Experimental investigation of exergetic efficiency of 3 side concave dimple roughened absorbers

Highlights: • Novel type 3 sides concave dimple roughened duct is exergetically investigated. • Detailed experimental and theoretical model has been discussed. • Exergy efficiencies of 1 and 3 sides roughened ducts are determined and compared. • Exergy efficiency is plotted against Reynolds No. and temperature rise parameter. • Optimized value of flow and roughness geometrical parameter was determined. The present work deals with evaluation of exergetic efficiency (η_II) of 3 side concave dimple roughened solar air heater duct system and the results were compared with 1 side roughened system experimentally and theoretically. Exergy analysis uses quality of useful energy i.e., what fraction of solar radiation is actually being converted into useful heat gain of air leaving beside possible losses. Experiments were performed under actual outdoor condition in open sun and relevant input data were recorded for exergy evaluation. Each day's data was recorded under different sets of parameters viz., Reynolds number (Re) 2500–13500, relative roughness pitch (p/e) 8–15, relative roughness height (e/D_h) 0.018–0.045 and dimple's depth to diameter ratio (e/d) 1–2. Variation in exergetic efficiency with Reynolds number and rise in temperature parameter (T_o-T_i)/I at varying p/e, e/D_h and e/d reveals that as Reynolds number changes, both 1 and 3 side roughness shows maximum exergetic efficiency at same Re but 1 side roughened duct shows maximum exergetic efficiency at (T_o-T_i/I) of 0.0172, 0.0125 and 0.017 for different p/e, e/D_h and e/d respectively. For 3 side roughened duct, maximum exergetic efficiency was seen at (T_o-T_i/I) of 0.023, 0.0212 and 0.022 for different p/e, e/D_h and e/d respectively.