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# Rayleigh Number

## Description

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 multiplied by the ratio of momentum and thermal diffusivities.

For most engineering purposes, the Rayleigh number is large, somewhere around 10^6 to 10^8.

Related formulas

## Variables

 Rax Rayleigh number for characteristic length x (dimensionless) g Standard gravity β thermal expansion coefficient (K-1) ν kinematic viscosity (m2/s) α thermal diffusivity (m2/s) Ts surface temperature (temperature of the wall) (K) Tinf quiescent temperature (fluid temperature far from the surface of the object) (K) x characteristic length (in this case, the distance from the leading edge) (m)