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Rayleigh and Nusselt numbers 
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#1
Jan2507, 09:01 AM

P: 747

Hi
Can anyone tell me anything about these numbers. I know the formulae I just want a qualitative description. As far as I have it the Rayleigh number tells you if you can have convection and how vigorous that convection is, and the Nusselt number tells you how efficient that convection is in transporting heat. I could well be wrong..... Cheers 


#2
Jun1109, 10:10 AM

P: 23

Check out the following reference:
Natural Convection in Enclosures Book: FreeConvective Heat Transfer Publisher: Springer Berlin Heidelberg Pages: 279343 


#3
Jun1109, 11:57 AM

Sci Advisor
P: 5,510

http://en.wikipedia.org/wiki/Rayleigh_number
http://en.wikipedia.org/wiki/Nusselt_number They both relate to thermal convection; there may be some reason (unknown to me) for using one over the other. The Rayleigh number seems to be for bulk fluids while the Nusselt number involves a boundary. 


#4
Jun1209, 03:06 PM

P: 23

Rayleigh and Nusselt numbers
sorry, I had a brain fart ...
Here's something I compiled for work: Rayleigh Number: Important in natural (free) convection Onset of natural convection occurs when the Rayleigh number reaches a critical value (~1700) Nusselt number: Ratio of convective to conductive heat transfer across a solidfluid interface Used to calculate heat transfer coefficient at the interface Depends upon Re and Pr in forced convection Depends upon Gr and Pr in natural convection Sometimes also interpreted as the dimensionless temperature gradient at the surface. 


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