Error in the Documentation and Modeling of the Polytropic Process in Positive Displacement Compressors

Hi, it seems that in the documentation of the Positive displacement compressor for all the domain there is an error in the formula used to compute the fluid power for polytropic process.

in particular the pressure ratio (p_in/p_out) is wrong, and from theory shoud be (p_out/p_in)...

After running some tests, I noticed that the issue is not only in the documentation but also in the model implemented in the block. Specifically, it seems that the block implements neither the incorrect formula with the inverted pressure ratio nor the correct one but rather a different incorrect formula.

[Torsten]

You can report a bug here:

http://www.mathworks.com/support/bugreports

[Yifeng Tang]

Hi @ROBERTO,

I agree it should be p_out/p_in.

Could you please share the test model you mentioned? I'd be interested in seeing the behavior you mentioned.

Thanks!

[ROBERTO]

Hi attached you have the test model

Answer 1

[Yifeng Tang]

Hi @ROBERTO,

This looks like an error in the documentation to me right now.

I measured a few more quantities from the model. Looks like the temperature change does follow the polytropic relationship (good!) of . And the measured energy flow difference of the fluid does match the reported fluid power of the compressor.

I'm not sure what the correct formulation should be yet. I'll look into that and reach out to the development & documentation team.

Many thanks!

[ROBERTO]

Thank you very much. Let me know when the problem will be fixed!

Best regards Roberto