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June 30, 2009, 09:53 
cellMixture: multicomponent mixture

#1 
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Maria
Join Date: Apr 2009
Posts: 12
Rep Power: 8 
Hi !
I think I have found a bug in the "calculate()" function of a multicomponent mixture. Let me go step by step: 1. For multiComponentMixtures OpenFOAM calculates the cellMixture as: cellMixture = ForAll (i) { cellMixture += Y[i] / species[i].W() * speciesData[i] }; 2. Enthalpy in hCombustionThermo is massbased defined, meanwhile in specieThermo h is molarbased and H is massbased. 3. In hMixtureThermo::h (massbased) h = ForAll(i) { += Y[i] / species[i].W() * speciesData[i].H() }; where, as far as I understand, this H[i] is the massbased enthalpy calculated in specieThermo. 4. Having a look at the units of the hMixtureThermo::h => [m^2 s^(2)] != [][Kmol_i Kg_i^(1)]*[m^2 s^(2)] Which IS DIMENSIONALLY INCORRECT!! ... It does the same for calculating alpha/mu of the mixture, which are defined also in a MASS basis. So: I guess it should be fixed replacing the current cellMixturedefinition of multiComponentMixture by: cellMixture = ForAll (i) { cellMixture += Y[i] * speciesData[i] }; Is there really a bug..or I am missing something?? Regards! Last edited by mgc; July 1, 2009 at 04:14. 

June 30, 2009, 12:18 

#2  
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Niklas Nordin
Join Date: Mar 2009
Location: Stockholm, Sweden
Posts: 693
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Quote:
Quote:
This is an inconsistency in the naming convention. Quote:
I cannot find it anywhere. It looks like you have replaced speciesData in Q1 with H, which is incorrect. All I can find in there that is fairly similar is this (which is correct): Code:
136 template<class MixtureType> 137 Foam::tmp<Foam::scalarField> Foam::hMixtureThermo<MixtureType>::h 138 ( 139 const scalarField& T, 140 const labelList& cells 141 ) const 142 { 143 tmp<scalarField> th(new scalarField(T.size())); 144 scalarField& h = th(); 145 146 forAll(T, celli) 147 { 148 h[celli] = this>cellMixture(cells[celli]).H(T[celli]); 149 } 150 151 return th; 152 } Quote:
but all the code is correct. I get the feeling you have started in the wrong end. Looking at how nasa polynomials are defined, which is the speciesData. (or it could just as well be janaf) http://www.me.berkeley.edu/gri_mech/data/nasa_plnm.html H/RT = a1 + a2 T /2 + a3 T^2 /3 + a4 T^3 /4 + a5 T^4 /5 + a6/T unit for R is J/mol*K and the right hand side is dimensionless. so H is in J/mol. and if you want to calculate the enthalpy for the mixture you just add these coefficients together on a molar fraction basis. Thus the a1 coefficient for the mixture is sum of all its components = sum (X_i*a1_i) That is what the forloop does in Q1. I hope all is clearer now 

June 30, 2009, 13:38 

#3 
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Maria
Join Date: Apr 2009
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Finally, I got it.
For those who could have the same doubt, I copy the keypoint (in my opinion) to understand how the "cellMixture" works: // template<class ThermoType> const ThermoType& multiComponentMixture<ThermoType>::cellMixture ( const label celli ) const { mixture_ = Y_[0][celli]/speciesData_[0].W()*speciesData_[0]; for (label n=1; n<Y_.size(); n++) { mixture_ *+=* Y_[n][celli]/speciesData_[n].W()*speciesData_[n]; } return mixture_; } // template<class equationOfState> inline void janafThermo<equationOfState>::*operator+=* ( const janafThermo<equationOfState>& jt ) { scalar molr1 = this>nMoles(); equationOfState:: operator+=(jt); molr1 /= this>nMoles(); scalar molr2 = jt.nMoles()/this>nMoles(); Tlow_ = max(Tlow_, jt.Tlow_); Thigh_ = min(Thigh_, jt.Thigh_); Tcommon_ = molr1*Tcommon_ + molr2*jt.Tcommon_; for ( register label coefLabel=0; coefLabel<janafThermo<equationOfState>::nCoeffs_; coefLabel++ ) { highCpCoeffs_[coefLabel] = molr1*highCpCoeffs_[coefLabel] + molr2*jt.highCpCoeffs_[coefLabel]; lowCpCoeffs_[coefLabel] = molr1*lowCpCoeffs_[coefLabel] + molr2*jt.lowCpCoeffs_[coefLabel]; } } Regards! María Last edited by mgc; July 15, 2009 at 05:57. 

May 12, 2013, 23:06 

#4 
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何刚
Join Date: Jan 2011
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So, we could only set janafThermo? And what about the hconstThermo?


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