1 Introduction
At present, the global environment is deteriorating, especially in the air environment closely related to human beings, which has seriously jeopardized human health. At the same time, certain key parts of modern high technology require extremely high levels of environmental purification. Therefore, modern society needs an air filter with advantages such as high efficiency and low resistance. Although nanomaterial photocatalysis technology is currently the most promising indoor air purification technology, it cannot purify suspended matter and fine particles in the air. The fiber filtration technology can effectively improve the performance of the conventional filter, prevent the fine particles from entering the room with the fresh air of the air conditioning system, and if the nano photocatalytic technology (such as surface spraying some nano TiO2) is comprehensively used, the toluene and formaldehyde can be effectively suppressed. The main indoor pollutants such as ammonia, volatile organic compounds and microorganisms pose a threat to people's health.
German scholars Albrecht and Kaufmann were the first to conduct a preliminary study on filters, and they explored the theoretical discussion of aerosol filtration mechanisms for fiber mats. Since then, many scholars in the world have carried out experiments and numerical studies on the performance of filters, but most of these studies are based on the two-dimensional filter model. However, in order to find out the relationship between filter structure and filter media filtration performance, these studies are not enough. Based on the above research, the numerical simulation of the internal gas phase flow field of three-dimensional staggered air filter is carried out to calculate the resistance of the filter and the resistance coefficient of the fiber under different operating conditions, in order to make some beneficial effects on optimizing the air filter structure. explore.
2. Empirical model of air filter resistance
At low flow rates and small Reynolds numbers, the resistance distribution at both ends of the porous medium obeys Darcy's law, where K is the Darcy constant:
3. Numerical calculation model
Figure 1 shows the calculation model and boundary conditions in this paper. It is assumed that air enters the calculation area as a velocity inlet and leaves the calculation area at the pressure outlet boundary condition. The length of the inlet of the calculation zone is L from the first row of the filter. In order to ensure the full development of the gas flow in the filter pipe, the calculation takes 10 df. For the boundary of the calculation area, the symmetry boundary condition is used in this paper. No slip boundary conditions are used at the fiber surface. In this paper, we calculate the unit shape of the mesh used to select Hex/wedge, and the meshing method selects Cooper, and the number of meshes is 386035. The residual is set to 10-5 when calculating.
4. Results and analysis
4.1 Flow field analysis
Figure 2 shows the flow field distribution of a section in the filter. It can be seen from the figure that when air flows through the fiber, the velocity at the front wall of the fiber decreases, and the speed increases with each fiber. However, since the fibers are staggered, they are different from the parallel arrangement. It is at the tail of the columnar fiber that no obvious vortex is formed. Moreover, it can also be seen from the figure that the flow field structure between the first row of fibers from the filter inlet and the last row of fibers changes substantially periodically.
4.2, resistance
The resistance is the differential pressure at the inlet and outlet of the filter, which is an important parameter reflecting the performance of the filter. Figure 3 shows the calculated values ​​of the filter resistance and the empirical model calculated values ​​for different headwind wind speeds. As can be seen from the figure, as the oncoming wind speed increases, the resistance of the filter increases linearly. The numerical calculations in this paper agree well with the Davies experimental correlation, and the error is less than 2%, while the other empirical formula prediction values ​​and experimental correlation errors are larger. It can be seen that the numerical calculations in this paper can predict the resistance of the air filter quite accurately.
Where F is the resistance to the fiber per unit length, which can be obtained by integrating the total stress on the surface of the cylindrical fiber.
Figure 4 shows the numerical calculation of the drag coefficient of different Re fibers, the empirical model prediction of Tritton and the theoretical calculations of Dennis and Chang [12]. It can be seen from Fig. 4 that the numerical calculation value agrees well with the empirical formula prediction value and the theoretical calculation value, and it can be seen from the figure that in the laminar flow region (Re ≤ 20), the resistance coefficient of the fiber follows the Re Increase and decrease.
5 Conclusion
In this paper, the numerical simulation technique is used to study the internal gas flow field of the staggered air filter. The results are as follows:
(1) When the air flows through the fiber, the velocity at the front wall of the fiber decreases, and the speed increases with each fiber. However, since the fibers are staggered, almost no formation is formed at the tail of the columnar fiber. Vortex. The flow field structure between the first row of fibers from the filter inlet to the last row of fibers varies substantially periodically.
(2) In terms of predicting filter resistance, the numerical results agree well with the experimental correlation of Daives, and the error is within 2%. Except for the Happel empirical model, other model prediction values ​​and experimental correlation errors are larger. The numerical results also show that the resistance increases linearly with the increase of wind speed.
(3) The numerical calculation values ​​of the resistance coefficient at different Re are in good agreement with the empirical model calculation values, and in the laminar flow region, the resistance coefficient of the fiber decreases with the increase of Re.
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