Use this node to model moisture transport in a porous medium filled with two phases: liquid water and moist air. Phases can be either in equilibrium or not, depending on the Equilibrium between liquid and gas phases option. The moisture transport occurs through vapor diffusion and convection in moist air, and convection and capillary flow in the liquid phase.

With this assumption, the variation of the total moisture content is expressed through the sum of the transport of vapor in moist air and liquid water. See Theory for Moisture Transport in Porous Media for details.

The liquid saturation, sl, which describes the amount of liquid water within the pores, is defined from the specification of a moisture storage function w(ϕw), which specifies the total moisture content (left hand side of the equation below) as a function of the relative humidity ϕw:

where εp (dimensionless) is the porosity.

In the equilibrium formulation, the evaporation and condensation source terms from the moist air and liquid water mass conservation equations cancel out. However, the Hygroscopic Porous Medium node stores the evaporation source term into the mt.G_evap variable, to be accounted for as a mass source in the fluid flow equations for the gas phase in a moisture flow model. This mass source is automatically handled by the Moisture Flow coupling node.

When there is no variation of the total pressure and under specific temperature conditions, the Building Material node may be used instead. See Theory for Moisture Transport in Building Materials for details.

When liquid water and water vapor are not in equilibrium, the equations of mass conservation of water are solved separately for both phases. They are coupled through a transfer term which accounts for evaporation, and is proportional to the difference between the partial vapor pressure and the equilibrium vapor pressure. See Theory for Moisture Transport in Porous Media for details.

This section has fields and values that are inputs to expressions that define material properties in the Liquid Water, Moist Air, and Porous Matrix subnodes.

The default Temperature Τ and Absolute pressure pA are User defined. When additional physics interfaces are added to the model, the temperature and absolute pressure variables defined by these physics interfaces can also be selected from the list. For example, if a Heat Transfer in Moist Porous Media interface is added, you can select Temperature (ht) from the list. If a Laminar Flow interface is added, you can select Absolute pressure (spf) from the list.

If the node was added automatically after selecting the Heat and Moisture Transport predefined multiphysics interface, the temperature of the Heat and Moisture multiphysics node is used by default and the input field is not editable. To edit the Temperature field, click Make All Model Inputs Editable ().

Select a coordinate system from the Coordinate system list for the interpretation of directions in anisotropic material properties. The default is the Global coordinate system, and the list contains any additional coordinate system (except boundary coordinate systems) added under the Definitions node.

The Hygroscopic Porous Medium node and its subnodes inherit these coordinate system settings. In particular, the Moist air velocity field (in Moist Air (Hygroscopic Porous Medium) subnode), the Permeability (in Porous Matrix (Hygroscopic Porous Medium) subnode), and the Capillary liquid flux density (in Hygroscopic Porous Medium node) should be set according to the coordinate system selected in this section.

See Coordinate Systems in the COMSOL Multiphysics Reference Manual for more details.

Select between the Equilibrium formulation and the Nonequilibrium formulation options in the Equilibrium between liquid and gas phases list.

Depending upon the selected option, further settings are required underneath: either the moisture storage function required by the single mass conservation equation, or the parameters used to couple the two mass conservation equations in each phase. See the Equilibrium Formulation and Nonequilibrium Formulation sections below for details.

When Equilibrium formulation is selected in the Equilibrium between liquid and gas phases list, this section defines the Moisture storage function w(ϕw). It characterizes the relationship between the amount of accumulated water and the relative humidity in the porous material. The default Moisture storage function is taken From material. For User defined, enter another value or expression.

When a Porous Material node is defined on the domains where the Hygroscopic Porous Medium feature is active, the Moisture storage function can be set in the Homogenized Properties section of the Porous Material node.

When Nonequilibrium formulation is selected in the Equilibrium between liquid and gas phases list, this section defines the equilibrium vapor pressure pv,eq (SI unit: Pa) and the evaporation rate Kevap (SI unit: 1/s).

In the Equilibrium vapor pressure list, select Water activity to define the equilibrium vapor pressure from the saturation pressure pv,sat (SI unit: Pa) and the water activity aw (dimensionless)

Enter a value for the Water activity in the porous medium. Default value is 1.

Select User defined to enter an expression for pv,eq. Default expression is mt.fpsat(mt.T), which gives the saturation pressure at the prescribed temperature.

Enter a value for the Evaporation rate. Default value is 0[1/s].

Physics tab with interface as Moisture Transport in Air, or any version of the Moisture Transport interface selected: