Thermo‐Hydraulic Performance of Roughened Solar Air Heaters Through Roughness Geometry: A Systematic Review With Quantitative Evidence Synthesis

The solar air heaters (SAHs) are simple and economic solar thermal technologies that are widely used for space heating, agriculture, industrial drying, and low‐grade process heating purposes. However, due to low convective heat transfer coefficients of air and viscous sublayer developed on the absorber surfaces, the thermal efficiency of these systems is restricted. The artificial roughness elements can increase the rate of heat transfer because they can create an adverse pressure gradient by flow separation, reattachment, and generation of vortex and turbulence in the near wall region. In this article, a comprehensive review and quantitative data synthesis of various geometries of artificial roughness have been presented for SAH applications. Reported performance was synthesized using heat‐transfer, friction, thermo‐hydraulic, thermal‐efficiency, effective‐efficiency, and exergy‐based indicators. The evidence demonstrates that maximizing heat‐transfer enhancement alone is insufficient for geometry selection because increased turbulence and blockage can impose substantial friction and pumping‐power penalties. Arc‐shaped, staggered, gap‐based, optimized V/W‐shaped, C‐shaped, and selected hybrid configurations frequently demonstrated favorable thermal–hydraulic trade‐offs under their respective operating conditions. The review therefore emphasizes geometry selection based on combined thermal, hydraulic, and thermodynamic performance rather than on heat‐transfer enhancement alone.