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-   -   units in hsEqn (http://www.cfd-online.com/Forums/openfoam/93484-units-hseqn.html)

 megacrout October 17, 2011 09:53

units in hsEqn

Hi,

I am confused about the parameters used in hsEqn (in my case, in reactingFoam: http://foam.sourceforge.net/docs/cpp/a02385_source.html) and their respective units.

No problem with the right side: both terms are in kg/(ms3). However, this does not seem to be the case of the terms on the left side. Firstly, hs is supposed to be in J/kmkol (see janafThermo, where it is computed) but kmol wonīt cancel out here. So I assume hs has here the same units as Hs (J/kg). This leaves the first two terms with units kg/(sm3) and the third one with units m2/s3.

1) What is wrong here? I did a similar unit check with UEqn and YEqn and got no problem.
2) Can someone show me why hs should suddenly be in J/kg instead of J/kmol? Where does it get converted?

Thx.

Tibo

 megacrout October 17, 2011 10:26

Ok, first two terms actually are in kg/(ms3) if we assume hs is J/kg, not in J/kmol. My bad.

Third term appers to be fine if we multiply it by density.

Still:
1) Can someone explain me why hs should suddenly be in J/kg instead of J/kmol? Where does it get converted?
2) Is there a mistake or is the density actually missing in the code?

Tibo

OpenFOAM's definition of Alpha corresponds to "rho times alpha" in terms of units.
hs is in units of energy/mass, and the conversion is done when computing hs at the cell level. I think the function hs(T) returns hs(in J/Kmol)/W.

 megacrout October 17, 2011 11:16

That would explain pretty well what I observed. Can you tell me what files to look at to see what you mention, i.e. can you proof what you wrote? I still have to learn quite a lot about C++ so I might have missed some code specificity too.

Thank you.

Tibo

Just check the units of alpha where it is defined.

\$FOAM/src/thermophysicalModels/basic/basicThermo

For the other thing, you might want to check the file specieThermo files.

\$FOAM/src/thermophysicalModels/specie/thermo

Hope this helps.

 megacrout October 18, 2011 08:00

It does. Thx.
I needed some time to understand how reactingFoam is related to basicThermo: it is used in basicPsiThermo which is in turn used in hsCombustionThermo on which reactingFoam is based.

I donīt really see how alphaEff ends up being called from basicThermo (i.e. with the properties of basicThermo) to hsEqn though. Can you help me on this?