## 4 Results and Discussion  
### The Effect of Crosswind on Cooling Performance  
In the air-cooling tower, heat transfers from hot water to the radiators and finally to the air. The air density varies with changing temperature. Surrounding air enters the cooling tower evenly from all sides depending on the density difference. Figure 11 shows the air velocity contours with crosswind when the crosswind speed is 0 m/s, 3 m/s, 4 m/s, 5 m/s, 6 m/s, and 7 m/s. The speed field inside and outside the tower is clearly depicted; the whole space presents a symmetrical distribution, and the contour map is essentially symmetrical from left to right in the absence of crosswind.  
The air velocity near the tower wall is smallest due to the viscous force effect of the wall. When the crosswind speed gradually becomes larger, the wind velocity in the tower becomes greater at low wind speeds. The air velocity flowing out of the tower is relatively close to that of ambient wind flow passing through the outlet of the cooling tower, suggesting that the air flow rate is not substantially affected by crosswind under a low wind speed. Meanwhile, the heat transfer of the radiators in the bottom of the cooling tower is enhanced because the air velocity of the tower inlet wind speed becomes larger. As the crosswind speed continues to increase, the air flow in the cooling tower is more complex, and the flow rate is slower. In this case, the air flow rate at the outlet of the cooling tower decreases, implying that the air flow rate entering the tower is smaller. Thus, heat transfer from the bottom radiator is weakened in the cooling tower.  
Figure 11 shows that the air flow rate into the tower becomes nonuniform with the increment of wind speed and declines on both sides. The air is forced to change direction around a cylinder for radiator resistance at the upwind. Therefore, a lower temperature exists upwind for stronger convective heat transfer compared to downwind. The heat transfer efficiency of the radiators declines accordingly. These results agree with those reported earlier by [PERSON] [35]. The temperature contours at the center of the \(xy\) plane at various crosswind speeds are shown in Figure 12.  
Figure 10: Comparison of thermal efficiency coefficients from numerical simulation with those from the experiment.  
Figure 11: Air velocity contour at crosswind speeds of (**a**) 0 m/s, (**b**) 3 m/s, (**c**) 4 m/s, (**d**) 5 m/s, (**e**) 6 m/s, and (**f**) 7 m/s.  
Figure 12: Temperature contour at the center of the mid-xy plane at a wind velocity of (**a**) 0 m/s, (**b**) 3 m/s, (**c**) 4 m/s, (**d**) 5 m/s, (**e**) 6 m/s, and (**f**) 7 m/s.  
However, as the crosswind speed increases, the heat transfer rate and air flow rate into the tower increase slightly once the wind speed exceeds 6 m/s. The air flow rate and heat transfer rate obtained by numerical simulation vary with crosswind velocity as shown in Figure 13. Moreover, the thermal efficiency coefficients of the cooling tower from the numerical and experimental results are shown in Figure 10 and are used to validate the numerical model. Figure 14 shows the variation of the temperature drop with the increment of wind velocity as obtained through the experiment.  
The cross-ventilation at the bottom of the tower reaches a minimum at the velocity of 6 m/s. The worst heat transfer performance occurs at the velocity of 5 m/s through comparing the numerical results with the experimental data; the heat transfer rate is reduced by 32.4% and the air flow rate is reduced by 62.5% according to Figures 13 and 14, versus results without crosswind. Figure 14 shows that the corresponding temperature drop is the minimum, and the crosswind at 5 m/s exerts a substantial impact on the overall performance of the tower.  
Figure 14: Water temperature decline from experiment versus dimensionless velocity.  
Figure 13: Air flow rate and heat transfer rate versus wind velocity.