Fluid flow/heat transfer Area dilemma

AI Thread Summary
In fluid flow calculations, the area used is the cross-sectional area, represented by A=(pi*D^2)/4, which is essential for determining fluid velocity. In contrast, for heat transfer calculations, the surface area is used, calculated as A=pi*D*L, which accounts for the entire tubular surface area. To choose the correct formula, one must consider whether the focus is on fluid dynamics or heat transfer. In scenarios involving heat transfer, knowing the length of the pipe is crucial, as it directly influences the surface area calculation. Understanding these distinctions helps in accurately applying the appropriate area formula for each context.
williamcarter
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Homework Statement


I have a confusion regarding areas.
Usually in fluid flow I am using the A=(pi*D^2)/4
However in heat transfer we usually use A=pi*D*L

Could you please explain this?I mean in first case(fluid flow) it is cross sectional area and in the 2nd case(heat transfer) is like the whole area.
Why is like that?

Could you please explain how to know when to use each formula of area?For example in this exercise regarding a pipe :
ImesfST.jpg

My question is which of the Area formulas we need to use in this problem?
We should use area A=pi*D*L right? Because it regards heat transfer.
However we do not know the Length L in the upper exercise.


Homework Equations


A=pi*D^2/4(fluid flow)
A=pi*D*L(heat transfer)

Where D=diameter
L=length

The Attempt at a Solution


I would like to be more confident in choosing the area formula , because I am confused.
Thank you in advance[/B]
 
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There are two parts to the calculations. When you are looking at the fluid flow to calculate the heat transfer coefficient for the flow within the tube, you need to know the fluid velocity. To get this, divide the volumetric flow rate by the cross sectional area of the tube. When you are looking at the heat transfer across a surface, you need to determine that area of the surface. For a tubular surface, that's ##\pi DL##.
 
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Chestermiller said:
There are two parts to the calculations. When you are looking at the fluid flow to calculate the heat transfer coefficient for the flow within the tube, you need to know the fluid velocity. To get this, divide the volumetric flow rate by the cross sectional area of the tube. When you are looking at the heat transfer across a surface, you need to determine that area of the surface. For a tubular surface, that's ##\pi DL##.
Thank you, yes it makes sense.
 
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