Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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 $$L_{c}=\frac{\pi DH}{2\pi r}=\frac{\pi DH}{\pi D}=H$$

share|cite|improve this question
What is P in the first equation? – Jason Davies Nov 13 '12 at 23:02
P must be perimeter? – Johannes Dec 14 '12 at 13:01
up vote 2 down vote accepted

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.

share|cite|improve this answer

both the above are right however these are the characteristic lengths for these shapes: sphere = R/3 = D/6 cylinder = Radius/2 = Diameter/4 plate (ie: flat cylinder) = Length/2 Cube = Length/6

share|cite|improve this answer
Please, use MathJax to write formulas or formula-like objects. It really improves your answer. – Victor Pira Oct 22 '15 at 9:26

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.

share|cite|improve this answer
does the equation $L_{c}=\frac{A_{s}}{P}$ not hold true? – Greg Harrington Nov 13 '12 at 23:13
Again, what is P? – Jason Davies Nov 14 '12 at 8:45

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.