Transport models are used to calculate concentrations or temperatures. The basis for every transport model is the flow calculation, which is why the definition of the corresponding flow boundary conditions is also a prerequisite for every transport model.

The following options are available for the nodes of the model edge for a transport model:

• The concentration of the node is set to a fixed value via the 1KON data type (1st type Dirichlet boundary condition).

• At the nodes where an inflow rate from RAND or KNOT or FLAE (in adjacent elements) is defined, or a mass flow rate from either a constant potential head (POTE) or from leakage boundary conditions (LEKN or LERA or LEEL and VORF), an inflow concentration KONZ must be specified. If no inflow concentration via KONZ is defined in the model files for a potential inflow node, the concentration is automatically assumed to be zero).

• At the nodes of the model boundary where an outflow from the model domain occurs, natural boundary conditions are automatically assigned, i.e. the dispersive flow across the boundary is assumed to be zero.

The calculation of concentrations and temperatures is mathematically equivalent, which is why the data types described in this chapter are used in both mass transfer and heat transfer calculations. The designations in menus and dialogues are always those of a mass transport calculation. In the case of heat transport calculations, the user must transfer the designations independently.

The unit of the concentrations attributes depends on the type of mass transport calculation:

• For a mass transport calculation with a tracer or density-independent flow, it is possible to enter the concentration with "any" unit (e.g. [g/l], [g/g], [mg/l], [kg/kg], etc.) The provided unit must be consistent within a model for all the concentration types.

• For a density-dependent calculation, the concentration must have the unit [kg (solute)/kg (solution)].

• For a calculation with adsorption, the concentration must have the unit [kg/m³].

• For a heat transfer calculation, all temperatures must be entered in the unit [°C].