Transport Properties
The Transport Properties is the main node used to model mass transfer in a fluid mixture with the Transport of Concentrates 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.
The options available in this feature differs between COMSOL products. (See https://www.comsol.com/products/specifications/).
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 8-4 (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.
Density
Define the density of the mixture and 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.
Convection
Select the source of the Velocity field u:
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Select User defined to enter manually defined 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 molecular and thermal diffusivities of the present species based on the selected Diffusion model.
When using a Maxwell–Stefan Diffusion Model or a Mixture-Averaged Diffusion Model, select the Binary diffusion input type (Table or Matrix) and specify the Maxwell–Stefan diffusivities in the table or matrix, then enter the Thermal diffusion coefficients .
When using a Fick’s Law Diffusion Model, specify the Diffusion coefficient and the Thermal diffusion coefficients for each of the species.
Maxwell–Stefan Diffusivity Matrix
Using a Maxwell–Stefan Diffusion Model or a Mixture-Averaged Diffusion Model, the Maxwell–Stefan diffusivity matrix Dik (SI unit: m2/s) can be specified by a table or matrix. For a simulation involving Q species the Maxwell–Stefan diffusivity matrix is a Q-by-Q symmetric matrix, where the diagonal components are 1. Enter values for the upper triangular components, Dij, which describe the interdiffusion between species i and j. For the table input type, only upper triangular components (Dij) are listed. The name of species pair consists of species in the first and second column. For the matrix input type, the numbering of the species corresponds to the order, from top to bottom, used for all the input fields for species properties (see for example the molar mass fields in the Density section).
Diffusion Coefficient
Using a Fick’s Law Diffusion Model, the diffusion is by default assumed to be isotropic and governed by one Diffusion coefficient (SI unit: m2/s) for each species. To allow for a general representation, it is also possible to use diffusion matrices (diagonal, symmetric, or anisotropic).
Thermal Diffusion Coefficient
To model thermal diffusion, prescribe the Thermal diffusion coefficients (SI unit: kg/(m·s)), by entering one thermal diffusion coefficient for each species in the corresponding field. In a multicomponent mixture, the sum of the thermal diffusion coefficients is zero. The default value for all thermal diffusion coefficients is 0.
Specify the molecular and thermal diffusivities of the present species based on the selected Diffusion model.
Migration in Electric Field
This section is available when the Migration in electric field check box is selected for the Transport of Concentrated Species interface.
Electric Potential
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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 used for the Mixture-averaged and Fick’s law transport models. By default the mobility is set to be calculated based on the species diffusivities and the temperature using the Nernst-Einstein relation. To manually specify the mobilities, select User defined for the mobility um,c (SI unit: s·mol/kg) and enter one value for each species.
The temperature (if you are using mobilities based on the Nernst–Einstein relation) is taken from the Model Inputs section.