Understanding reaction rates

vbchris · February 22, 2018, 16:02

Hello all,

I'm trying to implement a new reaction rate type in OF 5.0. I am having some trouble understanding the code and if anyone can provide clarity it would be appreciated.

In reactingFoam the species conversion is implemented in the YEqn.h file:

Code:

            fvScalarMatrix YiEqn
            (
                fvm::ddt(rho, Yi)
              + mvConvection->fvmDiv(phi, Yi)
              - fvm::laplacian(turbulence->muEff(), Yi)
             ==
                reaction->R(Yi)
              + fvOptions(rho, Yi)
            );

The function R I believe references the chemistryModel member RR which is calculated

Code:

 // Calculates the reaction rate
  // RR_[i] can then be called by chemistry.rr() in the YEqn.h file by the solver
  
  
  
  644 template<class CompType, class ThermoType>
  645 void Foam::chemistryModel<CompType, ThermoType>::calculate()
  646 {
  647     if (!this->chemistry_)
  648     {
  649         return;
  650     }
  651 
  
  // Gets the necessary property fields from thermo().  This is where all thermo properties are handled.
  
  652     tmp<volScalarField> trho(this->thermo().rho());
  653     const scalarField& rho = trho();
  654 
  655     const scalarField& T = this->thermo().T();
  656     const scalarField& p = this->thermo().p();
  657 
  
  // Main loop for calculating the reaction rates
  // The index celli refeers to a specific cell in the mesh
  
  658     forAll(rho, celli)
  659     {  
  660         const scalar rhoi = rho[celli];
  661         const scalar Ti = T[celli];
  662         const scalar pi = p[celli];
  663 
  
  // The index i referrers to a species int 
  
  664         for (label i=0; i<nSpecie_; i++)
  665         {
  666             const scalar Yi = Y_[i][celli];
  667             c_[i] = rhoi*Yi/specieThermo_[i].W();
  668         }
  669 
  
  // Omega modifies dcdt_, Ti and pi are copies and c_ isn't used in the following loop.
  
  670         omega(c_, Ti, pi, dcdt_);
  671 
  672         for (label i=0; i<nSpecie_; i++)
  673         {
  674             RR_[i][celli] = dcdt_[i]*specieThermo_[i].W();
  675         }
  676     }
  677 }

So RR = dcdt*MolecularWeight

dcdt is set by the omega function which is virtual

Code:

template<class CompType, class ThermoType>
   97 void Foam::chemistryModel<CompType, ThermoType>::omega
   98 (
   99     const scalarField& c,
  100     const scalar T,
  101     const scalar p,
  102     scalarField& dcdt
  103 ) const
  104 {
  105     scalar pf, cf, pr, cr;
  106     label lRef, rRef;
  107 
  
  //initializes dcdt to 0
  
  108     dcdt = Zero;
  109 
  
  // For each rxn
  
  110     forAll(reactions_, i)
  111     {
  112         const Reaction<ThermoType>& R = reactions_[i];
  113 
  114         scalar omegai = omega
  115         (
  116             R, c, T, p, pf, cf, lRef, pr, cr, rRef
  117         );
  118 
  119         forAll(R.lhs(), s)
  120         {
  121             const label si = R.lhs()[s].index;
  122             const scalar sl = R.lhs()[s].stoichCoeff;
  
  // dcdt = stoichCoeff*
  
  123             dcdt[si] -= sl*omegai;
  124         }
  125 
  126         forAll(R.rhs(), s)
  127         {
  128             const label si = R.rhs()[s].index;
  129             const scalar sr = R.rhs()[s].stoichCoeff;
  130             dcdt[si] += sr*omegai;
  131         }
  132     }
  133 }
  

template<class CompType, class ThermoType>
   97 void Foam::chemistryModel<CompType, ThermoType>::omega
   98 (
   99     const scalarField& c,
  100     const scalar T,
  101     const scalar p,
  102     scalarField& dcdt
  103 ) const
  104 {
  105     scalar pf, cf, pr, cr;
  106     label lRef, rRef;
  107 
  
  //initializes dcdt to 0
  
  108     dcdt = Zero;
  109 
  
  // For each rxn
  
  110     forAll(reactions_, i)
  111     {
  112         const Reaction<ThermoType>& R = reactions_[i];
  113 
  114         scalar omegai = omega
  115         (
  116             R, c, T, p, pf, cf, lRef, pr, cr, rRef
  117         );
  118 
  119         forAll(R.lhs(), s)
  120         {
  121             const label si = R.lhs()[s].index;
  122             const scalar sl = R.lhs()[s].stoichCoeff;
  
  // dcdt = stoichCoeff*
  
  123             dcdt[si] -= sl*omegai;
  124         }
  125 
  126         forAll(R.rhs(), s)
  127         {
  128             const label si = R.rhs()[s].index;
  129             const scalar sr = R.rhs()[s].stoichCoeff;
  130             dcdt[si] += sr*omegai;
  131         }
  132     }
  133 }

Here dcdt = sigma(+-stoichCoeff*omegai) where the lhs/rhs of the reaction defines the sign of the stoichCoeff

However I'm struggleing to understand the code for omegai

Code:

157 template<class CompType, class ThermoType>
  158 Foam::scalar Foam::chemistryModel<CompType, ThermoType>::omega
  159 (
  160     const Reaction<ThermoType>& R,
  161     const scalarField& c,
  162     const scalar T,
  163     const scalar p,
  164     scalar& pf,
  165     scalar& cf,
  166     label& lRef,
  167     scalar& pr,
  168     scalar& cr,
  169     label& rRef
  170 ) const
  171 {
  172     const scalar kf = R.kf(p, T, c);
  173     const scalar kr = R.kr(kf, p, T, c);
  174 
  175     pf = 1.0;
  176     pr = 1.0;
  177 
  178     const label Nl = R.lhs().size();
  179     const label Nr = R.rhs().size();
  180 
  181     label slRef = 0;
  182     lRef = R.lhs()[slRef].index;
  183 
  184     pf = kf;
  185     for (label s = 1; s < Nl; s++)
  186     {
  187         const label si = R.lhs()[s].index;
  188 
  189         if (c[si] < c[lRef])
  190         {
  191             const scalar exp = R.lhs()[slRef].exponent;
  192             pf *= pow(max(0.0, c[lRef]), exp);
  193             lRef = si;
  194             slRef = s;
  195         }
  196         else
  197         {
  198             const scalar exp = R.lhs()[s].exponent;
  199             pf *= pow(max(0.0, c[si]), exp);
  200         }
  201     }
  202     cf = max(0.0, c[lRef]);
  203 
  204     {
  205         const scalar exp = R.lhs()[slRef].exponent;
  206         if (exp < 1.0)
  207         {
  208             if (cf > SMALL)
  209             {
  210                 pf *= pow(cf, exp - 1.0);
  211             }
  212             else
  213             {
  214                 pf = 0.0;
  215             }
  216         }
  217         else
  218         {
  219             pf *= pow(cf, exp - 1.0);
  220         }
  221     }
  222 
  223     label srRef = 0;
  224     rRef = R.rhs()[srRef].index;
  225 
  226     // Find the matrix element and element position for the rhs
  227     pr = kr;
  228     for (label s = 1; s < Nr; s++)
  229     {
  230         const label si = R.rhs()[s].index;
  231         if (c[si] < c[rRef])
  232         {
  233             const scalar exp = R.rhs()[srRef].exponent;
  234             pr *= pow(max(0.0, c[rRef]), exp);
  235             rRef = si;
  236             srRef = s;
  237         }
  238         else
  239         {
  240             const scalar exp = R.rhs()[s].exponent;
  241             pr *= pow(max(0.0, c[si]), exp);
  242         }
  243     }
  244     cr = max(0.0, c[rRef]);
  245 
  246     {
  247         const scalar exp = R.rhs()[srRef].exponent;
  248         if (exp < 1.0)
  249         {
  250             if (cr>SMALL)
  251             {
  252                 pr *= pow(cr, exp - 1.0);
  253             }
  254             else
  255             {
  256                 pr = 0.0;
  257             }
  258         }
  259         else
  260         {
  261             pr *= pow(cr, exp - 1.0);
  262         }
  263     }
  264   
  265     return pf*cf - pr*cr;
  266 }

Can anyone help explain how the reaction rate is computed from Omegai?

Also, I see that the Arrhenius reaction rate is calculated in ArrheniusReactionRateI.H but I dont see how this is implemented in omegai?

Code:

inline Foam::scalar Foam::ArrheniusReactionRate::operator()
(
    const scalar p,
    const scalar T,
    const scalarField&
) const
{
    scalar ak = A_;

    if (mag(beta_) > VSMALL)
    {
        ak *= pow(T, beta_);
    }

    if (mag(Ta_) > VSMALL)
    {
        ak *= exp(-Ta_/T);
    }

    return ak;
}

Thanks!

nerv383 · February 26, 2018, 02:52

Hi,

I am having the same question. I suppose the code for omegai is to calculate the progress rate for one reaction as the expression below

$\Omega$ = $k_{f}$ * $\prod^{N}_{k=1}$ $(c_{kf})^{v_{kf}}$ - $k_{r}$ * $\prod^{N}_{k=1}$ $(c_{kr})^{v_{kr}}$

However, if that is the case, I cannot understand how the code achieve this calculation, especially for the snippet

Code:

        if (exp < 1.0)
        {
            if (cf > SMALL)
            {
                pf *= pow(cf, exp - 1.0);
            }
            else
            {
                pf = 0.0;
            }
        }
        else
        {
            pf *= pow(cf, exp - 1.0);
        }
    }

It would be very appreciated if anyone can share any ideas about it.

From my understanding, the Arrhenius law is applied when kf and kr is calculated.

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February 26, 2018, 02:52		#2
nerv383 New Member zhijie huo Join Date: Feb 2017 Posts: 1 Rep Power: 0	Hi, I am having the same question. I suppose the code for omegai is to calculate the progress rate for one reaction as the expression below $\Omega$ = $k_{f}$ * $\prod^{N}_{k=1}$ $(c_{kf})^{v_{kf}}$ - $k_{r}$ * $\prod^{N}_{k=1}$ $(c_{kr})^{v_{kr}}$ However, if that is the case, I cannot understand how the code achieve this calculation, especially for the snippet Code: if (exp < 1.0) { if (cf > SMALL) { pf = pow(cf, exp - 1.0); } else { pf = 0.0; } } else { pf = pow(cf, exp - 1.0); } } It would be very appreciated if anyone can share any ideas about it. From my understanding, the Arrhenius law is applied when kf and kr is calculated.