Immobile Fluids (Porous Medium)
This node should be used to model an unsaturated porous matrix whose pore space is not solely filled with liquid and contains one or more gases considered to be immobile fluids; or a porous matrix containing insulated enclosures.
The Immobile Fluids node is available only when Porous Medium type is set to Local thermal equilibrium in the Porous Medium parent node, with either Plane layers parallel to heat flow, Plane layers perpendicular to heat flow, Power law, or Equivalent thermal conductivity option selected in the Effective thermal conductivity list.
Model Input
This section contains fields and values that are inputs for expressions defining material properties. If such user-defined property groups are added, the model inputs appear here.
Temperature
This section is available when material properties are temperature-dependent. By default, the temperature of the parent interface is used and the section is not editable. To edit the Temperature field, click Make All Model Inputs Editable (). The available options are User defined (default), Common model input (the minput.T variable, set to 293.15 K by default) and all temperature variables from the physics interfaces included in the model. To edit the minput.T variable, click the Go to Source button (), and in the Default Model Inputs node under Global Definitions, set a value for the Temperature in the Expression for remaining selection section.
Immobile Fluids
In this section, the total Volume fraction of the immobile fluids, θimf, should be set.
The total volume fraction of immobile fluids is calculated from
The volume fraction available for mobile fluids (that is, the effective porosity) is then calculated from
where the total volume fraction of immobile solids is calculated from
Heat Conduction, Immobile Fluids
The default Thermal conductivity kimf use values From material. For User defined select Isotropic, Diagonal, Symmetric, or Full based on the characteristics of the thermal conductivity and other values or expressions in the fields or matrices.
When the thermal conductivity is taken From material and more than one Immobile Fluid subnode is added under the Porous Material node, the effective conductivity for the equivalent immobile fluid is calculated from
In the Porous Medium node, the effective thermal conductivity is defined as:
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Plane layers parallel to heat flow (default), which calculates the effective conductivity of the solid-fluid system as the weighted arithmetic mean (or volume average) of the conductivities of the fluids and the porous matrix:
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Plane layers perpendicular to heat flow, which calculates the effective conductivity of the solid-fluid system as the weighted harmonic mean (or reciprocal average) of the conductivities of the fluids and the porous matrix:
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Power law, which calculates the effective conductivity of the solid-fluid system as the weighted geometric mean of the conductivities of the fluids and the porous matrix:
Thermodynamics, Immobile Fluids
The specific heat capacity describes the amount of heat energy required to produce a unit temperature change in a unit mass of the immobile fluid.
The Density ρimf and the Specific heat capacity Cp, imf should be specified. When the density and specific heat capacity are taken From material and more than one Immobile Fluid subnode is added under the Porous Material node, the effective properties are obtained by applying a volume average model:
In the Porous Medium node, the effective volumetric heat capacity at constant pressure is defined as:
The Immobile Fluids node requires the Subsurface Flow Module. For a detailed overview of the functionality available in each product, visit https://www.comsol.com/products/specifications/
Location in User Interface
Context Menus
Heat Transfer in Porous Media>Porous Medium>Immobile Fluids
Ribbon
Physics tab with Porous Medium selected in the model tree:
Attributes>Immobile Fluids