In this paper, forced convection of a multiwalled carbon nanotube (MWCNT)–water nanofluid (NF) in a new flat plate solar collector (FPSC) equipped with elliptical pipes instead of circular ones is investigated. The three-dimensional conservation equations were solved in the domain with the finite volume method (FVM) based on the semi-implicit method for pressure linked equations (SIMPLE) algorithm. The laminar-turbulent range of the Reynolds number (Re) and the volume fraction of the NF (ϕ) were 50–12,000 and 0–0.1, respectively. The optimization process was accomplished through the comparison of diverse parameters to attain the optimal case with the highest exergy efficiency. In this study, it was concluded that, in the case of using elliptical pipes instead of circular tubes, the time that the fluid was inside the FPSC increased, which led to an increase in the outlet temperature, while the exergy efficiency of the FPSC increased. Additionally, it was observed that using elliptical pipes enhanced the outlet fluid temperature, energy efficiency, and exergy efficiency. Generally, while the trend of exergy efficiency variation with effective parameters was rising, applying elliptical pipes caused the efficiency to increase. In addition, the exergy efficiency variation decreased when these parameters were changed. The highest value of exergy efficiency was 7.1%. On the other hand, for each specific FPSC, there was a unique mass flow rate at which the exergy efficiency reached its maximum value, and for higher mass flow rates, the efficiency was slightly diminished and then remained unchanged. Finally, the highest exergy efficiency was achieved for ϕ = 0.10%.

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This study investigates the heat index by combining factors affecting the enhancement of heat transfer, including the modeling of three turbulators with 3D innovative geometries, hybrid nanofluid, including Cu and Al 2 O 3 , and high Reynolds numbers. Using the Eulerian method and the phase-coupled simple algorithm, a 3D tube in which turbulators with different sizes were embedded along with a hybrid nanofluid with a two-phase view were investigated. The geometry of rectangular turbulator is a = 1 cm, b = 3, 5, and 7 cm, and L = 40 cm, and geometry of twisted rectangular turbulator is like rectangular turbulator, and each part has a 5-cm length; also, the geometry of triangular turbulator is considered with b = 3, 5, and 7 cm, and L = 40 cm. In the two-phase view, because each phase can have a different velocity gradient and a different temperature gradient, the results were close to the experimental results. The results indicated that turbulators, hybrid nanofluid, and Reynolds number influenced heat transfer rate enhancement; for the rectangular turbulator, changing the turbulator size, volume fraction, and Reynolds number from the minimum values to the maximum values increased heat transfer by 49.17%. The same conditions increased heat transfer for the twisted rectangular and triangular turbulators by 44.28% and 45.19%, respectively. The proposed factors are practical and can be beneficial in heat transfer and fluid mechanics. Overall, at minimum conditions Re = 15,000 and φ = 0% up to maximum conditions Re = 25,000 and φ = 4% for rectangular, twisted rectangular, and triangular turbulators increasing heat transfer rates of 79.49%, 97.5%, and 82.45% have been observed, respectively.

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