What do Rayleigh and Nusselt numbers tell us about heat transfer?

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Discussion Overview

The discussion centers on the Rayleigh and Nusselt numbers, focusing on their qualitative descriptions and implications for heat transfer, particularly in the context of convection. Participants explore the roles these numbers play in understanding convection dynamics and heat transfer efficiency.

Discussion Character

  • Exploratory, Technical explanation

Main Points Raised

  • One participant suggests that the Rayleigh number indicates the possibility and intensity of convection, while the Nusselt number reflects the efficiency of heat transport via convection.
  • Another participant provides a reference to a book on natural convection, implying a deeper exploration of the topic may be found there.
  • A third participant shares links to Wikipedia articles, noting that both numbers relate to thermal convection and hinting at a distinction in their applications, with the Rayleigh number being more relevant for bulk fluids and the Nusselt number for boundary layers.
  • A later reply outlines specific characteristics of the Rayleigh number, mentioning its critical value for the onset of natural convection, and describes the Nusselt number as a ratio of convective to conductive heat transfer, dependent on various dimensionless numbers in different convection scenarios.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and interpretation of the Rayleigh and Nusselt numbers, with no consensus reached on their qualitative implications or applications.

Contextual Notes

Some statements rely on specific conditions or assumptions regarding the context of convection, such as the critical value for the Rayleigh number and the dependencies of the Nusselt number on other dimensionless numbers.

billiards
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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
 
Science news on Phys.org
Check out the following reference:

Natural Convection in Enclosures
Book: Free-Convective Heat Transfer
Publisher: Springer Berlin Heidelberg
Pages: 279-343
 
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.
 

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