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-   -   2 diferent fluids mixing (http://www.cfd-online.com/Forums/fluent/68753-2-diferent-fluids-mixing.html)

Figd84 September 30, 2009 16:44

2 diferent fluids mixing
 
I need to mix 2 diferent fluids (air-water) in a 3 way pipe (2 inlets, 1 outlet). Iīve activated the mixing model, but didnīt know how to define diferent fluids in the inlet boundary conditions.

Anyone knows how to do this??

Thanks in advance

herntan September 30, 2009 21:22

define>species transport

u need to edit ur material u want to mix there.

Philipov October 1, 2009 02:06

Try to read more about mixture models.... in case of such kind of fluids may be better to use vof...

herntan October 1, 2009 02:09

as i know u don't need vof if there is not reaction between 2 fluid.
Normal mixing is ok for just species transport.

Philipov October 1, 2009 04:52

As I said: Read more!!!!

The VOF model (described in Section 23.3) is a surface-tracking technique applied to a fixed Eulerian mesh. It is designed for two or more immiscible fluids where the position of the interface between the fluids is of interest. In the VOF model, a single set of momentum equations is shared by the fluids, and the volume fraction of each of the fluids in each computational cell is tracked throughout the domain. Applications of the VOF model include stratified flows , free-surface flows, filling, sloshing , the motion of large bubbles in a liquid, the motion of liquid after a dam break, the prediction of jet breakup (surface tension), and the steady or transient tracking of any liquid-gas interface.

herntan October 1, 2009 05:16

Opps.. i didn't saw its a multiphase flow.
I have no experience in VOF model..
Philipov should be right.

Figd84 October 1, 2009 08:53

Iīve thought about VOF, but I think that species model with low difusivity coefficient is better, because Iīm using a 2 inlet pipe with one exit. Air must flow trough one inlet and water in the other.

I need an air injection, defining an air and a water inlet that i canīt find how to define separately with VOF.

Thanks

Philipov October 1, 2009 09:00

Have no comment to the question.... you really do not understand the physics of vof.... try to read the provided help files and you will find that in vof method (in case of 2 phases) you get primary and secondary... All that is 100% filled with one phase i 0% filled with the other one.. so you can define from one inlet water to be 1 volume fraction and air to be 0 (zero).. for the other inlet - define the opposite... out let define as outflow or pressure BC... whatever....

Philipov October 1, 2009 09:03

Quote:

Originally Posted by herntan (Post 231099)
as i know u don't need vof if there is not reaction between 2 fluid.
Normal mixing is ok for just species transport.

Surprised..... you say no reaction between 2 fluids - do not use vof!!!!!!!!!!!
Just the opposite.....
To all the people using this forum.... pay attention to the things you write.

Figd84 October 1, 2009 12:34

Quote:

Originally Posted by Philipov (Post 231160)
so you can define from one inlet water to be 1 volume fraction and air to be 0 (zero).. for the other inlet - define the opposite...

thatīs the question, I canīt find where to do that. Where did you found it??

I canīt define the Volume fraction at the inlet, because the Multiphase tab in BC menu doesnīt allow to define Volume fraction there.

Figd84 October 1, 2009 17:52

maybe itīs a bug in FLuent 6.3.

I used VOF, but when Iīm trying to define volume fraction at BC, the first time I click the second-phase tab, to set the second phase volume fraction, the Multiphase option doesnīt appear at all. I have to set all other BC FIRST before the multiphase tab appears in the second Phase.

The simulation converged fine and there wasnīt another problem.

herntan October 1, 2009 20:32

haha.... i don't know what VOF all about.
but when goes for species transport with same phase... there is an difference on reaction and non reaction...

Philipov must be good in multiphase multi species. I would need ur help one day if i touch VOF or multiphase.

Philipov October 2, 2009 02:24

May be I was not clear... try to find vof tutorial...


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