The Fluid node is added by default and is used to model the mass transfer in a fluid with the Size-Based Population Balance interface. The node adds all equations governing the diffusive and convective mass transport of the population. It also provides inputs for material properties. The Fluid node is not available in 0D.

The temperature model input is always available. Select the source of the Temperature. To enter a value or expression for the temperature, select the User Defined option. The User Defined option is always available.

Under Convection, the velocity field for convective mass transfer can be specified. Select the source of the velocity field from the Velocity Field list. For User Defined, enter values or expressions for the velocity field components in the input fields. This option is always available.

In the Diffusion section, select the source of the diffusion from the list. It can be set to From material, User defined, or Define Individually.

For From material, select a material from the Material list to access diffusion coefficients defined within that material. Select a diffusion coefficient from the Diffusion coefficient list. If you do not select a material with a diffusion coefficient, you can enter a value or expression in the Diffusion coefficient field. The value or expression can be a scalar value for isotropic diffusion or a tensor describing anisotropic diffusion. Select the appropriate tensor type — Isotropic, Diagonal, Symmetric or Full.

When the diffusion source is set to User defined, enter a value or expression in the Diffusion coefficient field. When this option is selected, the text field also accepts an expression written as a function of the particle size, pop.L.

Select Define individually to set the diffusion coefficient for each discretized size interval individually.