Empirical correlations development for heat transfer and friction factor of a solar rectangular air passage with spherical-shaped turbulence promoters

A study was conducted to enhance the thermo-hydraulic performance of a rectangular solar air passage with integrated spherical-shaped turbulence promoters. The effects of the flow and geometric parameters on heat transfer along with the friction factor were studied. The experiments were carried out with a relevant range of the Reynolds number from 4500 to 16,500, and geometric parameters consisting of the dimensionless spherical-shaped diameter oDs=DHTHORN from 0.130 to 0.217, the stream-wise spacing parameter oXs=DsTHORN from 4.04 to 6.47 and the span-wise spacing parameter oYs=DsTHORN from 4.04 to 6.47. An analysis was further carried out to develop empirical correlations for the Nusselt number and the friction factor in terms of geometric parameters and the Reynolds number. Comparisons between results of correlations and the experimental data revealed good agreements. Results showed that, at a specific Reynolds number, the Nusselt number reached a peak at Ds=DH and Ys=Ds of 0.195 and 4.62, respectively. An increase in the value of Ds=DH increases the friction factor, while an increase in oXs=DsTHORN and Ys=Ds decreased the friction factor. Maximum friction factor for Ds=DH, oXs=DsTHORN and Ys=Ds was found at 0.217, 4.04 and 4.04, respectively. In addition, to evaluate the thermo-hydraulic performance, an efficiency parameter called the efficiency factor was calculated. The maximum value of the efficiency factor was 2.98, which occurred at the Reynolds number of 10,500 corresponding to the optimal parameters Ds=DH = 0.195, Xs=Ds = 4.04, and Ys=D-s = 4.62.