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Molar fraction in CFX Pre expression

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Old   June 12, 2014, 18:21
Default Molar fraction in CFX Pre expression
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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:

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.
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Old   June 13, 2014, 08:37
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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:

| |
| The CFX expression language was evaluating: |
| Additional Variable Value |
| |
| The problem was: |
| |
| |
| 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 |
| |
| |
| 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 |
| |

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.
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Old   June 13, 2014, 17:28
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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.
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