Convective Heat Loss from Upward and Downward-Facing Cavity Solar Receivers: Measurements and Calculations

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Abstract
Up to now, nearly all the experimental and theoretical work treating convective heat loss of cavity receivers had been effected for downward and sideward-facing receivers, as they are used in dish concentrators and solar power towers. With a test facility it was possible to show flow patterns and a temperature field in a hot cavity for a variety of parameter settings and over the whole range of tilt angles. Two types of geometries were examined: a spherical and a hemispherical cavity, both with an inner diameter of 400 mm, the aperture diameter ranging between 60 and 195 mm. Temperatures were varied between 573 K and 873 K. The dependence of losses on temperature, tilt angle, aperture-radius, geometry, ribs, and ventilation is reported. Special emphasis is invested to get an understanding of the air flow in an upward facing cavity. Based on the works of Clausing et al. (1987), and Stine and McDonald (1989), two algorithms are developed that allow the calculation of convective loss from upward-facing receivers. Over the whole range of tilt angles, calculated results are compared with measurements. Good agreement between the calculated and measured data is found.