# Thermal Performance Analysis of a Double-Pass Solar Air Collector: A CFD Approach
## 3 Results and Discussion
### Thermal Behavior
For configuration (c), a zone from z = 0.2 m to z = 1.0 m with lower temperatures is observed, and even though its outlet is from above; it also has a small region with low temperatures near the exit. Contrastingly, configuration (d) has most of its receiver flat-plate with high temperatures, except for its final part (around z = 1.7 m to z = 2.0 m). Still, it is essential to note that the high temperatures observed for the receiver plates of Figure 9 (around 345 K) are of lesser magnitude than those observed in Figure 8 (around 370 K), and thus higher heat removal was accomplished with the application of flow 2 (0.02 kg/s). Table 5 shows the average temperatures in the flat plate receiver (\(T_{p,avg}\)) for both airflow rates. The highest temperature, as expected, is obtained with flow 1, 362.2 K with configuration (a); the lowest is obtained with flow 2, with configuration (c), of 338.5 K. The flat-plate temperature differences between flows 1 and 2 are greater than 19.7 K. \begin{table}
\begin{tabular}{l l c c} \hline  & & \multicolumn{2}{c}{**Mean Temperature, \(T_{p,avg}\) (K)**} \\ \hline  & **Configuration** & **Flow 1: 0.01 kg/s** & **Flow 2: 0.02 kg/s** \\ \hline (a) & Down–Down & 362.17 & 340.77 \\ (b) & Down–Up & 360.83 & 341.08 \\ (c) & Up–Up & 362.03 & 338.51 \\ (d) & Up–Down & 359.92 & 340.00 \\ \hline \end{tabular}
\end{table} 
Table 5: Mean temperatures in the flat plate receiver of the U-shape double-pass CPC. Figure 10 displays the first and second pass air temperature profiles for the four analyzed configurations. The blue line represents the air temperature profile in the first pass, and the red line is the profile of the second pass. In addition, the scale of the horizontal axis of the graphs (z-position) is inverted to facilitate the interpretation of the results since it allows visualizing the air outlet of the collector on the far right. In the first pass for the configurations with the inlet from the bottom, (a) Down-Down and (b) Down-Up, the temperature profile has a slight increase near the entrance (z = 0.1 m),  
Figure 10: Air temperature profiles. (**a**) Down–Down; (**b**) Down–Up; (**c**) Up–Up; (**d**) Up–Down. then it continues to increase until near the end of the cavity (z = 1.9 m) where it has another sharp increase. In the configurations that have an entrance to the cavity from the top, (c) Up-Up and (d) Up-Down, it is observed that in the first pass, there is a sudden increase in temperature in the section close to the entrance of the cavity (z = 0.1 m). In addition, all the temperature profiles are smooth for both flow rates; nevertheless, the temperature profiles have a less pronounced slope for configurations (a), (c), and (d). Furthermore, in the second pass of all configurations, the temperature profile is smooth, with a sustained increase in temperature, reaching similar outlet temperatures of 327 K for flow 1 and 313 K for flow 2. In the second pass for all the studied configurations, the temperature profile steadily increases from z = 2.0 m to z = 0.0 m, where it is observed that the temperature profiles are smooth for both flow rates. The differences in the behavior among the configurations in the second pass are only slight differences in the temperatures at the inlet and outlet, where the lowest inlet temperature is found in configuration (c) Up-Up, and the highest in (a) Down-Down. On the other hand, the configurations that have an outlet from above, (b) Down-Up and (c) Up-Up, present a higher temperature at the outlet end of the cavity (end of the first pass) than at the entrance to the duct (beginning of the second pass). The phenomenon is caused by stratification in the cavity, so the air enters the connecting elbow at a temperature lower than that shown in Figure 10 (See Table 6). This occurs with both flows but is more significant with 0.01 kg/s due to the greater air stagnation in the cavity. Table 6 shows the elbow inlet and outlet temperatures for both air flow rates.