The Chemistry (chem) interface () is found under the Chemical Species Transport branch () when adding a physics interface.

Many of the fields and nodes described in this section are only made available when either a Reaction or a Species (or both) subnode is added to the Model Builder. All predefined constants and expressions can be overwritten by user-defined expressions. This makes it possible to go beyond the modeling assumptions set as defaults in this physics interface.

The following is a description of the features and fields available on the Settings window for the Chemistry interface.

The Label is the default physics interface name.

The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.

The default Name (for the first physics interface in the model) is chem.

This section sets the Temperature, Pressure, and Electrode potential (only available with a Battery Design Module, Corrosion Module, Electrochemistry Module, Electrodeposition Module, or Fuel Cell & Electrolyzer Module license) to be used by the current interface. Toggle the Enable electrode reactions check box to enable the Electrode Potential input in the Model Input section, as well as the Electrode Reaction and Electrode Reaction Group features. Use the lists to select a temperature, pressure or electrode potential defined and announced by another interface in the model. For example, when a heat transfer interface is also included, the temperature solved for is available in the Temperature list.

For Temperature or Pressure, you can also select Common model input to use a globally available common model input. In all three cases, select User defined to manually define the variable in question.

Use this setting to define specify what kind of mixture assumption is used. For Diluted Species the mixture is composed of low concentration solutes present in a solvent. For Concentrated Species a mixture where no single species is considered to be in excess is assumed.

Select Diluted species from the Type list to use the concentration variables from a Transport of Diluted Species interface in the Species Matching section. The same setting should be used for any other interface solving for molar concentrations using a diluted species assumption. Select Concentrated species from the Type list to use the mass fractions from a Transport of Concentrated Species interface.

All property parameters and property functions required by the interface can be automatically created by coupling to a system added to the Thermodynamics node. To do so, click the Thermodynamics check box and select an existing Thermodynamic System.

The Thermodynamics check box is enabled when the Thermodynamics node, including one or more systems, is available under Global Definitions. Also, the Chemistry interface needs to include at least one species.

Use the Phase list to specify the state of aggregation of the mixture.

Use this list to populate the inputs in the Bulk species table with dependent variables from a certain mass transfer interface. The Species solved for list contains all present interfaces of the Type selected in the Mixture Properties section. For example, when the Type is Diluted species, all Transport of Diluted Species interfaces or variants thereof, such as Transport of Diluted Species in Porous Media interface, are available in the list.

Use the Bulk species table to specify the concentrations to be used as arguments in reaction kinetics variables, for example the reaction rate. Reaction kinetics variables are generated by the species features (Species and Species Group) and by the reaction features (Reaction, Electrode Reaction, Reversible Reaction Group, Electrode Reaction Group, and Equilibrium Reaction Group).

When a mass transfer interface has been selected in the Species solved for list, use the Molar concentration column to select one of the dependent variables in the selected interface to the corresponding Species in Chemistry. When using a concentrated species interface (defined by the Type list in the Mixture Properties section), instead use the Mass fraction column to select a dependent variable for each species in Chemistry. For a consistent set up, the species molar mass in the Transport of Concentrated Species interface is updated to the one defined in the Chemistry interface.

The Molar concentration and Mass fraction columns also includes a User defined option, in which case a constant, parameter, or variable expression can be entered in the Value column.

When the Thermodynamics is enabled, the species in the Chemistry interface can be coupled to the species in the Thermodynamic System. This is needed to ensure that arguments for the thermodynamic functions are correctly defined. Use the drop-down lists in the From Thermodynamics column to match each species in the Chemistry to a species in the coupled thermodynamic system. For each thermodynamics-coupled species, the required property parameters and functions are added under the corresponding thermodynamic system. When all species are matched, the Chemistry is considered fully coupled and functions representing mixture properties, such as the density, are also added automatically under the same thermodynamic system.

The Surface Species table is available when there is at least one surface species (with “(ads)” as suffix).

Specify the concentration of surface species in the table in this section. In addition, if a surface species is present at the same time as an Electrode Reaction, the Density of sites input field becomes visible. The Density of sites input value will be used as the default reference concentration for surface species reacting in an Electrode Reaction.

The Solid Species table is available when there is at least one solid species (with “(s)” as suffix).

The Aqueous Species table is available when there is at least one aqueous species (with “(aq)” as suffix).

This table is only available when equilibrium reactions are present in the interface. Use the Reaction rate column to specify the rate for each equilibrium reaction. By doing so the postprocessing variable for the total reaction rate for each species, of form chem.Rsum_species, will be updated correctly. For a mass transport interface, say Transport of Diluted Species, the reaction rate needed for an equilibrium reaction is typically a dependent variable. In that case, the name of the variable can be found in the Shape Function section of the Equation View of the node.

When the Chemistry interface is created using from the Generate Space-Dependent Model the table is automatically set up in accordance with the added equilibrium reaction nodes.

Select the Calculate mixture properties check box (selected as default) to calculate mixture properties that can be picked up in the space-dependent model interfaces. The properties that can be calculated are shown beneath the check box. Consider also if the built-in Automatic expressions fit the model or if User defined expressions are more suitable. In general, the Chemistry interface calculates properties in the same way as the Reaction Engineering interface.

This section is available when the Thermodynamics check box is cleared.

Select the Use activity check box to solve for species activities instead of species concentrations, which is a common approach when non-ideal fluids are modeled.

An activity coefficient other than 1 can be set for each species for the Species node in the Species Concentration/Activity section.

It is available when the Thermodynamics check box is cleared.

This section enables CHEMKIN® import to simulate complex chemical reactions in gas phase.

Two types of CHEMKIN input files can be imported here: Thermo and Transport, for thermodynamic properties and transport properties, respectively. Properties for either volumetric or surface species are supported. Click Browse to locate the CHEMKIN file to be imported, then click Import. For Thermo, the imported data is directly entered in the NASA format fields in the Species node’s Thermodynamic Expressions section; for Transport, the imported data is entered in its Transport Expressions section.