In Fluent, you can specify faces with particular functions such as inlet, outlet, or symmetry.

Faces that are not specified are automatically walls, so you can leave all the spherical faces unspecified and they will be zero slip walls.

The faces of the fluid on the four long sides of the box are going to be symmetry, while the top and bottom face of the box are going to be outlet and inlet.

The spheres can have an internal heat generation.