Caedium with either the Panel Flow or Professional add-on can model both internal and external flows. For a tutorial on internal flow try "Internal Flow."

In essence you'll need to make the air flow path through the duct into a single water tight volume (or solid). Then assign a substance (e.g., Air from the Physics Tool Palette) to the volume and assign walls, inlets and outlets to the relevant boundaries. For our Panel Flow add-on make sure that the mass-flow rate through your inlets (velocity x area) matches that through your outlets.

If you have objects within the duct (e.g. a model within a wind tunnel) you will need to create a volume that includes the objects inside the duct. Usually the easiest way to achieve this is to subtract (Subtract tool in the Geometry Tool Palette) your objects (volumes) from the duct volume. For a tutorial using the Subtract tool try "Simple Volume Creation"

Alternatively, try capping the inlet and outlet (using the Fill tool) to the flow path and then stitching faces to form a closed volume. This approach is likely to be more reliable. For a tutorial on fill and stitch operations try "Loft, Fill and Stitch."

Hope this helps.