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Turbulence in a tube (large Re-range)

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Old   July 24, 2013, 03:01
Default Turbulence in a tube (large Re-range)
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Hi guys,

I need your help clarifying some thoughts about modelling turbulence.

The setup:
I am modelling a flash evaporation in a pipe with small diameter (capillary). A subcooled refrigerant enters the capillary and due to pressure drop evaporates. Of course, the velocity increases with evaporation. The capillary is rough.

Some numbers:
Diameter of capillary: ~ 0.5mm
Length of capillary: up to 4m
Roughness: 1.5E-06m
Device-Reynolds-Number at Inlet: ~ 3000
Device-Reynolds-Number at Outlet: ~ 250000

Turbulence Model (SST, automatic wall function):
  • I think, there are two aspects that do not let me profit from the wallfunction and I rather need to resolve the boundary layer (y+ <=1 and at least 10 nodes in the boundary layer).
    • I have a transition from kind of laminar to turbulent.
    • I am especially interested in the near wall area, because the pressure drop is happening there. So I need to resolve the boundary layer even in the turbulent region.
  • That means I need to resolve the boundary layer in the whole pipe.
  • I have a large range of y+ and boundary layer thickness in my simulation.
    • y+ is largest for high Re.
      • Therefore my y+-values at the beginning of my capillary will be smaller than 1. Is this a problem?
    • Boundary layer thickness is largest for small Re. So I need to resolve my boundary layer up to this thickness.

Is my thinking flawed or is it reasonable? Are there aspects I am not considering? I would really appreciate your input. Thank you.
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Old   July 25, 2013, 03:28
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Is this steady state or transient? Why are you doing the model - what are you trying to learn?
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Old   July 25, 2013, 03:51
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I am doing a steady state simulation. On a coarse mesh, that means I do not resolve the boundary layer, it converges and I get qualitative good results. Experiments provide values for the massflow and temperature profile along the capillary.

It is my master thesis and I basically try to set up a simulation that matches the experiments. So that in future, perhaps they can simulate new configurations instead of doing the experiment.
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