Hygroscopic Porous Medium
Use this node to model moisture transport in a porous medium filled with two phases in equilibrium: liquid water and moist air. The moisture transport occurs through vapor diffusion and convection in moist air, and convection and capillary flow in the liquid phase.
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 ww), 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.
Model Input
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 ().
Coordinate System Selection
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.
Moisture Transport Properties
The Moisture storage function ww) 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.
Then, the following options are available for the capillary flux:
When Capillary model is set to Kelvin’s law (default), a Darcy’s law based on the capillary pressure is used. The capillary pressure is derived from the relative humidity by using Kelvin’s law, using the assumption that the liquid and gas phases are in equilibrium.
When Capillary model is set to Diffusion model, the capillary flux is derived from the gradient of the moisture content, and the Moisture diffusivity Dw.
For User defined, set a value for the Capillary liquid flux density, glc, to characterize the liquid transport as a function of the moisture content.
When a Porous Material node is defined on the domains where the Hygroscopic Porous Medium feature is active, the Moisture storage function and the Moisture diffusivity can be set in the Homogenized Properties section of the Porous Material node.
See Porous Material in the COMSOL Multiphysics Reference Manual.
Location in User Interface
Context Menus
Ribbon
Physics Tab with interface as Moisture Transport in Air, Moisture Transport in Building Materials, or Moisture Transport in Porous Media selected: