# void fraction? volume fraction?

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 June 4, 2010, 07:18 void fraction? volume fraction? #1 New Member   tacaro Join Date: Jun 2010 Posts: 1 Rep Power: 0 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 granzer likes this.

 May 20, 2011, 03:35 #2 New Member   M. A. Join Date: Jul 2010 Posts: 27 Rep Power: 15 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 21, 2011, 08:46 #3 Member   David Stanbridge Join Date: Apr 2010 Location: Norwich, UK Posts: 59 Rep Power: 16 Tacaro, Void Fraction and Volume Fraction are the same

 May 21, 2011, 12:16 #4 New Member   M. A. Join Date: Jul 2010 Posts: 27 Rep Power: 15 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

 March 3, 2012, 16:29 void fraction #5 New Member   kahany Join Date: Nov 2010 Posts: 4 Rep Power: 15 depending on your multiphase modulus, you shoud find it under "phase". make sure that you present results for "phase-2" I hope I helped.

 March 12, 2012, 02:00 #6 New Member   M. A. Join Date: Jul 2010 Posts: 27 Rep Power: 15 Dear Talib, Thanks for your reply. 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...

 March 15, 2012, 11:32 two phase flow #7 New Member   zainab p Join Date: Mar 2012 Posts: 6 Rep Power: 14 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?

 March 15, 2012, 12:34 not enough info #8 New Member   kahany Join Date: Nov 2010 Posts: 4 Rep Power: 15 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.

 March 15, 2012, 13:17 #9 New Member   zainab p Join Date: Mar 2012 Posts: 6 Rep Power: 14 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

 July 4, 2016, 06:19 #10 New Member   Bahador Join Date: Jun 2016 Posts: 3 Rep Power: 9 Void Fraction and Volume Fraction are not the same.The gas volume fraction is usually larger than the gas void fraction.

 October 15, 2016, 12:11 #11 New Member   Elias Join Date: May 2012 Posts: 10 Rep Power: 13 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? Last edited by eliastorca; October 20, 2016 at 04:51.

 October 20, 2016, 02:32 #12 New Member   Roya.TR Join Date: Feb 2014 Posts: 9 Rep Power: 12 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. please help me. thank you

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