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I have a cold cylinder that is submerged in hot water and I need to find the convective heat transfer coefficient. I can do the whole process but I am stuck finding the characteristic length. I found that the characteristic length of an object is $$L_{c}=\frac{A}{P}$$

Now I assume that heat transfer area in this case includes the top and botom of the vertical cylinder. So does my characteristic length become $$L_{c}=\frac{2\pi r^{2}+\pi DH}{2\pi r}$$

Or do I neglect the surface area of the top and bottom to have $$L_{c}=\frac{\pi DH}{2\pi r}=\frac{\pi DH}{\pi D}=H$$

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    $\begingroup$ What is P in the first equation? $\endgroup$ Nov 13, 2012 at 23:02
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    $\begingroup$ P must be perimeter? $\endgroup$
    – Johannes
    Dec 14, 2012 at 13:01

3 Answers 3

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There is no unique definition for characteristic length. One can define the length scale as the square root of the surface area, as the third root of the volume, as the volume-to-surface ratio, etc. It all depends what length scale you want identify. For your problem (convective heat transport of an object submerged in a liquid at elevated temperature) the characteristic length is the vertical size of the object. This is because the convection will take place in the vertical direction (cold water flowing down, hot water rising).

If your object is a cylinder in vertical orientation, the convective length scale is the height H of the cylinder. If your object is a cylinder placed horizontally the characteristic length is determined by the diameter D of the cylinder.

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The characteristic length is commonly defined as the volume of the body divided by the surface area of the body, i.e.

$L_c = \frac{V_{body}}{A_{surface}}$

The surface area should include the top and bottom of the cylinder.

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  • $\begingroup$ does the equation $L_{c}=\frac{A_{s}}{P}$ not hold true? $\endgroup$ Nov 13, 2012 at 23:13
  • $\begingroup$ Again, what is P? $\endgroup$ Nov 14, 2012 at 8:45
  • $\begingroup$ P probably indicates the perimeter. Then they must be considering a 2D geometry. $\endgroup$
    – Infi
    Nov 16, 2021 at 13:55
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Both of the above answers are correct. The characteristic lengths for the following shapes are:

  • Sphere = $Radius/3$ = $Diameter/6$
  • Cylinder = $Radius/2$ = $Diameter/4$
  • Plate (ie: flat cylinder) = $Length/2$
  • Cube = $Length/6$
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    $\begingroup$ Please, use MathJax to write formulas or formula-like objects. It really improves your answer. $\endgroup$ Oct 22, 2015 at 9:26

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