# Research on the Influence of Rectifying Orifice Plate on the Airflow Uniformity of Exhaust Hood
## 2 Materials and Methods  
### Model Establishment, Evaluation Method, and Mesh  
Using CAD, we developed a 1:1 three-dimensional model of the exhaust hood in the push-pull ventilation system, based on similar theoretical foundations [17], to ensure meaningful comparisons between the systems are possible. As shown in Figure 1a, the characteristics of the exhaust hood are described as follows: the exhaust hood inlet length is 1200 mm, the width is 650 mm, and the exhaust hood outlet pipe diameter is 200 mm. Since the upper rectifying orifice plate of the exhaust hood usually serves as a workbench [7; 18], in order to prevent the upward diffusion of gasses containing dust and toxic pollutants from being inhaled by the experimenter, the airflow direction of the exhaust hood is from the inlet to the outlet of the exhaust hood, as indicated by the yellow arrow. The exhaust hood includes two rectifying orifice plates: the upper rectifying orifice plate is located at the inlet of the exhaust hood, with dimensions of L1 = 1200 mm and W1 = 650 mm. Meanwhile, the lower one is located inside the exhaust hood, with dimensions of L2 = 750 mm and W2 = 410 mm. For a rectifying orifice plate, the main factors affecting the airflow uniformity are the aperture (denoted by \(d\)), the porosity (denoted by \(\alpha\)), and the number of rectifying- orifice plates. Figure 1b shows the two parameters of the rectifying orifice plate, aperture and porosity. The aperture refers to the diameter of each hole on the rectifying orifice plate, and the porosity is the ratio of the total area of the holes to the area of the plate itself. For a rectifying orifice plate with a length of \(L\) and a width of \(W\), and assuming that the number of holes on the rectifying orifice plate is \(m\), then porosity \(\alpha\) can be calculated using the following Equation (1):  
\[\alpha=\frac{\text{Total area of holes}}{\text{Area of the plate}}=\frac{m\pi \left(\frac{d}{2}\right)^{2}}{L\cdot W}, \tag{1}\]  
Typically, the apertures range from 2 mm to 6 mm, and the porosity ranges from 4.91% to 62.98%. In this study, rectifying orifice plates with designated apertures of 2, 3, 4, 5, and 6 mm and porosities of 4.91%, 17.91%, 35.43%, 47.79%, and 62.98% are selected. Therefore, we studied the cases of single-layer and double-layer rectifying orifice plates with different apertures and porosities to determine which structure and characteristics yield the best rectifying effect. Since the upper rectifying orifice plate is used as a workbench [19], the area above the upper rectifying orifice plate is where dust and toxic pollutants are generated. Therefore, the wind velocity monitoring surface (parallel to the \(xy\) plane) and line (parallel to the \(x\)-axis) is set as shown in Figure 1a for simulation analysis, with a height of h = 100 mm from the upper rectifying orifice plate. The average wind velocity and wind velocity non-uniformity at the monitoring surface are used to evaluate the influence of the rectifying orifice plate on the airflow uniformity of the exhaust hood [20]. As shown in Figure 1c, assuming that the number of measuring points on the monitoring surface  
Figure 1: (**a**) Exhaust hood with double-layer rectifying orifice plate; (**b**) parameters of the rectifying orifice plate; (**c**) measuring points illustration; (**d**) overall mesh of the computational domain and the exhaust hood model; (**e**) exhaust hood mesh and face mesh at the exhaust hood outlet and the two rectifying orifice plates. is \(n\), then the formula for calculating the wind velocity non-uniformity is reported as Equation (2). \[\beta=\frac{\sqrt{\frac{\Sigma(n_{i}-\overline{n})^{2}}{n-1}}}{\overline{v}}, \tag{2}\]  
where \(\beta\) is the wind velocity non-uniformity; \(v_{i}\) is the measured wind velocity at the measuring point; \(\overline{v}\) is the average wind velocity; and \(n\) is the number of measuring points.