Use the Inflow node to specify the number densities of different sizes at an inlet boundary.

If you want to specify the number densities of only certain size intervals, this can be done using the Number Density node instead.

The Inflow node is not available in 0D.

Select User defined from the list to enter a value or expression that is applied to all size intervals. 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 instead to enter a separate value or expression for each size interval.

The option Number-density constraint constrains the number density values on the boundary.

The other option, Flux (Danckwerts), prescribes the total flux at the boundary defined by an upstream number density and fluid velocity at the boundary.

To display this section, click the Show More Option button () and select Advanced Physics Options. You can find details about the different constraint settings in the section Constraint Reaction Terms in the COMSOL Multiphysics Reference Manual.