Fluid flow/heat transfer Area dilemma

[williamcarter]

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 :

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]

 

[Chestermiller]

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 $\pi DL$.

 

[williamcarter]

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.