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Which Multiphase Model?

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Old   December 3, 2003, 05:40
Default Which Multiphase Model?
  #1
Bob
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Hi,

I am having trouble in deciding which multiphase model is best suited to solving my problem.

Basically I am a modelling a straight sided cyclone (vortex tube) that is placed in a large pressure vessel so a lquid level comes half way up the cyclone. Under operation the swirling gas will impart momentum onto the liquid phase and start it rotating so a "v shaped" profile shall be formed inside the cyclone. The gas should exit out the top of the device.

I have tried using the VOF implicit option under steady state and transient but I cannot get the liquid to take the correct profile. Am I using the correct model? What are your suggestions?

Many Thanks
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Old   December 3, 2003, 09:15
Default Re: Which Multiphase Model?
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mateus
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Hi!

I thik that only VOF model can solve this problem. You are clearly dealing with two noniterpreting fluids... The VOF model can be a bit stuberen to work with so it's normal to have problems at first. Try to use different time steps, and discretization schemes. 2nd order discretization can also do the trick...

Hope it helps

MATEUS
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Old   December 3, 2003, 09:46
Default Re: Which Multiphase Model?
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Bob
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Thanks mateus.

This problem is a bit like the Fluent tutorial of water in a spinnning bowl apart from instead of the wall imparting momentum to the fluid, in my problem the high velocity swirling flow is causing the water to rotate and move up the side of the walls.

I have to use a time dependant simulation because the final steady state solution will be dependant on the initial hieght of the liquid in the cyclone (half way up).

I am using PRESTO for pressure, PISO for pressure-velocity coupling and second order for all other variables. I shall try you suggestion of changing the time steps. If you have any other suggestions I would much appreciate it.

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Old   December 3, 2003, 13:04
Default Re: Which Multiphase Model?
  #4
manu
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Hi!

Some points which will maybe help you... -is your mesh strongly refine near the wall? - some time it's good to run first the model with a single phase (gaz) and then you change the part of the model you want in water.

Hope it can help... good luck..

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Old   December 3, 2003, 13:21
Default Re: Which Multiphase Model?
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Bob
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My mesh is quite course at the moment. Are you saying I will get a better result if I refine the mesh?

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Old   December 3, 2003, 17:39
Default Re: Which Multiphase Model?
  #6
Anton
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This is going to be a trial and error process with VOF. Because you need a transient solution and want to accurately capture the liquid-air interface, you'll need to use the geo-reconstruct scheme, which means small time-steps. You may also want to consider dynamic adapting the grid to refine the interface as it moves in time. Good luck.
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Old   December 4, 2003, 04:52
Default Re: Which Multiphase Model?
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Bob
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Hi Anton,

Can you please explain your reasons why the geo -reconstruct scheme would be better in this simultation?
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Old   December 4, 2003, 06:21
Default Re: Which Multiphase Model?
  #8
manu
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Well, I m not completly sure but you could try. It won't be worst and it could be better. The point is that it's better to have more nodes where the gradient of velocity are the higher which means near the wall or the cyclone (in you case)

In addition I don't know which values of velocity you use but the best things is to start with a low one and then increases it gradually.

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