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?????????
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[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.
[edit] in perpendicular
maybe the "in perpendicular to the flow direction" should be rephrased or something?
