# Turbulence in a tube (large Re-range)

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 July 24, 2013, 03:01 Turbulence in a tube (large Re-range) #1 New Member   Join Date: May 2013 Posts: 6 Rep Power: 3 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.

 July 25, 2013, 03:28 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 9,775 Rep Power: 77 Is this steady state or transient? Why are you doing the model - what are you trying to learn?

 July 25, 2013, 03:51 #3 New Member   Join Date: May 2013 Posts: 6 Rep Power: 3 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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