@wikipedia
Dimensionless quantity characterising the development of Natural Thermal Convection:
(1) | \mbox{Ra}_L = \mbox{Gr} \cdot \mbox{Pr} = \frac{g \, \beta \, \Delta T \, L^3}{\nu \cdot a} |
where
\Delta T | Temperature gradient across Natural Thermal Convection | \nu | Kinematic viscosity of the fluid |
---|---|---|---|
\beta | Thermal expansion coefficient of the fluid | a | Thermal Diffusivity of the fluid |
L | Characteristic length of Natural Thermal Convection | g | Standard gravity constant |
For the annulus between two concentric pipes:
(2) | \mbox{Ra}_D = \frac{8}{(D_o - D_i)^3} \cdot \frac{ \left[ \ln D_o/D_i \right]^4 }{D_i^{-3/5}+D_o^{-3/5}} \cdot \mbox{Ra}_L |
See also