charge density in flow through capacitor

Dear all,

I am relatively new to comsol. I am trying to solve a simple 2D flow through capacitor model with electrostatics module, laminar flow having 0.1 M NaCl as the dielectric medium.

In the problem do I have to give space charge density value (calculated manually) in the boundary condition?
It would be so kind of you if anyone could guide on how to model this system.

I am accessible to only comsol base pack (4.3) with CFD module. The problem concept is attached herewith.

Thanking in advance.

Tinto