How to find vapor quality (by isolating phases?) in Post

federernadal · November 2, 2015, 10:58

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?

JuPa · November 2, 2015, 11:53

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:

Tiny time steps (for boiling simulations, I've used 1e-5 s, I expect condensation to be the same order of magnitude).
A good mesh to resolve the interface.
Good knowledge of the interface structure to resolve the interfacial area density.
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).

federernadal · November 2, 2015, 13:36

Thanks Jupa!

How do I do an instantaneous simulation?

ghorrocks · November 2, 2015, 15:57

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?

federernadal · November 2, 2015, 16:08

Im not sure...Im just following up on what Jupa said, that instantaneous modeling would be a good start...

highorder_cfd · November 2, 2015, 17:08

Quote:

Originally Posted by federernadal

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.

JuPa · November 3, 2015, 07:23

Quote:

Originally Posted by federernadal

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.

federernadal · November 5, 2015, 08:59

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?

Opaque · November 5, 2015, 11:51

Have you looked at the tutorials ?

There is one about phase change. See Chapter 26: Axial Turbine Equilibrium and Non-Equilibrium Steam Predictions

federernadal · November 6, 2015, 23:35

Alright, I want to do the tutorials but where are the renders or meshes? I cant find them in the Help menu...

highorder_cfd · November 10, 2015, 09:38

Log in into the ANSYS Customer Portal and you will download the files from there

November 2, 2015, 10:58	How to find vapor quality (by isolating phases?) in Post	#1
federernadal Member Join Date: Oct 2015 Posts: 56 Rep Power: 10	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?

November 2, 2015, 11:53		#2
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	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: Tiny time steps (for boiling simulations, I've used 1e-5 s, I expect condensation to be the same order of magnitude). A good mesh to resolve the interface. Good knowledge of the interface structure to resolve the interfacial area density. 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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November 2, 2015, 13:36		#3
federernadal Member Join Date: Oct 2015 Posts: 56 Rep Power: 10	Thanks Jupa! How do I do an instantaneous simulation?

November 2, 2015, 15:57		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	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?

November 2, 2015, 16:08		#5
federernadal Member Join Date: Oct 2015 Posts: 56 Rep Power: 10	Im not sure...Im just following up on what Jupa said, that instantaneous modeling would be a good start...

November 5, 2015, 08:59		#8
federernadal Member Join Date: Oct 2015 Posts: 56 Rep Power: 10	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?

November 5, 2015, 11:51		#9
Opaque Senior Member Join Date: Jun 2009 Posts: 1,803 Rep Power: 32	Have you looked at the tutorials ? There is one about phase change. See Chapter 26: Axial Turbine Equilibrium and Non-Equilibrium Steam Predictions

November 6, 2015, 23:35		#10
federernadal Member Join Date: Oct 2015 Posts: 56 Rep Power: 10	Alright, I want to do the tutorials but where are the renders or meshes? I cant find them in the Help menu...

November 10, 2015, 09:38		#11
highorder_cfd Member Join Date: Jan 2015 Posts: 63 Rep Power: 11	Log in into the ANSYS Customer Portal and you will download the files from there