6 Replies Latest reply on Aug 25, 2013 11:12 PM by Jared Conway

# Flow Pressure Results Reversed???

I have created a centrifugal water/air separation vessel using Flow.  The primary flow media is water and the air was simulated by using a particle injection of low density polypropylene particles if 0.125 inch diameter.  The trajectories of the water and the particles (air) are exactly as desired.

However, when I do the pressure plot, the outlet pressure is greater than the inlet pressure. Has Flow just reversed my results??

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Are you sure about your pressure values? I mean the environnement pressure value is greater than the inlet pressure...?

Maybe you are working with relative pressure whereas SW uses absolute pressure..?

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Hi Loic,

The environmental pressure input was 14.7 psia, which is very close to atmospheric pressure.  So, I input the environmental pressure to 25 psia, but obtained the same result.

But the lights came on this morning.  The higher pressure at the outlet port is due to the hydrostatic head of the water in the vessel.  24 inches of water is .86 psi.  To be able to calculate the pressure drop through the separator due to flow, I will have to disable "gravity effect".  But Flow predicted what would be observed if pressure gauges were at the inlet and outlet ports.

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Try changing the outlet surface boundary condition to an Environmental Pressure and set it to be 25 psia.  You should keep gravity on then also.
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Hi Joe,

Changing the outlet boundary condition to 25 psia yields the same results with gravity on.  But it should yield the same result because the higher pressure at the outlet is due to the friction loss of the fluid plus the static head of the fluid at that point.  The pressure loss from the fluid flow is about 0.5 psi and the static head of the fluid is about .9 psi ( but I don't know how full Flow is thinking the vessel is).  This separator in real life will separate air from water and that it will pressurize the upper part of the separaton vessel.  A manual or automatic air vent will then release the air to the atmosphere.  But when I modelled it at 14.9 (atmospheric) with inlet mass flow = outlet mass flow, would Flow predict the liquid level in the vessel???

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I received this thread from tech support.  Perhaps you could shed some light on this statement: Flow Simulation does not have direct hydrostatic capabilities. The “Gravity” option is primarily used for natural convection problems to account for the heated air rising in the correct direction and velocity.

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Daniel, it might be good to start a thread with your specific problem.

But I can say for sure that flow has natural convection which is gravity driven and also will display hydrostatic pressure.