## 4 Results and Discussions
### Thermal Performance Results and Discussions
Moreover, the maximum ambient temperature for the same day was recorded at 30.8 \({}^{\circ}\)C. Figure 5b presents the radiation intensity and  
Figure 3: Effect of change in flow rate and inlet temperature on thermal efficiency. Figure 2: Validation of the present model against the experimental results reported by [PERSON] et al. [63]. ambient temperature distributions for a typical winter day when the maximum recorded values equaled 260 W/m\({}^{2}\) and 5.5 \({}^{\circ}\)C, respectively. Figure 6 shows the thermal properties of the base fluid and the HNFs versus temperatures at different concentrations. According to Figure 6, the thermal properties of the examined HNFs followed the trend of Therminol VP1. In detail, the thermal conductivity, viscosity, and density decreased as the inlet temperatures increased while the specific heat capacity increased linearly with the increase in inlet temperature. Moreover, the thermal conductivity, viscosity, and density were increased as concentration increased while the specific heat capacity decreased as concentration increased. Figure 4: Thermal efficiency with various inlet temperatures for different mono and hybrid nanofluids. Figure 5: Radiation intensity and ambient temperatures with time: (**a**) Summer and (**b**) winter. Based on evaluation of the thermal properties of the base fluid and HNFs under various input temperatures, their effects on the heat transfer coefficient results and the dimensionless Nusselt number, obtained using Equations (15), (18) and (19), are presented in Figure 7A,B. Figure 7A,B shows a 3D representation of the results, which reflect a similar trend for the heat transfer coefficient and dimensionless Nusselt number with an increase in inlet temperatures and concentrations. The highest heat transfer coefficient and dimensionless Nusselt number occurred at the high temperature and at high HNF concentrations, and those values were equal to 2236 W/m\({}^{2}\cdot\)K and 1427. For the sake of comparison, Figure 8 presents the enhancements that were evaluated for the heat transfer coefficient and Nusselt number. This study's maximum enhancement value was reached using a high concentration HNF volume equal to 169% for the heat transfer coefficient, and reached 138% for the Nusselt number. It is important to note that the main reason for the enhancement difference between the Nusselt number and the heat transfer coefficient is due to the definition of evaluating the heat transfer coefficient using Equation (15). Figure 9 shows the pressure drop trends of various TFs. The significant and variable effects of the different hybrid nanofluid concentrations versus the base fluid itself are clear and are basically linked to the change in density of the various concentrations. As shown in Figure 6A, the hybrid nanofluid concentration of 4% had a high density, thus it had a significant increase in the pressure drop compared with the base fluid and other concentrations. In addition, the pressure drop decreased with increasing temperatures, which was clearly visible and was linked to the dynamic viscosity decrease due to a high inlet temperature. Figure 6: Thermal properties versus temperatures for base fluid and HNFs. (**A**) Density and dynamic viscosity; (**B**) thermal conductivity and specific heat capacity. Figure 7: The 3D representation for the (**A**) heat transfer coefficient and (**B**) Nusselt number versus concentrations and temperatures. The obtained thermal and exergy efficiencies versus the variable inlet temperatures and concentration volumes of the HNF are presented under a constant volume flow rate equal to 150 L/min in Figure 10 and, respectively. Figure 10 shows the main result, that the highest thermal efficiency (74.273%) was obtained at a high concentration of HNFs and at a low temperature. This result can be attributed to the low heat losses at low temperatures. On the other hand, the maximum calculated enhancement result, which reached 0.39%, was obtained at a high temperature and a high concentration.