Learning Forum

[Swen Groen]

Hello,

Currently I work on a project related to the optimal designs of cryo-ablation catheters. Cryo-ablation uses the phase transition and gas expansion to freeze tissue locally. I have setup the model for the tissue part and but am struggling with the fluidic part.

Beneath is the mesh that I use for a axisymmetric model, where in the inner part is the inlet for liquid coolants, near the tip is a restriction to increase the pressure-difference between the outlet and the end of the inner tube.

Currently I am running into issues with setting up a real gas model for improved accuracy of the Joule-Thomson effect.
Here is a list of things that I have tried and with the issues arrising in different fluent version (2022R1, 2022R2 and 2023R1) :

1. User-defined real-gas-model 

I tried using the UDRGM provided from Ansys to check functionality and modify the constants. Compiling and loading the UDRGM is succesful. I try loading the UDRGM with:

define/user-defined/real-gas-models/user-defined-real-gas-model

I get the following error message:

I cant seem to figure out what this issue is.

2. Ansys Defined Real-gas models

I setup a defined Real-gas-model in the material properties:

With this method I run into convergence problems due to these models being less reliable in the liquid phase , I could select a multiphase for phase-transition but more on this later.

3. NIST Real gas tables

The third method I tried was using the NIST and create look-up tables. The issue with this method is due to the iterative process the temperature gets out of range of the lookup table and the simulation stops. To reduce this error I use an UDF to increase the pressure at the inlet gradually towards the desired temperature, to decrease high pressure drops in a specific cell during the iterative process. Even with the gradual increase I get the following error:

But when I look into the results of the previous iteration I see the following results:

So this is nowhere close to the mentioned 128 K or the boundary of 216K in that regard

 

So these are the methods that I have tried without much success.

4. Lookup tables

I have one other method that I still want to try using lookup tables to decrease the computational power while increasing the robustness of the simulation in itself.

For this I want to use the real gas models to determine the values at certain temperatures and densities, which has shown success in literature: Lookup tables in literature

For this I have a question: To ensure I use the same formulas as the NIST is it possible to locate the tables from the NIST lookup table outside of Ansys from this checkmark:

I can recreate the lookup tables with the formulas for the real gas model used by NIST, but would like to have a double check whether the values are correct.

Lastly, I was thinking of using a multiphase flow incorporating multispecies real gas models. However, I do have a theoretical question: is this really necessary? Since the real gas models should describe the state they are already in based on their density, so besides having the possibility to easily visuallize them what is the added benefit of modelling the phase transition in multiphase compared to a real gas model? I am new to the real gas models and might have misunderstood some of the physics behind it.

 

Hope somebody can help me with these issues.

 

Kind regards,

Swen

[Rob]

Real gas doesn't behave well around the phase change. You're generally safer with a multiphase model to handle the phase change. Unless you really need real gas I'd always favour ideal gas + phase change: note I don't tend to model much in this application area so can't comment in great detail (I'm also limited by the forum rules). 

I'm a little surprised the model ran with that mesh, you're going to need a lot more cells to pick up the boundary layer effects and all the other fun stuff when real gases start flowing.