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Thermophysical Property Units

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Old   November 20, 2013, 07:43
Default Thermophysical Property Units
  #1
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Johan Zietsman
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I am using OpenFOAM 2.2.0.

I am busy using the chtMultiRegionFoam solver to configure a heat transfer case that involves multiple materials. When I started editing the thermophysicalProperties files, I found it very hard to determine what the units of the properties were. I am referring to the following content of the file:

mixture
{
specie
{
nMoles 1;
molWeight 63.546; // molar weight, [kg/kmol]
}

transport
{
kappa 401; // thermal conductivity, [W/(m.K)]
}

thermodynamics
{
Hf 0; // ??? enthalpy of formation, [J/kmol]
Cp 24449; // heat capacity at constant pressure, [J/(kmol K)], ref. FactSage FactPS database
}

equationOfState
{
rho 8960; // density, [kg/m^3]
}
}

I delved into the header files to find comments where the units were documented. These parameters are implemented with the scalar data type, and not a dimensionedScalar data type. This makes me a bit worried.

Is there a specific method in that I can use to determine the units of quantities such as those above, or are the units implicitly implemented in the code with comments documenting the choice of units?

Was the implementation of units on thermophysical properties changed in version 2.2.0 to eliminate the units? If so, what is the reason for this.

Thanks for the help.

Johan
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Old   January 21, 2014, 05:01
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Joachim Herb
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Quote:
Originally Posted by johanz View Post
I am using OpenFOAM 2.2.0.
...
Code:
mixture
{
    ...
thermodynamics
    {
        Hf      0; // ??? enthalpy of formation, [J/kmol]
        Cp      24449; // heat capacity at constant pressure, [J/(kmol K)], ref. FactSage FactPS database
    }
...
...
Comparing the example given at http://www.openfoam.org/docs/user/thermophysical.php with the value (for air) from https://en.wikipedia.org/wiki/Heat_capacity it looks like cp is in J/(kg K) and not J/(kmol K).

In the bug report http://www.openfoam.org/mantisbt/view.php?id=1008#c2487 it is explained that:
Quote:
In the source code, typically upper case letters (e.g. Ha) indicate [J/kg] and lower case (e.g. ha) [J/kmol].
Internally in https://github.com/OpenFOAM/OpenFOAM...ThermoI.H#L129 the sensible enthalpy is calculated which has the units J/kmol (according to https://github.com/OpenFOAM/OpenFOAM...tThermo.H#L154) so cp has the units J/(kmol K). The conversion is from kg to kmol is done when reading it in https://github.com/OpenFOAM/OpenFOAM...stThermo.C#L52

The same seems to apply to Hf (so also given as J/kg in thermophysicalProperties)
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