In all three types of transport phenomena it is quite important to distinguish between external and internal flows.
A closed conduit is any structure for conveying fluids from one point to another where the flow is not open to atmosphere along the path except perhaps at the end of the conduit. Examples include pipes, tubes, ducts, etc.
An internal flow is a process where fluid flows through the inside of a closed conduit. Typically, a pressure force (i.e. pressure gradient, pressure drop) or gravitational force is used to move the fluid and overcome the viscous shear forces (friction) offered by the walls of the conduit.
A submersed object is an object whose entire external surface area is in contact with a fluid flowing past the object.
An external flow is a process where fluid flows around a submersed object or a submersed object moves through a fluid (same problem, different reference frames)
The reason these types of flow are different is that the walls have a different effect in both cases. For external flows the wall introduces friction, mass, or heat into the flow, but that effect can keep traveling outward from the object indefinitely.
In an internal flow the walls begin to "gang up" on the fluid so that when they introduce friction, mass, or heat into the flow, these effects begin to "overlap" at some point.
Distinguish between external and internal flows.