| Definition: | quantity of dimension 1 characterizing the relative importance of natural convection and thermal conduction, defined by , where l is a specified length, ρ is mass density, cp is specific heat capacity at constant pressure, g is acceleration of free fall, αv is the cubic expansion coefficient, T is thermodynamic temperature, η is dynamic viscosity, λ is thermal conductivity, and α is thermal diffusivity NOTE The Rayleigh number is the product of the Grashof number Gr and the Prandtl number Pr, thus Ra = Gr ⋅ Pr.
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