Thermo-hydraulic performance enhancement of nanofluid-based linear solar receiver tubes with forward perforated ring steps and triangular cross section; a numerical investigation

Cylindrical pipes are installed to a line-focusing solar system with a linear receiver tube for transmitting thermal energy to the working fluid. In this study, the effects of a novel forward ring step inside circular pipes on the heat transfer performance of linear solar receiver tubes were investigated using computational fluid dynamics. The rings are perforated, and their cross section is triangular. Although the applied heat flux is consistent with a solar collector with a linear receiver tube, the analysis can be performed for any given heat flux distribution on circular pipes. The model was verified by comparing the predicted Nusselt numbers to those of the Gnielinski correlation, and the numerical results were in good agreement with those of the correlation. First, the system performance was enhanced by choosing the best working fluid taking into account ethylene glycol, thermal oil, water as well as CuO-water and TiO2-water nanofluids with different volume fractions. Then, geometric parameters including the inner diameter of steps and the distance between steps were studied. Finally, thermal characteristics of the receiver with steps were compared to other enhanced structures. Results showed that using CuO-water as the working fluid led to the highest thermal performance for which, at volume fraction of 1%, the maximum Nusselt number and thermal efficiency are about 19% and 7.6% higher, respectively, compared to those of pure water. Thermal efficiency was also increased by about 3% while volume fraction of Cu-O increased from 1% to 4%. In addition, the maximum Nusselt number of the collector tube with steps occurred at d(r) = 0.2d which was 1.43 times more than that for the tube without steps. It was also found that a decrease in the distance between steps caused an increase in the Nusselt number significantly. Moreover, the highest efficiency was about 76.5% which occurred when the distance between steps was equal to the inner diameter of the tube. Finally, the best thereto-hydraulic performance of the tube occurred when the distance between steps was equal to the inner diameter of the tube (D = d), and the inner diameter of the steps was 60% of the inner diameter of the tube (dr = 0.6d) at lower Reynolds numbers while that was 80% of the inner diameter of the tube (dr = 0.8d) at higher Reynolds numbers.