If a simple 3D duct is modeled in a steady state pure Flow analysis, and it does not converge, check to see if the flow in equals the flow out of the duct.
In a Steady State Flow analysis, the mass that flows in must equal that mass that flows out. Otherwise a transient analysis must be run. This could occur if the inlet uses the inlet Flow Boundary Condition (BC) with a particular mass flow and an Outlet Flow BC is used with a different mass flow. The easiest method to test this issue is to replace the Outlet Flow BC with a Opening BC.