## 4 Results
### The Impact of the Schematic of the Proposed Collector's Fins  
In this section, the influence of the collector shape on thermal efficiency is investigated numerically. Three various models are considered, and the obtained results have been compared with the simple shaped collector. The considered models here are shown in Figure 2. The variation of heat transfer coefficient versus different inlet velocity for various models is illustrated in Figure 7. Hence, it can be concluded that firstly, all the proposed models show higher heat transfer coefficient more than the simple collector (Model 1). Secondly, it is depicted that among the studied models, the maximum heat transfer coefficient belongs to Model 2 in all studied inlet velocities. Additionally, the minimum heat transfer coefficient belongs to Model 4 in all considered inlet velocities. Figure 5: Validation betwirkt the available simulation and (**a**) [PERSON]–Boelter correlation (2016); (**b**) [PERSON]–Shimazaki correlation (2017). Figure 6: Results of the validation analysis between the present numerical work and experimental work of [PERSON] et al. (2018). To realize better the effect of geometry on the thermal performance of the proposed system, the streamline with the contour of temperature for all four studied models is shown in Figure 8. Hence, it can result that the schematic of the geometry and also the attendance of the inner axial helical fins have a significant impression on the rotation flow production and finally more heat transfer rate. Furthermore, contours show that Model 4 could enhance the heat transfer rate more than the other models. Additionally, all models depict better thermal performance in comparison with the collector equipped with simple channel (Model 1) because of the presence of swirl flows generated by the helical fins. Figure 8: The streamline with contour of temperature for various models at P = 500 mm. Figure 7: Heat transfer coefficient variation with the inlet velocity for various models at P = 500 mm. The variation of pressure drop and friction factor (f) versus various inlet velocities for various models are illustrated in Figure 9a,b, respectively. Figure 9a depicts that all models have more pressure drop compare to Model 1 (collector with simple channel, without helical fins) obviously because of presence of swirl flows. The maximum pressure drop belongs to Model 2 and the Model 4 shows lowest pressure drop among these four models. Additionally, as the inlet velocities rises, the pressure drop increases due to the effect of forced convection in the channel. This trend is the same for all models. Moreover, by increasing the inlet velocities, the differences between the studied models rises. This point could be important considering the pressure drop factor, as the differences between Model 4 and Model 1 (as base model) are very low. Figure 9: (**a**) Pressure drop and (**b**) friction factor variation with the inlet velocity for various models at P = 500 mm. Figure 9b shows that the trend of variation of fraction factor among the models is the same with pressure drop (Figure 9a). However, it should be focused that the variation of friction factor with inlet velocities is not the same with pressure drop, and they are exactly the opposite. By increasing the inlet velocity, the friction factor declines. According to Equation (5), the friction factor is directly related to the pressure drop; however, the squared velocity is placed at the denominator of the equation. Contours of temperature at outlet for various models and P = 500 mm are presented in Figure 10. The impact of changing the schematic of the collector on temperature distribution in the outlet port and also the heat transfer rate is presented clearly in this figure. Accordingly, all models show better temperature distribution than the Model 1 (simple one, without fins). The best parameter to comprehensively study the impact of efficient parameters on the performance of the proposed collector is thermal efficiency (\(\eta\)), which is calculated by Equation (6).