ABSTRACT Heat transfer augmentation in solar air heaters (SAHs) can be achieved economically and conveniently using artificial roughness elements on the absorber surface. Cubical roughness elements used in this investigation produces better heat transfer characteristics. However, this enhancement is particularly accompanied by increased pumping power due to increased friction. This study presents the results of the experiments carried out to determine the optimum design and operating conditions which gives a better thermohydraulic performance. Efficiency index () is often used as thermohydraulic performance parameter which is evaluated using correlation = (()/()). Experiments are conducted within the limits of Reynolds number () from 3000 to 8000, relative roughness pitch () of 10 to 20 and relative roughness gap () of 4 to 8. Relative roughness height () of 0.06 and aspect ratio () of 5 is maintained constants. Substantial enhancement in the Nusselt number (59.1–72.7%) accompanied by a large increase in the friction factor (138.7–179.4%) over SAH with smooth absorber plate is achieved. Optimum thermohydraulic performance is obtained for lower values of the Reynolds number, relative roughness pitch () value 20 and relative roughness gap () of 8.

中文翻译：

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吸收体表面设置立方粗糙度的太阳能空气加热器的最佳热工水力性能

摘要 太阳能空气加热器 (SAH) 中的传热增强可以通过在吸收器表面上使用人造粗糙度元件经济而方便地实现。本研究中使用的立方粗糙度元件产生更好的传热特性。然而，由于摩擦增加，这种增强特别伴随着泵送功率的增加。本研究介绍了为确定最佳设计和操作条件而进行的实验结果，从而提供更好的热工水力性能。效率指数 () 通常用作热工水力性能参数，使用相关性 = (()/()) 进行评估。实验在雷诺数 () 为 3000 至 8000、相对粗糙度间距 () 为 10 至 20 和相对粗糙度间隙 ( ) 为 4 至 8 的范围内进行。0.06 的相对粗糙度高度 () 和 5 的纵横比 () 保持恒定。Nusselt 数显着提高（59.1-72.7%）伴随着摩擦系数的大幅增加（138.7-179.4%）比使用光滑吸收板的 SAH 实现。对于较低的雷诺数值、相对粗糙度间距 () 值为 20 和相对粗糙度间隙 () 为 8，可获得最佳热工水力性能。