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-   -   multiphase B.C (http://www.cfd-online.com/Forums/fluent-multiphase/120583-multiphase-b-c.html)

mehdimoradi. July 10, 2013 06:50

multiphase B.C
 
Hi
in a multiphase project, inlet of each phase is separated from others. also for outlet, each phase exit from a outlet separate from other phases.
how can define these boundary conditions in fluent software?
please help me.
thanks

RodriguezFatz July 11, 2013 02:59

Do the phases separate "automatically" by bouyancy inside your domain, or how does this work? Can you post a picture?

mehdimoradi. July 11, 2013 14:47

1 Attachment(s)
Quote:

Originally Posted by RodriguezFatz (Post 439081)
Do the phases separate "automatically" by bouyancy inside your domain, or how does this work? Can you post a picture?

Thanks RodriguezFatz.
a meshed layout of my project has been attached as a picture format.
Please help me.

Attachment 23427

RodriguezFatz July 12, 2013 01:44

So does it mean, the heavy particles entering on the two inlets at the top will fall down, whereas the air moves up and leaves at the top?
What I would do:
Three velocity inlets and two pressure outlets. Set operating density to zero, so you will include static pressure in your calculations. Then, set the pressure of your top outlet to zero and estimate your bottom outlet pressure with hydrostatic pressure drop.
Also, you could use a "degasing" outlet for the top. But I am not sure, what that physically means...

mehdimoradi. July 12, 2013 02:30

Quote:

Originally Posted by RodriguezFatz (Post 439286)
So does it mean, the heavy particles entering on the two inlets at the top will fall down, whereas the air moves up and leaves at the top?
What I would do:
Three velocity inlets and two pressure outlets. Set operating density to zero, so you will include static pressure in your calculations. Then, set the pressure of your top outlet to zero and estimate your bottom outlet pressure with hydrostatic pressure drop.
Also, you could use a "degasing" outlet for the top. But I am not sure, what that physically means...

It must be mentioned that in first of test, heavy particles enter bin to fill it completely and then they are exiting the bin with known mass flow rate. although new grain and inert enter the bin with outlet mass flow rate simultaneously. in other word the bin will be loaded completely during the tests.
about pressure out let at bottom, what setting in fluent software must be done to air (phase1) do not exit from the bottom?

RodriguezFatz July 12, 2013 04:38

You can not create a simple outlet, that prevents air from exiting, as far as I know. How do you think this could work in a real experiment?

mehdimoradi. July 12, 2013 07:00

Quote:

Originally Posted by RodriguezFatz (Post 439329)
You can not create a simple outlet, that prevents air from exiting, as far as I know. How do you think this could work in a real experiment?

In real experiment we use a screw conveyor with small pitch in out let of grain. Thus filled screw conveyor(with grains...) prevent from air exit.

shubham208011 July 18, 2013 12:09

See you can assume that there is no air but in practical air is always there.

To visualize this lets assume spherical particle closely packed in a container (stand in vertical direction) which has conical bottom. How closely you pack these particle they end up leaving space between and around them which is called as void (air pockets) and this is the basis of solid bulk density.
Now in my personal opinion (which could be wrong) in your problem from macroscopic point of view we can assume that as air enters the system it rises upward and during upward motion it absorbs moister from only solid and exited from top. Since very small amount of air in compression to solid comes out from bottom and hence it is neglected.

But in CFD (finite volume/fluent) whole volume is divided into very very small control volume (microscopic level) and for each control volume continuity, heat transfer, mass transfer & other equations are solved and here volume fraction of different phases in each small control volume is a significant parameter and hence it is not possible in Fluent/CFD to say that only solid will come out from bottom

mjgraf July 18, 2013 12:59

not that familiar with Fluent, but in CFX that bottom outlet you can set the volume fraction of air to be 0 and no air will exit. The top Air outlet needs to be set to degassing which will let the air out but act as a wall to the other phases.

mehdimoradi. July 25, 2013 11:28

Question
 
Hello mjgraf
If in Fluent, gravity force is activated, secondary phases exit from top outlet?

mjgraf July 25, 2013 11:33

guess that depends on the dynamics and buoyancy.


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