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sieve tray simulation(very essential and important)

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Old   May 19, 2011, 11:52
Default sieve tray simulation(very essential and important)
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Ehsan
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hi guys,i have simulated a air-liquid system with CFX but i have some problem in this.
my system consist of two perforated plate and one downcomer which connects them together.
i have considered this conditions:
steady state conditions
air and liquid as incompressible fluids.
laminar flow for air and turbulant flow for water.
in liquid inlet: parabolic velocity(ULin) and air volume fraction=0
in gas inlets(probes):Vairin and air volume farction=1
my question is about what BC do i have to set for gas outlet and liquid outlet location.
if i set velocity which is exist in liquid inlet(ULin) and zero air volume fraction,the liquid height in downcomer will
drop very much,but if i consider whatever pressure like atmosphere pressure(opening) instead of velocity and zero
air volume fraction,the liquid height in downcomer will grow very much
to the first plate.
is this problem related to geometry?because i drew my volume with one domain.
do i set another boundary condition for liquid outlet location?
please help me guys,it is very necessaary and important for me.
i m waiting for your answers.
thank you



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Old   May 19, 2011, 19:06
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Glenn Horrocks
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The boundary condition type is set by the physics. Something external sets the fluid level in this device, so you have to model its effects. If you do not know how to describe it then you cannot model it. You might be able to extend the simulation domain to include more downstream features which define the fluid level properly.

The top boundary can be either a degassing boundary or an opening I suspect. That one should be easy.

Why do you assume the air flow is laminar? It probably is turbulent.
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Old   May 20, 2011, 04:12
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Ehsan
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Quote:
Originally Posted by ghorrocks View Post
The boundary condition type is set by the physics. Something external sets the fluid level in this device, so you have to model its effects. If you do not know how to describe it then you cannot model it. You might be able to extend the simulation domain to include more downstream features which define the fluid level properly.

The top boundary can be either a degassing boundary or an opening I suspect. That one should be easy.

Why do you assume the air flow is laminar? It probably is turbulent.
Thank you for reply.
yes,you are right,but i considered the physic of the problem which i didnt mentioned in pervious post,for example i defined drag coffecient that belongs to air-water systems.what do you mean by physics?
do you suggest me to devide my geometry ino two volume and connect them with intergace?can it be useful?
the top boundary is as opening boundary.
do you think that to be turbulat of air flow has any effect on liquid level?
i have to say here that my CFD results on down plate is validated with experminetal data but my main problem is level of the liquid in downcomer which not validated.
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