# Volume fraction problems

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August 12, 2014, 03:09
Volume fraction problems
#1
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Join Date: Dec 2012
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I'm a beginner with CFD, Fluent, Multiphase & VOF. So please bear with me if my question is too obvious.
I am modeling a distributor, with 1 inlet & 4 outlets, trying to find the volume fraction of my 2nd phase & have been following the instructions I found here & in the manual. But somehow I ended up with this (attached pic): 2 distributors instead of one!!!!
& the gradient of results is stuck in 3.5e-1 for all colors!!
I don't know what I did wrong
Please help me
Attached Images
 confusing_volume fraction.jpg (82.0 KB, 60 views)

 August 13, 2014, 08:52 #2 New Member   Join Date: Dec 2012 Posts: 24 Rep Power: 5 I'm simulating the 2phase flow inside an air conditioning distributor, using water as the working fluid in 2phases (liquid & vapor) I want to show the contours of the volume fraction of phase 2, but I have a problem where it appears as constant throughout the whole domain (with the gradient on the left stuck on the same value, my original value of 0.35), the same thing happens with the 1st phase but with the value of 0.65. I read on the topic & I found about patching, although all the tutorials, examples & forum posts suggest using 1 or 0 for the initial value of the 2nd phase, and it's always a simulation of water & air. So I am not sure if it applies to my case. However, my domain has water in both phases simultaneously, from the very first point of the domain. I don't know if the problem is my solution method : Coupled & PESTO or that I'm not doing the patching properly, or there's something wrong with my time steps? Or should I use a UDF? I don't know how to program those, "yet", but if u think that's the solution to my problem, then I may give it a try. Please help me out. Any input will be helpful.

 August 20, 2014, 03:35 #3 New Member   Join Date: Sep 2012 Posts: 26 Rep Power: 6 Hello rgd, the volume fraction shown in you picture is right after initialising the case without any calculation (time = 0). Regarding this time it is perfectly normal to look like this due to Fluents settings. Regarding the two domains, did you set any view settings? (mirror etc.) It should be possible to patch any other values than 0 or 1 for the phases (I am not sure on this one i never used it) The vF value is a percent of the phases (35% Phase 2 and 65% Phase 1 in your case) You are using the Euler-Multiphase model right? If I am correct are your particles smaller than your cells? (This is a must for the Eulerian modell). Is there any problem if you start with 0% of your Phase 2 insinde the domain? As a boundary condition set the vF for you Phase 2 at the inlet to 0.35 and see what happens. Greets Nigirim rgd likes this.

August 22, 2014, 06:37
#4
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Dear Nigirim,

Thanks a lot for ur reply. I really appreciate it.

Quote:
 Originally Posted by Nigirim Hello rgd, the volume fraction shown in you picture is right after initialising the case without any calculation (time = 0). Regarding this time it is perfectly normal to look like this due to Fluents settings.
I actually had already run the simulation, but I guess my time step was very large (I didn't know how much I should set it to, so I set it to 1 second , I know it sounds stupid, but I am still learning )
I started using smaller time steps in the order of 1e-5 to 1e-7.
I started getting nicer profiles for the volume fractions at different time steps by: Read>Data>time step, usually until 120-140 I get good profiles with a good color map showing the change in the volume fraction. After that it goes back to a similar profile, with the whole color map stuck at 0.35.

The thing is I am trying to find the mass flow rate at the outlets, so it means it should reach equilibrium, when Qin=Qout, right? so that the sum of mass flow rate at inlet + mass flow rate at outlet = zero (approximately), this only happens when the color map is stuck on 0.35.

I also want to find the volume fractions at the outlets, at the same time, in order to show the maldistribution at the different outlets.

But when I try to find the volume fractions at the equilibrium state it shows as the picture.

I don't know what to do to find both, to be the final values, not the time step values.

Quote:
 Originally Posted by Nigirim Regarding the two domains, did you set any view settings? (mirror etc.)
That was another stupid mistake I made , I wanted to create a symmetry plane to show the values at the middle portion, so instead of using an iso-surface, I used a plane.

Quote:
 Originally Posted by Nigirim It should be possible to patch any other values than 0 or 1 for the phases (I am not sure on this one i never used it) The vF value is a percent of the phases (35% Phase 2 and 65% Phase 1 in your case)
Yes, I used patch at the inlet with 0.35 value for the secondary phase, it seems that the only way to do it.

Quote:
 Originally Posted by Nigirim You are using the Euler-Multiphase model right? If I am correct are your particles smaller than your cells? (This is a must for the Eulerian modell). Is there any problem if you start with 0% of your Phase 2 insinde the domain? As a boundary condition set the vF for you Phase 2 at the inlet to 0.35 and see what happens. Greets Nigirim
No, I am using VOF model, I am following this paper I found about a similar study, they used VOF model and it seemed to work fine.

But if u think I should use Eulerian model, could u please explain to me why?
I really appreciate ur input.
And thanks a lot for all ur help.

Regards,
Raghda

 September 4, 2014, 05:44 heat pipe #5 Member   Muhammed Asmail Join Date: Nov 2009 Location: Iraq Posts: 40 Rep Power: 8 Hi all Please i need the boundary condition of volume fraction in heat pipe by using ANSYS-FLUENT program

 September 4, 2014, 14:36 #6 New Member   Join Date: Dec 2012 Posts: 24 Rep Power: 5 Hi Muhammed, u want to know how to set it, right? u can go to boundary conditions> highlight ur inlet from the zone menu & choose ur secondary phase from the phase drop down list> then press edit > & choose the multiphase tab: there u can set ur volume fraction of ur secondary phase. Hope this helps.

September 4, 2014, 15:54
heat pipe
#7
Member

Muhammed Asmail
Join Date: Nov 2009
Location: Iraq
Posts: 40
Rep Power: 8
Quote:
 Originally Posted by rgd Hi Muhammed, u want to know how to set it, right? u can go to boundary conditions> highlight ur inlet from the zone menu & choose ur secondary phase from the phase drop down list> then press edit > & choose the multiphase tab: there u can set ur volume fraction of ur secondary phase. Hope this helps.
Thank you so much rgd of course it is good information
i want to know the boundary condition for volume fraction of thermosyphon heat pipe that it is enclosed with this message,as the same the path that you wrote it to me? and how much value of the volume fraction for this case?

Many Thanks
Attached Images
 400px-HPfig7.png (42.8 KB, 24 views)

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