How to calculate the wetted perimeter

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The wetted perimeter refers to the area of a surface that is in contact with a fluid. In the context of internal forced convection in a thick-walled pipe, the correct approach involves considering the geometry of the flow, specifically in annular channels formed by concentric cylinders. The calculation should not simply use the formula for a solid cylinder's perimeter, as this does not account for the flow dynamics in annular spaces. Understanding the distinction between thick-walled pipes and annular flow is crucial for accurate calculations. Properly identifying the geometry is essential for determining the wetted perimeter in fluid dynamics applications.
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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:
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Does anyone know why?
 
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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
 
Thread 'Correct statement about size of wire to produce larger extension'

Thread 'Correct statement about size of wire to produce larger extension'

The answer is (B) but I don't really understand why. Based on formula of Young Modulus: $$x=\frac{FL}{AE}$$ The second wire made of the same material so it means they have same Young Modulus. Larger extension means larger value of ##x## so to get larger value of ##x## we can increase ##F## and ##L## and decrease ##A## I am not sure whether there is change in ##F## for first and second wire so I will just assume ##F## does not change. It leaves (B) and (C) as possible options so why is (C)...
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