In fluid mechanics, internal flow is a flow for which the fluid is confined by a surface. Hence the boundary layer is unable to develop without eventually being constrained. The internal flow configuration represents a convenient geometry for heating and cooling fluids used in chemical processing, environmental control, and energy conversion technologies.

An example includes flow in a pipe.

In fluid mechanics, external flow is such a flow that boundary layers develop freely, without constraints imposed by adjacent surfaces. Accordingly, there will always exist a region of the flow outside the boundary layer in which velocity, temperature, and/or concentration gradients are negligible.It can be defined as the flow of a fluid around a body that is completely submerged in it.

An example includes fluid motion over a flat plate (inclined or parallel to the free stream velocity) and flow over curved surfaces such as a sphere, cylinder, airfoil, or turbine blade,air flowing around an airplane and water flowing around the submarines.

关于内流，查到了从MIT毕业的三位学者的巨作《Internal Flow - Concepts and Applications》。

此外，《Fundamentals of heat and mass transfer》一书的 7、8章也分别对其有所介绍。

若有其他详细的资料，望朋友们来分享~