# Research on the Influence of Rectifying Orifice Plate on the Airflow Uniformity of Exhaust Hood
## 4 Experiment and Discussion  
The simulation analysis of the single-layer and double-layer rectifying orifice plates reveals that the two parameters, aperture and porosity, crucially affect the airflow uniformity of the exhaust hood. Firstly, we found varying the apertures of the rectifying orifice plate from 2 mm to 6 mm considerably affects the airflow uniformity of the exhaust hood. Our results indicate that near the exhaust hood outlet, the airflow uniformity is negatively correlated with the aperture; conversely, near the exhaust hood inlet, the airflow uniformity is positively correlated with the aperture. Additionally, the impact of the aperture on the wind velocity non-uniformity is more pronounced in the rectifying orifice plate located near the exhaust hood outlet. The average wind velocity of the exhaust hood increases with the aperture. Regarding the influence of the porosity on the airflow uniformity, we achieved the best results with a porosity of 35.43%. It is noteworthy that using a higher porosity for the lower layer than for the upper layer in the double-layer rectifying orifice plate causes noticeable wind velocity fluctuations, as shown in Figure 3d. Indeed, under certain conditions of wind velocity, the porosity of the rectifying orifice plate affects the total airflow volume through a given section per unit time. Thus, when the porosity of the upper layer of the rectifying orifice plate is too small, the total airflow volume through the section reduces, which generates a vortex at the section that provokes the fluctuation of the wind velocity. Moreover, when the porosity of the upper rectifying orifice plate is set to 62.98%, the airflow uniformity of the exhaust hood is impaired. Additionally, exhaust hoods with double-layer rectifying orifice plates exhibit better wind velocity non-uniformity than those with single-layer plates, demonstrating that rectifying orifice plates effectively enhance airflow uniformity. Therefore, a model of the exhaust hood with a double-layer rectifying orifice plate was constructed, featuring an upper rectifying orifice plate with an aperture of 6 mm and a porosity of 35.43% and a lower rectifying orifice plate with an aperture of 2 mm and the same porosity. Figure 4a shows the isopleth map of the wind velocity located vertically to the monitoring surface along the center line. It can be observed that the isopleths of the wind velocity ranging from 0.5 m/s to 0.1 m/s are essentially parallel to the surface of the exhaust hood, indicating that the airflow above the exhaust hood does not diffuse horizontally. The isopleth at 0.5 m/s is located approximately 100 mm above the exhaust hood, suggesting that the wind velocity near the exhaust hood exceeds 0.5 m/s, meeting the design specifications of the exhaust hood. In order to verify that our simulation results accurately reflect the experimental device, we built an exhaust hood device with an upper rectifying orifice plate with aperture of 6 mm and a lower rectifying orifice plate with aperture of 2 mm, with a porosity of 35.43% for both plates, which were made using 2 mm thickness steel plates. The flow field within the exhaust hood was visualized by an artificially generated water mist. As shown in Figure 4b, to avoid interference caused by the initial velocity of the water mist, the mist outlet was arranged parallel to the surface of the exhaust hood. It can be observed that the horizontally released water mist was collected by the airflow and moved vertically downward, parallel to each other. This demonstrates that the airflow inside the exhaust hood was distributed in parallel, which confirms that the double-layer rectifying orifice plate structure provided excellent airflow uniformity. To further quantify the airflow uniformity of the exhaust hood, a multi-channel anemometer (1560, Kanomax, Osaka, Japan) was used to measure the wind velocity at a distance of approximately 30 mm from the surface of the exhaust hood with a high-precision wind velocity probe (0963, Kanomax, Osaka, Japan), as depicted in Figure 4c.