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Fluent simulation of Hydrocyclone.. need help pls...

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Old   October 19, 2009, 08:33
Default Fluent simulation of Hydrocyclone.. need help pls...
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Hi guys,

I am trying to predict my hydrocyclone (HC) model using fluent and i have bumped into several problems. Here, i'm trying to model water liquid rotating through the HC, so i have one inlet and two outlets (overflow and underflow). The HC inlet has a pressure of 60psig and mass flow of 0.988kg/s, overflow having a pressure of 30psig and underflow of 45psig.

The problem is the inlet pressure. I've set the operating pressure to 0Pa to analyse just the gauge pressure. But, when i set the inlet pressure to 60psig (414000Pa), the mass flow inlet gives me approximately 4.5kg/s, which is more than i need. Furthermore, the velocity when initialized is somehow incorrect as i predicted using the simple (m=rho*velocity*cross-sectional area).

Aside from that, i specify the outlets as 200000Pa and 300000Pa, then try to run first using RSM model with second order upwind, as stated in most journals. However the residual has a diverging nature, even though the solutions have been relaxed (decreased around 0.2-0.3).

Can someone please provide any suggestion to me? Thx in advance.
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Old   October 19, 2009, 23:29
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You are over-specifying the boundary conditions. I think you should remove the inlet pressure specification and only enforce the flow rate. Since you are fixing the pressure at the outlets, inlet pressure should follow from the solution.
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Old   October 19, 2009, 23:54
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Oh..k i get it. So i've changed my inlet to msas flow inlet. However i have another question about the operating pressure. Default is 101325, which i presume is the atmospheric condition. the coordinates probably indicates where this pressure would be. So if i set it to 0Pa, does it mean i would just be looking at gauge pressure? If i set it to my inlet, will fluent calculate my inlet based on gauge?
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Old   October 20, 2009, 00:07
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Well, I don't know about fluent specifics. But in an incompressible simulation, the pressure values in your simulation doesn't mean anything. All you care about is the pressure variations. You can add or subtract any pressure value from the whole domain. So I guess:

with reference pressure set to 0, you will have absolute pressure results

with reference pressure set to 1 atm, your results would be gauge pressure.

The flow fields should be exactly the same regardless of this choice. Just be consistent with your BC's and initial conditions.
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