Use the Reaction node () to define a chemical reaction where one or several species form other chemical species.

To add a Reaction node either right-click the Chemistry node, or click Reaction in the Chemistry toolbar.

Enter a chemical equation in the Formula field. The chemical equation should be of the format “A + B arrow C + D”. Here, A and B are reactants, C and D are product species, and arrow denotes a reaction arrow. The participating species should be written with Valid Species Names. Valid reaction arrows are “<=>” for reversible reaction, “=>” for irreversible reaction, and “=” for equilibrium reaction. Species can be given trivial names, or their chemical formulas can be used as names. Examples of valid formulas are; “carbon+oxygen=>carbondioxide”, and “C(s)+O2(g)=>CO2(g)”.

Click Apply to make the interface examine the species taking part in the chemical equation, and automatically add the associated Species features to the Model Builder.

Use the Balance button to automatically calculate the stoichiometric coefficients such that the number of atoms of each kind are the same on both sides of the reaction. This turns, for example, the formula “H2+O2=>H2O” into “2H2+O2=>2H2O”. The balancing involves parsing all participating species for elements in the periodic table. It therefore requires that all species in the Formula field are written either using their chemical formula, say “H2O”, or that each species in the reaction have an enabled Chemical Formula field in their Species node. For example, it is possible to balance the formula “H2+O2=>water” as long as there is a Species node with the name “water” already present, and an enabled Chemical Formula.

For automatic reaction balancing to be successful, requires that the problem is well posed. One example of a problem that is not well posed is “C+H2=>CH4+C2H6”, in which case any ratio of CH4/C2H6 could be obtained. Another example is “H2=>O2” where not all elements are present on both sides.

Select the Reaction type — Reversible, Irreversible, or Equilibrium — or edit the expression directly in the Formula field. In the latter case, specify the reaction type with a delimiter separating the two sides of the equation:

Each Reaction type has its own set of reaction kinetics:

where νij is the stoichiometric coefficient.

This section is available when the Reaction type is either Reversible or Irreversible.

When Mass action law is selected (default), the rate expression is automatically derived from the stoichiometric coefficients in the reaction formula:

The deduced overall reaction order is shown in text below the respective equation in the Reaction Rate section.

If the reaction order differs from the stoichiometric coefficients, or if an arbitrary rate expressions is applicable, change Reaction Rate to User defined. An expression field r appears with the default expression being that from the mass action law. Below this there are fields to set the reaction order. For a reversible reaction the reverse reaction order may be specified in addition to the forward one. The unit of the rate constant k (or frequency factor A in the case of Arrhenius behavior), is derived from the reaction order, in SI units: (m3/mol)α − 1/s, where α equals the order with respect to volumetric species. When surface species are present — identified by their “(ads)” suffix — the unit is instead given by m3α+2β − 2/molα+β−1/s, where β is the order with respect to surface species.

This section applies for Reversible or Irreversible reactions and defines the reaction rate constants used in the reaction rates.

The Forward rate constant kf is used for both Reversible and Irreversible reactions. The Reverse rate constant kr is only used for Reversible reactions (Equation 2-99).

The SI units of the rate constants are automatically based on the order of the reaction with respect to the concentrations, as defined in the Reaction formula.

The Specify equilibrium constant checkbox is available for Reversible reactions. If the checkbox is selected the rate constants are defined in a different manner with the reverse rate constant being computed from the following expression:

Thus, in this case, the forward rate constant and equilibrium constant for the reaction are needed. The Equilibrium constant is edited in the Equilibrium Settings section.

When the Use Arrhenius expressions checkbox is selected the Arrhenius parameters are automatically used in predefined expressions for the forward and reverse rate constants kfand kr, respectively.

Specify the activation energy and the frequency factor in the Arrhenius expressions to account for temperature variations. The reference temperature, Tref equals 1 K. The available fields are based on the Reaction type chosen in the Reaction node. Enter values or expressions for each of the following (reverse expressions are only available for reversible reactions):

This section is available for equilibrium reactions, and for reversible reactions when the Specify equilibrium constant checkbox has been selected.

For an equilibrium reaction, specify the Equilibrium expression. When the Equilibrium expression is set to Automatic the following expression is used:

Select User defined from the Equilibrium expression list to instead enter a manually defined equilibrium expression.

Specify the Equilibrium constant Keq0 for an equilibrium reaction, or for a reversible reaction when the Specify equilibrium constant checkbox has been selected (in the Rate Constants section).

The Equilibrium constant can either be User defined, or automatically defined when set to Automatic or Thermodynamics.

Use the Automatic option to compute the equilibrium constant for an ideal system.

The Thermodynamics option is available when all reactions in the interface are equilibrium reactions, and the interface is fully coupled to a Thermodynamic System (see Species Matching). Use this setting to automatically compute the equilibrium constant for an ideal or nonideal system, dependent on the thermodynamic model applied for the coupled system.

Using Automatic or Thermodynamics, Keq0 is calculated from the Gibbs free energy of the reaction. For more details see The Equilibrium Constant and the Automatically Defined Equilibrium Constants section therein.

This section contains information about thermodynamic properties that relate to a selected reaction. Several Automatic definitions are available here.

The Enthalpy of reaction H (SI unit: J/mol) is calculated by the interface from species properties and the related stoichiometric coefficients:

The Entropy of reaction S (SI unit: J/(mol·K)) comes from a similar expression:

In Equation 2-101 and Equation 2-102, hi and si are the species’ molar enthalpy and molar entropy, respectively.

Enter these quantities in the Species Thermodynamic Expressions section for the Species node either by using the predefined polynomial or by providing a custom expression or constants.

The stoichiometric coefficients, νij, are defined as being negative for reactants and positive for products. Using Equation 2-101 and Equation 2-102 to equate the Gibbs free energy of reaction enables the equilibrium constant to be expressed according to Equation 2-101.

The Heat source of reaction (SI unit: W/m3) is automatically computed from the heat of each reaction j, given by

When the Turbulent-reaction model is set to None, laminar flow is assumed and the reaction source terms are defined from the prescribed reaction rates.

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 Turbulent Reactions section, and in The Reaction Source Term for Turbulent Flow, in the CFD Module User’s Guide. 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.

This checkbox is available when a valid turbulence model is selected. With the checkbox selected, the reaction rate is limited by the turbulent mixing, and the settings for kinetic reaction rates in the Rate Constants and Reaction Orders is not available.

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