COMSOL 6.4 - The Aqueous Electrolyte Transport Interface

The Aqueous Electrolyte Transport Interface

The interface (

), found under the branch (

), is used to solve for the mass transport of an arbitrary number of species in water-based systems subject to potential gradients. The species transported can be any combination of weak and strong acids and bases, ampholytes, and uncharged species. The transport of mass and charge is based on the Nernst–Planck equations for molecular transport, in combination with electroneutrality and dissociation equilibria for weak acids, bases, and ampholytes as well as the water autoionization reaction.

The physics interface can simulate mass transport in aqueous electrolyte systems containing several equilibrium reactions, which is the case in, for instance, atmospheric corrosion.

The interface supports simulation in 1D, 2D, and 3D as well as for axisymmetric components in 1D and 2D.

The dependent variables are the electrolyte phase potential and the molar concentrations of the included species, added individually by each species node at the electrolyte node.

Settings

The is the default physics interface name.

The 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 field. The first character must be a letter.

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

Domain Selection

If any part of the model geometry should not partake in the mass transfer model, remove that part from the selection list.

Out-of-Plane Thickness


	For 2D components, enter a value or expression for the Out-of-Plane Thickness dz (SI unit: m). The value of dz determines the size of the domain perpendicular to the modeled 2D cross section. This value yields, for example, the correct total current when the current density is obtained from a 2D simulation.

Cross-Sectional Area


	For 1D components, enter a Cross-Sectional Area Ac (SI unit: m2) to define a parameter for the area of the geometry perpendicular to the 1D component. The value of this parameter is used to, among other things, automatically calculate the total current from the current density vector. The analogy is valid for other fluxes. The default is 1 m2.

The (or ) checkbox (depending on component dimension) is by default selected. When selected, any time-dependent out-of-plane expansion or contraction will change the species concentration in the domain. Since the concentration is uniform in the out-of-plane direction, this setting can be useful to simulate thin films that grow or shrink; for instance, water evaporation or condensation.

Transport Mechanisms

Mass transport due to diffusion and migration is included by default. If the checkbox is cleared, the potential can be set manually to a constant value (the default is 0 V) in the node.

Use the checkbox available under to control convective transport. By default, the checkbox is cleared.

Consistent Stabilization

To display this section, click the button (

) and select in the dialog.

•

When the checkbox is selected, a weak term that reduces spurious oscillations is added to the transport equation. The resulting equation system is always nonlinear.

•

For both and , the is always . Derivatives of the diffusivity are thus neglected.

Inconsistent Stabilization

To display this section, click the button (

) and select in the dialog. By default, the checkbox 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 button (

) and select in the dialog. Normally these settings do not need to be changed. Select a — (the default) or . Use the conservative formulation for compressible flow. Note that the setting not only dictates the convective term but also the accumulation term in the material balance for porous media.

Discretization

To display all settings available in this section, click the button (

) and select in the dialog.

The concentration variables are set to use elements by default.

The potential variable is set to use elements by default.

The checkbox 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 checkbox is cleared, the COMSOL Multiphysics 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:

•

<name>.nIl, where <name> is the name of the interface (default is el), set on the interface top node. This is the normal electrolyte current density.

•

<name>.ntflux_<species_name> is the (see Species Nodes in the Aqueous Electrolyte Transport Interface below). This is the normal total flux for each species.

Also the checkbox is available if the previous checkbox 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 in the COMSOL Multiphysics Reference Manual.

Regarding the , see Splitting Complex-Valued Variables in the COMSOL Multiphysics Reference Manual.

Dependent Variables

The dependent variable name for the electrolyte potential variable is aqt.phil by default.

The concentration-dependent variables are named as aqt.xxx, where aqt is the name of the interface as set above, and the xxx string is controlled by the setting on the individual species nodes.