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

Molar fraction in CFX Pre expression

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

Reply
 
LinkBack Thread Tools Display Modes
Old   June 12, 2014, 18:21
Default Molar fraction in CFX Pre expression
  #1
New Member
 
Keal
Join Date: Jun 2014
Posts: 2
Rep Power: 0
Niekjuh is on a distinguished road
Hi all,

I'm currently busy with an expression for the chemical timescale (which i want to use in the Damkohler number).

In that expression I have to use the molar fraction of CH4 and O2.

Expression (allready filled in all the constants):

(2.21E-14)*e^(22659/T)*(CH4.Molar Fraction + 1.0E-12)^0.33*(O2.Molar Fraction + 1.0E-12)^(-1.05)

And I have this error:

WARNING
Unrecognized system error.
An error was encountered while trying to resolve the name 'CH4.Molar Fraction' referenced by 't chem exp'.


Do I to specify something for CH4 and O2?

Thanks in advance.
Niek
Niekjuh is offline   Reply With Quote

Old   June 13, 2014, 08:37
Default
  #2
New Member
 
Keal
Join Date: Jun 2014
Posts: 2
Rep Power: 0
Niekjuh is on a distinguished road
I've fixed it by adding another material library.

I'm doing two analyses: one with the eddy dissipation model and one with the burning velocity model.

The BVM starts running without any errors but for the Eddy model I get this error after a few seconds:

+--------------------------------------------------------------------+
| PROBLEM ENCOUNTERED WHEN EXECUTING CFX EXPRESSION LANGUAGE |
| |
| The CFX expression language was evaluating: |
| Additional Variable Value |
| |
| The problem was: |
| INVALID-EXPONENTIATION |
| |
| FURTHER INFORMATION |
| |
| The problem was encountered in executing the expression for: |
| t chem exp |
| The complete expression is: |
| 2.21e-14 [m^-2.160 mol^0.720 s]* exp(22659[K]/Temperature) * (m \|
| in(CH4.Molar Conc |
| The error occurs on sub-expression: |
| (min(O2.Molar Concentration ,1e-12 [mol m^-3]))^-1.05 |
| |
| BACKGROUND INFORMATION |
| |
| The error was detected at one location. The same problem may be |
| present at other locations - that has not been investigated. |
| The following values are for the first location which has the |
| problem. |
| |
| In this expression, these sub-expressions have been calculated: |
| |
| Sub-expression = Value |
| |
| 2.21e-14 [m^-2.160 mol^0.720 s]* exp(22659[K]/Temp= 0.00000 |
| |
| END OF DIAGNOSTIC OUTPUT FOR CFX EXPRESSION LANGUAGE


Does someone knows what it means? Otherwise I'll just start without the expressions in CFX-pre and add them in post.

Last edited by Niekjuh; June 13, 2014 at 10:54.
Niekjuh is offline   Reply With Quote

Old   June 13, 2014, 17:28
Default
  #3
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,824
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
When you use empirical fits like this the units calculator goes bezerk. It cannot handle units like m^-2.16. I would adjust your equation so the calculation is done unitless so it can accept raising to non-integer powers.
ghorrocks 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
Setting Pressure Boundary Conditions in ANSYS CFX Pre saisanthoshm88 CFX 19 April 26, 2014 20:36
writing execFlowFunctionObjects immortality OpenFOAM Post-Processing 30 September 15, 2013 06:16
CFX Expression Language help RicochetJ CFX 3 April 21, 2013 07:46
How to install CGNS under windows xp? lzgwhy Main CFD Forum 1 January 11, 2011 19:44
CGNS lib and Fortran compiler manaliac Main CFD Forum 2 November 29, 2010 07:25


All times are GMT -4. The time now is 14:43.