The Chemical Species Transport Interfaces > The Transport of Diluted Species Interface

The Transport of Diluted Species Interface
Mass transfer is an important part of chemical engineering because this field considers the conversion of one type of substance into another. A lot of this occurs through chemical reactions, although separation and other unit operations are an important part. You can use the Transport of Diluted Species interface to model transport of a diluted species in chemical systems by convection and diffusion.
In the Transport of Diluted Species interface, Fick’s law describes the diffusive transport in the flux vector. Fick’s law is adequate when the diffusing species is dilute with respect to a solvent. Assuming a binary mixture of solute A in solvent B, concentrations of up to 10 mol% of A can be considered dilute.
The Transport of Diluted Species (tds) interface (), found under the Chemical Species Transport branch (), is used to calculate the concentration field of a dilute solute in a solvent. Transport and reactions of the species dissolved in a gas, liquid, or solid can be handled with this interface. The driving forces for transport can be diffusion by Fick’s law, convection when coupled to a flow field, and migration, when coupled to an electric field.
The interface supports simulation of transport by convection and diffusion in 1D, 2D, and 3D as well as for axisymmetric components in 1D and 2D. The dependent variable is the molar concentration, c. Modeling multiple species transport is possible, whereby the physics interface solves for the molar concentration, ci, of each species i.
Settings
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 tds.
Domain Selection
If any parts of the model geometry should not partake in the mass transfer model, remove that part from the selection list.
Transport Mechanisms
Diffusion is always included. By default, the Convection check box is selected under Additional transport mechanisms.
Note: Not all additional transport mechanisms listed below are available in all products. For details see http://www.comsol.com/products/specifications/.
Select the Migration in electric field check box to activate the migration transport of ionic species. See further the theory section Adding Transport Through Migration in the Chemical Reaction Engineering Module User’s Guide.
Select the Adsorption in porous media check box to activate the adsorption of solutes in porous media. See further Adsorption in the Chemical Reaction Engineering Module User’s Guide.
Select the Dispersion in porous media check box to activate the dispersion mechanism in porous media. See further Dispersion in the Chemical Reaction Engineering Module User’s Guide.
Select the Volatilization in partially saturated porous media check box to model volatilization in partially saturated domains.
Consistent Stabilization
To display this sections, click the Show button () and select Stabilization.
When the Crosswind diffusion check box is selected, a weak term that reduces spurious oscillations is added to the transport equation. The resulting equation system is always nonlinear. There are two options for the Crosswind diffusion type:
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Do Carmo and Galeão—the default option. This type of crosswind diffusion reduces undershoots and overshoots to a minimum but can in rare cases give equation systems that are difficult to fully converge.
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Codina. This option is less diffusive compared to the Do Carmo and Galeão option but can result in more undershoots and overshoots. It is also less effective for anisotropic meshes. The Codina option activates a text field for the Lower gradient limit glim. It defaults to 0.1[mol/m^3)/tds.helem, where tds.helem is the local element size.
For both consistent stabilization methods, select an Equation residual. Approximate residual is the default and means that derivatives of the diffusion tensor components are neglected. This setting is usually accurate enough and is computationally faster. If required, select Full residual instead.
Inconsistent Stabilization
To display this section, click the Show button () and select Stabilization. By default, the Isotropic diffusion check box is not selected, because this type of stabilization adds artificial diffusion and affects the accuracy of the original problem. However, this option can be used to get a good initial guess for under-resolved problems.
Advanced Settings
To display this section, click the Show button () and select Advanced Physics Options. Normally these settings do not need to be changed. Select a Convective termNon-conservative form (the default) or Conservative form. The conservative formulation should be used for compressible flow. See Convective Term Formulation for more information.
Discretization
To display this section, click the Show button () and select Discretization.
The Compute boundary fluxes check box is activated by default so that COMSOL Multiphysics computes predefined accurate boundary flux variables. When this option is checked, the solver computes variables storing accurate boundary fluxes from each boundary into the adjacent domain.
If the check box is cleared, the COMSOL software instead computes the flux variables from the dependent variables using extrapolation, which is less accurate in postprocessing results but does not create extra dependent variables on the boundaries for the fluxes.
The flux variables affected in the interface are:
ndflux_c (where c is the dependent variable for the concentration). This is the normal diffusive flux and corresponds to the boundary flux when diffusion is the only contribution to the flux term.
ntflux_c (where c is the dependent variable for the concentration). This is the normal total flux and corresponds to the boundary flux plus additional transport terms, for example, the convective flux when you use the non-conservative form.
Also the Apply smoothing to boundary fluxes check box is available if the previous check box is checked. The smoothing can provide a more well-behaved flux value close to singularities.
For details about the boundary fluxes settings, see Computing Accurate Fluxes.
The Value type when using splitting of complex variables setting should in most pure mass transfer problems be set to Real, which is the default. It makes sure that the dependent variable does not get affected by small imaginary contributions, which can occur, for example, when combining a Time Dependent or Stationary study with a frequency-domain study. For more information, see Splitting Complex-Valued Variables.
Dependent Variables
The dependent variable name is Concentration c by default. The names must be unique with respect to all other dependent variables in the component.
Add or remove species variables in the model and also change the names of the dependent variables that represent the species concentrations.
Enter the Number of species. Use the Add concentration () and Remove concentration () buttons as needed.
Further Reading
Numerical Stabilization in the COMSOL Multiphysics Reference Manual.
See Table 2-3 for links to common sections and Table 2-4 for common feature nodes. You can also search for information: press F1 to open the Help window or Ctrl+F1 to open the Documentation window.
Effective Diffusivity in Porous Materials: Application Library path COMSOL_Multiphysics/Diffusion/effective_diffusivity
Thin-Layer Diffusion: Application Library path COMSOL_Multiphysics/Diffusion/thin_layer_diffusion