ABSTRACT

The solar cavity receiver performance is considerably influenced by wind specifications in the parabolic dish system. This paper performs experiments to study the effects of wind direction and speed on the heat losses from a bicylindrical cavity receiver at different inclinations. A range of wind speeds from 0.3 to 5.7 m/s and the three head-on, back-on, and side-on wind directions for the side-facing and downward-facing cavity situations are examined. The cavity surface temperature distribution shows a jump due to the particular shape of the cavity and the sharp corner between two cylinders. The results show that the head-on wind yields a higher combined convection heat loss in contrast to the back-on and side-on wind directions for all wind speeds and cavity inclinations except 90° (vertically downward-facing) where the effects of wind directions are the same. For the lower side-on wind speeds, increasing cavity inclination results in decreasing combined convective loss, while on the contrary the change in the combined convective loss is very small regardless of the cavity inclination at higher wind speeds above 4 m/s. Moreover, for the back-on wind, comparing to the other wind directions, the combined convective loss is lower. For all wind directions and speeds, the horizontal cavity receiver gives lower forced convective loss compared with the natural convective loss. For the downward-facing cavity, an empirical correlation has been developed for the combined Nusselt number with respect to the Reynolds number, Grashof number, cavity inclination, wind direction, and the absolute temperature ratio. The proposed correlation supports the experimental results by a deviation within ±20%. Moreover, the uncertainty analysis revealed that the relative expanded uncertainty of combined Nusselt number varies between 2.1% and 16.3%, depending upon the cavity inclination, wind speed and direction.

摘要

太阳能腔接收器性能受抛物面碟形系统中风规格的影响很大。本文通过实验研究风向和风速对不同倾角的双圆柱腔接收器热损失的影响。检查了从 0.3 到 5.7 m/s 的风速范围以及面向侧面和向下的空腔情况的三个正面、背面和侧面风向。由于型腔的特殊形状和两个圆柱体之间的尖角，型腔表面温度分布显示出跳跃。结果表明，对于所有风速和空腔倾角，除了 90°（垂直向下），风的影响与背向和侧向风向相比，迎面风产生更高的组合对流热损失方向是一样的。对于较低的侧向风速，增加空腔倾角会降低组合对流损失，而相反，在高于 4 m/s 的较高风速下，无论空腔倾角如何，组合对流损失的变化都非常小。此外，对于逆向风，与其他风向相比，组合对流损失较低。对于所有风向和风速，与自然对流损失相比，水平空腔接收器的强制对流损失更低。对于朝下的空腔，针对雷诺数、格拉肖夫数、空腔倾角、风向和绝对温度比的组合努塞尔数建立了经验相关性。所提出的相关性支持实验结果的偏差在 ±20% 以内。此外，不确定性分析表明，组合努塞尔数的相对扩展不确定性在 2.1% 和 16.3% 之间变化，具体取决于空腔倾角、风速和风向。