# Thermal Performance Analysis of a Double-Pass Solar Air Collector: A CFD Approach
## 3 Results and Discussion
### Thermal Behavior  
Figures 6 and 7 show the temperature fields of the four analyzed configurations. In Figure 6, for flow 1, it is observed that there is an extended region at low temperatures for all configurations in the cavity inlet, which is related to the air inlet in the form of a jet described in Figure 4. In all configurations, a region with low temperature is generated related to the jet of cold air that enters the cavity. For the configurations with an inlet from the bottom, the zone is located in the upper left corner. This is due to the presence of the primary eddies observed in the hydrodynamic behavior of the fluid (Figures 4 and 5). In configurations with a top inlet, the area is also extended towards the middle of the cavity, which is related to less turbulence in the movement of the fluid. Figure 5: Streamlines at the collector cross-section with air mass flow 2: 0.02 kg/s. (**a**) Down–Down; (**b**) Down–Up; (**c**) Up–Up; (**d**) Up–Down. When the air enters from above, a zone of hot air is generated in the region near the entrance and another near the exit, while configurations with a bottom inlet have a heat recovery since the air enters the cavity. Figure 6: Air temperature fields at the collector cross-section with air mass flow 1: 0.01 kg/s. (**a**) Down–Down; (**b**) Down–Up; (**c**) Up–Up; (**d**) Up–Down. Figure 7: Air temperature fields at the collector cross-section with air mass flow 2: 0.02 kg/s. (**a**) Down–Down; (**b**) Down–Up; (**c**) Up–Up; (**d**) Up–Down. Figure 8 (flow 1: 0.01 kg/s) shows that when the air enters from below (configurations a and b), it removes heat from the first part of the receiver since it enters the cavity. In contrast, when the air enters from above (configurations c and d), the zones with lower temperatures are displaced from around positions z = 0.30 m to z = 0.65 m. The displacement of the presence of the lower temperature zones is a direct consequence of the formation of the main eddy, which helps to remove heat from the receiver. On the other hand, in configuration (a) Down-Down, a higher temperature of around 370 K is observed, indicating that the heat would not be uniformly removed in the first pass. This high-temperature zone is explained by contrasting with the hydrodynamic behavior observed in Figure 4, since when the air enters from the bottom, it heats up when it comes into contact with the receiver and rises, then continues its movement mainly through the upper part and then descend when looking for the exit that is in the lower part of the cavity. In contrast, configurations (b), (c), and (d) have a medium-high temperature zone in the center of the receiver. When the air leaves the collector cavity (z = 2.0 m), low-temperature zones are generated in the configurations with an outlet from below (configurations (a) and (d)). In contrast, in configurations (c) and (d), the existence of two zones of low and medium-high temperature indicates that although heat removal is heterogeneous, energy is recovered in the zone between z = 0.30 m and z = 0.8 m, which is related to the presence of a large eddy in the inlet zone surroundings. In configuration (b), two low-temperature zones are observed at the inlet and outlet of the collector and a medium-high temperature zone in the center. In Figure 9, similar temperatures but lesser magnitude can be observed, corresponding to the prevalence of high velocities due to the application of a large mass flow rate (0.02 kg/s). Low-temperature regions are observed near the inlets in the receiver plates of configurations (a) and (b), where the inlet is from below. Configurations (a) and (d), with outlets from below, have low-temperature regions near the outlets meaning that heat removal mainly occurs in those sections. On the other hand, configurations (a) and (b) have a sizeable high-temperature area in their central zone, noting that this area comprises most of the receiver plate extension.