In fluid mechanics, 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 the 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 named after Lord Rayleigh and is defined as the product of the Grashof number, which describes the relationship between buoyancy and viscosity within a fluid, and the Prandtl number, which describes the relationship between momentum diffusivity and thermal diffusivity. Hence the Rayleigh number itself may also be viewed as the ratio of buoyancy forces and (the product of) thermal and momentum diffusivities.
For free convection near a vertical wall, this number is
In the above, the fluid properties Pr, ν, α and β are evaluated at the film temperature, which is defined as
For most engineering purposes, the Rayleigh number is large, somewhere around 106 and 108.
In geophysics the Rayleigh number is of fundamental importance: it indicates the presence and strength of convection within a fluid body such as the Earth's mantle. The mantle is a solid that behaves as a fluid over geological time scales. The Rayleigh number for the Earth's mantle, due to internal heating alone, RaH is given by
where H is the rate of radiogenic heat production, k is the thermal conductivity, and D is the depth of the mantle.
A Rayleigh number for bottom heating of the mantle from the core, RaT can also be defined:
 Where ΔTsa is the superadiabatic temperature difference between the reference mantle temperature and the Core–mantle boundary and c is the specific heat capacity, which is a function of both pressure and temperature.
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