Brayton cycle in helium atmosphere with existing gas turbines?

[alexisgros]

Hello everyone,

I was wondering what would the impact be if an existing gas turbine made to run in air at atmospheric pressure was run in an atmosphere of helium (let's say in a closed loop heated through a heat exchanger). How would the power output and efficiency be impacted, and what would be the impact of the helium pressure on the system?
For example, would the power output be equivalent if the helium pressure was such that its density matched that of air?
How would the higher conductivity of helium impact the plant?

Thanks in advance for your ideas and input!

 

[BvU]

A bit hard to imagine a combustion chamber with helium only. Can you provide a sketch ?

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[alexisgros]

Yes, it would be in the case of a closed brayton cycle with a heat exchanger instead of a combustion chamber. These systems were studied for power production in spacecraft.

The reason I asked this question in the aerospace engineering forum is that I wanted to know the impact of such a cycle when using it with existing, for example aeroderivative gas turbines to be run in a helium environment.

 

[jack action]

Why don't you tell us what you find?

Here, you have all the necessary equations to evaluate a Brayton cycle.

Here, you have the physical properties of helium, and here the ones for air.

 

[BvU]

Yes, it would be in the case of a closed brayton cycle with a heat exchanger instead of a combustion chamber. These systems were studied for power production in spacecraft.

The reason I asked this question in the aerospace engineering forum is that I wanted to know the impact of such a cycle when using it with existing, for example aeroderivative gas turbines to be run in a helium environment.

I see. Your 'run in He' got me confused when opposed to 'run in air'. You mean He as a working fluid.

 

[Flyboy]

I think they already have helium working fluid Brayton turbine designs, if not necessary in hardware form then certainly as highly refined designs.

As for “aeroderivative gas turbines”, I think that’s not viable. The differences between an internal combustion gas turbine and a Brayton turbine optimized for the kind of application you’re proposing are numerous and costly enough to warrant a clean sheet design.

 

[alexisgros]

Thank you Flyboy and jack action, I have the answers I was looking for. The equations for the brayton cycle are nice but the real question is how would the efficiency of the compressor and turbine stage be affected by the use of helium, and it appears that it is affected enough to warrant redesigning the whole plant. Helium leak issues also make it challenging to reuse an existing air breathing engine.

 

[Benjies]

Maybe I'm misunderstanding something, but given the existence of entropy throughout this system, I don't see what this theoretical cycle you are proposing "does". In the engine you are trying to convert into a helium heat exchanger, the turbine exists to drive the compressor. This is the sole focus and function of the turbine- to drive compression.

So, it appears to me that you are proposing an engine in which we compress helium, then expand it, and introduce a turbine to drive the compression process... Due to temperature difference between the engine core and the exterior of the engine, and imperfect turbomachinery losses, you will experience entropy and energy loss throughout this engine.

With a traditional combustion engine, these losses are compensated for through converting chemical/potential energy into thermal energy; however, in your system, there is no addition of energy to compensate for these losses. Therefore, it appears to me that all you've done is introduced an "entropy-generating system". Now, if you wanted to impart energy to something exterior to the system through the transfer of heat from the heat exchanger/chamber, that's a different story, but this comes back to my original question: What exactly are you trying to achieve with this when you say it will "generate power" for your larger system? Because as it stands, Ein > Eout, and in the absence of converting chemical energy to useful energy through combustion, you don't seem to be achieving anything at all.

I'm probably just missing some context for your application.

 

[Flyboy]

Maybe I'm misunderstanding something, but given the existence of entropy throughout this system, I don't see what this theoretical cycle you are proposing "does". In the engine you are trying to convert into a helium heat exchanger, the turbine exists to drive the compressor. This is the sole focus and function of the turbine- to drive compression.

So, it appears to me that you are proposing an engine in which we compress helium, then expand it, and introduce a turbine to drive the compression process... Due to temperature difference between the engine core and the exterior of the engine, and imperfect turbomachinery losses, you will experience entropy and energy loss throughout this engine.

With a traditional combustion engine, these losses are compensated for through converting chemical/potential energy into thermal energy; however, in your system, there is no addition of energy to compensate for these losses. Therefore, it appears to me that all you've done is introduced an "entropy-generating system". Now, if you wanted to impart energy to something exterior to the system through the transfer of heat from the heat exchanger/chamber, that's a different story, but this comes back to my original question: What exactly are you trying to achieve with this when you say it will "generate power" for your larger system? Because as it stands, Ein > Eout, and in the absence of converting chemical energy to useful energy through combustion, you don't seem to be achieving anything at all.

I'm probably just missing some context for your application.

He was asking if it was practical to take an existing gas turbine engine and modify it to be a Brayton cycle unit working with helium. The answer was a pretty resounding “no”.