What is rayleigh's?

Answer 1

For free convection near a vertical wall, the Rayleigh number is defined as

where

• 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

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.