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Old   March 16, 2014, 02:57
Question Water diffusion into air
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Mahmoud Gabr
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Dear experts;
I have a problem in modelling my first multiphase model. simply, it is a static water in a closed volume with air (volume fraction is 0.3 for liquid water and 0.7 for air), All volume walls have 80 C0 temperature. volume is closed so no inlet or outlet.
The required is the volume fraction of water vapor due to diffusion (there is no boiling in this problem) in the steady state case and then it would be converted to transient case for calculating diffusion rate.
I have tried to model it by define fluid phases as follow; Air , water and water vapor. I had selected homogeneous multiphase model with homogeneous heat transfer model. turbulence model is K-Epsilon and fluid pair model was defined for Air and liquid water phase for the interphase transfer is free surface. Also defined for air and water vapor as mixture. initialization condition for the volume fraction for each fluid was defined (air =0.3 water liquid =0.7 and water vapor =0)
I had searched in ANSYS help for diffusion schemes in expert parameters but i didn't find enough data explaining each scheme so until now I didn't use any scheme in solving.
Could someone suggest ideas for improving this model and give me links explaining diffusion schemes better as until now I have this like for explaining this schemes but it is also not enough [http://www.arc.vt.edu/ansys_help/cfx...SolvDisc.html].

Thanks
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Old   March 16, 2014, 05:42
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Glenn Horrocks
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You will have to describe what you are doing more thoroughly for us to help.

Is there a single air/water interface? Is the bottom 30% of the box filled with water and the top 70% filled with air? Does it have a distinct interface between the air and the water? Are there water droplets or air bubbles? Is the humidity of the air important? Is water evaporation important?
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Old   March 16, 2014, 07:02
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Thanks ghorrocks
following is answer for your questions
Is there a single air/water interface? yes, water upper surface.
Is the bottom 30% of the box filled with water and the top 70% filled with air? yes
Does it have a distinct interface between the air and the water? condition for a simple interface between water and air.
Is the humidity of the air important? Is water evaporation important? both yes, because I want to develop this model for determine the effectiveness of water desalination system in evaporating water and also water diffusion due to concentration into air.
please let me know if there are more questions for clarifying my model.

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Old   March 16, 2014, 18:06
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So it appears you have a multiphase model (liquid water versus air), and the air is a multicomponent gas (air+water vapour). The water vapour in air is not a multiphase model as the water vapour is the same phase as the air.

The liquid water/air multiphase bit should be straight forward. A multicomponent gas is also straight forward. The tricky bit in this simulation is to get the conditions right on the interface so water evaporates (and condenses if necessary) at the correct rate and adds to the water vapour.

But before you work on the evaporation on the interface do a multiphase model (water/air) and a multicomponent model (water vapour/air) first with no evaporation at the interface to make sure the basic stuff is working first.
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Old   March 17, 2014, 00:30
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So how can model this ghorrocks,
1- should I build 2 volumes one for water and another for air and make an interface between them?
2- if Yes ;How can I define fluid in a volume (domain) , and define another fluid in another domain because when I divided the total volume into 2 volume, I couldn't define one or more fluid for each one differ from the other volume.
3- what would be the interface type between the 2 volumes (assuming the first question answer is Yes) for modelling the evaporation and diffusion on water free surface.

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Old   March 17, 2014, 04:41
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Is the interface location fixed? Do you care about the flows in the water and the air?
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Old   March 17, 2014, 06:19
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-when I had defined the interface, it was fixed related to named selected face . can I make it variable or imaginary, as the free surface of the water volume would not fixed while water vapor is evaporating from the water liquid into air.
-Yes I care about both flow in the water and the air

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Old   March 17, 2014, 17:25
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It is still not completely clear but I suspect a free surface multiphase model for the air/water interface will work, and multicomponent mixture for the air/water vapour.

So have a look at the CFX tutorials for setting up free surface simulations. It also has examples on multipcomponent mixtures. But combining them together you will have to work out yourself.
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Old   March 17, 2014, 17:47
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Okay, but could you tell me how to define one or more fluid for a domain and another one fluid or more for another domain. I have tried many times for doing that and unfortunately couldn't also I have tried to find a tutorial for that and also couldn't.
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Old   March 17, 2014, 17:48
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Your simulation is a single domain. Have a look at the flow over a bump tutorial for how to set up free surface multiphase simulations.
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Old   March 18, 2014, 17:17
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Thanks ghorrocks; I am looking at it right now

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Old   April 14, 2014, 03:55
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Hi ghorrocks;
I have tried many tutorials and finally I can model 2 phase flow . Now I have a new problem in defining condensation of water vapor on low temperature vertical wall. could you help me in defining it correctly or give me a tutorial for this case. my case is closed volume (2D) , VOF , transient , water vapor at 400K and cold wall at 280K. no inlet or outlet just closed volume.

Best regards
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Old   April 14, 2014, 07:22
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I do nto have any tutorials for the wall condensation model. Try the ANSYS community page as it might have one. Otherwise ANSYS support will have some I suspect, so get in contact with them.
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Old   April 14, 2014, 14:39
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Thanks ghorrocks for your guidance. I will try both of them. thank you
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