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In fluid mechanics, the Rayleigh number (Ra) 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 and viscosity forces times the ratio of momentum and thermal diffusivities.
For free convection near a vertical wall, the Rayleigh number is defined as
- x = Characteristic length (in this case, the distance from the leading edge)
- Rax = Rayleigh number at position x
- Grx = Grashof number at position x
- Pr = Prandtl number
- g = acceleration due to gravity
- Ts = Surface temperature (temperature of the wall)
- T∞ = Quiescent temperature (fluid temperature far from the surface of the object)
- ν = Kinematic viscosity
- α = Thermal diffusivity
- β = Thermal expansion coefficient (equals to 1/T, for ideal gases, where T is absolute temperature)
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 to 108.
For a uniform wall heating flux, the modified Rayleigh number is defined as 
- q"o = the uniform surface heat flux (W/m2)
- k = the thermal conductivity (W/m•K)
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
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.
High values for the Earth's mantle indicates that convection within the Earth is vigorous and time-varying, and that convection is responsible for almost all the heat transported from the deep interior to the surface.
- M. Favre-Marinet and S. Tardu, Convective Heat Transfer, ISTE, Ltd, London, 2009
- Bunge, Hans-Peter; Richards, Mark A.; Baumgardner, John R. (1997). "A sensitivity study of three-dimensional spherical mantle convection at 108 Rayleigh number: Effects of depth-dependent viscosity, heating mode, and endothermic phase change". Journal of Geophysical Research 102 (B6): 11991–12007. Bibcode:1997JGR...10211991B. doi:10.1029/96JB03806.
- Turcotte, D.; Schubert, G. (2002). Geodynamics (2nd ed.). New York: Cambridge University Press. ISBN 0-521-66186-2.