Chemical kinetics
Contents
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
is the pre-exponential factor, is the temperature exponent and the activation energy
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 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
- and the high pressure limit
.
The expression taken at any pressure is based on a combination of both low and high-pressure Arrhenius expressions. The term is here equivalent to a pressure and 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
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.
Species production/consumption source terms
Species source terms are deduced from