Flow structure and convective heat transfer in a bladed structure under wind conditions

• Published in: The International journal of heat and fluid flow Vol. 85; p. 108676
• Main Authors: Ghanadi, Farzin, Torres, Juan F., Arjomandi, Maziar, Pye, John
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.10.2020
• Subjects: Convective heat transfer; PIV measurements; Solar receiver; Wind tunnel; Convective heat transfer; Solar receiver; Wind tunnel; PIV measurements
• ISSN: 0142-727X; 1879-2278
• DOI: 10.1016/j.ijheatfluidflow.2020.108676

Bladed receivers have been proposed as an alternative to conventional external receivers for high-temperature concentrating solar power systems. Instead of flat banks of tubes arranged on the receiver surface, the tube-banks are oriented into the form of blades protruding from the back-wall of the receiver. This modified arrangement improves light-trapping, without the size and cost implications of a large cavity receiver, and also acts to trap hot air between the blades, hence reducing convective transfer per-tube-area. The present paper investigates the convective heat transfer from a scaled model of a bladed receiver and its correlation with the flow behavior between the blades. Wind tunnel measurements were undertaken to investigate the effects of the wind speed, the blade length and the receiver orientation on the convective heat transfer. It was observed that increasing the pitch angle from 26° to 70°, measured downwards from the vertical in a headwind configuration, leads to a significant reduction in the convective heat transfer coefficient of up to 60%, for the investigated blade lengths. To identify the effect of flow structure on the rate of the convective heat transfer at low Richardson umbers, a second experimental study with isothermal conditions was conducted in a water channel matching the Reynolds number of the wind tunnel experiment. The mean velocity fields and the associate streamline topologies within an open cavity with different pitch angles and blade length to spacing ratios (RBS) were also investigated through Particle Image Velocimetry (PIV) measurements. The streamlines revealed that the shear layer at a pitch angle of 70° for the receiver with the longest blade,RBS = 3, bridges the cavity opening without a strong interaction with the counter-rotating vortices inside the cavity, which resulted in an enlargement of the stagnation zone close to the cavity back-wall. This stagnation zone was identified to be the cause of the convective heat transfer reduction observed in the earlier wind tunnel experiments. It was also found that the Reynolds number has a strong impact on the heat transfer rate, as increasing the wind speed from 3 m/s to 6 m/s enhanced the heat transfer coefficient by up to 50%. As the corresponding Reynolds number in the water channel increased from 1.4 × 104 to 2.8 × 104, PIV data showed that the shear layer was drawn into the cavity and consequently, the velocity magnitude near the back-wall of the cavity reached up to 90% of the freestream velocity. The results provide a good understanding of flow behavior in the vicinity of the blades and its impacts on the convective heat transfer from the receiver.