*2.4. Measurement Error*

*2.4. Measurement Error*  The *unbalance rate* refers to the ratio of the difference between the maximum and minimum measured value to the average value of multiple measured values. In this experiment, we used a hot wire anemometer to measure the average wind speed in the air duct. We found that the average wind speed at each testing point fluctuated widely, which was not conducive to experimental measurement. Therefore, to improve and ensure the meas-The *unbalance rate* refers to the ratio of the difference between the maximum and minimum measured value to the average value of multiple measured values. In this experiment, we used a hot wire anemometer to measure the average wind speed in the air duct. We found that the average wind speed at each testing point fluctuated widely, which was not conducive to experimental measurement. Therefore, to improve and ensure the measurement accuracy of the experimental data, we acquired multiple measurements and checked the *unbalance rate*.

$$
\Delta Inbalance\,\,rate = \frac{v\_{\text{max}} - v\_{\text{min}}}{\overline{v}}\tag{5}
$$

$$
\overline{v} = \frac{1}{n} \sum\_{i=1}^{n} v\_i \tag{6}
$$

1 1 *<sup>n</sup> i i v v n* <sup>=</sup> <sup>=</sup> (6) where *v*max is the maximum measured value in the air duct, m/s; *v*min is the minimum measured value in the air duct, m/s; and *v* is the average value of the measured data in the air duct, m/s.

### where *v*max is the maximum measured value in the air duct, m/s; *v*min is the minimum meas-**3. Results**

### ured value in the air duct, m/s; and *v* is the average value of the measured data in the air *3.1. Blade Number Optimization*

checked the *unbalance rate*.

duct, m/s. **3. Results**  *3.1. Blade Number Optimization*  The number of fan blades was set to three, four, five, and six. We compared the performance of the natural ventilator with different number of axial fan blades with that of the original natural ventilator without fan blades. To avoid the influence of other factors, the radius, inclination angle, bending direction, and the position of the fan blades were unchanged. The experimental conditions are listed in Table 3.

The number of fan blades was set to three, four, five, and six. We compared the performance of the natural ventilator with different number of axial fan blades with that of the original natural ventilator without fan blades. To avoid the influence of other factors, the radius, inclination angle, bending direction, and the position of the fan blades were

unchanged. The experimental conditions are listed in Table 3.


**Table 3.** Experimental setting of optimization of number of fan blades.

*Fire* **2022**, *5*, x FOR PEER REVIEW 11 of 22

Figure 15a–c shows the ventilation speed and wind speed at different testing points in the air duct. Figure 15a–c shows the ventilation speed and wind speed at different testing points in the air duct.

**Figure 15.** Relationships between wind speed in air duct and natural ventilator speed with different blade numbers at different testing points. (**a**) Testing point 3; (**b**) Testing point 4; (**c**) Testing point 5. **Figure 15.** Relationships between wind speed in air duct and natural ventilator speed with different blade numbers at different testing points. (**a**) Testing point 3; (**b**) Testing point 4; (**c**) Testing point 5.

Figure 15a–c shows that the ventilation volume, from high to low, was found for five, six, four, and axial flow fan blades at the same speed. When the number of axial fan blades was increased, the air volume, and thus ventilation performance, generated by the natural ventilator increased first and then decreased. With five axial flow fans, the natural ventilator produced the largest air volume, and its ventilation performance was the best. Figure 15a–c shows that the ventilation volume, from high to low, was found for five, six, four, and axial flow fan blades at the same speed. When the number of axial fan blades was increased, the air volume, and thus ventilation performance, generated by the natural ventilator increased first and then decreased. With five axial flow fans, the natural ventilator produced the largest air volume, and its ventilation performance was the best.

According to the principles of fluid mechanics, when a natural ventilator works, the fluid in the duct is pushed by an axial force, which increases the energy of the fluid [37]. When an axial flow fan blade is added, it rotates with the ventilator. In addition, an axial thrust is generated, which increases the ventilation capacity of the whole natural ventila-According to the principles of fluid mechanics, when a natural ventilator works, the fluid in the duct is pushed by an axial force, which increases the energy of the fluid [37]. When an axial flow fan blade is added, it rotates with the ventilator. In addition, an axial thrust is generated, which increases the ventilation capacity of the whole natural ventilator.

tor. When axial fan blades are used, they should reduce the air collision caused by the rotation of the fan blades as the ventilation rate of the ventilator increases. To ensure the When axial fan blades are used, they should reduce the air collision caused by the rotation of the fan blades as the ventilation rate of the ventilator increases. To ensure the ventilation volume, the fan speed of the ventilator must be increased, which increases

ventilation volume, the fan speed of the ventilator must be increased, which increases

energy consumption. After adding three fan blades, the maximum ventilation volume was not obtained in the experiment (as shown in Figure 15c, the maximum wind speed was 0.525 m/s with three blades). However, the addition of five fan blades resulted in the maximum experimental ventilation volume due to the moderate number of blades generating the minimum friction loss in air at a relatively low speed. The air entering the blade could gain more kinetic energy due to the lesser friction, resulting in a higher ventilation rate. energy consumption. After adding three fan blades, the maximum ventilation volume was not obtained in the experiment (as shown in Figure 15c, the maximum wind speed was 0.525 m/s with three blades). However, the addition of five fan blades resulted in the maximum experimental ventilation volume due to the moderate number of blades generating the minimum friction loss in air at a relatively low speed. The air entering the blade could gain more kinetic energy due to the lesser friction, resulting in a higher ventilation rate.
