I would draw your attention to look at the design of the recuperator of a gas turbine. The recuperator not only works as a heat exchanger but its design circulates in-coming compressed air like a diffuser.

Recuperator - Wikipedia, the free encyclopedia

What do you think I have done this afternoon- I read all those articles in wikipedia, and few more.

Anyway-thank you very much.

See my reply to Joshi

As I understand it, you recover some of the lost heat in the exhaust and increase the energy value of the inlet gasses. That way you don't waste energy heating your air.

I don't have the best understand of it. In some engines you want to cool inlet air so you get a more dense mixture with more oxygen, in some you want to pre-heat the air so fuel isn't wasted increasing the temperature.

Drew k

'...the temperature is raised while the pressure is constant (actually decreases a small amount due to frictional losses), so the volume increases....'

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I'm a little confused here. Help me out? Lets say our gas turbine is using a thermal wheel to perform recuperation. The thermal wheel transfers heat from somewhere post combustion to somewhere post compression and pre-combustion.

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Am I reading your statement correctly, in that you are saying the addition of heat (post compression/pre combustion) is causing pressure to stay constant (or reduce slightly due to friction), and is causing volume to increase?

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If that is the case, if we stop the thermal wheel from rotating (effectively stopping the most of the heat transfer), are you saying the pressure will remain roughly unchanged or maybe increase slightly, and that the main effect will be a decrease in volume?

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Where is this increasing/decrease in volume accommodated?

I was trying to remind OP that the gas law, P*V/T=constant, has a third component. And, in the gas turbine (Brayton) Cycle, that the heat addition, whether it's by fuel combustion or recuperation, is at (ideally) constant pressure. I assumed he was referring to steady state conditions (but after reading his post again, I may be wrong).

Your question, on the other hand, is related to an un-steady state condition where the heat input is changed. This changes most (or all) of the operating parameters throughout the turbine. This involves RPM, compressor characteristics, turbine characteristics and nozzles. But, after steady state is regained with lower heat input, the combustion pressure will be lower.

I think I misunderstood your original comment.

Consider is as simple combustion air preheater ( Air heater) A shell and tube you normally find to heat water with steam or thermal oil (heat exchanger) and heat water with exhaust of boiler (Economizer) or generate steam/hot water from exhaust of gas engines(Waste Heat Recovery Boiler) or gas turbines( Heat Recovery Steam Generators).

These are its closest family members.

<...If the compressed air is more heated, its pressure will increase, and therefore it will resist the incoming air from the compressor....>

This is incorrect. The gases passing through a recuperator do not increase in pressure, as there is no shaft work introduced into it. What does happen is that, as the temperature increases, the volume increases/the density drops.

My question was very bad formulated, because I wonder how doesn't the air that was heated by the recuperator and is placed in a closed environment-the tube that leads to the combustor which has no free space,behind is the compressor, and in front the combustor ,so maybe that all depends on 2 diffusers. The first blocking the way back to the compressor, and the second open the door to the combustor.

Or- am I totaly wrong , and this is working quite different!

Can you show a design that you are drawing this description from? I am not sure I am understanding what you are describing.

Drew K

You are almost there in your thinking.

This is a continuous operation: Air goes into the recuperator--gets heated-- pressure increases--pushes the air forward to the combustor--air pressure is releaved a bit--new air comes from the compressor line since the pressure has droped (being released out to combustor). The air cannot go back into the compressor easily since the compressors pressure will always be higher: a balance is reached so that the combustor gets the required air to operate at the rate required. To increase air intake, the compressor pressure is imcreased or the diffusers are opened (???).

If the compressed air is more heated, its volume will increase, and therefore it will resist the incoming air from the compressor.

My understanding when the air is heated it will require more volume. Similar to ACFM and SCFM. this usually is compensated with the design requirements and is only minor but still be considered.

One of the the issues at hand on recuperation which is also known as regeneration efficiencies aka regenerated effectiveness.

In the dairy inductries we try to recover some of this energy in pastuerization. A sketch is shown below, as you can see is the formula for determining its effectiveness, I just added some arbitary numbers in here for an example, as you can see, its effectiveness is not good, one would like to see above 80%, and rarely do you see above 92%:

In a process such as a Gas Turbine the recoup will look like this:

There is a very good book with this information I would suggest.

Jones, J.B., Dugan, R.E., (1996), 'Engineering Thermodynamics', Prentice Hall, Englewood Cliffs, New Jersey, 07632