The present investigation is performed outdoors to explore the thermal performance (eta(th)) of a rectangular pyramid roughened solar air heater. The experiment was conducted on the roof of the Mechanical Department, MANIT Bhopal (latitude 23.25 degrees, longitude 77.43 degrees) during February-June 2023 between 10:00 AM and 3:00 PM at a test rig kept flat on the ground using six different mass flow rates ((m) over dot) while keeping one mass flow rate fixed for the entire day. The presence of rectangular pyramid roughness induces disturbances and enhances the mixing of primary and secondary flows. Consequently, this leads to a greater improvement in eta(th). The roughness parameters varied and investigated are the relative roughness height (e/D-h) varied from 0.033 to 0.054, the relative roughness pitch (p/e) varied from 8 to 20, and two novel geometrical parameters, the relative base length to width ratio (b(L)/b(W)) and the roughness height to base length ratio (e/b(L)) are introduced and varied as 1.25-2 and 0.35-0.63, respectively. The Reynolds number (Re) is varied in the range of 3350-13,350. eta(th) is evaluated in terms of the collector heat removal factor (f(hrf)) and the collector efficiency factor (f(cef)). A maximum eta(th) of 79.8% has been reported at e/D-h = 0.047, p/e = 16, e/b(L) = 0.58, b(L)/b(W) = 1.5, and at (m) over dot = 0.0476 kg/s. Fhrf and Fcef are maximum at p/e = 16, e/D-h = 0.047, bL/b(W) = 2, and e/b(L) = 0.63. Maximum and minimum F-hrf were found to be 0.79 and 0.65, respectively Maximum and minimum F-cef were found to be 0.87 and 0.68, respectively as 0.87 and 0.68 corresponding to the optimum parameters.
