for heat recovery and use costlier heat resistant material in the turbine. So instead why not send the compressed air directly through the turbine blades?

As an example, in wind turbines I understand we try to work on factors to improve the wind speed which in turn would mean more mechanical energy of the blades and in turn more electricity. I am trying to understand the purpose of the combustor similarly. Thanks for your help!

Answer 1

The turbine blades are what turns the compressor blades in the first place. They do this by extracting work from the hot gas shooting out of the combustors at high speed, and feeding it forward on a spinning shaft to the compressor blades. Those blades then suck air into the engine and force it into the combustors hard enough that the hot gas generated in the combustors (by the burning fuel) can only exit the combustors in the direction of the hot section turbine blades.

If no fuel was burned in the combustors, there would be no way to turn the compressor, and the engine would not produce any thrust.

The thermodynamic cycle upon which a jet engine operates is called the brayton cycle and I suggest you study it in order to have a full understanding of how a jet engine works.

Answer 2

The best way to approach this question is through basic thermodynamics.

Suppose you have a compressor and a turbine but no combustion in between. Then we can define processes in this machine as follows:

1 to 2: isentropic compression (for simplicity)

2 to 3: nothing

3 to 4: isentropic expansion (in turbine)

I urge you to draw a P-V diagram at this point. You will see that your cycle will have no closed area on a P-V curve- hence the cycle will do no work on the air coming at it.

Now add a combustion chamber in between 2 and 3. Then in this region, by using ideal gas, you’ll see that you will be able to increase pressure at constant volume and thus actually enclose a nontrivial area under your curve in the P-V diagram. Thus, the machine will be able to do work on the flow, which is its primary intended use.