The numerical study of the natural convection loss occurring from cylindrical solar cavity receivers is reported in this communication. These cavity receivers can be used with solar dish concentrators for process heat applications at medium temperature levels. Three cylindrical cavity receivers of diameter 0.2, 0.3, and 0.4？m with aspect ratio equal to one and opening ratios of 1 and 0.5 are used for the analysis. Fluent CFD software is used for the analysis of the three-dimensional (3D) receiver models. In this study the receiver tubes within the cylindrical cavity are modeled as a helical coil similar to those existing in actual systems. The flow of the working fluid within the helical coil is also modeled. The simulations are performed for fluid inlet temperatures of 150°C and 250°C and for receiver inclination angles of 0 (sideways-facing cavity), 30, 45, 60, and 90 degree (vertically downward-facing receiver). It is found that the convective loss increases with increasing mean fluid temperature and decreases with, increase in receiver inclination. The convective loss is found to increase with, opening ratio. These observations are true for all cavity receivers analysed here. A Nusselt number correlation involving Rayleigh numbers, receiver inclinations, and opening ratios is proposed for the convective loss. 1. Introduction Cavity receivers are widely used along with solar dish systems for providing industrial process heat [1, 2] generating electric power [3, 4] and for thermochemical reactions [5]. The overall efficiency of such systems is dependent on the thermal losses occurring from the cavity receiver. It is observed from the literature that the convective heat transfer constitutes a major share of the thermal losses [6–8]. The convective losses from these solar cavity receivers are found to be dependent on various parameters like receiver inclination ( ), receiver wall boundary condition, aspect ratio ( ), opening ratio ( ), and external wind. This is the reason due to which analysis of convective losses from solar cavity receivers is complicated when compared to that of heat transfer due to radiation and conduction [9, 10]. Experimental and numerical investigations on natural convection losses in cavity receivers used with solar dish concentrators have been performed on different cavity shapes. Stine and McDonald [9] have performed experimental studies on conical frustum-cylindrical cavity receivers while Kugath et al. [1] has performed field studies on a similar receiver shape at medium temperature levels. Cylindrical-shaped cavity
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