CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

ANSYS CFX volume fraction definition for multiphase simulation

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree1Likes
  • 1 Post By juliom

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 29, 2011, 09:25
Default ANSYS CFX volume fraction definition for multiphase simulation
  #1
Senior Member
 
Julio Mendez
Join Date: Apr 2009
Location: Fairburn, GA. USA
Posts: 289
Rep Power: 17
juliom is on a distinguished road
Send a message via Skype™ to juliom
Dear Collegues,
I have been working on my thesis of master degree, It is related to multiphase fluid flow simulation, especifically in: gas-liquid vertical gravitational separator.
Im working in ANSYS CFX and the most difficult part is to setup the volume fraction at the inlet.
The main goal of the thesis is to evaluate the separation process within the vessel, but the data used is from the real process, for example:
Oil rate, gas rate, pressure, densities, viscocity and so on..
THE VOLUMEN FRACTION DOES NOT APPEAR IN NO DATA SHEET.
How can I get this volume fraction?
please help me
Zahid Ibrahim likes this.
juliom is offline   Reply With Quote

Old   November 6, 2013, 00:47
Default
  #2
New Member
 
Zahid Ibrahim Mohammed
Join Date: Jul 2013
Posts: 4
Rep Power: 12
Zahid Ibrahim is on a distinguished road
Quote:
Originally Posted by juliom View Post
Dear Collegues,
I have been working on my thesis of master degree, It is related to multiphase fluid flow simulation, especifically in: gas-liquid vertical gravitational separator.
Im working in ANSYS CFX and the most difficult part is to setup the volume fraction at the inlet.
The main goal of the thesis is to evaluate the separation process within the vessel, but the data used is from the real process, for example:
Oil rate, gas rate, pressure, densities, viscocity and so on..
THE VOLUMEN FRACTION DOES NOT APPEAR IN NO DATA SHEET.
How can I get this volume fraction?
please help me
Hi I'm new in use (cfx-14) I want the different volume fraction. Actually my problem is water and air flow through straight pipe. Can somebody tell how I can get the lower half of pipe fill in water and the upper half fill in air.

Help urgently required

Thanks
Zahid Ibrahim is offline   Reply With Quote

Old   November 6, 2013, 05:30
Default
  #3
Senior Member
 
Join Date: Feb 2011
Posts: 495
Rep Power: 17
Antanas is on a distinguished road
Quote:
Originally Posted by Zahid Ibrahim View Post
Hi I'm new in use (cfx-14) I want the different volume fraction. Actually my problem is water and air flow through straight pipe. Can somebody tell how I can get the lower half of pipe fill in water and the upper half fill in air.

Help urgently required

Thanks
You should refer to CFX tutorial 9. Look at expressions for UpVFAir, UpVFWater and DownVFAir, DownVFWater
Antanas is offline   Reply With Quote

Old   November 7, 2013, 13:19
Default
  #4
New Member
 
Zahid Ibrahim Mohammed
Join Date: Jul 2013
Posts: 4
Rep Power: 12
Zahid Ibrahim is on a distinguished road
Hi, Sorry but I read Tutorial 9: Flow Through a Butterfly Valve and I don't found any expressions for UpVFAir, UpVFWater and DownVFAir, DownVFWater.

Help urgently required

Thanks
Zahid Ibrahim is offline   Reply With Quote

Old   November 7, 2013, 14:23
Default
  #5
Senior Member
 
Julio Mendez
Join Date: Apr 2009
Location: Fairburn, GA. USA
Posts: 289
Rep Power: 17
juliom is on a distinguished road
Send a message via Skype™ to juliom
You have two options. You can either define the volumen fraction through an user define function (UDF) or during your meshing process and your modelling at Design modeler or whichever you use, you can have the inlet Split by two parts. In each part you will have a boundary condition and there you will be able to define the volumen fraction at each one!!
juliom is offline   Reply With Quote

Old   January 9, 2018, 04:43
Default Volume fraction- multiphase flow
  #6
New Member
 
Maren
Join Date: Dec 2017
Posts: 4
Rep Power: 7
Mara26 is on a distinguished road
Hello,
i'm new to multiphase flow and ANSYS CFX.
I am trying to simulate a multiphase flow in CFX turbomachinery with water and air. I have chosen air as a dispersed fluid and water as a continous one. The inlet boundary condition is the bulk mass flow rate and the volume fraction of water is 0.99 and air 0.01. The Outlet boundary condition is a static pressure condition.
I'm trying to calculate the head of the turbine by using the expression:
Head=
(massFlowAve(Total Pressure in Stn Frame )@Outlet-massFlowAve(Total Pressure in Stn Frame )@Inlet)/(ave(Water. Density)@Outlet*ave(Water. Volume Fraction )@Outlet+(ave(Air. Density)@Outlet*ave(Air.Volume Fraction)@Outlet)*g)

But i'll get an error and no head value. Can anyone help me?
I would be very thankful
Mara26 is offline   Reply With Quote

Old   January 10, 2018, 16:08
Default
  #7
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,474
Rep Power: 140
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Set all the sub-functions in your expression to a variable which you output to a monitor point. Then you might find which sub-function is not working. In other words:

MFA_In = massFlowAve(Total Pressure in Stn Frame )@Inlet
MFA_Out = massFlowAve(Total Pressure in Stn Frame )@Outlet

etc

Another point: You are using the ave() function to evaluate your properties. This just does a simple nodal average which is probably not what what you want. areaAve() is probably a better function.
ghorrocks is offline   Reply With Quote

Old   January 15, 2018, 02:02
Default
  #8
New Member
 
Maren
Join Date: Dec 2017
Posts: 4
Rep Power: 7
Mara26 is on a distinguished road
Thank you for your reply!
I set up all variables to a monitor point and it seemed that all of the variable worked.
In CFX Post there ist the error for my Head Calculation:
"ERROR The following unrecognised names were referenced: Air. Volume Fraction (on 'Outlet'), Water. Volume Fraction (on 'Outlet')."

But in the Outputfile there are the variables Air. Volume Fraction and Water. Volume Fraction.
I really don't understand, why CFX Post couldn't calculate the Head value (H).
Mara26 is offline   Reply With Quote

Old   January 23, 2018, 09:02
Default
  #9
New Member
 
Maren
Join Date: Dec 2017
Posts: 4
Rep Power: 7
Mara26 is on a distinguished road
Unfortunately i still try to simulate a multiphase flow through a centrifugal pump.
The fluid is water(continous fluid) and air(dispersed fluid, mean diameter 0.5mm).
I choose the bulk mass flow rate as the inlet BC, with volume fraction 0.01 for air and 0.99 for water, so there is 1% gas in the flow. For my outlet BC i choose: static pressure 0Pa.
Then i set the expression for calculating the head of the pump as:

(massFlowAve(Total Pressure in Stn Frame )@Outlet-massFlowAve(Total Pressure in Stn Frame )@Inlet)/((areaAve(Water. Density)@Outlet*0.99+(areaAve(Air. Density)@Outlet*0.01))*g)


Unfortunately the head is way too high...(much better than without any gas- which doesn't make sense).
Does somebody know how to fix it?



I would be grateful if somebody would help me!
Mara26 is offline   Reply With Quote

Old   January 23, 2018, 16:39
Default
  #10
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,474
Rep Power: 140
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Have you read the FAQ on accuracy? https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F
ghorrocks is offline   Reply With Quote

Old   January 24, 2018, 03:08
Default
  #11
New Member
 
Maren
Join Date: Dec 2017
Posts: 4
Rep Power: 7
Mara26 is on a distinguished road
Yes i did. Unfortunately i couln't find a solution for my issue.
It looks like that CFX "loosing" the air fraction directly after the inlet. Therefore I have only a two phase flow at the inlet and in the rest of the pump the air is vanished.
CFX set the default value for minimum volume fraction of 1.0E-15.
But if i choose in CFX-Pre to set up a value for the minimum volume fraction the simulation diverged and i get a "fatal overflow" error...
Air.VolumeFraction.png

Last edited by Mara26; January 25, 2018 at 03:05.
Mara26 is offline   Reply With Quote

Old   January 24, 2018, 05:32
Default
  #12
Senior Member
 
Gert-Jan
Join Date: Oct 2012
Location: Europe
Posts: 1,768
Rep Power: 25
Gert-Jan will become famous soon enough
If 'In' is not equal to 'Out', then your CFX calculation is incorrect.

Convergence of multiphase simulation should not be judged based on residuals only. You should monitor imbalances of all equations. Then if all are within 5% (meaning in=out), then come back.

Also, create multiple monitors points where you monitor velocity, pressure ,tke, and volume fractions. And see how these develop during your iteration process. If they still develop, your case is not steady, and conclusions are hard to take.

Especially, close to the outlet where your vol fraction should reach 1%. Monitor the value during the iteration process,
Gert-Jan is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Simulation of a single bubble with a VOF-method Suzzn CFX 21 January 29, 2018 00:58
Compressible Flow in Ansys CFX bcheruk CFX 15 July 6, 2017 06:30
air bubble is disappear increasing time using vof xujjun CFX 9 June 9, 2009 07:59
interDyMFoam - change in volume fraction gopala OpenFOAM Running, Solving & CFD 0 April 27, 2009 10:46
CFX Solver Memory Error mike CFX 1 March 19, 2008 07:22


All times are GMT -4. The time now is 22:00.