Three dimensional simulation and performance enhancement of evacuated U–Tube solar collectors filled with nanofluid

In this paper, forced convection flow and heat transfer of Cu-water nanofluid in U-Tube collector are studied. The three-dimensional governing equations are numerically solved in the domain by the control volume approach based on the SIMPLE algorithm. Reynolds numbers are considered in laminar-turbulent range of 2000<Re<8000. The most efficient model was achieved by comparison of different parameters to reach the optimal case with the maximum exergy efficiency. From this study, it is concluded that in the case of using U-tube, instead of shell and tubes, the time that the fluid is inside the collector increases and leads to outlet temperature increase from the collector the exergy efficiency increases. Also, it is realized that enhance the outlet fluid temperature, energy efficiency and exergy efficiency. Generally, while the trend of exergy efficiency variation with effective parameters is increasing, applying the mixers precipitate the efficiency increment. In addition, for the case that the trend of exergy efficiency variation with changing these parameters is decreasing, the decreasing trend gets slow. The exergy efficiency of studied UTC at 14:00 has the maximum exergy efficiency among all studied times and about 71.54%. The received energy always reduces by increasing of operating temperature. The value of received energy at operating temperature of 23°C is about 0.347 kW/m2. The nanofluid flow has higher thermal conductivity than the base fluid and can absorb more solar irradiances. But the nanofluid has also more dynamic viscosity than base fluid which increases the pressure drop penalty and friction factor in system. Finally, the highest exergy efficiency was obtained for the nanoparticle volume fraction of ϕ=4%. Therefore, the UTC with tube diameter of 8nm filled with Cu-water nanofluid with 4% volume fraction and 40nm nanoparticle diameter at Re=2000 is introduced as the best model in present study.