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

Check for mass conservation

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

Like Tree1Likes
  • 1 Post By vinerm

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 10, 2020, 03:23
Unhappy Check for mass conservation
  #1
New Member
 
Shuyan
Join Date: Feb 2020
Posts: 8
Rep Power: 6
shuuyaan is on a distinguished road
Hello all,

I am doing a transient 2d multiphase simulation with evaporation-condensation. There are no inlets/outlets, the initial volume of water is patched into the region. My primary phase is water and my secondary phase is mixture-template (air + water vapor). I am reporting the mass from the reports>volume-integrals>mass option.

At t=0,
Total Mass = 1.6632497117e+00 kg
Mass of water = 1.6609126049e+00 kg
Mass of mixture-template= 2.3371067885e-03 kg

At t=0.62s,
Total Mass = 1.3104626319e+00 kg
Mass of water = 1.3081499147e+00 kg
Mass of mixture-template= 2.3127171514e-03 kg

Please let me know if I'm checking it correctly, and why it is incorrect, and how can I improve my simulation.
shuuyaan is offline   Reply With Quote

Old   April 10, 2020, 03:40
Default Closed Domain
  #2
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 36
vinerm will become famous soon enough
If the domain is closed, you need to ensure that the density of mixture of air and water-vapor is governed by either ideal or real gas. Constant density mixture cannot exist in a closed domain.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Old   April 10, 2020, 05:07
Default
  #3
New Member
 
Shuyan
Join Date: Feb 2020
Posts: 8
Rep Power: 6
shuuyaan is on a distinguished road
Hi @vinerm

I can confirm that I am using the incompressible-ideal-gas method. Any other thoughts of what I can check?

Btw, appreciate your reply!
shuuyaan is offline   Reply With Quote

Old   April 10, 2020, 05:16
Default Incompressible
  #4
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 36
vinerm will become famous soon enough
It has to be ideal gas and not incompressible ideal gas.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Old   April 10, 2020, 10:03
Default
  #5
New Member
 
Shuyan
Join Date: Feb 2020
Posts: 8
Rep Power: 6
shuuyaan is on a distinguished road
Hi @vinerm,

Thank you. That solved my issue.

Please allow me to extend my question as this continues to baffle me -

I am reporting the mass fraction data as follows:

At t=0,
Mass Fraction of Air = 0.8131
Mass Fraction of Vapour = 0
Total Mass Fraction = 0.8131

At t=1,
Mass Fraction of Air = 0.3021
Mass Fraction of Vapour = 0.2111
Total Mass Fraction = 0.5132

Since the mass is already shown to conserve, is there any reason the total mass fraction continues to decrease with time?
shuuyaan is offline   Reply With Quote

Old   April 10, 2020, 10:10
Default Mass Fraction
  #6
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 36
vinerm will become famous soon enough
Total mass fraction is always conserved and equal to 1. It cannot change. Probably, you are reporting something else, may be mass.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Old   April 10, 2020, 10:55
Default
  #7
New Member
 
Shuyan
Join Date: Feb 2020
Posts: 8
Rep Power: 6
shuuyaan is on a distinguished road
Hi @vinerm,

I am reporting the mass fraction from surface report>area-weighted average>species>mass fraction of air, phase>mixture-template

My workflow is as below-

I initialized the simulation with volume fraction of 1 for mixture-template. Then, I patched a volume fraction of 0 in the region where water will be located. Hence, the initial mass fraction of 0.8131 is logical to me and I would expect the max value to be capped at 0.8131.

Please advice, thank you.
shuuyaan is offline   Reply With Quote

Old   April 10, 2020, 11:03
Default Mass Fraction
  #8
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 36
vinerm will become famous soon enough
Area-Weighted Averaging is used for boundaries, not for cell zones. Use volume average or mass average for cell zones.

It does not matter whether the domain is completely filled with mixture or partially filled, mass fraction will always be 1 because of the way it is defined.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Old   April 10, 2020, 12:41
Default
  #9
New Member
 
Shuyan
Join Date: Feb 2020
Posts: 8
Rep Power: 6
shuuyaan is on a distinguished road
Hi @vinerm,

Once again, thanks for your help! I would never have caught these details myself. It's always the basics that give me such a headache.

Please just clarify my statements below for my understanding.

1. In order to report values from the interior, I have to pick volume average and the cell zones. This applies to 2D and 3D simulations. (I have been using area average and the "interior" boundary)

2. For values such as heat flux, which is to be reported from the wall "boundary", it is okay to pick area average and the respective walls.

3. What about the contour plots? Are the values defined by the 'volume' or the 'area'? Would it be accurate to portray the mass fraction with contour plots and the respective animations?

Thank you
shuuyaan is offline   Reply With Quote

Old   April 10, 2020, 12:48
Default Suggestions
  #10
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 36
vinerm will become famous soon enough
1. For volumetric averages, always use volume or mass averaged.

2. For boundaries, you can always use either area or mass weighted. Mass weighted cannot be used for boundaries that have no mass crossing them, such as walls. So, for walls, always use area-weighted.

3. Contours are not averages.
juliom likes this.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm 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
Mesh check failed subhas.hunasikatti@gmail. FLUENT 0 January 19, 2014 04:09
[ICEM] Understanding check mesh output kotimari ANSYS Meshing & Geometry 0 July 15, 2012 08:33
faceZones and topological check mturcios777 OpenFOAM Programming & Development 0 June 15, 2012 13:48
Automated check for convergence Chander CFX 3 May 2, 2012 18:07
Cell check and Boundary check errors AB Siemens 4 October 28, 2004 13:04


All times are GMT -4. The time now is 21:35.