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-   -   void fraction? volume fraction? (https://www.cfd-online.com/Forums/fluent/76780-void-fraction-volume-fraction.html)

 tacaro June 4, 2010 07:18

void fraction? volume fraction?

Two phase pipe flow is ofter featured with "void fraction" in the open literature.

How does it relate to "volume fraction" value obtained from post-processing function of Fluent?

Thanks

 motahar May 20, 2011 03:35

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.

Thanks Everybody

 swiftaircraft May 21, 2011 08:46

Tacaro, Void Fraction and Volume Fraction are the same

 motahar May 21, 2011 12:16

Hi swiftaircraft,
Yes you're right.
I've worked in single phase for more than 1 year.
But I'm beginner in multiphase flow.
Can you help me how to define multiphase in CFX-Pre?
Thanks

 talib March 3, 2012 17:29

void fraction

depending on your multiphase modulus, you shoud find it under "phase". make sure that you present results for "phase-2"
I hope I helped.

 motahar March 12, 2012 03:00

Dear Talib,

Fortunately I could find how to use 2-phase flow in CFX.
But it didn't work well.

I asked a CFX worker and he told me that ANSYS CFX does not support 2-phase.
There are options in CFX-Pre to define the 2-phase, but the software doesn't understand it well.

For example, I'm defining a coolant tube located in a cube made of cu and an armour layer of Carbon on it. The heat flux deposits on the armour layer and then via condution, reaches to the coolant channel (The coolant is water).
By increasing the heat flux up to the 5 MW/m^2, we see phase change in the channel.
When the heat flux reaches 10 MW/m^2, and the coolant velocity is 1 m/s, the temperature is so hot that we have melting of the body. But CFX does not stop running and in CFX-Post I can see the outlet temperature of the coolant is more than 3000 K and obviously all the fluid changes to vapour. But the void fraction analysis yet shows that most of the coolant is yet fluid!!!

I couldn't find any solution for this matter yet.

I would be so thankful if any friend can help me in this issue!!!

Thanks all...

 zeybi1604 March 15, 2012 12:32

two phase flow

hello.i am actually working on a two phase model having a volume fraction of 0.5316.the problem is that the solution does not converge.can anyone help please?

 talib March 15, 2012 13:34

not enough info

hi zeybi1604 , I would be happy to try and assist, but this is not enough information.
please elaborate on your problem (models, geometry, materials, etc.) and how exactly it is not converged. Where is the problem? Is it via mass conservation? maybe only via average void fraction, etc.

 zeybi1604 March 15, 2012 14:17

thank you for replying Mr Talib. i am doing a multiphase simulation using the mixture model inside a packed bed with the beads being solid inside the bed. the materials inserted are air(primary phase) and fines(secondary phase). the volume fraction for the fines is 0.5316. during the iteration process i get an error message "divergence in AMG solver x momentum". my aim is to find the fines accumulation inside the bed.take care

 bss July 4, 2016 06:19

Void Fraction and Volume Fraction are not the same.The gas volume fraction is usually larger than the gas void fraction.

 eliastorca October 15, 2016 12:11

Hello everyone. Bss has reason about volume fraction and void fraction, they are not the same. Actually, I understand from what I read, that the volume fraction is equal to the void fraction only when the two phases (gas and liquid) travel at the same velocity. If my assumption is true the volume fraction represented by Fluent actually is what is called the volumetric void fraction. I'm in the correct assumptions?

 Roya.TR October 20, 2016 02:32

Hi everybody
i'm new in multiphase fluid flow
i want to solve a 4 phases flow (gas-oil- water and sand) in a cyclender.
which methods is better for my simulation?
I want to use eulerian model. is it appropriate?
i chose air as the primary phase but I don't know how define the diameter of liquids.