CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > STAR-CCM+

Problem: Convergence in unsteady EBU combustion model

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 21, 2014, 05:33
Default Problem: Convergence in unsteady EBU combustion model
  #1
New Member
 
Alexey
Join Date: Apr 2014
Posts: 5
Rep Power: 3
Defend is on a distinguished road
Hello!

For monthes I've tried to solve the problem with the combustion and then decided to ask for a help on this forum =)

I used non-premixed Standart EBU combustion model for chemical reaction C2H5OH+3O2=2CO2+3H2O to verify the experimental data. In experiment I have a burning liquid C2H5OH in closed volume room. To exclude multiphasing I used wall with normal mass flow of gas C2H5OH from its surface (not mass flow rate on inlet). Also I added small pressure outlet bondary condition.

I tried to use different turbulent models, time-steps, Courant number, other solver parameters and all the time I have the same problem: residuals of CO2, H2O at every time-step becomes flat without any value decreasing. First time at value 10e-4, then 10e-2... and after many time steps at value 10e+7... I tried to increase the number of cells - I made 100000 polyhedral cells for simple 2d geometry. Also I found, that Green-Gauss gradient method instead of Hybrid Gauss-LSQ gives me better convergence. But again and again I have nondecreasing value of CO2 and H2O residual 10e+7.
It would be the great luck if anyone of you could explain me, how to get the residual convergence 10e-5 at any time-step of such task.

Thank you in advance.
Defend is offline   Reply With Quote

Old   April 21, 2014, 13:01
Default
  #2
Senior Member
 
Join Date: Nov 2010
Posts: 418
Rep Power: 8
me3840 is on a distinguished road
Are those the normalized or unnormalized residuals?
me3840 is offline   Reply With Quote

Old   April 21, 2014, 13:07
Default
  #3
New Member
 
Alexey
Join Date: Apr 2014
Posts: 5
Rep Power: 3
Defend is on a distinguished road
what does it mean - normalize/unnormalize residual?..
Defend is offline   Reply With Quote

Old   April 21, 2014, 13:49
Default
  #4
Senior Member
 
Join Date: Nov 2010
Posts: 418
Rep Power: 8
me3840 is on a distinguished road
The residuals are normalized by default to the average value of the given residual over the first few iterations in order to group them closer together. You can disable this and view their true values by turning the normalization option to off in the monitors folder. If the values you're looking at are very low, you may be hitting a precision limit or just seeing noise.
me3840 is offline   Reply With Quote

Old   April 21, 2014, 14:39
Default
  #5
New Member
 
Alexey
Join Date: Apr 2014
Posts: 5
Rep Power: 3
Defend is on a distinguished road
Hmm... Ok, thank you, I'll try tomorrow!
Defend is offline   Reply With Quote

Old   April 22, 2014, 04:02
Default
  #6
New Member
 
Alexey
Join Date: Apr 2014
Posts: 5
Rep Power: 3
Defend is on a distinguished road
me3840, thank you! I had normalized values of residuals. I changed them to unnormalized and the true value is 1e-8!..

Maybe you could answer to my question about coefficient A of EBU combustion model:

Everywhere in literature I see, that this coefficient for modeling must be A=4. And in my task to reach the temperature values that accords to experimental data this coeff must be A=0.1. Is it normal to change this coeff or I have some mistakes in solving?..
Thank you in advance.
Defend is offline   Reply With Quote

Old   April 22, 2014, 10:25
Default
  #7
Senior Member
 
Join Date: Nov 2010
Posts: 418
Rep Power: 8
me3840 is on a distinguished road
I don't really know anything about the EBU models to say, sorry. From what I've read the coefficients are tuned for most cases, but can be changed in some.
me3840 is offline   Reply With Quote

Old   April 22, 2014, 14:48
Default
  #8
New Member
 
Alexey
Join Date: Apr 2014
Posts: 5
Rep Power: 3
Defend is on a distinguished road
Anyway, thank you for solving my problem with residuals!
I hope someone else know more about EBU model. =)
Defend is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
unsteady heat transfer model problem Beryl FLUENT 0 July 12, 2011 23:26
convergence problem in RNG k-e model pj21aug FLUENT 0 October 14, 2010 11:43
Convergence problem for P1 & Energy HP FLUENT 5 May 21, 2005 15:01
Unsteady Problem Convergence Sham FLUENT 2 March 17, 2005 05:34
Convergence Problem in k-e model Rajeev Kumar Singh FLUENT 3 April 15, 2002 07:13


All times are GMT -4. The time now is 18:14.