Thermal Dispersion
This subnode should be used to model the heat transfer due to hydrodynamic mixing in a fluid flowing through a porous medium. It adds an extra term ∇ ⋅ (kdispT) to the right-hand side of the heat equation in porous media, through the modification of the effective thermal conductivity keff with the dispersive thermal conductivity kdisp:
and specifies values for the longitudinal and transverse dispersivities.
Dispersivities
This section defines the Longitudinal dispersivity λlo and Transverse dispersivity λtr used for the computation of the tensor of dispersive thermal conductivity:
where Dij is the dispersion tensor
and λijkl is the fourth-order dispersivity tensor
When the Horizontal and vertical transverse dispersivities option is selected in the Define transverse dispersivity from list, the Transverse horizontal dispersivity λtrh and Transverse vertical dispersivity λtrv are defined instead of the Transverse dispersivity λtr.
In this case, it is assumed that z is the vertical direction and it defines the dispersion tensor as
 
The Thermal Dispersion node is only available with some COMSOL products. For a detailed overview of the functionality available in each product, visit https://www.comsol.com/products/specifications/
Location in User Interface
Context Menus
Ribbon
Physics Tab with Porous Medium selected in the model tree: