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modeling tracer transport in liquid

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Old   March 5, 2013, 08:17
Default modeling tracer transport in liquid
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Hi all,

I wish to simulate tracer solute within moving liquid using FLUENT. The tracer is inert ion/molecule solule in liquid water. I have difficulty in matching what I understand about a liquid solution with CFD model(s). I didn't get much clue from the user manuals, and I am still unsure about the power of UDF or other hidden techniques, if applicable.

The first part of my question:
Conceptually, the molecule/ion is soluble in liquid water. Since they are not too much by volume and is not "continuous", so it should not be Eulerian model. Since they are actually a huge amount of molecules, which size is too small and amount is too large, it seems that it is also not Lagrangian model. I checked the species transport model and found a list of gaseous mixture build-in, including a mixture of inert gas model. But my system is in liquid water, and I am thinking only a single species of tracer solute, I doubt that can be used.

How can I "define" a new species, and let it "goes" with water?
From CFD perspective, is this a multiphase problem or a species transport problem?

The second part of my question:
After I get this new species get along with water, I would like to alter the BC. The command list in "Calculation Activities" is what I am thinking, as I came over lines like "define bc mass-flow-inlet inlet water y n 0" when going through some tutorials. I looked around for explanation about the syntax and arguments for it but in vain. Anyone know where shall I look for it? (I see some examples in the forum, but would like to learn how to use them)

Thank you in advance!
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Old   March 5, 2013, 10:52
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Hi
Species transport model is what you need. however, you can use UDS too, but you don't need any UDF. If you use species transport, you also should define a tracer. The tracer properties are the same as water, so there is no need to define new material. You need to define a mixture and adjust parameters for the defined mixture. The only thing remains is to define inlet and out let and suitable boundary conditions. The trick for a puls injection is that you set tracer mass fraction in inlet equal to 1 then do 1 or 2 iterations, then set it again to 0 and keep going to get convergency. To get a curve for the tracer behavior, usually mass fraction vs iteration or time, you should go to monitor -surfaces and define appropriate surfaces and set whether you need time or iteration. Finally, turn it on as simulation begins.
I might give you more info later if you need.
Good luck
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Old   March 5, 2013, 12:30
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Thank you very much!! I am going to try it out. Hopefully I will be back here with some good news next time.

And, yes. I would like to know more about this in near future.
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Old   March 6, 2013, 06:40
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Hi,

I have tried it out. I set the inlet BC as told. However, from the results it seems that the tracer mass fraction is also originally 1 within the interior cell zone. So the simulation looks more like there were constant tracer going in and out before simulation starts and then until 0.02 s later there is no more from the inlet. If I set the cell zone condition to fluid (not mixture) then the 'species' part dissapeared totally. Can you tell me how to set the interior tracer concentration to zero when I start the simulation? Thank you!
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Old   March 6, 2013, 12:08
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I suppose you patch a constant mass fraction for the zone. It's not what I meant. You have to change the mass fraction as I told you in boundary condition. Go to
define/ boundary condition, species transport and set the value for the tracer equal to 1, then do one iteration and again reset it to 0 and start your simulation till get the converged result. So I think you went the wrong path. If you have any more questions contact me:
aboozar.azarafza@gmail.com

Last edited by A CFD free user; March 6, 2013 at 13:57.
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