The Reacting Flow, Diluted Species Interface e () merges the functionality of the Transport of Diluted Species and the Free and Porous Media Flow physics interfaces into a multiphysics interface. In this way coupled mass and momentum transport in free and porous media can be modeled from a single physics interface, with the model coupling for the velocity field set up automatically. In addition, the effective transport coefficients in a porous matrix domain can be derived based on the corresponding values in for a non-porous domain.

These topics are briefly discussed in this section:

•Coupling to Other Physics Interfaces

•Adding a Chemical Species Transport Interface

Coupling to Other Physics Interfaces

When you are simulating applications that can be described by the material transport physics interfaces in the Chemical Species Transport branch, there is often a need to couple the material transport to other physics. Convection is often the cause of the material transport, so couplings to fluid-flow interfaces are required. The CFD Module includes physics interfaces for laminar flow and porous media flow as well as more advanced descriptions of fluid flow, such as turbulent and multiphase flow.

Moreover, most chemical reactions or other type of material processing, such as casting, either require or produce heat, which in turn affects both the reaction and other physical processes connected to the system. The CFD Module includes physics interfaces for heat transfer through conduction and convection as well as through porous media. More extensive description of heat transfer, such as surface-to-surface radiation, can be found in the Heat Transfer Module.

Finally, COMSOL Multiphysics supports simulations of electrostatics or DC-based physical phenomena, even if conductivity is nonlinear. If the electric field is AC/DC in nature, or if your system is affected by electromagnetic waves, then the AC/DC Module and RF Module include appropriate physics interfaces for such phenomena. Furthermore, some applications of electrochemical reactions, particularly in electrochemical power source applications, are better handled by the Batteries & Fuel Cells Module.

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Adding a Chemical Species Transport Interface

A chemical species transport interface can be added when first creating a new model, or at any time during the modeling process.

1For a new model, select physics interfaces as the second step in the New Model window (after specifying the space dimension).

In an active model, right-click the Physics node in the Model Tree and select Add Physics ().

2Expand the Chemical Species Transport node in the list of physics interfaces and select one of the available chemical species transport interfaces.

3Click the Add Selected button (). The physics interface displays under Added to model.

4Under Dependent variables, specify the number of species (concentrations or mass fractions) and their names.

5Continue by adding more interfaces and specifying the number of species (concentrations or mass fractions) that are to be simulated in a mass transport physics interface when adding that interface. Under Dependent variables, enter the

Number of species. To add a single species, click the Add Concentration button () underneath the table or enter a value into the Number of species field. Click the Remove Concentration button () underneath the table if required.

The Transport of Concentrated Species interface needs to contain at least two species (the default). Also edit the strings or names directly in the table. The names must be unique for all species (and all other dependent variables) in the model.

6Click the Next button () and chose a Study type.

7In the upper-right corner of the Select Study Type page, click the Finish button ().

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The Transport of Concentrated Species interface (), found under the Chemical Species Transport branch () in the Model Wizard, has the equations, boundary conditions, and reaction terms for modeling chemical species transport in mixtures by solving for the mass fractions. It supports the simulation of transport by convection, diffusion, and migration in 1D, 2D, and 3D as well as for axisymmetric models in 1D and 2D. The interface defines the equations for the species mass fractions, including a diffusion model (Mixture-averaged or Fick’s law).

Some examples of what can be studied with this interface include:

•The evolution of chemical species transported by convection and diffusion.

•The migration in an electric field in the case of ionic species, in mixtures and solutions that cannot be deemed as being diluted.

•Concentrated solutions or gas mixtures, where the concentration of all participating species are of the same order of magnitude, and therefore their molecular and ionic interaction with each other must be considered. This implies that the diffusive transport of a single species is dependent on the mixture composition, and possibly on the temperature, the electric potential, the pressure, or any combination.

The default transport mechanism is the Convection and Diffusion node, which is dynamic and is derived from which transport mechanism is activated.

When this interface is added, these default nodes are also added to the Model Builder— Convection and Diffusion (which applies a Mixture-average diffusion model), No Flux, and

Initial Values. Right-click the main node to add other features that implement, for example, boundary conditions and reactions.

I N T E R F A C E I D E N T I F I E R

The interface identifier is a text string that can be used to reference the respective physics interface if appropriate. Such situations could occur when coupling this interface to another physics interface, or when trying to identify and use variables defined by this physics interface, which is used to reach the fields and variables in expressions, for example. It can be changed to any unique string in the Identifier field.

The default identifier (for the first interface in the model) is chcs.

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D O M A I N S E L E C T I O N

The default setting is to include All domains in the model to define the species equations. To choose specific domains, select Manual from the Selection list.

E Q U A T I O N

The base equation for an individual species i is given in the Equations section and is:

The displayed formulation changes depending on the active transport mechanisms and the selected diffusion model.

T R A N S P O R T M E C H A N I S M S

In this section, choose a diffusion model and additional active transport mechanisms. The interface includes the following transport mechanisms:

•Diffusion is always active. Select from different diffusion models available from the

Diffusion model list.

•Convection is active by default. To activate or deactivate convection, select or clear the Convection check box. The second term on the left-hand side of Equation 3-2 represent mass transport by convection.

•Migration of ionic species is not active by default. To activate or deactivate migration, select or clear the Migration in electric field check box. The migration term is part of the relative mass flux vector.

Select a Diffusion model—Mixture-averaged (the default) or Fick’s law.

The Mixture-averaged model is less computational expensive but also requires the Maxwell-Stefan diffusivities. The Fick’s law model is a general model to be used when the diffusion is known to be Fickian, or when molecular diffusion is not the dominating transport mechanism and a robust but low order model is wanted.

Mixture-Averaged Diffusion Model

When using the Mixture-averaged diffusion model the relative mass flux vector is

where the last term on the right-hand side is the migratory flux, which is added by selecting the Migration in electric field check box. The mixture-averaged diffusion coefficient Dmi is computed as

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where Dik (SI unit: m2/s) are the multicomponent Maxwell-Stefan diffusivities, which are supplied as an input to the model.

Fick’s Law Diffusion Model

When using the Fick’s law diffusion model the relative mass flux vector is

where DFi (SI unit: m2/s) is a user defined diffusion coefficient (isotropic, diagonal, or symmetric). The last term on the right hand side is the migratory flux, which are added by selecting the Migration in electric field check box.

S P E C I E S

Select the species that this interface solves using the mass constraint in Equation 3-5 (that is, its value comes from the fact that the sum of all mass fractions must equal 1). Select the preferred species in the From mass constraint list. To minimize the impact of any numerical errors, use the species with the highest concentration. By default, the software uses the first species.

D E P E N D E N T V A R I A B L E S

Add or remove species in the model and also change the names of the dependent variables that represent the species concentrations.

Specify the Number of species. There must be at least two species. To add a single species, click the Add Concentration button () under the table. To remove a species, select it in the list and click the Remove Concentration button () under the table. Edit the names of the species directly in the table.

The species are dependent variables, and their names must be unique with

respect to all other dependent variables in the model.

Important

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