# Rayleigh number

The Rayleigh number (Ra, after Lord Rayleigh ) is a dimensionless parameter that describes the nature of the heat transfer within a fluid.

When the Rayleigh number is below a critical value for the fluid, the heat transfer is given primarily by conduction; if it exceeds the critical value, the heat transfer is given primarily by convection.

In which

- Ra = Rayleigh number (dimensionless)
- Gr = the Grashof number (dimensionless)
- G = gravitational acceleration (eg gravity in SI units 9.81 m / s ² )
- Ts = characteristic temperature ( in SI units K)
- T ∞ = resting temperature ( in SI units K)
- ν = kinematic viscosity of fluid ( in SI units m² / s)
- A = thermal diffusivity ( in SI units m² / s)
- β = coefficient of thermal expansion ( in SI units 1 / K)
- L = characteristic length (m in SI units)

- Code ( Fluid Mechanics )
- Code ( thermodynamics)