Fluids

Specific heat calculation in supercritical fluids – FLUENT

• franciscocarvalho
Subscriber

Good morning,

I am simulation the injection of a liquid at supercritical pressure but at a temperature bellow the pseudo-boiling temperature into a warm chamber. It is expectable that as the temperature of the injected fluid increases, the specific heat increases sharply and then decreases. I simulated the conditions with the REFPROP library instead of an equation of state and the results are good. But when I use the Peng-Robinson or the Soave-Redlich-Kwong equation of state, the peak does not appear (the figures in attachment show what I mean). I used a UDF for the ideal cp in the material properties menu so we can assume that that is not the problem. It looks like the problem is with the residual specific heat calculated by FLUENT. It seems that it is limited to a maximum value but I do not know where this is set.

The last picture I send you is from different test conditions but you can see once again the limitation of the max cp when using the Peng-Robinson equation of state.

Any help would be very appreciated. Thank you!

• DrAmine
Ansys Employee

You mean with pseudo-boiling the critical temperature right?

The peak you are talking about is by crossing the critical point region (increase and drop). It is expected that NIST or REFPROP much more accurate here.

Can you please insert pictures instead of attaching them? Add any other information which can help us to understand the issue?

• franciscocarvalho
Subscriber

Dear Amine,

Thanks for the quick reply. I actually mean the pseudo-boiling temperature or Widom-line. Beyond the critical point, there is a like that separates the liquid like from the gas like supercritical fluids and I am simulating this transition. The cp in the Widom line is much higher on this line than in the surroundings and tends to infinity as the Widom line approaches the critical point. Injection of nitrogen at 126.9 K into a chamber filled with nitrogen at 300K. Chamber pressure is 4MPa.

I understand that the NIST database is far more accurate, but the results I obtain with the Peng-Robinson equation of state are just unrealistic. The three pictures I mentioned are bellow.

This is with the REFPROP database:

And this is the same case with the Peng-Robinson EoS:

This last case is with the Peng-Robinson EoS but at a lower injection temperature. As you see, the max cp is still limited at the same value as in the previous case.

• DrAmine
Ansys Employee

Can we think the position to correlate with temperature?

• DrAmine
Ansys Employee

Or plot the temperature along the position.

• franciscocarvalho
Subscriber

Dear Amine,

Here you have the axial temperature distribution for the real gas library:

and here for the PR EoS:

• franciscocarvalho
Subscriber

Dear Amine,

Do you have any clue what the problem could be?

Best regards

• DrAmine
Ansys Employee

First of all: you need to be patient on this open forum.

We expect that with CEOS one will see the widow lines.

1/Is it possible to try in CFX?

2/In Fluent: Can you contact ASC so that he can communicate with local support about a possible issue?

• franciscocarvalho
Subscriber

Dear Amine,

My apologies for rushing things.

I have not used CFX yet but I can try.

I am not sure if this is what you mean but the address of my university is Calçada Fonte do Lameiro 10, 6200-358 Covilhã, Portugal

Best regards

• DrAmine
Ansys Employee

We need to check if this is really an issue and we need to communicate outside of this forum.

• franciscocarvalho
Subscriber

Dear Amine,

• DrAmine
Ansys Employee

Thanks!

• franciscocarvalho
Subscriber

No problem. I really appreciate your help!

I'll be waiting for an email from you.

Best regards

• DrAmine
Ansys Employee

Please mark this as "Is Solved". For the community members: the heat capacity is limited to certain value based on the departure heat capacity when using real cubic EOS. This behavior will be enhanced for the future so that one can adjust the thresholds.