Why does air have to be compressed first in gas turbines?

[Science20]

In all gas turbines air is first compressed using a compressor and then head added in a combustor and finally the hot pressurized air is expanded in nozzles to convert air contained energy to kinetic energy which can be harvested and transferred in the form of thrust or shaft rotation.

My question is what does exactly the compression step do? besides pushing the air through the turbine does it have other purposes? and how increasing the compression ratio increase turbine output power?

The question rephrased: Does increasing the air pressure ahead of the turbine expanding nozzles results in higher nozzle exit velocities and why?

 

[anuttarasammyak]

Gas should be contained in small volume so that it inflates to work.

 

[anorlunda]

If there was no compressor, what is there to stop the expanding gas from flowing out the front instead of the back? One solution to that is the pulse Jet.

Another question. Is there more oxygen in one liter of compressed air or one liter of unpressureized air?

 

[Science20]

So it is all about combustion temperature and heat added to the air?! ...more air = more oxygen = more fuel can be injected = more heat added to the air = air volume increase = expansion of higher volume air produces higher velocities = more work produced

right?

well, but know that the gas turbine operate with very lean air mixture which means even if reduced amount of air was introduced to the combustion section all the fuel will still be fully burnt so the combustion temperature will not be reduced ...what am I misunderstanding here?!

 

[anorlunda]

even if reduced amount of air was introduced to the combustion section all the fuel will still be fully burnt

At some point, all the oxygen will be used up and more fuel cannot be burned.

The factors mentioned above are not the whole story. Momentum is conserved. The momentum of exhaust gasses is calculated as mass times velocity. You have been focused only on velocity. What about the difference in mass flow with and without a compressor?

But word exchanges are not the best way to gain understanding. You need to look at the equations and the numbers. Below is a simplified thrust calculation from the Wikipedia article on turbojet engines.

 

[russ_watters]

You really need to look at the thermodynamics to properly answer the question. While it is true that you must have a pressure increase to drive air through the engine and higher [compressed] volumes mean better thrust to weight ratio, that's not the main issue and doesn't explain why efficiency (for all thermodynamic engines) increases with pressure ratio. From wikipedia:

Pressurizing is what creates the difference between the two halves of the cycle. Without a compression step 1-2, there is no power extraction step 3-4, you just move back and forth on the heat input and rejection steps 2-3 and 4-1 without generating any power.

It's worth noting too that the energy of compression isn't lost, it stays in the working fluid, to be extracted back out later.

 

[Lnewqban]

... well, but know that the gas turbine operate with very lean air mixture which means even if reduced amount of air was introduced to the combustion section all the fuel will still be fully burnt so the combustion temperature will not be reduced ...what am I misunderstanding here?!

A rich mix is not desirable, more fuel than air needed to fully and timely burn it is a no-no.
All combustion engines try to reduce incomplete combustion as much as possible.

In some cases, excess of oxygen is deliberate in order to avoid non-burned fuel that reduces the temperature and useful work of the combustion, and increases de amount of harmful chemical subproducts (bad for the engine itself and for the environment).

Please, see:
https://en.m.wikipedia.org/wiki/Combustion#Complete_and_incomplete