Experimental analysis of free convection heat loss in a bicylindrical cavity receiver

Convection heat loss from solar cavity receivers in parabolic dish concentrators significantly reduces the performance of the system. In this paper, steady-state experiments were conducted to investigate heat losses from an electrically heated bicylindrical cavity receiver, which consists of two coaxial cylinders of diameters 0.06 and 0.09m with an aspect ratio (length to diameter) of one. The cavity surface was exposed to constant heat fluxes from 785 W/m(2) to 1770 W/m(2) at different inclination angles from -90 degrees (vertically upward-facing) to +90 degrees (vertically downward-facing). The temperature distribution of the cavity surface depicts that due to the presence of a sharp corner between the two cylinders, the flow instability intensifies, and the temperature of the top surface rises. The results showed that the maximum convective loss occurs approximately at -30 degrees that corresponds to 63% of the total heat loss while the minimum convective loss occurs at +90 degrees, which accounts for about 19% of the total heat loss. A convective heat loss relation was developed for cavity surface temperatures up to 300 degrees C and used to estimate convective losses at higher temperatures. Flow visualization experiments were also performed to investigate heat losses from the cavity receiver. It was observed that increasing the inclination angle from 45 degrees to 60 degrees results in a decrease in the induced buoyant force. Empirical correlations were also developed for Nusselt number as a function of Rayleigh number, inclination angle, and the ratio of cavity surface temperature to the ambient temperature for both upward and downward-facing situations. Comparing correlated Nusselt numbers with experimental results for upward-facing positions shows +/- 5% deviation, while for downward-facing inclinations, the correlation predicts the experimental data by a deviation within +/- 12%. Moreover, a comparison of experimentally measured convective losses with literature shows discrepancy at lower inclinations but depicts good agreement when the inclination angle increases. For the vertically downward-facing situation, the experimental data shows a close fit where the effect of aperture size becomes negligible. The analyses also revealed that the relative combined standard uncertainty of convective Nusselt number varies between 2% and 14%, depending upon the cavity inclination and the heat flux applied.