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?????????

[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.

[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.

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