Heat transfer and friction factor correlation for rhombus-shaped roughness geometry on the absorber plate of solar air collector

Breaking laminar sub-layer and creating turbulence flow is the main characteristic of solar air heater to increase heat transfer. Several geometries have been employed, and their effect on heat transfer has been studied by researchers. These effects are studied in form of Nusselt number (N_u) and friction factor (F_r) rise and correlations for them are developed. In current experimental study, a novel geometry in shape of rhombus is used as roughness. Rhombus shape is selected because of its feature of v shape and converging nature on its end sides which helps to guide the flow of air and offer more secondary flow and less resistance. The geometrical and flow parameters are varied as number of elements (W/V_r), relative diagonal length (d_r/e), Reynolds number (Re), relative roughness height (e/D_h), relative roughness pitch (P/e) within range of 8–12, 6–13.33, 2500–14200, 0.0202–0.0337 and 6–14, respectively. Variation of 258–378% and 215–148% more compared to smooth plate is achieved for N_u and F_r, respectively. N_u increases when values of P/e increases and attains maxima at P/e = 10, e/D_h = 0.03375, W/V_r = 10 and d_r/e = 9.6. Experimental correlations are grounded in actual data making them applicable to real-world scenarios and practical applications. The deviation between N_u and F_r calculated from the correlation and experimentation was ± 12% and ± 10%, respectively.