where ci denotes the concentration of species
i (SI unit: mol/ m
3),
Di is the diffusion coefficient of species
i (SI unit: m
2/s),
u is the solvent velocity (SI unit: m/s),
F refers to Faraday’s constant (SI unit: s·A/mol),
V denotes the electric potential (SI unit: V),
zi is the charge number of the ionic species (dimensionless), and
um,i its ionic mobility (SI unit: s·mol/kg).
There are three physics interfaces for this type of transport. In the Nernst-Planck Equations (npe) interface (
), found under the
Chemical Species Transport branch (
) when adding a physics interface, the transport of every charged species is accounted for, and is solved for in combination with the electroneutrality condition.
When this physics interface is added, these default physics nodes are also added to the Model Builder—
Convection, Diffusion and Migration,
No Flux (the default boundary condition for species concentrations),
Electric Insulation (the default boundary condition for the electric field), and
Initial Values. Then, from the
Physics toolbar, add other nodes that implement, for example, boundary conditions and rate expression terms. You can also right-click
Nernst-Planck Equations to select physics features from the context menu.
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
npe.
Select the species that the physics interface computes from the electroneutrality condition in Equation 3-95. That is, its value comes from the fact that the net charge in every control volume is zero. Select the preferred species in the
From electroneutrality list. To minimize the impact of any numerical errors, use the species with the highest concentration. By default, the software uses the first species.
Specify the Number of species. There must be at least two species. To add a single species, click the
Add species button (
). To remove a species, select it in the list and click the
Remove species button (
).Edit the names of the species directly in the table.
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 term—
Non-conservative form (the default) or
Conservative form.
To display this section, click the Show button (
) and select
Stabilization. Any settings unique to this physics interface are listed below.