Fluid (Porous Medium)
This node defines the velocity field and material properties of the mobile fluid used in the heat transfer equation 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.
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.
Volume Reference Temperature
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 Common model input is used. This corresponds to the variable minput.Tempref, which is set to 293.15 K by default. To edit it, click the Go to Source button (), and in the Default Model Inputs node under Global Definitions, set a value for the Volume reference temperature in the Expression for remaining selection section.
The other options are User defined 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.
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.
Absolute Pressure
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 Absolute pressure pA is taken from Common model input. It corresponds to the variable minput.pA, set to 1 atm by default. To edit it, click the Go to Source button (), and in the Default Model Inputs node under Global Definitions, set a value for the Pressure in the Expression for remaining selection 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 Laminar Flow interface is added, you can select Absolute pressure (spf) from the list. The last option is User defined.
Concentration
This section can be edited whenever a material property is dependent on concentration, for example, when the Fluid type is set to Moist air with the Input quantity set to Concentration.
In the Concentration c (SI unit: mol/m3 or kg/m3) list, select an existing concentration variable from another physics interface, if there are concentration variables, User defined to enter a value or expression for the concentration, or Common model input that corresponds to the minput.c variable.
Heat Convection
The default Velocity field u is User defined. For User defined, enter values or expressions for the components based on space dimensions. Or select an existing velocity field in the component (for example, Velocity field (spf) from a Laminar Flow interface). The Common model input option corresponds to the minput.u variable. To edit it, click the Go to Source button (), and in the Default Model Inputs node under Global Definitions, set values for the Velocity components in the Expression for remaining selection section.
Heat Conduction, Fluid
The thermal conductivity kf describes the relationship between the heat flux vector q and the temperature gradient T in q = −kfT, which is Fourier’s law of heat conduction. Enter this quantity as power per length and temperature.
The default Thermal conductivity kf is taken From material. For User defined, select Isotropic, Diagonal, Symmetric, or Full based on the characteristics of the thermal conductivity, and enter another value or expression. For Isotropic, 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.
Note that this section is not available when Porous medium type is set to Local thermal equilibrium and Equivalent thermal conductivity is selected in the Effective thermal conductivity list of the Porous medium parent node.
Thermodynamics, Fluid
This section defines the thermodynamics properties of the fluid.
The heat capacity at constant pressure Cp,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 ideal gas law when Ideal gas is selected.
The ratio of specific heats can be calculated automatically when set to Automatic by using Mayer’s relation:
considering that:
αp is the coefficient of thermal expansion (SI unit: 1/K):
χt is the isothermal compressibility (SI unit: 1/Pa):
The available options for the Fluid type are Gas/Liquid (default), Moist air, or Ideal gas. After selecting a Fluid type from the list, other settings are displayed below.
Gas/Liquid
This option specifies the Density, the Heat capacity at constant pressure, and the Ratio of specific heats for a general gas or liquid.
Ideal Gas
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 Gas constant type: Specific gas constant Rs or Mean molar mass Mn. If Mean molar mass 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 Heat capacity at constant pressure Cp,f or the Ratio of specific heats γ by selecting the option in the Specify Cp or 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.
Moist Air
If Moist air is selected, the thermodynamics properties are defined as a function of the amount of vapor in moist air. The Input quantity options available to define this amount are as follows:
Vapor mass fraction to define the ratio between vapor mass and total mass.
Concentration to define the amount of water vapor in the total volume. If this option is selected, a Concentration model input is automatically added to the Model Inputs section.
Moisture content (default, also called mixing ratio or humidity ratio) to define the ratio between the mass of water vapor and the mass of dry air.
Relative humidity ϕw, a quantity defined between 0 and 1, where 0 corresponds to dry air and 1 to air saturated with water vapor. The Relative humidity, temperature condition and Relative humidity, absolute pressure condition should be specified.
When the Porous Medium type is set to Local thermal nonequilibrium in the Porous Medium parent node, the Initial Values, Heat Source, Thermal Insulation, Symmetry (Heat Transfer Interface), Temperature, Heat Flux, Lumped System Connector, Phase Change Interface, Continuity, Inflow, Outflow, Open Boundary, Boundary Heat Source, Surface-to-Ambient Radiation (Heat Transfer Interface), and Deposited Beam Power features are available under the Fluid subnode.These subnodes allow the definition of domain and boundary conditions specific to the fluid phase temperature Tf.
Dynamic Viscosity
When the Local thermal nonequilibrium option is selected in the Porous medium type list of the Porous Medium parent node, and Interstitial convective heat transfer coefficient is set to Spherical pellet bed, the Dynamic viscosity, μ, should be set to evaluate the Nusselt number. Note that when Fluid type is set to Moist air, the moist air viscosity is defined by the node, and this section is not present.
Location in User Interface
Context Menus
Ribbon
Physics tab with Porous Medium selected in the model tree: