## 4 Results and Discussions
### Numerical Results  
The results of numerical study are given in this section. In Figures 6 and 7, a YZ plane is created 2 cm above the obstacles. Velocities of different absorber surfaces are given in Figure 6. The velocity streamlines are given in Figure 7. The velocity contours of the complex absorber surface (C1), the less complex absorber surface (C2), and the smooth absorber surface (C3) are respectively presented in the subfigures of Figure 6a-c. When the C1 type collector was used, the maximum speed was obtained as 0.19 m/s, while when the C2 type collector was used, the air speed was determined as 0.186 m/s. This shows that the air velocity is slightly higher on the more complex surface compared to the less complex surface. This is because elements that add more complexity increase the turbulence value. Obstacles to the flow increase the velocity by causing the formation of vortices in the flow. The collector type without any elements on it, namely C3, has the lowest velocity distribution on the surface, which is already an expected situation. It  
Figure 6: Velocity contours on each absorber surface; (**a**) C1, (**b**) C2, (**c**) C3. Figure 7: Velocity streamlines on each absorber surface; (**a**) C1, (**b**) C2, (**c**) C3. can be concluded that adding surface-enhancing elements causes a nearly 35% increase in air velocity. The temperature values of the three absorber surfaces, C1, C2, and C3, are respectively shown in Figure 8a-c. As can be clearly seen in the figure, the maximum temperature is the same in all three cases. However, the collector area where the maximum temperature is effective is largest in C1 and smallest in C3. In other words, the temperature effects appear towards the exit of the channel for C3 type collector, but appear at the center parts of the channel for C1 and C2 types. This is an indication that the stainless steel scouers not only create turbulence, but also affects temperature distribution. It is also worthy to note that, especially on the complex surface (C1), the temperature in the center region takes the shape of a zigzag and shows a change that is almost proportional to the shape of the array. The efficiency of the collectors depends on the structure of the solar air heater and the solar irradiance value. The material, shape, and dimensions of the collector affect the performance improvement. Surface-enhancing elements such as fins, obstacles, curved surfaces, and the different shapes given in the literature when placed on the absorber surface increase the temperature and thus increase the efficiency, because they increase the heat transfer area [2,3]. The number and arrangement style of the stainless-steel scouers placed above and below the absorbing surface creates turbulent flow. This provides the highest heat transfer coefficient, the highest absorber temperature, and reduced thermal heat loss. Therefore, the highest efficiency is achieved when C1 is used. On the other hand, C3 has less efficiency than the other collectors. This is because this collector has a flat absorbing surface, and the heat transfer area is smaller than the others. The temperature contours of the glass surface of the tested surfaces C1, C2, and C3 are respectively presented in Figure 9a-c. Note that the glass surface started to cool earlier on complex surfaces. It cannot be said that the turbulence-enhancing elements on the surface have much effect on the glass surface temperature. However, when you look at the figures carefully, it can be seen that the cooling occurs more uniformly on the C1 surface compared to the other C2 and C3 surfaces. The velocity and temperature data were collected while considering that a path (line) passes through the center of the channel at a height of 30.4 cm from the bottom and 10.4 cm from the absorber surfaces. The average velocities on that path are given in Figure 10a and  
Figure 8: Temperature contours on each absorber surface; (a) C1, (b) C2, (c) C3. the average temperatures are given in Figure 10b.