A dimensionless number used in heat transfer in general and free convection calculations in particular. It is equivalent to (Gr.Pr).

Ra = (L3 rho2 g beta (T2-T1) CP) / (mu k)

Ra = (L3 rho g beta (T2-T1)) / (mu alpha)

alpha: thermal diffusivity, beta: coefficient of expansion, CP: heat capacity, (T2-T1): temperature difference, g: gravitational acceleration, k: thermal conductivity, L: characteristic length, mu: viscosity, rho: density

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