A combined experimental and analytical study of natural convection heat transfer in the enclosure formed between a convex shaped, isothermal heated body and its concave shaped, surrounding isothermal enclosure is presented. An experimental test program is conducted at atmospheric and reduced pressure conditions using a transient measurement technique for various enclosure configurations, including concentric cubes, cube-in-sphere, sphere-in-cube, and other geometries. Measurements are reported for a wide range of Rayleigh number, including the diffusive limit. An analytical modeling procedure is developed based on a composite solution of three asymptotic relationships, the conduction limit, the laminar boundary layer and transition flow convective limits. The composite model is in excellent agreement with the data, with an average RMS difference of 2–7% for all enclosure configurations and test conditions.

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