Abstract:

Objective In filtration simulation calculation for internal structure of fiber assemblies, the fiber filtration medium model established is limited to a single cylinder. Although this simplified model is conducive to the rapid solution of the problem, its limitation is that it cannot fully demonstrate the influence of multi-fiber structure on the filtration performance. In practical engineering applications, fibers in the assembly used for air filtration have axially curved structures, and are not completely cylindrical. If this factor is ignored, the simulated results will be inevitably different from the actual test results, and the filtration characteristics and internal pressure loss of the filter material can not be accurately described. In order to simulate and predict the properties of fiber filter media more accurately, more complex models need to be built to describe the internal structure of fiber assembly. This includes considering the bending and random arrangement of the fibers. The refined model will help understand furtherthe flow characteristics and filtration behavior of fiber filter media, improve the consistency of simulation results and actual test results, and provide a reliable scientific basis for the design and optimization of filter media in engineering applications.

Method A three-dimensional fiber filter media model with curvature K=0, 2, 4 and 6, solid volume fraction of 8%, randomly distributed in space was established by Digimat modeling software. Combined with computational fluid dynamics methods, based on Lagrange discrete model and Laminar flow field, similarity principle and Reynolds similarity criterion were used. The inlet velocities were set as 0.05, 0.142, 0.5, 1 and 2 m/s, respectively, and the average particle size was 0.25, 0.5, 1, 1.5, 2.5, 4 and 5 mm. The gas-solid two-phase flow inside the micron fiber model was numerically simulated.

Results As the fiber curvature K increases from 0 to 6, when the inlet velocity is 0.5 m/s, the distribution of the fiber in the filter region is irregular, random and asymmetrical, and the flow velocity distribution is also irregular, and the influence of curvature on the velocity field distribution is not obvious. With the increase of the curvature of the fiber model, the internal pressure loss of the fiber model increases linearly. The pressure value of the windward side of the fiber is greater than that of the leeward side, and the pressure loss when the curvature K is 6 is significantly greater than that when the curvature K is 0. The filtration efficiency of fiber filtration medium increases with the increase of model curvature. When the inlet velocity is 1 m/s and the curvature K is 6, the filtration efficiency of particles with an average particle size of 5 mm is close to 90 %, which is significantly higher than the filtration efficiency of K=0 under the same particle size. This is because the curvature of the fiber increases and more bending and folding will appear on the fiber surface, which will increase the surface area of the fiber. The larger surface area of the fiber provides more opportunity for the fiber to contact with the dusty air stream, which makes it easier for the fiber to collide with particles in the air stream and trap them. In addition, the greater the curvature of the fiber body, the greater the degree of deformation and distortion of the fiber body. Thus, a more complex and disorganized stacking and crossing structure can be formed between adjacent fibers, and the pore size between adjacent fibers is smaller, so that the interception and trapping capability of a single fiber on particles will be greatly improved, and the overall filtration efficiency of the filter medium will also be improved.

Conclusion The filtration efficiency and pressure loss of the fiber assembly increase with the increase of fiber curvature. In practical engineering applications, in order to improve the filtration capacity of the filter material, the filter material with more fold structure is used as far as possible to increase the filtration area and dust holding capacity of the filter material while ensuring the pressure loss as little as possible. The folded structure makes the fibers in the filter material show a high degree of non-uniformity. More interception sites and bending paths are created, effectively enhancing the interaction between particles and fibers, thereby improving the filtration efficiency of the filter material.

Key words: computational fluid dynamics, fiber curvature, pressure loss, numerical simulation, filtration efficiency