Improvements of Advanced Designs of Solar Air Heaters with Roughness Elements: A Review Approach

The solar air heater (SAH) is a simple device manufactured from locally available materials and used to heat air for various purposes, including heating, drying, and industrial applications. Despite its simplicity and low manufacturing cost, it has low thermal performance. Therefore, it has undergone several modifications in an attempt to improve its thermal performance. Among these attempts is the use of roughened air-duct to create turbulent flow of air and then improves heat transfer rates. The challenge was to improve heat transfer rates while simultaneously reducing frictional properties. This review focuses on recent advances in roughened surface used for improving the performance of SAHs. Comparison between different roughened surfaces in in terms of thermohydraulic performance, friction factor (f) is also of great interest in this work. The review results recommended using perforated rectangular fins as the roughened elements as it achieves the highest thermohydraulic performance parameter (THPP = 5.1) among the several roughened surfaces considered in this review. While the highest improvement in Nusselt number was achieved (47.7 times compared to smooth-duct) when punched baffles of delta-shaped winglets were used. The frustum jet impingement roughness showed the highest improvement in f. Based on the previous conclusions, combining perforated fins, punched baffles and frustum jet impingement roughness can result in the best performance of the solar air heaters.