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

Ra = (L^{3} rho^{2} g beta (T_{2}-T_{1}) C_{P}) / (mu k)

Ra = (L^{3} rho g beta (T_{2}-T_{1})) / (mu alpha)

alpha: thermal diffusivity, beta: coefficient of expansion, C_{P}: heat capacity, (T_{2}-T_{1}): temperature difference, g: gravitational acceleration, k: thermal conductivity, L: characteristic length, mu: viscosity, rho: density

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