Chemical kinetics

This chapter reports the principles that drive the computation of combustion chemistry in most CFD softwares.

Chemkin (.scheme .therm .trans), Cantera (xml)

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Arrhenius law

${\mathcal {A}}_{j}$ is the pre-exponential factor, ${\mathcal {\beta }}_{j}$ is the temperature exponent and $E_{a_{j}}$ the activation energy

$k_{j}={\mathcal {A}}_{j}T^{{\mathcal {\beta }}_{j}}\exp \left(-{\frac {E_{a_{j}}}{RT}}\right)$

Three-body reactions

In the forward direction, three-body reactions involve two species A and B as reactants and yield a single product AB. In that case, the third body ${\text{M}}$ is used to stabilize the excited product AB*. On the contrary, in the reverse direction, heat provides the energy necessary to break the link between A and B.

$......$

The third body M can be any inert molecule.

Falloff reactions

Under specific conditions, some reaction rate expressions are dependent on pressure and temperature. This is especially true for the rate associated to unimolecular/recombination fall-off reactions which increases with pressure. In such cases, if the chemical process takes place in a high or low pressure limit typical Arrhenius laws are applicable to the reactions that are described. However, if the pressure is in between, an accurate description of the phenomenon requires a more complicated rate expression. In such a case, the reaction is said to be in the ”fall-off” region.

Lindemann

Several formulas (derived from the Lindemann description) are available to smoothly relate the limiting low and high-pressure rate expressions. With the Lindemann approach, Arrhenius parameters need to be given for both

the low pressure limit

$k_{0}={\mathcal {A}}_{0}T^{{\mathcal {\beta }}_{0}}\exp \left(-{\frac {E_{a_{0}}}{RT}}\right)$

and the high pressure limit

$k_{\infty }={\mathcal {A}}_{\infty }T^{{\mathcal {\beta }}_{\infty }}\exp \left(-{\frac {E_{a_{\infty }}}{RT}}\right)$ .

The expression taken at any pressure is based on a combination of both low and high-pressure Arrhenius expressions. The term $P_{r}$ is here equivalent to a pressure and ${\text{M}}$ represents the concentration of the mixture, possibly estimated from third-body efficiencies.

Troe


   <reaction reversible="yes" type="falloff" id="0012">
     <equation>O + CO (+ M) [=] CO2 (+ M)</equation>
     <rateCoeff>
       <Arrhenius>
          <A>1.800000E+07</A>
          <b>0</b>
          <E units="cal/mol">2385.000000</E>
       </Arrhenius>
       <Arrhenius name="k0">
          <A>6.020000E+08</A>
          <b>0</b>
          <E units="cal/mol">3000.000000</E>
       </Arrhenius>
       <efficiencies default="1.0">AR:0.5  C2H6:3  CH4:2  CO:1.5  CO2:3.5  H2:2  H2O:6  O2:6 </efficiencies>
       <falloff type="Lindemann"/>
     </rateCoeff>
     <reactants>CO:1 O:1.0</reactants>
     <products>CO2:1.0</products>
   </reaction>

Reaction rates

The concentration of the species involved within the studied reaction are considered for

${\mathcal {Q}}_{f,j}=k_{j}*[A]*[B]$

${\mathcal {Q}}_{r,j}={\frac {k_{j}}{EQ_{j}}}*[C]*[D]$

The global rate of a reaction j (evolution in concentration per unit of time) varies depending on the proportion of the rates associated to the forward and backward directions.

${\mathcal {Q}}_{j}={\mathcal {Q}}_{f,j}-{\mathcal {Q}}_{r,j}$

Species production/consumption source terms

Species $Y_{k}$ source terms are deduced from

${\dot {\omega }}_{k}=W_{k}\sum _{j=1}^{N_{R}}\nu _{k,j}{\mathcal {Q}}_{j}$

Chemical kinetics

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