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Numerical oscillations in CFX with water boiling problem.

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Old   December 13, 2011, 03:59
Default Numerical oscillations in CFX with water boiling problem.
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Hi everyone, this is my first post here so nice to meet you.

I'm a CFX user and I'm trying to calculate a water boiling problem, but I'm getting weird results. The problem looks like this:


- Water enters an annular tube (space between two coaxial cylinders) where it is heated up through the inner cylinder wall until it starts to evaporate (see attached picture).

- The cylinder is 340 mm long, diameters of inner and outer cylinders are 12.7 mm and 25.4 mm. Water enters 31 K below the saturation temperature at a velocity of 0.4 m/s. Inlet Reynolds number is Re_inlet = 5071.

- A heat flux is applied through the inner cylinder wall.

- I believe the computational mesh is pretty good, what do you think? (see attached picture)

- Computational parameters: I'm using upwind scheme for the advection terms. I did not set the Wall Boling model, phase change is expected to occur when the liquid temperature reaches the saturation temperature.

THE PROBLEM: when water starts to evaporate I get spurious oscillations! I tried different things to try and suppress them but they're still there! (attached picture shows water vapour volume fraction on several cut planes, saturated at volume_fraction_vapour = 0.1)

THE QUESTION: Is there anyone among you who got this same problem in the past? Can you give me any hint as to what might be the reason of this problem?

I thank you guys for your help, this is driving me crazy...
Cheers.
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Old   December 13, 2011, 16:15
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Glenn Horrocks
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Why do you say this is spurious? Boiling is a very lumpy process and often ends up in slug flows and things like that.

Upwinding for the advection terms will cause excessive damping. Use a second order scheme.

Are you doing this steady state or transient?
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Old   December 15, 2011, 03:42
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Dear Glenn, I'm running it as a steady state-calculation.
I also tried the high order scheme available in CFX with similar results ...

I was trying to perform this simulation in order to validate this boiling model, so I took this case from the literature. In the experiment (and their simulations with CFX!) they get smooth solutions where vapour volume fraction grows along the tube so I was hoping to get similar results.

Thanks for your reply anyway.
Any other hint you might think of?
Thanks!

Cheers.
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Old   December 15, 2011, 16:18
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This FAQ covers some issues of relevance here:
http://www.cfd-online.com/Wiki/Ansys...gence_criteria
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Old   December 16, 2011, 09:00
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Dear Glenn,

thanks for your reply. I will have a look at the document.

Cheers.
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