An experimental investigation of heat transfer by natural convection from a smooth, isothermal cubic cavity with a variety of side-facing apertures is described in this paper. The study was motivated by the desire to predict the convective loss from large solar thermal-electric receivers and to understand the mechanisms which control this loss. Hence, emphasis is placed on the large Rayleigh number, Ra, regime with large ratios of the cavity wall temperature Tw to the ambient temperature T∞. A cryogenic wind tunnel with test section temperatures which are varied between 80 K and 310 K is used to facilitate deduction of the influences of the relevant parameters and to obtain large temperature ratios without masking the results by radiative heat transfer. A 0.4-m cubic cavity, which is mounted in the side wall of this tunnel, is used. The area of the aperture Aa and its location are key variables in this study. The data which are presented cover the ranges: 1 < Tw/T∞ < 3, L2/18 ≤ Aa ≤ L2, and 3 × 107 < Ra < 3 × 1010.

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