Experimental investigation of relative roughness height effect in solar air collector with convex dimples

This study focuses on the effect of dimple depth on improving thermal performance in a solar air collector (SAC) with huge convex dimples placed staggered on an absorber. In this regard, convex dimple absorbers with two different types of relative roughness heights, δ/D = 0.16 and δ/D = 0.32, and a flat plate absorber were tested for back-pass and front-pass applications at air mass flow rates of 0.013, 0.027, and 0.036 kg/s. The highest average energy efficiency of 37.9% and 53.5% were achieved for dimpled absorber plates in back-pass and front-pass, respectively, with δ/D = 0.32 at an air mass flow rate of 0.036 kg/s. The exergy efficiency ranged from 8.1% to 12.4% for the back-pass application, whereas it fluctuated from 14.6 to 19.7 in the front-pass application. In comparing δ/D = 0.32 with the flat plate in the back-pass application, Nud/Nuf varied from 1.2 to 1.48 with an increase in Re, while in the front-pass Nud/Nuf changed from 1.13 to 1.37. In the back-pass, Nud/Nuf/(fd/ff) achieved 1.24 in favor of δ/D = 0.32 at Re = 4000, on the other hand, it reached 1.15 in the front-pass. Similarly, the highest Colburn factor values in all Re numbers were obtained in the case of relative roughness height of δ/D = 0.32. As a result, a relative roughness height equal to 0.32 is the optimal geometry for this investigation of a dimpled collector. Accordingly, the results showed that SACs with convex dimples are more successful than the flat plates, thanks to the surface area expansion and turbulence generating ability, which significantly increases the thermal efficiency. This study also recommends the use of a front-pass application.