Natural convection inevitably occurs in solar thermal systems, e.g., open cavity receivers in high concentrating solar dishes and solar towers. In most applications, it can contribute a significant fraction of total energy loss, and hence it is an important determining factor in system performance. The present study investigates both experimentally and numerically the natural convection loss from the aperture of open cavity receivers used in paraboloidal dish concentrators. The aim of this study is to quantitatively determine the relationship between convection loss to various pertinent parameters such as ambient temperature, operating temperature, cavity inclination and cavity geometry. The experimental investigation was based on an isothermal electrically heated model receiver, with a test temperature of 450°C. The model receiver is tested at inclinations varying from 0° (cavity facing the side) to 90° (cavity facing straight down). With the numerical work the commercial CFD software package, Fluent 6.0 was used to model cavity convection losses. The range of input parameters used is similar to those of the experimental model. In addition, two cases of full-scaled receivers currently used in ANU dishes are considered. The accuracy concerning grid structure and grid resolution of the numerical model is also discussed. The numerical results obtained are qualitatively in good agreement with those predicted by various previously proposed correlations. The Clausing model [1981] shows the closest prediction to both numerical and experimental results despite its original use for bigger-scale central receivers.

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