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

Laminar gas flow under slight suction - transient works but not steady

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 8, 2011, 15:53
Default Laminar gas flow under slight suction - transient works but not steady
  #1
New Member
 
Join Date: Jan 2011
Posts: 21
Rep Power: 15
audrey is on a distinguished road
Hi all,

I am working on a large CFD project and this portion of the work is just a small step to help me build the final model. I was assuming that this would be quite a straight forward and easy simulation, but here I am...

Essentially, I have a low flow of gas entering a rotary kiln (long horizontal pipe) under laminar conditions. The walls on the kiln are rotating at 2rpm (although I've also tried the simulation without rotation). The inlet condition specifies the normal gas velocity to 0.00336 m/s based on lab conditions. I am trying to run this as a steady simulation because it will get quite complex once fully built.

The outlet here is a series of 8 openings around the pipe wall at a distance of 3m from the inlet. I originally assumed (before lab work started) an outlet static pressure of 0 Pa and my simulations converged well and quickly, plus the results seemed perfectly reasonable (not fully checked against analytical solution yet).

Now, when doing experiments, we noticed that the pressure condition at the outlet is actually under a negative pressure of 29 Pa (caused by the afterburner that follows the kiln). I thought this would just be a matter of changing the pressure at the bc and voila, but no. I couldn't get convergence. Tried different time step, tried decreasing the pressure slowly, tried starting from different previous simulations, nothing helped.

I decided to run a transient analysis of the system. After fiddling with the time step, I was able to get the residuals to decrease to very low levels. I had a good look at the results from the transient analysis, and variations in velocities and pressure during the simulation are very small - can't be noticed by just looking at figures. I tried running a steady simulation from the results of the transient analysis, but no luck.

I don't think this should be a transient situation, but I may be wrong. I'm including a figure that shows the system layout (mesh is tet mesh with 1.2 million elements and passing all CFX tests). The other 2 figures show the RMS history plots for my steady and transient runs. I'm a bit puzzled by the behaviour of the transient run RMS (very bouncy, steps are caused by changes in timestep size), if anybody can say anything about that I'd be interested.

Any advice/suggestions/comments are welcome here, I'm at a loss.
Attached Images
File Type: jpg Residuals of transient analysis.jpg (28.9 KB, 12 views)
File Type: jpg Residuals of steady analysis.jpg (24.3 KB, 9 views)
File Type: jpg System layout.jpg (17.9 KB, 15 views)
audrey is offline   Reply With Quote

Old   September 8, 2011, 20:43
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,862
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Quote:
I am trying to run this as a steady simulation because it will get quite complex once fully built.
The decision as to run steady state of not has little to do with complexity. If it is transient, then you have to bite the bullet and run it transient.

Quote:
the pressure condition at the outlet is actually under a negative pressure of 29 Pa
If the gas is incompressible then simply move the reference pressure down by 29Pa and the solution remains the same. In fact, even if it is compressible you should set the reference pressure equal to the exit pressure in your case.

But your general question seems well covered by this FAQ:
http://www.cfd-online.com/Wiki/Ansys...gence_criteria
ghorrocks is offline   Reply With Quote

Reply

Tags
convergence stall, steady, suction, transient

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
CFX Treatment of Laminar and Turbulent Flows Jade M CFX 18 September 15, 2022 08:08
Validation for laminar Disperse phase flow shashwat Main CFD Forum 0 April 4, 2008 03:20
How to determine whether flow is steady or unstead Maliya,Barth Main CFD Forum 7 October 7, 2006 03:26
Gas pressure question Dan Moskal Main CFD Forum 0 October 24, 2002 23:02
Laminar gas flow at low Re in tubes Glenn Price Main CFD Forum 0 October 8, 1998 13:21


All times are GMT -4. The time now is 14:22.