Experimental Investigation of Natural Convection Heat Loss From a Model Solar Concentrator Cavity Receiver

Natural convection heat loss inevitably occurs in cavity-type receivers in high concentrating solar dishes, downward focusing systems and solar towers. In most applications, it can contribute a significant fraction of total energy loss, and hence it is an important determining factor in system performance. To investigate natural convection losses from cavity type receivers, an electrically heated model receiver, was tested at inclinations varying from −90 deg (cavity facing up) to 90 deg (cavity facing straight down), with test temperatures ranging from 450 to 650 deg C. Ratios of the aperture diameter to cavity diameter of 0.5, 0.6, 0.75, 0.85 and 1.0, were used. In addition to measurements of overall heat loss, the Synthetic Schlieren technique was used to visualize the flow pattern out of the cavity. Numerical modeling of the convection losses from the cavity was carried out for positive angles with the commercial computational fluid dynamics software package, Fluent 6.0. Good agreement was found between the numerical flow patterns at the aperture region with the schlieren images and between measured and predicted values for heat loss. Of the previously published work that has been reviewed, a model proposed by Clausing, A. M., 1981, “An Analysis of Convective Losses from Cavity Solar Central Receivers,” Sol. Energy 27 (4) pp. 295–300 shows the closest prediction to both numerical and experimental results for downward facing cavities despite its original use for bigger-scale central receivers.