Everything about Rayleigh Number totally explained
In
fluid mechanics, the
Rayleigh number for a fluid is a
dimensionless number associated with the heat transfer within the fluid. 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.
For
free convection (aka
natural convection) near a vertical wall, this number is
»
For most engineering purposes, the Rayleigh number is large, somewhere around 10
6 and 10
8.
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, which is a solid but which behaves as a fluid over geological time scales. The high value 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.
Further Information
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