
[Sponsors] 
November 22, 2002, 03:15 
void fraction!!!

#1 
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Hello,
I'm in confusion.I hope someone can help me.I'm trying to model steadystate air/water dispersed flow (not stratified) in a tank. Since the flow is incompressible, 2fluid, 2phase, the mass conservation equation becomes: div(u)=0 So i want to ask you that, how can I numerically compute the void fraction? when will alpha(void fraction) appear in my equations? 

November 22, 2002, 05:34 
Re: void fraction!!!

#2 
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Yes and No! Two phases in a flow domain indicates that two different fluids each with its own set of transport equations share the flow domain. The portion occupied by each fluid is indicated by a fraction (you call it void fraction for the gas phase). Thus the fluid fraction will be one minus the void fraction.
The two sets of equations should be solved simultaneously as amongst other variables the pressure in the flow domain is shared. Assumptions, such as the incompressible condition is now used to derive a new set of governing equations to be solved iteratively. Usually div(u)=0 is used to solve for the pressure. It's not mass conservation, In variable density multiphase flow it means volume conservation. By using div(u)=0 together with a socalled void fraction equation you will satisfy mass conservation of each of the phases. 

May 20, 2011, 03:33 

#3 
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M. A.
Join Date: Jul 2010
Posts: 27
Rep Power: 7 
Hi friends,
I'm simulating a tube with water flow. The tube encounters boiling near the wall. I intend to calculate 'void fraction versus enthalpy' along the channel. Can you help me how to calculate void fraction? I'm in an emergency condition. Waiting for your comments!!! Thanks Everybody 

May 20, 2011, 09:49 
VF computation

#4  
Senior Member

Hi,
if you are doing multiphase flows (boiling flows), vf is an output. If it is your own code, you will need to output volume fraction. Does this answer your question? if you are asking how to "compute" vf  then, we are talking about the solution to boiling flows. Are you discussing about boiling flow simulations then? Either case  are you using your own code or commercial? Thanks, CFDtoy Quote:
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CFDtoy 

May 20, 2011, 10:54 

#5 
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M. A.
Join Date: Jul 2010
Posts: 27
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Hi CFDtoy,
Thanks for your quick reply. I'm using commercial code, ANSYS CFX. First, in ICEM CFD I simulated the geometry. I work in simulating ITER divertors in magnetised confinement fusion (MCF). The tube is surrounded by copper. When plasma heating heats the Cu, the heat conduction of Cu conducts the heat to the cooling tube with water coolant. When the water heats, the boiling bubbles appears near the tube wall. I intend to simulate this geometry. One of the plots I should draw, is plotting the 'void fraction versus enthalpy' along the channel. In CFDPost in CFX media I've seen 'volume fraction' in CFD processor but I don't know how to calculate the 'void fraction'??? Can you help me in this issue?? Thanks a lot. 

May 20, 2011, 11:06 

#6 
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M. A.
Join Date: Jul 2010
Posts: 27
Rep Power: 7 
Hi again,
I think I have made a mistake. I've used fluid phase in CFXPre. So you meant if I use multiphase flow, void fraction will be an output like volume fraction. Am I right??? How should I define multiphase flow in CFXPre??? I think I should go through this process: First, do I have to define it in 'Analysis Type' then in 'External Solver Coupling'? In the 'option' in 'External Solver Coupling', I can choose 'ANSYS MultiField' or ''ANSYS MultiField via Prep7' instead of 'None'. And then in 'Domain', I should define the 'Domain Type' as 'Fluid Domain'. Am I proceeding right? 

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