Natural Convection and Radiation Heat Loss from Open Cavities of Different Shapes and Sizes Used with Dish Concentrator

Abstract

Numerical three dimensional studies of the combined natural convection and radiation heat loss from downward facing open cavity receiver of different shapes is carried out in this paper. The investigation is undertaken in two categories: same inner heat transfer area and aperture area (case I) and same aspect ratio and aperture area (case II). These studies are carried out for five isothermal wall temperatures (523 to 923 K in steps of 100K). The effect of inclination is studied for seven inclinations from 0° (cavity aperture facing sideways) to 90° (cavity aperture facing down), in steps of 15°. The cavity shapes used are: cylindrical, conical (frustum of a cone), cone-cylindrical (combination of frustum of cone and cylindrical shape), dome-cylindrical (combination of hemispherical and cylindrical shape), hetro-conical, reverse-conical (frustum of a cone in the reverse orientation) and spherical. For both cases, conical cavity yields the lowest convective loss among the cavities investigated whereas spherical cavity results in the highest convective loss. Convective heat loss from cavities of different shapes and sizes are characterized by using different internal zone areas of the cavity (Acw, Acz, Acb and Aw). Acb is found to be better parameter for characterization of the convective heat loss. Nusselt number correlation is developed using convective zone area (Acb). It correlates 91% of data within ±11% deviation, 99% of data within ±16% deviation. Radiative losses (Qrad) have been determined numerically from cavities of both cases. The ratio of Qrad/Aap is found to be more or less constant (variation within 5%) for all types of cavities and for 0 ? epsilon ? 1. Thus radiative loss is dependent on aperture area and effective emissivity of cavity rather than the shape of the cavity. Further, it also matches well with the analytical formula based on effective emissivity.