modeling tracer transport in liquid
 

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!

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

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.

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!

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