# Vacuum Pressure at Outflow

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 March 6, 2012, 08:32 Vacuum Pressure at Outflow #1 Member   Join Date: Nov 2011 Posts: 42 Rep Power: 5 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?

 March 6, 2012, 10:30 #2 Senior Member   Join Date: Aug 2011 Posts: 114 Rep Power: 6 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?

 March 8, 2012, 11:15 #3 Member   Join Date: Nov 2011 Posts: 42 Rep Power: 5 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.

 March 8, 2012, 11:20 #4 Senior Member   Join Date: Aug 2011 Posts: 114 Rep Power: 6 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 ).

March 8, 2012, 12:24
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 Originally Posted by scipy 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 ).
That is brilliant! I never realised this! Thank you very much

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