[leon_jar]

Hi guys,
I try to figure out how to simulate multiphase flow but I’m a bit confused. I read manual/theory guide plus posts form the forum but still not sure if I understand correct and what models/options should I applied. I want to simulate the flow as attached picture. Case is initialized with air volume fraction=1 for both domains. Then, water flows to rotating pipe. The pipe has two outflows. The purpose of the flow is to cool down stationary walls of grey cylinder. Water outflows from the domains through the pipe at the bottom. I build two domains, one contains rotating pipe and another one is a stationary cylinder. Now, I have some doubts regarding setting up the multiphase flow:
- I defined two fluids: air and water, both as Continuous Fluid,
- I set the gravity and Buoyancy reference density equal to air density,
- In Fluid models tab , for heat transfer I use homogeneous model
- I’m not sure what models of multiphase to use homogeneous or inhomogeneous?
- Does the free surface model should be used (in my opinion yes)?
- In Fluid Pair Models I'm not sure if Surface Tension Coefficient, Surface Tension Model, and Interphase Transfer model should be used?

Thank you for any help and advice.

[ghorrocks]

You need to know what your flow is doing before you can answer your questions. First of all, read the CFX documentation on multiphase flow. You need to know the background to choose the right options.

Questions you need to ask are:
* Is the water small droplets? A continuous stream? Does it form a big puddle?
* Is the air continuous? Bubbles? Does foam form?
* How big is this system? What is the flow rate? How hot is it?

[leon_jar]

The answers are:
-it is a continuous stream of water, mass flow of water is 0.2 kg/s
- yes, air is also continuous. When the simulation gets steady, air should be trapped at the top of the cylinder and form free surface interface with the water
- diameter of the cylinder is h=20 in, d=10 in

[ghorrocks]

It appears a homogeneous free surface multiphase model will be suitable for this. Have a look at the flow over a bump tutorial (in the CFX tutorial documentation) for an example of how to set this up.

[leon_jar]

Thank you for your help.
But the fact, that there will be probably some splashing due to rotating pipe isn’t it better to use inhomogeneous model? In mentioned tutorial, I can find a note that since mass transfer in not modeled I do not need to model surface tension. But what about the Interphase transfer model. I can’t find any information about it. Could you briefly explain me what is it?

[ghorrocks]

You did not mention any splashing....

Is the splashing significant? Is your mesh going to be fine enough to resolve it?

What do you need interphase transfer for? Is it boiling or condensing?

[leon_jar]

Yes, splashing is significant and mesh is fine at this place.
About the Interphase transfer model. The problem is that I can’t find any information about this model. It isn’t mass transfer model, so I don’t know what exactly it is. There are three options to choose one of them: none, Mixture model and free surface. If it is a model to simulate boiling or condensation so I don’t need. There is no mass transfer. Just heat transfer from walls to air/water and from air to water.
thank for your help

[ghorrocks]

Then choose the "None" option.

[leon_jar]

Thank you very much