Natural Convection Measurements for a Concentric Spherical Enclosure

An experimental test program is described for the measurement of natural convection for an isothermal, heated sphere centrally located in an isothermal, cooled spherical enclosure. A transient test method is used in a reduced pressure environment to provide data for a wide range of Rayleigh number, from the limiting case of laminar boundary layer convection to the diffusive limit. Tests are performed using a fixed outer diameter for four different inner sphere diameters, resulting in diameter ratios in the range 1.5⩽do∕di⩽4.8. The data are in excellent agreement with the exact solution for the conductive limit and are shown to be bounded by a model for the isolated, isothermal sphere.