Talk:Nusselt number

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I have went through some research papers. The equation h=K * Nu /L, all the research papers are saying, L is the characteristic length. But they did not give any equation to relate that characteristics length with geometry of the convection surfaces.

In one research paper L is considered as diameter. Then, how can we calculate nusselt number for a flat plate?????????

Contents 1 Characteristic Length 2 Nusselt number vs Biot number 3 how to calculate nusselt no. 4 in perpendicular 5 Ratio of convection to conduction transfer

[edit] Characteristic Length

The characteristic length depends on the geometry.

For a circular pipe the characteristic length would be the diameter.

For non circular ducts, the characteristic length would be:

L=4A/p

where A is the cross-sectional area of the duct, P would be the wetted perimeter.

Note that for a circular pipe:

L=4*(pi*D^2/4)/(pi*D) = D

For flat plates, the characteristic length is usually the distance from the leading edge for a local Nusselt number. Average Nusselt numbers over a distance 0 to L use the L as the characteristic length. Note that most of these Nusselt number relationships (based on Reynolds and Prandlt numbers) are empirical.

• As above, Nu will in general depend on the geometry under consideration. Selection of the significant length dimension will obviously change the value of Nu. I cannot say for certain as I am not a professor of fluid mechanics (merely a mechanical engineering undergrad), but I believe that the selection of the significant length dimension should be the direction of the growth of the boundary layer. An example would be that of a horizontal cylinder and a vertical cylinder in natural convection. For the vertical cylinder, the significant length dimension is the height L. For the horizontal cylinder, the significant length dimension is the diameter D.
  • I just got an e-mail back from the TA for my heat transfer class. He confirmed that the above is correct, so I'll be updating the Nusselt Number main page.

[edit] Nusselt number vs Biot number

Could someone explain what the difference between the Nusselt number and the Biot number is? They have very similar expressions.

The Nusselt number is most useful in determining the convective heat transfer coefficient, whereas the Biot number is used in unsteady problems.

This is a typical exam question. The "k" in the Biot numer is of a solid, that in the Nusselt number of a fluid. Different meaning, different use... see a heat transfer textbook.

[edit] how to calculate nusselt no.

For finding heat loss in natural convection we require heat transfer coefficeint(h). For finding h we require nusselt no. Now how to calculate nusselt no. There are different equations for different cases.

[edit] in perpendicular

maybe the "in perpendicular to the flow direction" should be rephrased or something?

[edit] Ratio of convection to conduction transfer

This equality is far too general to be of any help. For instance, in natural convection in cavities is literal, in laminar internal flows can be descriptive of the situation and in turbulent and external flows is simply false. To explain fully what it means would require several pages. In addition, it tends to confuse students. For instance, they then mix-up Nusselt and Biot numbers, since the ratio is much more meaningful for the latter.

Xtobal (talk) 12:02, 14 April 2008 (UTC)

Fair enough. I have no problem with you removing that part of the equation now that you have explained that, but before it appeared more like vandalism because there was no explanation to why it was deleted. In the future I recommend that you used the edit summary. Thanks! --Wizard191 (talk) 12:55, 14 April 2008 (UTC)