The Rayleigh number for a fluid is a dimensionless number associated with buoyancy-driven flow, also known as free convection or natural convection. When the Rayleigh number is below a critical value for that fluid, heat transfer is primarily in the form of conduction; when it exceeds the critical value, heat transfer is primarily in the form of convection.

The Rayleigh number is the product of the Grashof number $Gr$ and the Prandtl number $Pr$.

Symbol
$\mathrm{Ra}_{\mathrm{L}}$
Unit
-
Formulae
$\mathrm{Gr}_{\mathrm{L}} \mathrm{Pr}_{\mathrm{gas}}$
Related
$\mathrm{Gr}_{\mathrm{L}}$
$\mathrm{Pr}_{\mathrm{gas}}$
Used in
$\mathrm{Nu}_{\mathrm{L}}$