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Vermischung zweier Fluide über Skalare

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Message 1 of 5
uc16axom
383 Views, 4 Replies

Vermischung zweier Fluide über Skalare

Guten Tag zusammen,

 

ich bin derzet Student und arbeite an meiner Masterarbeit.

In dieser soll unter anderem ein Versuchsaufbau am Computer simuliert werden. Ich entschloss mich das Programm Autodesk CFD zu verwenden.

 

Der Aufbau besteht nach Reduzierung auf das Nötigste aus 3 Edelstahlrohren mit einem Innendurchmesser von circa 1 mm. Durch das erste Rohr (circa 2000 mm lang) strömt flüssiges CO2 mit 5 kg/h und einem Druck von 60 bar und 20 Grad Celcius. Durch das zweite Rohr (300 mm lang) strömt Öl mit 0,06 l/h und ebenfalls mit dem selben Druck und der selben Temperatur. Beide Rohre gehen in ein T-Stück. Das dritte Rohr (3000 mm lang) ist der Auslass in die Atmosphäre.

 

Nachdem das Strömungsvolumen erstellt wurde, lege ich unter Material und Dichte - Variationsmethode, stückweise linear fest und gebe hier die Skalare und deren Dichte ein. Nach Festlegung aller Randbedingungen gebe ich unter "Start", "Physik" und "Erweitert" den Diffusionskoeffizient ein.

 

Folgende Fragen:

1. Wird hierbei das komplette erstellte Volumen (also bereits im Rohr mit nur CO2 oder im Rohr nur mit Öl) mit dem Diffusionskoeffizient gerechnet oder erst nach dem T-Stück, wenn beide Volumen sich miteinander vermischen?

 

2. Welche Randbedingung nehme ich am Auslassquerschnitt? "Unbekannt"?

 

Grüße

Tobias

4 REPLIES 4
Message 2 of 5
Jon.Wilde
in reply to: uc16axom

Hi,

 

The volume should be the entire flow volume.

What state are both fluids in the though, are the densities pretty similar? If not, this might not be OK as a scalar analysis.

 

You would likely need a P=0 at the outlet and no pressure at the inlet (otherwise this is over constrained). You could use the material definition or environment reference properties to raise it to 60 bar though.

 

Thanks,

Jon

Message 3 of 5
uc16axom
in reply to: Jon.Wilde

The density of oil is 850 kg/m³ and the density of CO2, 781 kg/m³. Is it okay for using the scalar analysis?

 

Near the outlet, I measure the pressure, temperature and density according to the simulation. I noticed that at a pressure of about 56 bar and a temperature of CO2 of 29 degrees Celcius the density must be 184 kg / m³.
Unfortunately, the result is still 781 kg / m³?

I have set the steam pressure in the material settings. In addition, I set the environment as variable and have the heat transfer activated.

Near the outlet, I also have the normal ambient pressure of 1 bar, I have in the results still 56 bar.

Can you help me?

 

Message 4 of 5
Jon.Wilde
in reply to: uc16axom

Hi,

 

The more I think about this and discuss with my peers, the less I think we can do it.

At the inlets, we should be able to set the material up so that both density and viscosity change with scalar to give us the correct properties.

 

The problem is, what happens when they both mix? In CFD, we really only have a single fluid with the properties changing based on scalar. In reality though, you have very different fluids mixing, which I do not think is something we can model.

It seems like you would need to make a large assumption at that mixing point, is that something you are comfortable with?

 

Do you also see a change in temperature? The material properties would also change with temperature right? If we use scalar, we then cannot account for property changes with temp - another assumption.

 

In summary, I am not sure if what CFD will provide you with is going to be too useful, it depends what you need.

 

Hope that helps,

Jon

Message 5 of 5
uc16axom
in reply to: Jon.Wilde

Hello Mr. Wilde,

Thanks for the detailed answer!

Probably I will assume the whole volume as CO2 because the proportion of oil is low.

best regards
Tobi

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