Thermal-hydraulic characteristics and exergy performance in tube-on-sheet flat plate solar collectors: Effects of nanofluids and mixed convection

Flat plate solar collector (FPSC) is one of the most popular equipment among solar energy systems which can be utilized for heating of domestic or public buildings where the demands for hot water are quite indispensable. Although they have a number of benefits such as no need for sun tracking and low maintenance cost, their low thermal performance is considered as a hindrance in their extensive development. In this paper, a three-dimensional inclined tube-on-sheet flat plate solar collector is numerically modeled. The FPSC system is deemed to work under conjugated laminar mixed convection heat transfer mechanism while the operational fluid is selected to be Al2O3/water nanofluid. The influence of several parameters on the performance of the present FPSC, namely inlet alumina/water nanofluid temperature, and volume concentration, heat flux absorbed by the absorber plate, dimensionless numbers consist of Reynolds (Re), Grashof (Gr), Prandtl (Pr), and Richardson (Ri) are discussed on the heat transfer, fluid flow, and entropy generation. Furthermore, the effects of friction factor inside the riser as well as the pressure drop are taken into the account in this research. Results revealed that increasing the volume fraction of nanofluid at a fixed Reynolds number declines the outlet temperature whereas lowering the Reynolds number at a same volume fraction gives rise to higher outlet temperature. From heat transfer standpoint, it is observed that in a constant Reynolds number by increasing nanoparticle volume fraction, the Nusselt number falls while the heat transfer coefficient rises. In addition, rising the Reynolds number eventuates in friction factor reduction while the pressure drop augments. Moreover, from the obtained outcomes, it is figured out that generally using alumina/water nanofluid provides lower total entropy generation, in comparison with the water. Simulation data reveal that alteration in the heat flux does not have any sensible effects on the friction factor.