Heat transfer in flows created by buoyancy, or natural convection, is a widely studied topic across various disciplines spanning natural flows as well as those with engineering applications. The convective heat transfer rate on a surface is commonly represented by the Nusselt number (Nu), a ratio of convective to diffusive transport, expressed often as , where is the Rayleigh number, the buoyancy forcing parameter, and the Prandtl number. Motivated by the observation that for turbulent convection, which implies the heat flux is independent of the length scale ( , characteristic length related to the geometry), we propose an alternate and physically more meaningful non-dimensional heat transfer parameter, denoted by . is derived using only the near wall variables and does not contain . For , is constant. Even for laminar convection, where , , a weak function of . We show that for natural convection over several geometries and a wide range of Ra, the values within a narrow range while the corresponding Nu values span several orders of magnitude. We also show that is akin to the non-dimensional representation of wall shear stress, skin friction coefficient . We believe that just like , will be an equally useful non-dimensional measure of heat transfer in natural convection flows.
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