Design and Development of a Fluidic Barrier for Solar Cavity Receivers

Abstract

Convective heat losses from solar cavity receivers are highly non-linear and account for about 60% of total heat losses from cavity receivers. Considering the growing role of cavity receivers in concentrating solar thermal plants with a tower, minimising convective losses from these cavities enhances the thermal efficiency of the whole plant. In this project, the use of fluidic barriers to minimize convective losses for both conventional and hybrid combustion-solar receivers is assessed. Systematic experimental and numerical studies have been conducted to devise new feasible mitigation strategies while ensuring maximum solar collection efficiency. The first phase of experiments was performed on a purpose-built, electrically heated, cavity receiver, which was placed in a large wind tunnel to directly measure the convective heat losses under a variety of operating and wind conditions. The cavity was equipped with different aerodynamic barriers to control the flow across the aperture including air blowing, air suction and a combination of air blowing and air suction curtains at variable velocity, discharge angle and position. The influence of various tilt angles (𝜃 = 0°, 15°, 30° and 45°), wind speeds (uw = 0, 3, 6 and 9 m/s) and wind directions (𝛼= 0° and 45°) were examined for two fixed internal temperatures of the cavity (Tcav = 300℃ and 400℃). The experimental results are presented as a function of dimensionless numbers such as Richardson number, Reynolds number and relative momentum fluxes of curtain flow to wind flow wherever possible to provide a suitable comparison base. The commercial CFD package, ANSYS, was also utilized to conduct a computational study to further understand the flow features. The model was validated using available experimental data and has helped to confirm and complement knowledge generated from equivalent experimental campaigns. The results showed that directing a blown air curtain outward, toward the wind direction, offers tangible advantages over flow parallel to the aperture plane. It is also found that higher air curtain velocity, up to an optimum value, results in a higher effectiveness of the air curtains. The assessment of the effect of the orientation of the air curtain on the effectiveness demonstrated that for buoyancy dominant flow an upward blowing air curtain has a better performance than a downward blowing air curtain with a maximum difference of 47%. It is also revealed that a suction nozzle, mounted at the bottom of the aperture, is more effective than a blowing nozzle mounted at the top of the aperture, for the title angle of 𝜃 = 45°, while the measured effectiveness was 76% and 43% for wind speeds of 0 and 9 m/s, respectively. At a yaw angle of 45°, a remarkable difference in the effectiveness was reported for various suction nozzle configurations for a wind speed of 9 m/s, highlighting the need to activate different nozzles depending on wind direction. The combination of air blowing and air suction fluidic barrier was also assessed for a tilt angle of 45° and wind speeds of 0 and 9 m/s. It is found that this approach enhances the effectiveness by 5% over the suction only approach for wind speed of 9 m/s whereas at no wind condition it reduces the effectiveness compared with suction only approach. The second phase of the experimental study involved the use of a water tunnel and aimed at investigating the flow structure in a scaled down hybrid cavity receiver incorporating multiple internal jets, simulating fuel and air nozzles for a combustion-solar hybrid receiver. Particle Image Velocimetry, PIV, laser based technique was used to capture the instantaneous planar velocity field at multiple axial positions inside and outside the model. The effects of jet configuration, aperture size and variation of external flow velocity and direction on the internal jet decay, recirculation pattern and water ingress and egress via the aperture were recorded. The results showed high dependency of the flow structure, within the cavity, on the internal jet configuration and minor dependence on the velocity and direction of the external flow. It is also found that the mass entrainment of external flow into the cavity is significant and is strongly dependant on the aspect ratio of the aperture to the cavity, yaw angle and external flow speed. These findings helped quantify the controlling parameters and inform future strategies to mitigate the ingress of external air into the hybridised cavity receiver. It is concluded that an adaptive air curtain system should be devised so that it could sense the wind direction and wind speed and activate one or more of the curtain nozzles, induce suction or blowing, and regulate the air speed to achieve the best performance. This work provided the foundation for utilisation of a dynamic aerodynamic active flow for real world application of solar cavity receivers to improve the thermal efficiency of concentrating solar tower plants.