Confusion about energy conversion mechanisms in gas turbines

[Science20]

TL;DR Summary

Confusion about energy conversion mechanisms in gas turbines.

Hello Gents,

I am very confused about the working principle of gas turbines. I understand that air is first compressed in a compressor to obtain high pressure air at the inlet of turbine, but before air is introduced to the turbine, first it has to be heated to very high temperatures through mixing with fuel and combustion process.

The turbine consists of alternating rows of stator vanes and rotor blades. The vanes or nozzles expand the high pressure high temperature air and hence convert the pressure energy to kinetic energy (high velocity jets of air), the high velocity air is directed to strike the turbine wheel causing it to rotate and produce shaft work.

What I am not understanding well is what is exactly the function of combustion step between the compressor and the turbine? Does heating cause air pressure to rise so the air gains more pressure energy and hence higher velocity air jets are produced by nozzles? If no, what happens then? and why they say that turbine efficiency is improved with increasing turbine inlet temperature and also by increasing pressure ratio of the compressor?

 

[Chestermiller]

Are you sure the air is mixed with the hot combustion gases, or just contacted with these gases to allow heat exchange with them in a heat exchanger?

 

[anorlunda]

Does heating cause air pressure to rise so the air gains more pressure energy and hence higher velocity air jets are produced by nozzles?

I assume that you are trying to understand jet engines, rather than gas turbines used to generate electric power.

Modify your thoughts slightly.

Mass is conserved. Therefore the mass of air in plus the mass of fuel in must equal the mass of exhaust out.

The volume of the gases are greatly increased by burning and high temperatures. Therefore the volume flow of gasses out at the exhaust is much larger than the volume flow of air plus fuel coming in.

What does that tell you about the velocity of the exhaust gases compared to the velocity coming in? There is no need to even mention compressors, turbines, blades, or pressure to answer that question.

In fact, it may be simpler for you to first understand the ramjet engine that has no compressor, no turbine, yet still follows the same principles of physics.

 

[Science20]

Are you sure the air is mixed with the hot combustion gases, or just contacted with these gases to allow heat exchange with them in a heat exchanger?

Well, I guess non combustion air is heat exchanged with combustion gases so that the temperature of the air at turbine inlet is very high. But why?

 

[Science20]

I assume that you are trying to understand jet engines, rather than gas turbines used to generate electric power.

Modify your thoughts slightly.

Mass is conserved. Therefore the mass of air in plus the mass of fuel in must equal the mass of exhaust out.

The volume of the gases are greatly increased by burning and high temperatures. Therefore the volume flow of gasses out at the exhaust is much larger than the volume flow of air plus fuel coming in.

What does that tell you about the velocity of the exhaust gases compared to the velocity coming in? There is no need to even mention compressors, turbines, blades, or pressure to answer that question.

In fact, it may be simpler for you to first understand the ramjet engine that has no compressor, no turbine, yet still follows the same principles of physics.

View attachment 282703

No I am concerned with gas turbines only.

This is mind opening. Thank you. So the volume of air greatly increases due to heat gain in the combustor, and when this big volume of air pass through stator nozzles it exits at higher velocity than if it was not heated in the first place. It does make sense now.
Do you have an idea why turbine efficincy still increases with air inlet temperature ?

 

[anorlunda]

Do you have an idea why turbine efficincy still increases with air inlet temperature ?

That's very complicated. It has to do with the airfoil shapes of the buckets (also called blades.) and with energy differences between the combustion chamber and the exhaust. I don't think I can give you a simple answer. Only a complete analysis considering all variables gives a complete answer.

Any turbine, even steam turbines in power plants, has higher efficiency if we have higher temperature and more pressure at the inlet.

By the way, in combined cycle plants, the hot exhaust from the gas turbine goes to a boiler to create steam which drives a steam turbine. They have two generators, on on the GT shaft, and one on the ST shaft. In that case, the best design does not extract all the energy from the GT exhaust, because that energy can be useful in the steam boiler. The designer optimizes the whole plant, not just the GT.

 

[Lnewqban]

Do you have an idea why turbine efficincy still increases with air inlet temperature ?

Higher air inlet temperature is always bad for the efficiency of any internal combustion engine, since less mass of oxygen for combustion is available.

Copied from
https://en.m.wikipedia.org/wiki/Engine_efficiency

"The efficiency of internal combustion engines depends on several factors, the most important of which is the expansion ratio. For any heat engine the work which can be extracted from it is proportional to the difference between the starting pressure and the ending pressure during the expansion phase. Hence, increasing the starting pressure is an effective way to increase the work extracted (decreasing the ending pressure, as is done with steam turbines by exhausting into a vacuum, is likewise effective).
...
One other factor negatively affecting the gas turbine efficiency is the ambient air temperature. With increasing temperature, intake air becomes less dense and therefore the gas turbine experiences power loss proportional to the increase in ambient air temperature."

 

[jack action]

Do you have an idea why turbine efficincy still increases with air inlet temperature ?

With any combustion engine, a higher temperature before expanding the gas means more energy has been extracted from the combustion process (assuming you are using the same amount of fuel, that is).

Then, the expansion process will convert the energy from heat to mechanical energy. Obviously, the greater the expansion ratio, the more energy is converted, therefore the more efficient will be the engine (again, more mechanical work for the same amount of fuel consumed).

 

[anorlunda]

Also, I was referring only to the turbine section of the gas-turbine engine. In other words, the temperature at the inlet of the turbine stage after combustion. That's a general statement for any turbine. But I didn't make it clear. It sounded like the inlet temperature for the whole engine.