Heat transfer mechanism and energy efficiency of artificially roughened solar air heaters—A review

Roughness applied on a broad wall of a solar air heater significantly enhances the heat transfer to the flowing fluid with the moderate rise in fluid friction. It is imperative to select the roughness pattern and its geometrical parameters, which are responsible for the change in fluid flow behaviour steering the level of heat transfer and friction. With a view to survey the mechanism of heat transfer governed by the fluid flow pattern over the roughened wall, the distinct roughness patterns used in solar air heaters are reviewed with a fresh perspective. The interpretation of the fluid turbulence and heat transfer mechanism in case of different rib geometries has been expatiated based on available literature. Optimally usable range of Reynolds number and Temperature rise parameter for roughness geometries are proposed on the basis of effective efficiency of roughened collector.