# Research on the Influence of Rectifying Orifice Plate on the Airflow Uniformity of Exhaust Hood
## 1 Introduction  
Due to safety and indoor air quality requirements, laboratory buildings usually need to have local ventilation facilities. The push-pull ventilation system, a method of local ventilation, can effectively improve air quality and provide a safe environment for experimenters by capturing dust and toxic pollutants generated within the laboratory. This system consists of two main components: the air supply hood and the exhaust hood [1]. Research has demonstrated that an exhaust hood with parallel distributed uniform airflow can enhance the capture efficiency of dust and toxic pollutants [2]. Consequently, the study of parallel-flow push-pull ventilation systems is becoming increasingly crucial. Over time, parallel-flow push-pull ventilation systems have proven to be endowed with the advantages of effectively capturing dust and toxic pollutants [3; 4; 5; 6; 7]. As such, this type of ventilation system is being adopted more widely at present. To improve the parallel-flow push-pull ventilation system, one promising approach that has yet to be explored is to design parallel-flow exhaust hood that promotes airflow uniformity. A large number of scholars have conducted research on the principle of the uniform airflow of exhaust hoods. The research shows the geometry of exhaust hoods, such astheir flange size, pulling channel size, offset distance, etc., greatly affect the airflow characteristics [8]. For example, [PERSON] et al. found that the airflow uniformity of the exhaust hood significantly influences the effectiveness of capturing pollutants, in a study of the effect of variations in wind velocity [9] and the size [10] of the exhaust hood using the dimensionless method. In another experiment, [PERSON] et al. improved the airflow uniformity of the exhaust hood by optimizing the exhaust hood elbow [11]. Meanwhile, [PERSON] et al. studied the effect of wind velocity uniformity on the mixing characteristics of push-pull ventilation systems [12] and carried out a series of experiments on the parallel-flow outlet with a double-layer rectifying orifice and a honeycomb push-pull ventilation system [13]. Research conducted by [PERSON] et al. used CAD to establish four exhaust hood models with different structures and used Fluent (2014) to simulate and study the method of generating uniform airflow at the exhaust hood [5]. Finally, [PERSON] et al. studied the push-pull ventilation system with a double-layer rectifying orifice plate in different parameters and concluded that the double-layer rectifying orifice plate structure leads to a better capture efficiency of dust and toxic pollutants than the single-layer rectifying orifice plate structure, but they did not analyze in detail how the rectifying orifice plate should be selected [14]. From this body of work, it clearly appears that setting up a rectifying orifice plate is an effective measure to improve the airflow uniformity of the exhaust hood. Typically, two parameters are considered in the design of a rectifying orifice plate: the aperture size of the holes (abbreviated as aperture here) and the porosity of the plate (abbreviated as porosity here) of the rectifying orifice plate. However, despite the wide adoption of this solution, the influence of the parameters of rectifying orifice plates on the airflow uniformity of exhaust hoods has not been clearly analyzed or characterized. This matter, i.e., the influence of different parameters of the rectifying orifice plate of the exhaust hood on the airflow uniformity, should be promptly addressed. The use of computational fluid dynamics (CFD) methods to conduct research on push-pull ventilation systems has been demonstrated in numerous studies to be both a rational and efficient approach [2; 8; 9; 10]. More recently, this computational method has also been broadened to simulate and analyze the effects of the rectifying orifice plate on the exhaust hood [5; 12]. It has previously been shown that wind velocity non-uniformity is a key factor in evaluating the airflow uniformity of exhaust hoods [11; 15; 16].