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Old   August 23, 2011, 08:08
Default Multiphase simulation
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Hello there.

I'm fairly new to CFX, and there's a lot things which are not clear to me.

Recently I'm trying to simulate a gas-solid exchanger and I'm having some troubles applying my boundary conditions.

I have a cylindrical column (tower like) with one lateral inlet.

In this column I inject hot air (1500 K) throught the lateral inlet and cold solids (300 K) through the top of the column. And I collect the heated solids in the bottom of the column and the cooled gas in the top of the column.

My questions are:

1 - In the lateral inlet I know the normal velocity and the temperature of the hot air. So I would set a inlet boundary condition, and apply the normal velocity value for the hot air, and set the volume fraction of the air to 1 and then set a zero normal velocity vector to the solids and its volume fraction to zero. Is this correct?

2 - In the bottom of the furnace I know only that no air comes out from there and the mass flow rate of the solids passing through it. So I would set an outlet boundary condition, and then set the mass flow rate of the solids equal to a known value and the mass flow rate of the gas equal to zero. Once again, is this correct?

3 - In the top of the furnace lie my biggest doubts. I know the pressure acting on the gas phase and solid mass flow rate entering the column, as well as its temperature.
I don't know what to do with this boundary. I thought about setting it as an opening, but this way I'm not able to specify the pressure acting at the gas phase and the mass flow rate of solids coming in the furnace. Also it would require me to set the values for the volume fractions, which I don't know. Actually one of the goals of this models is to find out the mass flow rate of cooled air coming out of the furnace. What could I do?

Thanks a lot for your help
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Old   August 23, 2011, 20:09
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1 - Sounds good.
2 - Are you sure no air comes out? How is the air sealed from coming out here?
3 - You could use either a degassing boundary or a simple pressure outlet/opening with no solids flow. You could impose the solids flow as a source volume a little way into the domain.

And as a general guide - when a boundary is too complex to define what is going on that generally means it is not a good location for a boundary. Maybe you should move the top boundary further away to where the gas and solids are separated and can be dealt with more easily.
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