Abstract 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 ( R B S ) 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, R B S = 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.

中文翻译：

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风力条件下叶片结构的流动结构和对流传热

摘要 叶片接收器已被提议作为用于高温聚光太阳能系统的传统外部接收器的替代方案。代替布置在接收器表面上的扁平管束，管束被定向成从接收器的后壁突出的叶片的形式。这种改进的布置改善了光捕获，而没有大腔接收器的尺寸和成本影响，并且还用于在叶片之间捕获热空气，从而减少每管区域的对流传递。本论文研究了叶片接收器的比例模型的对流传热及其与叶片之间流动行为的相关性。进行风洞测量以研究风速的影响，叶片长度和对流传热的接收器方向。据观察，对于所研究的叶片长度，将桨距角从 26° 增加到 70°，在逆风配置中从垂直向下测量，导致对流传热系数显着降低高达 60%。为了确定低理查森数下流动结构对对流传热速率的影响，在与风洞实验雷诺数匹配的水道中进行了第二次等温条件下的实验研究。还通过粒子图像测速 (PIV) 测量研究了具有不同桨距角和叶片长度与间距比 (RBS) 的开腔内的平均速度场和相关流线拓扑。流线表明，对于具有最长叶片 RBS = 3 的接收器，俯仰角为 70° 的剪切层桥接了空腔开口，而与空腔内的反向旋转涡流没有强烈的相互作用，这导致了靠近腔体后壁的停滞区。在早期风洞实验中观察到的对流热传递减少的原因被确定为该停滞区。还发现雷诺数对传热速率有很大影响，因为将风速从 3 m/s 增加到 6 m/s，传热系数提高了 50%。随着水道中相应的雷诺数从 1.4 × 104 增加到 2.8 × 104，PIV 数据显示剪切层被拉入腔体，因此，腔体后壁附近的速度大小达到自由流速度的 90%。结果提供了对叶片附近流动行为及其对来自接收器的对流传热的影响的良好理解。