Porous Media Transport Properties
The Porous Media Transport Properties node is used to model mass transfer in porous media using the Transport of Concentrated Species interface. The node adds the equations governing the mass fractions of all present species, and provides inputs for the transport mechanisms and for the material properties of the fluid mixture.
The settings in this node are dependent on the check boxes selected under Transport Mechanisms in the Settings window of the Transport of Concentrated Species interface.
This feature is only available in a limited set of add-on products. See https://www.comsol.com/products/specifications/ for more details on availability.
Model Inputs
Specify the temperature and pressure to be used in the physics interface. The temperature model input is used when calculating the density from the ideal gas law, but also when thermal diffusion is accounted for by supplying thermal diffusion coefficients. The pressure model input is used in the diffusional driving force in Equation 3-123 (that is, when a Maxwell–Stefan Diffusion Model is used) and when calculating the density from the ideal gas law.
Temperature
Select the source of the Temperature field T:
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Select User defined to enter a value or an expression for the temperature (SI unit: K). This input is always available.
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If required, select a temperature defined by a Heat Transfer interface present in the model (if any). For example, select Temperature (ht) to use the temperature defined by the Heat Transfer in Fluids interface with the ht name.
Absolute Pressure
Select the source of the Absolute pressure p:
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Select User defined to enter a value or an expression for the absolute pressure (SI unit: Pa). This input is always available.
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In addition, select a pressure defined by a Fluid Flow interface present in the model (if any). For example, select Absolute pressure (spf) to use the pressure defined in a Laminar Flow interface with spf as the name.
Matrix Properties
The default selection for Porosity is From material. In order to use a porosity defined in a material, specify the material using the Porous material list, and select From material from the Porosity list.
Select From pellet bed densities to compute the porosity using the (dry bulk) Bed density ρb and the (single phase) Pellet density ρpe (for example, when there is a reactive pellet bed in the same domain). The porosity is then defined from
For User defined, enter a value or expression for the Porosity, εp (dimensionless), of the porous media.
Density
Use this section to define the mixture density, and to specify the molar masses of the participating species.
Mixture Density
Select a way to define the density from the Mixture density list — Ideal gas or User defined:
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For Ideal gas, the density is computed from the ideal gas law in the manner of:
Here M is the mean molar mass of the mixture and Rg is the universal gas constant. The absolute pressure, p, and temperature, T, used corresponds to the ones defined in the Model Inputs section.
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For User defined enter a value or expression for the Mixture density ρ.
Molar Mass
Enter a value or expression for the Molar mass Mw for each species. The default value is 0.032 kg/mol, which is the molar mass of O2 gas.
Convection
Select the source of the Velocity field u:
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Select User defined to enter values or expressions for the velocity components. This input is always available.
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Select a velocity field defined by a Fluid Flow interface present in the model (if any). For example, select Velocity field (spf) to use the velocity field defined by the Fluid Properties node fp1 in a Single-Phase Flow, Laminar Flow interface with spf as the Name.
Diffusion
Specify the species molecular and thermal diffusivities in nonporous media in the manner described for the Transport Properties node.
To account for the effect of porosity in the diffusivities, select an Effective diffusivity model — Millington and Quirk model, Bruggeman model, Tortuosity model, or No correction. Using one of the first four models, the effective transport factor, fe, is defined from the porosity and the fluid tortuosity factor in the manner of:
(3-126)
For No correction, the effective transport factor is set to one.
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For the Millington and Quirk model, the effective transport factor is .
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For the Bruggeman model, the effective transport factor is .
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For the Tortuosity model, specify the tortuosity factor is τF.
The species diffusivities and mobilities are automatically adjusted for porous media transport using the effective transport factor.
Migration in Electric Field
This section is available when the Migration in electric field check box is selected in the Transport Mechanisms section of the interface, select the source of the Electric potential V:
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Select User defined to enter a value or expression for the electric potential. This input is always available.
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If required, select an electric potential defined by an AC/DC interface that is present in the model (if any). For example, select Electric potential (ec) to use the electric field defined by the Current Conservation node cucn1 in an Electric Currents interface ec.
Settings for the mobilities are needed for the Mixture-averaged and Fick’s law diffusion models. By default the mobility is set to be calculated based on the species diffusivities (adjusted by the Effective diffusivity model in the Diffusion section) using the Nernst-Einstein relation. To manually specify the mobilities, select User defined for the mobility um,w (SI unit: s·mol/kg) and enter one value for each species.
Enter the Charge number zc (dimensionless, but requires a plus or minus sign) for each species.
The temperature (if you are using mobilities based or the Nernst-Einstein relation) is taken from the Model Inputs section.