Mass Balance Equation
The default node attributed to the Transport of Diluted Species interface models chemical species transport through diffusion and convection and solves the mass conservation equation for one or more chemical species i:
(4-6)
Equation 4-6 in its form above includes the transport mechanisms diffusion and convection. If Migration in Electric Field is activated (only available in some add-on products), the migration mechanism will be added to the equation as well. See more details in the section Adding Transport Through Migration.
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ci is the concentration of the species (SI unit: mol/m3)
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Di denotes the diffusion coefficient (SI unit: m2/s)
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Ri is a reaction rate expression for the species (SI unit: mol/(m3·s))
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 u is the velocity vector (SI unit: m/s)
The flux vector N (SI unit: mol/(m2·s)) is associated with the mass balance equation above and used in boundary conditions and flux computations. For the case where the diffusion and convection are the only transport mechanisms, the flux vector is defined as
(4-7)
If Migration in Electric Fields is activated, the flux vector is amended with the migration term as show in the section Adding Transport Through Migration.
The first term on the left-hand side of Equation 4-6 corresponds to the accumulation (or indeed consumption) of the species.
The second term accounts for the diffusive transport, accounting for the interaction between the dilute species and the solvent. A user input field for the diffusion coefficient is available. Anisotropic diffusion coefficient tensor input is supported.
The third term on the left hand side of Equation 4-6 describes the convective transport due to a velocity field u. This field can be expressed analytically or be obtained from coupling this physics interface to one that computes fluid flow, such as Laminar Flow.
On the right-hand side of the mass balance equation (Equation 4-6), Ri represents a source or sink term, typically due to a chemical reaction, or /desorption on a porous matrix. To specify Ri, another node must be added to the Transport of Diluted Species interface—the Reaction node, which has a field for specifying a reaction equation using the variable names of all participating species.