Peng Robinson EOS: Unit auf critical volume

shock77 · May 26, 2020, 10:28

Hello everyone,

I have noticed, that the unit of the critical volume is odd in PengRobinGas.H:

Code:

 

               scalar Vc_;     //- Critical volume [m^3/kmol]

Why is the unit m^3/kmol and not m^3/mol? Wouldn't that be contradicting to the unitlessness of OpenFOAM?

Kind regards,
shock77

geth03 · May 27, 2020, 05:37

Vc_ is defined as scalar and not dimensionedScalar, so the dimension is somewhat hidden i guess.
i think how this value fits in overall in the equation is ultimately decisive.

shock77 · May 27, 2020, 07:00

Hi,

yes, I agree.

I have found out, that in PengRobinsonGas.C the compressibility is defined by:

Code:

           Zc_ = Pc_*Vc_/(RR*Tc_);

RR is definied in thermodynamicConstants.C as:

Code:

const scalar RR = 1e3*physicoChemical::R.value();

R is a dimensioned scalar, with the unit [J/(mol*K)].

So basically, the universal gas constant is multiplicated by 10^3, so the unit kmol makes sense. Still I dont know why it is done.

In PengRobinsonGas.H I have also seen:

Code:

 //- Return cp departure [J/(kmol K]


 inline scalar cp(scalar p, scalar T) const;

But the unit of cp should be [J/(kg K)], isnt it? At least I have definied cp always as 1004 for air in thermophysicalPropterties, when I have used the perfectGas EOS.
In perfectGas.H cp is definied the same.

Has anyone used PengRobinson?

Kind regards, shock77

shock77 · May 27, 2020, 08:08

Since the molWeight is in kg/kmol as well, there must be somewhere a statement like CP = cp/molWeight... but I cant find it.

Does anyone know, where to find this statement?

May 26, 2020, 10:28	Peng Robinson EOS: Unit auf critical volume	#1
shock77 Senior Member Join Date: Dec 2019 Posts: 215 Rep Power: 8	Hello everyone, I have noticed, that the unit of the critical volume is odd in PengRobinGas.H: Code: scalar Vc_; //- Critical volume [m^3/kmol] Why is the unit m^3/kmol and not m^3/mol? Wouldn't that be contradicting to the unitlessness of OpenFOAM? Kind regards, shock77

May 27, 2020, 07:00		#3
shock77 Senior Member Join Date: Dec 2019 Posts: 215 Rep Power: 8	Hi, yes, I agree. I have found out, that in PengRobinsonGas.C the compressibility is defined by: Code: Zc_ = Pc_Vc_/(RRTc_); RR is definied in thermodynamicConstants.C as: Code: const scalar RR = 1e3physicoChemical::R.value(); R is a dimensioned scalar, with the unit [J/(molK)]. So basically, the universal gas constant is multiplicated by 10^3, so the unit kmol makes sense. Still I dont know why it is done. In PengRobinsonGas.H I have also seen: Code: //- Return cp departure [J/(kmol K] inline scalar cp(scalar p, scalar T) const; But the unit of cp should be [J/(kg K)], isnt it? At least I have definied cp always as 1004 for air in thermophysicalPropterties, when I have used the perfectGas EOS. In perfectGas.H cp is definied the same. Has anyone used PengRobinson? Kind regards, shock77

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May 27, 2020, 05:37		#2
geth03 Senior Member Join Date: Dec 2019 Location: Cologne, Germany Posts: 369 Rep Power: 9	Vc_ is defined as scalar and not dimensionedScalar, so the dimension is somewhat hidden i guess. i think how this value fits in overall in the equation is ultimately decisive.

May 27, 2020, 08:08		#4
shock77 Senior Member Join Date: Dec 2019 Posts: 215 Rep Power: 8	Since the molWeight is in kg/kmol as well, there must be somewhere a statement like CP = cp/molWeight... but I cant find it. Does anyone know, where to find this statement?