CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM > OpenFOAM Running, Solving & CFD

chtMultiRegionSimpleFoam: inconsistency between BCs and results

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   February 21, 2017, 10:28
Default chtMultiRegionSimpleFoam: inconsistency between BCs and results
  #1
New Member
 
Andrea Di Ronco
Join Date: Nov 2016
Location: Milano, Italy
Posts: 9
Rep Power: 4
Diro7 is on a distinguished road
Hello to everyone,

I'm trying to simulate conjugate heat transfer with the chtMultiRegionSimpleFoam solver.
My case consists in 2 fluid channels that exchange heat in counterflow through a solid matrix (you can see a section view of the solid region in the first picture).
The channels are small in section (diameter is about 2-3 mm) but very long (2.5 m).

The inlet and outlet BCs for the fluids are quite straightforward to assess, as well as the ones for the 2 fluid/solid coupling surfaces.

Regarding temperature in the solid region, since the heat exchanger is supposed to consist in a very large number of channels, I set zeroGradient BCs on the left/right faces (due to pattern symmetry) and cyclicAMI BCs on the top/bottom faces (due to the pattern periodicity).
The only "arbitrary" choice is the BC on the front/back solid faces (i.e. the ones where the fluid inlets and outlets lie). I couldn't find any reasonable boundary condition (from the physical modelling viewpoint), so I decided to use zeroGradient also here.

The simulation after 1500/2000 steps seems to reach convergence (residuals are very low for each quantity) and most of the results seem to be in agreement with predictions (fluid T differences, total heat transfer rates), but something is not convincing:

1) In the second picture you can find a T plot over an axial line in the solid region (at the exact center of the section; you can see the point in the first picture): the temperature derivative along the axial direction at the surfaces is not 0, in contrast with the zeroGradient BCs!
I tried different meshes (I use the Sweep method of the Fluent meshing tool) with different axial refinements but, as far as I know, the results are always quite the same.

2) Also, as you can see, the four corners show temperature values that are quite probably wrong (much higher/lower than the rest of the section).

Did anyone experience the same issues, or does know a possible explanation?
Should I use different BCs, or try to improve the mesh?

Thank you very much in advance for the help.

Andrea

PS: if you are wondering, the temperature field in the first picture is taken from a section exactly at half the channel length.

EDIT1: I added a picture of the mesh over the face.

EDIT2: it seems that the strange values at the corners were only a matter of visualisation in ParaView. I'm not an expert on how it works, but I realised that when plotting the original Cell Field rather than the interpolated (or smoothed, I don't know how to say it) Point Field the corner values are ok. Probably - and reasonably - in order to show correct values ParaView should account for the cyclic-ness of the boundary conditions, but it can not and some plotting error occurs.
Attached Images
File Type: png section.png (39.4 KB, 24 views)
File Type: png line_plot.png (33.5 KB, 14 views)
File Type: jpg face_mesh.jpg (59.9 KB, 16 views)

Last edited by Diro7; February 24, 2017 at 11:50. Reason: 2nd issue solved
Diro7 is offline   Reply With Quote

Old   March 2, 2017, 05:36
Default
  #2
New Member
 
Andrea Di Ronco
Join Date: Nov 2016
Location: Milano, Italy
Posts: 9
Rep Power: 4
Diro7 is on a distinguished road
No hints?
Diro7 is offline   Reply With Quote

Reply

Tags
chtmultiregionsimplefoam

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
BCs compatibility modified simpleFoam Tensian OpenFOAM Running, Solving & CFD 3 September 26, 2013 07:18
The effects of BCs and the initial conditions on the final results mrji8011 Main CFD Forum 4 August 28, 2013 11:39
The effects of BCs and initial condition on final results mrji8011 FLUENT 0 August 27, 2013 23:16
Dealing with BC's in OF 1.6 vkrastev OpenFOAM Running, Solving & CFD 5 September 4, 2012 12:58
bc's of a komegaSST case Zymon OpenFOAM 11 July 25, 2010 10:36


All times are GMT -4. The time now is 15:54.