How to calculate the wetted perimeter

[theBEAST]

Homework Statement

So I have been taught that the wetted perimeter is the perimeter in which the surface is wet. So in the case for internal forced convection of a thick wall pipe I would have thought the perimeter would be 2*pi*r where r is the inner radius of the pipe. However, doing a bit of googling, this answer is clearly wrong as shown in the image below:

Does anyone know why?

 

[Chestermiller]

Yes. This is not a thick walled pipe calculation. It is a calculation of axial flow along an annular channel (between two concentric cylinders).

Chet