This node defines the coupling condition between the fluid phase and the pellets, when the Packed bed option is selected in the Porous medium type list of the Porous Medium parent node.

This section is invisible for the default Pellet-Fluid Interface node.

Select All or one of existing pellets for the nondefault Pellet-Fluid Interface nodes. To define properties of a pellet phase in the porous medium use a Pellet subnode of a Porous Material node (see Pellet in the COMSOL Multiphysics Reference Manual).

Select an option in the Coupling condition list to couple the macroscale heat equation solved in the fluid phase and the microscale equation solved in the pellets to account for radial thermal conduction. The Coupling condition list is selectable for the default Pellet-Fluid Interface node, but it is disabled for nondefault Pellet-Fluid Interface nodes because it is supposed that there should be only one type of boundary condition in a packed bed.

With the Continuous temperature option (default), the temperature at the outer surface of the pellets and the fluid temperature are enforced to be equal, and the resulting heat flux is applied in the fluid and pellets equations. See the Constraint Settings section below for advanced settings relative to the continuity constraint.

With the Convective heat flux option, a heat transfer term, proportional to the temperature difference between the fluid phase and the outer surface of the pellets, is applied in the equations of both phases. The heat transfer term is applied in an averaged way in the fluid equation, by multiplying it by the Specific surface area, Sb. Either use the Automatic expression for Sb, defined as a function of the average pellet diameter dpe, or set a User defined value or expression. Either use the Automatic option for the Interstitial heat transfer coefficient, hpe,f, defined as a function of the average pellets diameter dpe and the fluid-to-solid Nusselt number for which the fluid dynamic viscosity μ is needed (and should be set in the Fluid (Porous Medium) subnode). Or set a User defined value or expression for hpe,f.

To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog. The settings of this section have no effect when the Coupling condition is Convective heat flux in the Pellet-Fluid Interface section.

The Constraint settings can be set to Pointwise constraints (default) or Weak constraints. Select Weak constraints to replace the standard constraints with a weak implementation.

Physics tab with Porous Medium selected in the model tree: