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Old   August 17, 2011, 15:46
Default Pressure Drop Calculation
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Dear all,

In my 3D model I have a pipe with some bends and I would like to measure the pressure drop across the pipe. At the inlet I know flow rate and pressure of the gas; however, I don't know the pressure at the outlet.

This pipe is connected to another component; hence, the outlet pressure is not atmospheric.

What sort of boundary conditions would you use?

Mass flow inlet + pressure outlet: with Mass flow inlet I can specify the gas inlet flow and the pressure can be defined with the use of reference pressure. How about pressure at the outlet, by default it is set to zero - does that mean the pressure will drop to zero?!

I may have laminar and turbulent flows in different cases and gas operating temperature is around 700C.

Any comments would be much appreciated.

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Old   August 27, 2011, 05:18
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A mass flow inlet / pressure outlet combination just defines the mass flow at the inlet and the pressure at the outlet. The inlet pressure will be outlet pressure + pressure drop and will be set automatically. It can NOT be adjusted by changing the reference pressure!

Another possibility could be using a pressure outlet as inlet boundary conidtion and a flow split outlet for outlet. Flow split means "which fraction of the total inflow exits through this boundary". When there is only one outlet boundary, set the split ration to 1.
In this case, you will fix your inlet pressure to the desired value and the outlet pressure will be inlet pressure - pressure drop.

When it's an incompressible sim, it doesn't matter which method you use. When it's compressible and you expect bigger pressure differences in your system, you should refrain from my first suggestion, as the pressure level fixed at the pressure outlet will have an influence on the fluid density.
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Old   August 28, 2011, 10:21
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Thanks for the reply.

If I use pressure outlet at the inlet of my domain I can specify only the inlet pressure; how about the gas flow rate?
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Old   August 29, 2011, 02:06
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if you don't know the output pressure, you could do two things:
a) mass flow in, set reference pressure value and position at the inlet location and use a flow split outlet (ensure you have no backflow) - then the pressure field function will be relative to your inlet pressure of zero relative pressure (if not happy with this create a new field function offseting relative pressure to required value.
b) use negative flow velocity inlet at the outlet and pressure inlet set to know inlet pressure.
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