## Fluids

#### Warnings in Real Gas Simulation

• Stoki
Subscriber

Hello everyone!

I’m simulating the flow of CO2 condensation. I use the Non-equilibrium Phase Change with CO2 liquid and gas defined in RGP file. The first time I didn’t select the nucleation model. The solver always alert that some variables went out of bounds like this:

It puzzled me when I examed it in details.

For example, the second variable CO2l.Density(liquid CO2 Density) is listed becasuse the CO2l.Static Temperature(Max value 347.69K) is upper than tabluated.  But actually in the Liquid CO2 RGP table, temperature range is up to 450K. And in my opinion, when the temperature is 347.69K, the solver should read properties from Gas table rather than Liquid table.

The same questions occurs to other variables like CO2g.Saturation Temperature.It is Absolute Pressure as large as 16.98MPa that result in the error. But when the pressure is up to 16.98MPa, it is far away from the critical pressure(7.3MPa), which means that it is in supercritical region and there shouldn’t be any saturation data, let alone CO2g.Saturation Temperature.

Is there any reasonable explanation?

Best regards

Stoki

• Stoki
Subscriber

Any idea? Need Help!

• DrAmine
Ansys Employee

1/can you elaborate more on b.c. and especially the domain interfaces and which models are activated here

2/does single phase flow work fine

3/Which EOS is used?

It might be related to clipping and interpolation of proporties along the saturation line or above the critical pointo but not sure at this moment.

• Stoki
Subscriber

Thanks a lot for your reply!  I'm  simulating an impeller working in CO2. I'll  try my best to explain the simulation.

1.  The object of the simulation is an impeller. For B.C.,details is in follow:

Inlet: Total Pressur and Total Temperature(307K) for Gas CO2 phase, as well as volume fraction and droplet number as 0 for Liquid CO2 phase.

Outlet: Total pressure

Interface: the interface here is a clearance between impeller and shroud surface. Details are in picture.

The fluid model setting is here:

2.  Yes, a single phase of Gas CO2 works fine and I use it as an initialization for multiphase simulation. I try the multiphase because there seem to be a very small region where condensation may occur.

3.  I use the RGP file to define the material. The file is generated by the tool NISTtoRGP.  The Gas table, Liquid table and Saturation table are all from NIST data.

Best regards

Stoki

• DrAmine
Ansys Employee

The question is there is problem at the interface. Do have strong non-overlapping there? Can you perhaps try your operating pressure and fluid to the non-equilibrium tutorial to check if it is because of the geometry and mesh and to check if the RGP table is fine or use IAPWS EOS of status with same operating range as in the tutorial on your geometry.

• Stoki
Subscriber

1.Use IAPWS library rather than RGP file. There are no warnings.

2.Remove the clearance from the geometry, so that no interface setting is needed. However while using CO2 RGP table, the warnings remain as follow:

Perhaps it is the RGP file that matters. But since it works well in single phase(liquid or gas) simulation, I’m confused why they don’t work in combination.

• DrAmine
Ansys Employee

Bear in mind that even if the range tells you that you expecting vapor or liquid to be there the solver is always calculating properties for both components. Sometime the warnings might be ignored as they probably disappear as the solution proceed.

Which ranges require for your CO2?  Can you extend the range?

• Stoki
Subscriber

Thanks for your patience and understandings!

I enlarge the range(Temperature: 220K-550K, Pressure: 3MPa-40MPa,500x500 points) to make sure the everything is included. But the warning remains.

Best regards

Stoki

• DrAmine
Ansys Employee

Hi,

Please check what I also mentioned in my last message.

• Stoki
Subscriber

The range of Temperature in 220K-550K and Pressure in 3MPa-40MPa will be OK. Of course it is a file for test and I'll never spread it.

Again thanks for your patience and understandings!

Best regards,

Stoki

• DrAmine
Ansys Employee

Perhaps the RGP tables are not proper.

• DrAmine
Ansys Employee

Provided some information to help and perhaps other community members might chime in.

• DrAmine
Ansys Employee

• abbas
Subscriber

Hi, dear Amine

Could you please send me your RGP table for CO2. I generated two different RGP table for both liquid and vapor phase CO2. I am trying to use Homogenous Binary Mixture. But in saturation property, I have to add a saturation table, even though I put the saturation in the liquid RGP file.

Thanks

• DrAmine
Ansys Employee

Hi,

I have just highlighted the necessity of enlarging the range for RGP generation.

There is actually a method which involves AIM and its AFD solver. This method is clarified for commercial customer on the customer portal.

Please create a new thread for your question and add more details: perhaps you do not need any other table or RGP files.