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.

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.

When the Use Arrhenius expressions checkbox is not selected, input custom expressions or constants for the Forward rate constant kf and Reverse rate constant kr.

When the Use Arrhenius expressions checkbox is selected, enter values for the following parameters of the forward and reverse reactions:

Note this section is only available when licensed to the CFD Module (see 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 the manner described in The Reaction Source Term for Turbulent Flow. 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.

Select Rate expressions to regularize the individual rate expressions added to each species. If the mass fraction for a reactant species ωi becomes smaller than its Damping limits, ωidl, the rate expression, for 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 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 limits, ωidl, is 10−6, which is appropriate for most applications, but can require adjustment when working with for example catalytic trace species.

Select a heat source for this reaction from the H list. The listed heat sources usually come from reactions of Chemistry interfaces. If there is no heat source for the reaction in the list, select User defined, then enter the heat source.