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ANSYS CFX volume fraction definition for multiphase simulation 

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July 29, 2011, 09:25 
ANSYS CFX volume fraction definition for multiphase simulation

#1 
Senior Member

Dear Collegues,
I have been working on my thesis of master degree, It is related to multiphase fluid flow simulation, especifically in: gasliquid vertical gravitational separator. I´m 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 

November 6, 2013, 00:47 

#2  
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Zahid Ibrahim Mohammed
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Quote:
Help urgently required Thanks 

November 6, 2013, 05:30 

#3  
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November 7, 2013, 13:19 

#4 
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Zahid Ibrahim Mohammed
Join Date: Jul 2013
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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 

November 7, 2013, 14:23 

#5 
Senior Member

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!!


January 9, 2018, 04:43 
Volume fraction multiphase flow

#6 
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Maren
Join Date: Dec 2017
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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 )@OutletmassFlowAve(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 

January 10, 2018, 16:08 

#7 
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Glenn Horrocks
Join Date: Mar 2009
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Set all the subfunctions in your expression to a variable which you output to a monitor point. Then you might find which subfunction 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. 

January 15, 2018, 02:02 

#8 
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Maren
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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). 

January 23, 2018, 09:02 

#9 
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Maren
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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 )@OutletmassFlowAve(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! 

January 23, 2018, 16:39 

#10 
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Glenn Horrocks
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Have you read the FAQ on accuracy? https://www.cfdonline.com/Wiki/Ansy..._inaccurate.3F


January 24, 2018, 03:08 

#11 
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Maren
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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 1.0E15. But if i choose in CFXPre 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. 

January 24, 2018, 05:32 

#12 
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GertJan
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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, 

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