Reaction
Use the Reaction node to specify the reaction kinetics for a single homogeneous chemical reaction. The resulting mass source terms for the species involved in the reaction are automatically defined and added on the selected domains. For turbulent flow, the Reaction node includes the Eddy-dissipation turbulent-reactions model.
Reaction Rate
Select a Reaction rate — Automatic (the default), or User defined. Selecting Automatic the laminar flow reaction rate is computed using the mass action law
For User defined, input a custom expression or constants for the Reaction rate r.
Specify the reaction stoichiometry by entering values for the stoichiometric coefficients (dimensionless) of each species. Enter negative values for reactants and positive values for products.
Rate Constants
When the Use Arrhenius expressions check box is not selected, input custom expressions or constants for the Forward rate constant kf and Reverse rate constant kr.
When the Use Arrhenius expressions check box is selected, enter values for the following parameters of the forward and reverse reactions:
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Forward frequency factor Af and Reverse frequency factor Ar (dimensionless)
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Forward activation energy Ef and Reverse activation energy Er
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Forward temperature exponent nf and Reverse temperature exponent nr (dimensionless)
Turbulent Flow
Note this section is only available when then licensed to the CFD Module (see https://www.comsol.com/products/specifications/).
When the Turbulent-reaction model is set to None, laminar flow is assumed and the reaction source terms are defined from the reaction stoichiometry and reaction rates prescribed.
When the Turbulent-reaction model is set to Eddy-dissipation, turbulent flow will be accounted for in the reaction mass sources. In this case, enter values for the Turbulent reaction model parameters αED and β ED (dimensionless).
The Eddy-dissipation model also requires an estimation of the turbulent mixing time of the fluid flow turbulence. When a Fluid Flow interface defining it is present in the model, it can be selected from the Turbulence time scale list. For example, select Turbulence time scale (spf/fp1) to use the time scale defined by the Fluid Properties node fp1 in a Turbulent Flow, k-ε interface with the Name set to spf.
Regularization
Select Rate expression in order to regularize the individual rate expressions that are added to each species. If the mass fraction for a reactant species ωi becomes smaller than its damping limit, ωidl, the rate expression added to species ωi is reduced linearly. If ωi ≤  0 for a reactant species, the reaction rate contribution to that species is completely removed. Similarly, if the mass fraction for a product species ωj becomes larger than 1 − ωjdl, the rate expression added to that species is damped linearly. If ωj ≥  1 for a product species, the reaction rate contribution to that species is completely removed.
The default value for the damping limit, ωidl, is 10−6, which is appropriate for most applications, but can require adjustment when working with for example catalytic trace species.
Regularization of the rate expressions adds considerable stability to the reaction expressions, but should optimally only be used as a mean to reach convergence. If the regularization is active in too large parts of the domain, the mass balance can become affected since the regularization acts on the contributions to each individual species, not the reaction as a whole.