*3.2. Blade Inclination Angle Optimization*

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The ventilation performance of the ventilator was the best with five axial flow fans. However, when an axial flow fan works with a natural ventilator, the flow distribution on the rotating plane of the fan blade is not uniform due to the blade inclination angle, which reduces the effect of a ventilator. By comparing the changes in the air volume of the natural ventilator for different fan blade inclination angles, we determined the optimal inclination configuration of the fan blades; the working conditions are shown in Table 4. *3.2. Blade Inclination Angle Optimization*  The ventilation performance of the ventilator was the best with five axial flow fans. However, when an axial flow fan works with a natural ventilator, the flow distribution on the rotating plane of the fan blade is not uniform due to the blade inclination angle, which reduces the effect of a ventilator. By comparing the changes in the air volume of the natural ventilator for different fan blade inclination angles, we determined the optimal incli-

nation configuration of the fan blades; the working conditions are shown in Table 4.


**Table 4.** Experimental setting for the optimization of blade inclination angle.

The inclination angles of fan blades used in experiments were 5◦ , 15◦ , 20◦ , 25◦ , and 30◦ , as shown in Figure 16. The inclination angles of fan blades used in experiments were 5°, 15°, 20°, 25°, and 30°, as shown in Figure 16.

**Figure 16.** Different blade inclination angles used in the experiment. **Figure 16.** Different blade inclination angles used in the experiment.

Figure 17 shows that when other system parameters of the natural ventilator remained unchanged, the inclination angle of the fan blade and the ventilation rate of the ventilator did not show an linear increasing relationship. When the axial fan blade inclination angle was 25°, the air volume produced by natural ventilator was the largest; the Figure 17 shows that when other system parameters of the natural ventilator remained unchanged, the inclination angle of the fan blade and the ventilation rate of the ventilator did not show an linear increasing relationship. When the axial fan blade inclination angle was 25◦ , the air volume produced by natural ventilator was the largest; the lowest was produced when the inclination angle of fan blade was 5◦ .

lowest was produced when the inclination angle of fan blade was 5°.

**Figure 17.** Relationships between wind speed in the air duct and natural ventilator speed with different blade inclination angles at different testing points. (**a**) Testing point 3; (**b**) Testing point 4; (**c**) Testing point 5. **Figure 17.** Relationships between wind speed in the air duct and natural ventilator speed with different blade inclination angles at different testing points. (**a**) Testing point 3; (**b**) Testing point 4; (**c**) Testing point 5.

With the increase in the fan blade angle, the pressure difference between the upper and lower surfaces of the fan gradually increased, which enhanced the air disturbance around the fan blade. The ventilation rate was not the maximum when the blade angle was 30° because the pressure between the upper and lower surfaces of the axial flow fan is too large when the fan blade angle is too large. Some friction occurs between the fan blade and the air, and part of the energy is lost. The formation of a flow field on the rotating blade surface is caused by air inflow. When the air collides with the blade surface, part of a secondary vortex may be produced, which reduces the air volume produced by the ventilator and reduces ventilation performance. Therefore, we determined that the optimal blade inclination is 25°. With the increase in the fan blade angle, the pressure difference between the upper and lower surfaces of the fan gradually increased, which enhanced the air disturbance around the fan blade. The ventilation rate was not the maximum when the blade angle was 30◦ because the pressure between the upper and lower surfaces of the axial flow fan is too large when the fan blade angle is too large. Some friction occurs between the fan blade and the air, and part of the energy is lost. The formation of a flow field on the rotating blade surface is caused by air inflow. When the air collides with the blade surface, part of a secondary vortex may be produced, which reduces the air volume produced by the ventilator and reduces ventilation performance. Therefore, we determined that the optimal blade inclination is 25◦ .

### *3.3. Blade Bending Direction Optimization 3.3. Blade Bending Direction Optimization*

Combined with the analysis of the previous experimental results, we found that the performance of the ventilator was the best with five axial flow fans and a 25° angle on the fan blades. Then, we studied the influence of the bending direction of the axial fan blade on the ventilation effect. Axial flow blades' bending direction can be divided into forward and backward curved blades. The rotation direction of the forward curved blade is the same as that of the ventilator, whereas that of the backward curved blade is opposite to that of the ventilator. The experimental conditions are shown in Table 5. Combined with the analysis of the previous experimental results, we found that the performance of the ventilator was the best with five axial flow fans and a 25◦ angle on the fan blades. Then, we studied the influence of the bending direction of the axial fan blade on the ventilation effect. Axial flow blades' bending direction can be divided into forward and backward curved blades. The rotation direction of the forward curved blade is the same as that of the ventilator, whereas that of the backward curved blade is opposite to that of the ventilator. The experimental conditions are shown in Table 5.


**Table 5.** Experimental setting of optimization of blade bending directions.

According to Figure 18, we found the flow velocity in the air duct changed with the ventilator speed under different working conditions. Under the same rotating speed, the air volume of the ventilator with forward curved blades was significantly larger than that with the backward curved blade, indicating a better ventilation effect. According to Figure 18, we found the flow velocity in the air duct changed with the ventilator speed under different working conditions. Under the same rotating speed, the air volume of the ventilator with forward curved blades was significantly larger than that with the backward curved blade, indicating a better ventilation effect.

**Figure 18.** Relationships between wind speed in the air duct and natural ventilator speed with different blade bending directions at different testing points. (**a**) Testing point 3; (**b**) Testing point 4; (**c**) Testing point 5. **Figure 18.** Relationships between wind speed in the air duct and natural ventilator speed with different blade bending directions at different testing points. (**a**) Testing point 3; (**b**) Testing point 4; (**c**) Testing point 5.

When the axial fan blade rotates with a natural ventilator, the forward blade of the axial fan blade contacts the air in the duct. Forward curved blades have a relatively short leading edge within a streamlined fan section, which reduces air drag. The rotating blade strikes the air, with relatively little kinetic energy lost to the fluid. The torsion angle of the back edge of a forward curved fan blade within a wind duct is significantly larger than that of the forward edge, so relatively high wind pressure can be generated. Therefore, this type of fan blade shape reduces the energy loss when a natural ventilator is rotating and improves its ventilation performance. When the axial fan blade rotates with a natural ventilator, the forward blade of the axial fan blade contacts the air in the duct. Forward curved blades have a relatively short leading edge within a streamlined fan section, which reduces air drag. The rotating blade strikes the air, with relatively little kinetic energy lost to the fluid. The torsion angle of the back edge of a forward curved fan blade within a wind duct is significantly larger than that of the forward edge, so relatively high wind pressure can be generated. Therefore, this type of fan blade shape reduces the energy loss when a natural ventilator is rotating and improves its ventilation performance.

### *3.4. Relationship between Wind and Ventilation Speeds 3.4. Relationship between Wind and Ventilation Speeds*

Based on the above experimental analysis, we found that the ventilation effect of the natural ventilator was the best when forward curved axial flow fans are used with a blade inclination of 25° and five blades. Therefore, the experiment conditions were set as described in Table 6. Based on the above experimental analysis, we found that the ventilation effect of the natural ventilator was the best when forward curved axial flow fans are used with a blade inclination of 25◦ and five blades. Therefore, the experiment conditions were set as described in Table 6.


As shown in Figure 19, we found the speed of the original ventilator increased with

increasing ambient wind speed. When the original ventilator rotated with the rated speed

**Table 6.** Comparison of fan blade settings between the original and optimized ventilators. **Table 6.** Comparison of fan blade settings between the original and optimized ventilators. **Table 6.** Comparison of fan blade settings between the original and optimized ventilators. **Condition Number Number of Blades Number of Blades Bending Direction** 

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As shown in Figure 19, we found the speed of the original ventilator increased with increasing ambient wind speed. When the original ventilator rotated with the rated speed of 60 rpm, the wind speed with the fan outlet was 5.18 m/s. However, the same wind speed applied to the optimized ventilator caused rotation at 57.06 rpm. Therefore, compared to the original ventilator, the rotation speed of optimized ventilator was reduced by about 3 rpm. increasing ambient wind speed. When the original ventilator rotated with the rated speed of 60 rpm, the wind speed with the fan outlet was 5.18 m/s. However, the same wind speed applied to the optimized ventilator caused rotation at 57.06 rpm. Therefore, compared to the original ventilator, the rotation speed of optimized ventilator was reduced by about 3 rpm. of 60 rpm, the wind speed with the fan outlet was 5.18 m/s. However, the same wind speed applied to the optimized ventilator caused rotation at 57.06 rpm. Therefore, compared to the original ventilator, the rotation speed of optimized ventilator was reduced by about 3 rpm.

**Figure 19.** Relationships between wind speed in the air duct and natural ventilator speed before and after optimization. **Figure 19.** Relationships between wind speed in the air duct and natural ventilator speed before and after optimization. after optimization.

As mentioned above, when a ventilator works, it can be approximately regarded as a fan system, and the speed of the ventilator is proportional to the environmental wind speed. In the experiment, when fan blades were added, the overall approximation could be regarded as a series superposition of two fan systems. Because the added fan blades are small, the natural ventilator consumes little energy for rotation. When the same wind speed acts on the ventilator, the speed of the ventilator decreases, but the curved slope of ventilator speed with the change in ambient wind speed remains unchanged. As mentioned above, when a ventilator works, it can be approximately regarded as a fan system, and the speed of the ventilator is proportional to the environmental wind speed. In the experiment, when fan blades were added, the overall approximation could be regarded as a series superposition of two fan systems. Because the added fan blades are small, the natural ventilator consumes little energy for rotation. When the same wind speed acts on the ventilator, the speed of the ventilator decreases, but the curved slope of ventilator speed with the change in ambient wind speed remains unchanged. As mentioned above, when a ventilator works, it can be approximately regarded as a fan system, and the speed of the ventilator is proportional to the environmental wind speed. In the experiment, when fan blades were added, the overall approximation could be regarded as a series superposition of two fan systems. Because the added fan blades are small, the natural ventilator consumes little energy for rotation. When the same wind speed acts on the ventilator, the speed of the ventilator decreases, but the curved slope of ventilator speed with the change in ambient wind speed remains unchanged. Figure 20 compares the relationships between wind speed in the air duct and natural

Figure 20 compares the relationships between wind speed in the air duct and natural ventilator speed before and after optimization. Figure 20 compares the relationships between wind speed in the air duct and natural ventilator speed before and after optimization. ventilator speed before and after optimization.

**Figure 20.** *Cont*.

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**Figure 20.** Relationships between wind speed in the air duct and natural ventilator speed before and after optimization at different testing points. (**a**) Testing point 3; (**b**) Testing point 4; (**c**) Testing point 5. **Figure 20.** Relationships between wind speed in the air duct and natural ventilator speed before and after optimization at different testing points. (**a**) Testing point 3; (**b**) Testing point 4; (**c**) Testing point 5.

According to the above experimental results, compared to the original natural ventilator, the ventilation capacity of the newly designed natural ventilator is higher. For example, at point 3, when the original natural ventilator works at 60 rpm, the wind speed into the duct is 0.33 m/s. When the same energy is consumed, the rated speed of the newly designed ventilator is 55.5 rpm and the wind speed in the duct is 0.37 m/s. According to the above experimental results, compared to the original natural ventilator, the ventilation capacity of the newly designed natural ventilator is higher. For example, at point 3, when the original natural ventilator works at 60 rpm, the wind speed into the duct is 0.33 m/s. When the same energy is consumed, the rated speed of the newly designed ventilator is 55.5 rpm and the wind speed in the duct is 0.37 m/s.
