Fluid (Porous Medium)

This node defines the velocity field and material properties of the mobile fluid used in Equation 6-13 of the Porous Medium parent node, to model heat transfer in a porous matrix, possibly consisting of several solids, and filled with a mobile fluid, and one or more immobile fluids.

The fluid can be specified as a general gas or liquid, as an ideal gas, or as moist air.

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.

This section is available when a temperature-dependent density defined in a material is used. On the material frame, the density is evaluated in relation to a reference temperature in order to ensure conservation of the mass in the presence of temperature variations. By default the is used. This corresponds to the variable minput.Tempref, which is set to 293.15 K by default. To edit it, click the button (

), and in the node under , set a value for the in the section.

The other options are and all temperature variables from the physics interfaces included in the model.

	This model input does not override the Reference temperature Tref set in the Physical Model section of the physics interface, which is used to evaluate the reference enthalpy, and a reference density for incompressible nonisothermal flows.

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 field, click (

). The available options are (default), (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 button (

), and in the node under , set a value for the in the section.

Absolute pressure is used in certain predefined quantities that include enthalpy (the energy flux, for example).

It is also used if the ideal gas law is applied. See Thermodynamics, Fluid.

The default pA is taken from . It corresponds to the variable minput.pA, set to 1 atm by default. To edit it, click the button (

), and in the node under , set a value for the in the section. When additional physics interfaces are added to the model, the absolute pressure variables defined by these physics interfaces can also be selected from the list. For example, if a interface is added, you can select from the list. The last option is .

This section can be edited whenever a material property is dependent on concentration, for example, when the is set to with the set to .

In the c (SI unit: mol/m3 or kg/m3) list, select an existing concentration variable from another physics interface, if there are concentration variables, to enter a value or expression for the concentration, or that corresponds to the minput.c variable.

The defaultu is . For , enter values or expressions for the components based on space dimensions. Or select an existing velocity field in the component (for example, from a interface). The option corresponds to the minput.u variable. To edit it, click the button (

), and in the node under , set values for the components in the section.

The thermal conductivity kf describes the relationship between the heat flux vector q and the temperature gradient ∇T in q = −kf∇T, which is Fourier’s law of heat conduction. Enter this quantity as power per length and temperature.

The default kf is taken . For , select , , , or based on the characteristics of the thermal conductivity, and enter another value or expression. For , enter a scalar which will be used to define a diagonal tensor. For the other options, enter values or expressions into the editable fields of the tensor.

This section defines the thermodynamics properties of the fluid.

The heat capacity at constant pressureCp,f describes the amount of thermal energy required to produce a unit temperature change in a unit mass.

The ratio of specific heats γ is the ratio between the heat capacity at constant pressure, Cp,f, and the heat capacity at constant volume, Cv,f. When using the ideal gas law to describe a fluid, it is sufficient to specify γ to evaluate Cp,f. For common diatomic gases such as air, γ = 1.4 is the standard value. Most liquids have γ = 1.1 while water has γ = 1.0. γ is used in the streamline stabilization and in the variables for heat fluxes and total energy fluxes. It is also used if the ideal gas law is applied.

The available options for the are (default), , or . After selecting a from the list, other settings are displayed below.

This option specifies the , the , and the for a general gas or liquid.

This option uses the ideal gas law to describe the fluid. Only two properties are necessary to define the thermodynamics of the fluid:

•

The gas constant, with two options for the : Rs or Mn. If is selected, the software uses the universal gas constant R = 8.314 J/(mol·K), which is a built-in physical constant, to calculate the specific gas constant.

•

Either the Cp,f or the γ by selecting the option in the γ list. For an ideal gas, it is sufficient to specify either Cp,f or the ratio of specific heats, γ, because these properties are interdependent.

If is selected, the thermodynamics properties are defined as a function of the amount of vapor in moist air. The options available to define this amount are as follows:

•

to define the amount of water vapor in the total volume. If this option is selected, a model input is automatically added to the section.

•

(also called mixing ratio or humidity ratio) to define the ratio between the mass of water vapor and the mass of dry air.

•

, a quantity defined between 0 and 1, where 0 corresponds to dry air and 1 to air saturated with water vapor. The

and

should be specified.