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Old   November 2, 2015, 11:58
Default How to find vapor quality (by isolating phases?) in Post
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Hey all,

Still pretty new to CFX. Suppose I am modeling an expansion valve in the refrigeration cycle. As we all know, refrigerant enters at saturated liq and becomes 2 phase at the outlet. How do I determine the quality (mass flow of sat vapor/ total) using the ANSYS results?
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Old   November 2, 2015, 12:53
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Do you want to simulate condensation from saturated vapour at the inlet to two phase phase saturated vapour & condensed liquid at the outlet?

If so, good luck.

Condensation modelling is extremely difficult in CFD. It requires:
  1. Tiny time steps (for boiling simulations, I've used 1e-5 s, I expect condensation to be the same order of magnitude).
  2. A good mesh to resolve the interface.
  3. Good knowledge of the interface structure to resolve the interfacial area density.
  4. A user defined expression written in CFX pre to describe the interfacial area density.

It's not easy.

You can do an equilibrium phase change simulation which assumes instantaneous phase change. It's quite rudimentary but I recommend this as the first step. Using this you don't need points 3 and 4, and points 1 and 2 become more relaxed (allows for coarser time steps and meshes).
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Old   November 2, 2015, 14:36
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Thanks Jupa!

How do I do an instantaneous simulation?
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Old   November 2, 2015, 16:57
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CFX can do steady state simulations and transient simulations. Steady state simulations have no time derivative, transient simulations model the flow as it develops through time.

Is that what you mean by instantaneous simulations?
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Old   November 2, 2015, 17:08
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Im not sure...Im just following up on what Jupa said, that instantaneous modeling would be a good start...
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Old   November 2, 2015, 18:08
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Quote:
Originally Posted by federernadal View Post
Im not sure...Im just following up on what Jupa said, that instantaneous modeling would be a good start...

Homogeneous model...open the CFX Help and look for the Homogeneous and Inhomogeneous Models for phase change. There is a nice tutorial in the help about a turbine which illustrates how to set up this kind of analysis.
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Old   November 3, 2015, 08:23
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Quote:
Originally Posted by federernadal View Post
Im not sure...Im just following up on what Jupa said, that instantaneous modeling would be a good start...
You misunderstand me. A homogeneous phase change model is an equilibrium phase change model which assumes instantaneous phase change.

I.e. start with a homogeneous phase change simulation.

It's like modelling a single fluid with variable properties (liquid and gas) which switch to those properties depending on the enthalpy.
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Old   November 5, 2015, 09:59
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Jupa, Im curious...how did you model boiling for a non-instantaneous phase change? In your transient results, were you able to see how much vapor is coming out and how much is still liquid (as a function of time)? So at t=35s, the air above the pan of water is 30% vapor or something of that nature?
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Old   November 5, 2015, 12:51
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Have you looked at the tutorials ?

There is one about phase change. See Chapter 26: Axial Turbine Equilibrium and Non-Equilibrium Steam Predictions
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Old   November 7, 2015, 00:35
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Alright, I want to do the tutorials but where are the renders or meshes? I cant find them in the Help menu...
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Old   November 10, 2015, 10:38
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Log in into the ANSYS Customer Portal and you will download the files from there
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