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-   -   Vacuum Pressure at Outflow (https://www.cfd-online.com/Forums/fluent/98225-vacuum-pressure-outflow.html)

 Leepox March 6, 2012 08:32

Vacuum Pressure at Outflow

Hi guys,

Basically I am injecting air in at atmospheric pressure from one end. Whilst at the other end there is a vacuum pump sucking out the air. I take it I will have to use a pressure outlet BC.

However its asking for a gauge pressure and this is boggling me because absolute = atmospheric + gauge. (thus my operating pressure is at 0 for gauge pressure)

Since i am operating below atmospheric that means gauge has to be negative but then hysys does not allow me to do that.

Or am I completely missing the point? :confused:

 scipy March 6, 2012 10:30

Why not put the operating/enviroment pressure at 0, then on the inlet define 101325 Pa and 0 at the outlet.. wouldn't that give the same result? :)

 Leepox March 8, 2012 11:15

I'm just a bit worried that by increasing the pressure I will change the air density (as it is now effectively above atmospheric pressure) when the experimental equipment is actually working at atmospheric pressure.

 scipy March 8, 2012 11:20

If your CFD case involves compressibility then you're probably using a density based solver and that uses the ideal gas law to determine the density.. and since it's pV=mRT, it shouldn't really matter to it if you're calculating with absolute values of pressure or gauge pressure. In fact, it's always more thermodynamicaly correct to calculate with absolute pressures (at least from what I remember about thermodynamics and fluid mechanics :D).

 Leepox March 8, 2012 12:24

Quote:
 Originally Posted by scipy (Post 348389) If your CFD case involves compressibility then you're probably using a density based solver and that uses the ideal gas law to determine the density.. and since it's pV=mRT, it shouldn't really matter to it if you're calculating with absolute values of pressure or gauge pressure. In fact, it's always more thermodynamicaly correct to calculate with absolute pressures (at least from what I remember about thermodynamics and fluid mechanics :D).
That is brilliant! I never realised this! Thank you very much :D

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