Research on the Dust Suppression Effect of Three Types of Baffles on the Particle Flow Horizontal Projectile Falling Process in Transverse Airflow

. Industrial progress is the source of social development. As one of the important links in the industrial production process, industrial particulate transportation systems such as coal, cement, mineral materials and food transportation cause a large number of dust diffuse problems, which have become the pollution source of the deterioration of the environment quality in the workplace. In the production process, the initial velocity of the conveyor belt makes the particle flow form a horizontal projectile motion, which causes the dust pollution and transverse airflow often further aggravates the dust diffusion. Firstly, the author verified the applicability of Discrete Phase Model by using experimental result. Then, this paper studies the control effect of dust emission in the particles flow horizontal projectile fall process with different types baffle and different baffle positions under transverse airflow. The results show that the semicircular baffle has a better dust suppression effect, followed by the V-shaped baffle, the straight baffle is the worst. The semicircular and V-shaped baffles are moved closer to the particles source out, the better the dust suppression effect. The dust suppression effect of the straight baffle gradually improves as the distance increases, and reaches the best at a distance of L=1 m. The research results provide a theoretical basis for ventilation dust removal design of industrial particulate transport systems.


Introduction
In the process of transporting and unloading industrial particulate matter such as coal, cement and mineral material, due to the initial velocity of particulate matter, and this process is usually done outdoors, this forms a particles flow horizontal projectile fall process under the transverse airflow. Particles flow has entrainment effect on the air during the fall process. Under the influence of the transverse airflow, dust escapes from the material stream core area is key pollution source in the workplace [1].
The earliest research on the particles flow fall process can be traced back to the 1960s. Hemeon [2] pioneered a single particle model based on the particles fall in quiescent air to predict the volume of entrained air generated by the free-fall particle flow. Tooker [3] modified Hemeon's theory, and the obtained a new model to predicts the entrained air volume. It is found that the entrained air volume calculated by the new model is significantly less than the single particle model. Arnold [4] further studied the entrained air volume caused by the particles flow fall process. Studies have shown that the area of the material stream core zone is decreases with the increase of the fall height, while the influence radius of the boundary layer is increases with the increase of the fall height. When particle Reynolds * Corresponding author: sunhf1212@xtu.edu.cn number Re<500, Ogata [5,6] proposed a particles flow entrained air model. Uchiyama [7] used vortex methods to study the slip outlet particles flow free fall process. By analyzing the influence of particle diameter and density on the entrained air and particle velocity distribution, it is found that the particle velocity in the particles flow is greater than the single particle fall velocity, and the entrained air velocity at the material stream core area is the largest. Liu [8][9][10] verified the theoretical formula of entrained air volume obtained by Hemeon and Cooper. Based on the law of energy conservation, the formula of entrained air volume is derived by establishing a cylindrical coordinate system for the particles flow fall process according to the finite volume method. Ansart [11,12] established a new calculation formula in order to avoid the influence of empirical parameters on the entrained air volume in the particles flow fall process. Esmail [13,14] analyzed the characteristics of the entrained air flow field caused by the particles flow fall process based on the law of volume conservation, and found that the entrained air volume can be obtained by using the cone volume formed by the particles flow fall process over time. Li [15,16] used the π theory to analyze the factors affecting the entrained air velocity at the transfer point, and obtained the empirical formula for entrained air velocity, and then used the entrained air velocity and the diffusion area to obtain formula of entrained air volume. Sun [17][18][19][20] studied the entrained air volume during the particles flow fall process under different initial velocities and found that the initial velocity has little effect on the entrained air volume.
As mentioned above, the theoretical results of current research are mainly aimed at the particles flow free fall in quiescent air, while the horizontal projectile fall process under transverse airflow is still rare. In fact, the dust escape during the particles flow fall process is greatly affected by the transverse airflow. In view of this, the author takes the particles flow horizontal projectile fall process under transverse airflow as research object, simulating the dust diffusion and the results providing a theoretical basis for the ventilation dust removal design of industrial particulate transport systems.

Physical model
For numerical calculation of the particles flow horizontal projectile fall process under transverse airflow, an unrestricted airflow space with dimensions x × y × z=8000mm × 6000mm × 6000mm is created through ICEM CFD software. As shown in Figure 1

Boundary conditions
The initial velocity is 2m/s according to the belt running velocity. The particle diameter is 0-500μm, which is more common in industrial particles. The particle diameter is R-R distribution, the distribution index n is 2.154, and the particle average diameter is 320μm. The particles source outlet is the width of common industrial particles conveyor belt and the height of particles flow accumulation of 1000×200mm. The mass flow rate is 0.01 kg/s. The specific parameters are shown in Table 1:

Model validation
In order to verify the numerical model, the author used experimental compared to the numerical calculation results. The experimental is used the PIV system to test the characteristics of the entrained air flow field caused by the particles flow vertical fall process [19]. Then use ICEM CFD to establish the corresponding physical model, and choose the numerical calculation method used in this research to calculate the process. The simulation and experimental models are shown in Figure 2. Figure 2 The model of simulated and experimental. As shown in Figure 3, the numerical calculation entrained airflow velocity on the four sections of h=0.1m, 0.3m, 0.5m, 0.7m and experimental results are dimensionless processed. The ordinates is va/vcore. va is the entrained airflow velocity at different positions. vcore is the entrained air maximum velocity at different heights. The abscissa is (y-ycore)/Ra. y is the different positions at different heights. ycore is the particle flow core positions at different heights. Ra is the entrained air radius at different heights. It can be seen from Figure 3 that the experimental results and the numerical calculation results are in good agreement. . The particle concentration distribution is shown in Figure 4 in the X/Y direction under different V-shaped baffle positions. From particle concentration distribution we can know, when L=0.75m, the particles diffuse weakly in the X direction, followed by L=0m. In addition, as increases of the distance between the baffle and the particles source outlet, the particles diffusion becomes more serious in the X direction. when L=1m, the particles diffuse weakly in the Y direction, followed by L=0m. as increases of the distance between the baffle and the particles source outlet, the particles diffusion becomes more serious in the Y direction. It can be seen from the figure that the position of the V-shaped baffle is moved closer to the particles source outlet, the stronger the retention capacity of the particles flow core zone during the particles flow fall process. That is, the less likely it is for particles to diffuse outward. From the perspective of particle diffusion in the X/Y direction, when the V-shaped baffle L=0m, the particle diffusion is weakly, and the particle diffusion gradually increases with the increase of the distance between the baffle and particles source outlet.
The airflow velocity distribution is shown in Figure  5 in the X/Y direction under different V-shaped baffle positions. It can be seen from the airflow velocity Figure 4 Particles concentration distribution under Vshaped baffle distribution in the X direction that when L=0m the transverse airflow bypasses the V-shaped baffle, the larger the area of the low-velocity area of the air flow formed at the particles flow fall position, the more unfavorable the particles diffuse from the particles flow core area. It can be seen from the airflow velocity distribution in the Y direction that when L=0m, the airflow velocity at the particles flow core area presents an obvious Gaussian distribution. This phenomenon is similar to that of the particle flow falling in quiescent airflow. It can be seen that when L=0m, the particle flow projectile falling process is less affected by the transverse airflow. Figure 5 Airflow velocity distribution under V-shaped baffle The particle concentration distribution is shown in Figure 6 in the X/Y direction under different semicircular baffle positions. From the particle concentration distribution we can know, when L=1m, the particles diffuse weakly in the X direction, followed by L=0m. In addition, as increases of the distance between the baffle and the particles source outlet, the particles diffusion becomes more serious in the X direction. when L=0m, the particles diffuse weakly in the Y direction. As increases of the distance between the baffle and the particles source outlet, the particles diffusion becomes more serious in the Y direction. From the perspective of particle diffusion in the X/Y direction, when the semicircular baffle L=0m, the particle diffusion is weakly, and the particle diffusion gradually increases with the increase of the distance between the baffle and the particles source outlet. Figure 6 Particles concentration distribution under semicircular baffle The airflow velocity distribution is shown in Figure  7 in the X/Y direction under different semicircular baffle positions. It can be seen from the airflow velocity distribution in the X direction that when L=0.75m, the flow field formed by transverse airflow bypasses the semicircular baffle is the most stable at the back of the baffle, followed by L=0m. The more stable the flow field is, the less conducive to the particles diffuse from the particles flow core area. It can be seen from the airflow velocity distribution in the Y direction that when L=0m, the airflow velocity at the particles flow core area presents an obvious Gaussian distribution. This phenomenon is similar to that of the particle flow falling in quiescent airflow. It can be seen that when L=0m, the particle flow projectile falling process is less affected by the transverse airflow. Figure 7 Airflow velocity distribution under semicircular baffle The particle concentration distribution is shown in Figure 8 in the X/Y direction under different straight baffle positions. From the particle concentration distribution we can know, when L=1m, the particles diffuse weakly in the X direction, and the particles flow core area has the strongest retention capacity. As decreases of the distance between the baffle and the particles source outlet, the particles diffusion becomes more serious in the Y direction. From the perspective of particle diffusion in the X/Y direction, when the straight baffle L=1m, the particle diffusion is weakly, and the particle diffusion gradually decreases with the increase of the distance between the baffle and the particles source outlet.  Figure  9 in the X/Y direction under different straight baffle positions. It can be seen from the airflow velocity distribution in the X direction, when L=1m the airflow velocity is relatively small on the back side of the baffle. The smaller the airflow velocity is, the less conducive to the particles diffuse from the particles flow core area. It can be seen from the airflow velocity distribution in the Y direction that when L=1m, the flow field formed by transverse airflow bypasses the straight baffle is the most stable at the back of the baffle. The less conducive to the particles diffuse along the Y direction. In general, when L=1m, the particles flow projectile falling process is less affected by the transverse airflow.

Comparison of different baffles dust suppression effects
The particle concentration distribution in the X/Y direction of the optimal dust suppression positions of different baffles is shown in Figure 10. From the particle concentration distribution in the X direction, the Vshaped baffle has a better dust suppression effect, followed by a semicircular baffle, and the worst effect is a straight baffle. From the particle concentration distribution in the Y direction, the dust suppression effect of the semicircular baffle is significantly better than the other two baffles. Therefore, from the perspective of particle diffusion in the X/Y direction, the semicircular baffle has the best dust suppression effect.

Figure 10 Particles concentration distribution
The airflow velocity distribution in the X/Y direction of the optimal dust suppression positions of different baffles is shown in Figure 11. It can be seen from the airflow velocity distribution in the X direction, the flow field formed by transverse airflow bypasses the semicircular baffle and the V-shaped baffle are the most stable at the back of the baffle. The more stable the flow field is, the less conducive to the particles diffuse from the particles flow core area. It can be seen from the airflow velocity distribution in the Y direction, the airflow at the back of the semicircular baffle and the Vshaped baffle are Gaussian distribution. This phenomenon is similar to that of the particle flow falling in quiescent airflow. Therefore, considering the distribution of airflow velocity in the X/Y direction, the semicircular baffle has the strongest restraint effect on the transverse airflow during the particle flow projectile falling process.

Conclusion
Through the study of the control effect of dust emission in the particles flow horizontal projectile fall 1. Different types baffle are differ in the selection of the optimal position from the particles source outlet. The semicircular and V-shaped baffles are located at the position of the particles source outlet L=0m, and the dust control effect is better. With the increase of distance, the dust control effect of the semicircular baffle first sharply deteriorates. In addition, the semicircular baffle is more effective. The straight baffle gradually improves the dust control effect as the distance increases, and reaches the best at L=1m.
2. When the airflow field distribution is Gaussian, it means that the particles flow is less affected by the transverse airflow in the horizontal projectile fall process.