[Niekjuh]

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]

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.

[ghorrocks]

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.

[Bdew8556]

Hey guys,

I had a similar error:

ERROR PROBLEM ENCOUNTERED WHEN EXECUTING CFX EXPRESSION LANGUAGE The CFX expression language was evaluating: Input from CFX-5 The problem was: INVALID-EXPONENTIATION FURTHER INFORMATION The problem was encountered in executing the expression for: HSOut The complete expression is: HIn+cpRef*tIn*((pOut/pIn)^((gamma-1)/gamma)-1) The error occurs on sub-expression: (pOut/pIn)^((gamma-1)/gamma) BACKGROUND INFORMATION In this expression, these sub-expressions have been calculated: Sub-expression = Value cpRef*tIn = 165221. These values were set before reaching the current expression: Name = Expression = Value tIn = massFlowAveAbs(Temperature)= 164.497 pOut = 1 [atm] + massFlowAveAbs(Pr= -50895.4 pIn = 1 [atm] + massFlowAveAbs(Pr= 117734. gamma = 1.40000 cpRef = massFlowAveAbs(Specific Hea= 1004.40 HIn = massFlowAveAbs(Static Entha= -134241. END OF DIAGNOSTIC OUTPUT FOR CFX EXPRESSION LANGUAGE

Any thoughts?

Brett

[ghorrocks]

All I can suggest is to break the equation up. It does not like the exponent, so define it as a separate variable and check it is the value you expect.