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
Check out the following reference: Natural Convection in Enclosures Book: Free-Convective Heat Transfer Publisher: Springer Berlin Heidelberg Pages: 279-343
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.
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 solid-fluid 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.