Modeling of convective heat loss from a cavity receiver coupled to a dish concentrator

Abstract

The more heat losses from the cavity are associated with the higher temperature achieved inside the cavity receiver of a parabolic dish system. The heat loss from the cavity receiver takes place by all three modes, but the determination of convection heat losses are the difficult task due to complexity of flow and temperature profile near the cavity. Due to diverse wind effects in real scenario, the increase in the convective losses significantly impact the overall performance of the parabolic dish systems. Numerous studies have investigated the heat loss from various receiver geometries under natural and forced convection conditions. However there is a marked absence of studies that take into account the effect that the dish may have on the heat loss; the interaction between wind and the dish structure can affect the local air speed at the cavity inlet and thus the heat loss as well. This work aimed to investigate the heat losses from the coupled dish-cavity receiver system particularly for forced convection conditions. Numerical simulation were performed using the ANSYS CFX package to estimate the convective heat losses from the cavity receiver at various wind directions and dish-cavity orientation. The heat losses were assessed considering wind velocities from 0 to 20 m/s, together with a range of wind incidence angles and dish tilt angles. The results showed that the orientation and position of the parabolic dish has a significant effect on the heat loss from the cavity receiver. The dish structure reduced the heat loss from the cavity at most of the working orientations except in the case of flow parallel to the aperture of the dish. A critical velocity was perceived to separate the dominance of mixed/forced convection over natural convection. At lower velocities than the critical velocity, natural convection was found to be dominant. But due to orientation and tilt angle, the dish structure provided a significant impact on the forced convection. Nusselt number correlations is proposed to calculate the natural convection and combined convection from the coupled dish-cavity system. A relatively less strength of wind incidence angle to the tilt angle was observed in the presence of the dish structure in the flow field.