How to calculate the wetted perimeter

1. The problem statement, all variables and given/known data
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?

http://ift.tt/1p41ZwQ

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