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Helium relief line - 'clip pressure for properties'

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Old   July 29, 2010, 10:35
Default Helium relief line - 'clip pressure for properties'
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Stephen Watson
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Hi,

I'm modelling the flow of cryogenic helium across a heat exchanger (a coil inside an annulus). The assumed boundary conditions are approx sonic flow in and 1 bar out; this represents a volume of liquid helium abruptly vapourising and flowing through a relief valve to atmosphere. I'm looking for a max pressure.

I have the helium set as an ideal gas which is probably not entirely correct but it's closer than having it as incompressible like I had before... I'm only really looking for a ballpark figure in any case. I got an error saying that the absolute pressure is out of range, or something along those lines. A bit of reading suggested that my reference pressure was incorrect and resulting in a negative absolute pressure, so I turned on the 'clip pressure for properties' in solver control. It appeared to work, although the solution converged after a worryingly short period of time (only 10 or so iterations). Even though the results in post seem to be reasonable, I don't really understand what I've just done, why it's worked and what the original problem was...so I don't trust the results at all!

Any advice/help much appreciated.

Steve
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Old   July 29, 2010, 18:59
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Glenn Horrocks
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If you are getting negative absolute pressures it is more likely than not that your setup is wrong and fudges like clipping the pressure is just covering up a major flaw.

Check your material model and boundary conditions. They are almost certainly not physically possible.
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absolute pressure, clip pressure, compressible flow, cryogenics, helium

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