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On the basis of the consideration of air property variation with temperature, a three-dimensional (3-D) numerical investigation has been performed to attain insight into the cavity aspect ratio on the natural convection heat loss of a cavity receiver for a high-temperature solar dish system. Temperature and velocity contours as well as the variation curves have shown that the expansion of convection zone together with the augmentation of velocity magnitude is mainly responsible for the increase in natural convection heat loss with decreasing cavity aspect ratio, that is, shallower cavity. Moreover, the dependency of natural convection heat loss on the cavity aspect ratio is related to the tilt angle. Also, a modified definition of aperture ratio has been introduced, aiming to reflect the combined impact of the cavity aspect ratio and aperture size. Finally, two correlations of Nusselt number have been proposed by incorporating the cavity aspect ratio and the newly defined modified aperture ratio, respectively.