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Boundary conditions at inlet/outlet of a pipe with compressible flow

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Old   November 19, 2015, 06:04
Exclamation Boundary conditions at inlet/outlet of a pipe with compressible flow
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Hi Gentlemen,

a question for you:

I have a compressible gas flowing in a pipe (with in the middle of it a valve), I'm trying to impose the boundary coinditions at the inlet/outlet of the pipe, but something is wrong.

Operating conditions:

- 29.6 bar
- 30 C
- 20.96 kg/m3 (@ 29.6 bar and 30 C)
- 5694 m3/h (@ 29.6 bar and 30 C)

I'm imposing for the INLET CONDITION a static pressure of 29.6 bar with 30 C, and for the OUTLET CONDITION a mass flow rate (20.96*5694).

The results are completely wrong, it results a velocity of 400 m/s, etc..

What is wrong? Where is the mistake?

Can someone please help me?

Thanks
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Old   November 19, 2015, 07:14
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Laurent DASTUGUE
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Hi Nicola (Yvelines?),
you speak about wrong results, but what tool are you using to obtain these results ?
Laurent
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Old   November 19, 2015, 07:32
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I'm using, not by choice, Flow Works (SolidWorks).

Results have no sense, I know that, for example, velocity should be around 10/20 m/s and not 400 m/s.
Maybe there's some initial wrong setting.

I also tried another way to resolve the problem:
I imposed a mass flow rate at the INLET and a pressure of about 0 bar (1*^(-10)) at the OUTLET, but using a fluid with the properties calculated at my operating conditions (@29.6 bat and 30 C)
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Old   November 19, 2015, 08:12
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Filippo Maria Denaro
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compressible, unsteady subsonic flows require special care in setting the BC.s.
You need to take into account the role of the characteristic curves (defined for inviscid flows) that say you have one BC at inlet coming from the interior.
Similar care must be taken for the outlet where not all the BC.s can be taken from the interior but one has to be fixed.
Many textbooks and papers illustrate the correct BC.s
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Old   November 19, 2015, 08:30
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I took care about the things you said.
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Old   November 19, 2015, 09:30
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If you fix p and T at inlet you are also fixing the BC.s for the density = p /RT.
What about the condition you let from the interior, velocity?
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Old   November 19, 2015, 10:03
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I didn't impose any velocity, shouldn't it be automatically evaluated if I impose a boundary at the outlet?
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Old   November 19, 2015, 11:49
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Since your velocity is so high. The first thing that I would check it that the diameter of the pipe is in the right units.
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Old   November 19, 2015, 12:15
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The diamensions are correct
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Old   November 19, 2015, 12:18
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What is the numerical value of the area with units ?
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Old   November 19, 2015, 12:20
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Maybe I have not fully understand how your code set the BC.s, I did some simple evaluation:

fixind the static pressure p, assuming a crude approximation in terms of energy you have that the total pressure p0 and the kinetic energy are not prescribed individually but only their difference.
Therefore, your solution can get any level of kinetic energy (and therefore of velocity magnitude) according to your setting
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