How to define a fan in Caedium for PC cabinet cooling

I seek advice on how to efficiently model a small axial fan (92x92x32 mm) used to cool a PC cabinet.

I assume the "Fan" pressure step function is needed, but what about the swirl? Can that part of the real problem be ignored in the model without much fidelity loss?

The pressure step is also dependent on the flow rate. For high end PC fans the flow vs pressure graph is usually supplied, for example like this:
(For low end fans often only max flow and max pressure is supplied - and as independent values.)

Is there a way to define the connection between flow and pressure build-up inside Caedium or would this typically be handled by an external script?

Fan type supported for inlet and outlet BCs

Caedium supports Type = Fan for both Physics:Inlet and Physics:Outlet boundary conditions, where each supports a Fan Curve (volume flow rate vs pressure drop / rho). Neither condition type supports swirl.

If you want to define swirl then you will need to use a default inlet type, set U = Reference or Fixed Value, specify the inlet velocity vector, and add Conditions->Angular Velocity. Whether swirl is important for your simulation is something you have to decide.

is this an internal fan?

But is this an internal fan (meaning that it is only moving the air inside the volume), or its bringing new "fresh" fluid into the system what otherwise the inlet would do.

Inlet, outlet, and internal fan conditions

The previous comment concerns inlet and outlet fan conditions. There is also a internal fan condition.