Simulation Of Microstructure And Filtration Characteristics Of Fibrous Filter Media

The safety and environmental issue attracted broad attention recently due to the transmission of bioaerosol,continuously deteriorating environment and air pollution,which also drives the investigation of air filtration technology and application.As the key of air filtration technology,fibrous filter media with tortuous interconnected pore channels and fiber networks,formed by the overlapped and entangled fibers,could allow the effective passage of the air,while capturing the pollutants contained in the air stream However,traditional method based on a large amount of experiment couldn’t respond promptly to the requirement of innovative filter media with improved performance from the society and industry.In this context,it is necessary to study the microstructure characteristics and filtration performance of filter media with simulation method,and propose a promising option to drive the development of innovative filter media.In this dissertation,the microstructures of filter media material were firstly analyzed with a combination of fiber analyzer,SEM,X-ray CT and other characterization methods,then the morphological parameters such as diameter（width）,length,orientation,curliness and torsion angles of fibers commonly used in wet-laid filters were extracted.Based on the determined parameters,random parametric filter media models were generated correspondingly.The permeabilities of the virtual filter media were simulated by solving the Navier-Stokes or Stokes equations with Geo Dict,and compared with the results from simulations of CT models and experimental tests.This dissertation tried to explore the feasibility of establishing a database of fiber models.In the following chapter,virtual single-layer composite filter media model containing 3 natural fibers and PET fiber were created based on the determined fiber models.Considering the particle inertia,fluid drag force,Brownian motion,adhesion force and elastic collision between moving particles and fibers,the initial filtration efficiency against 0.26μm Na Cl particles were simulated and compared with test results from TSI 8130.It may furtherly confirm the established database of fiber models could be used for designing the combinations of fibers and microstructure of the filter media,and then optimizing the filtration performance.For the study of multi-layer filter media containing multiscale fibers,the digital models of electrospun nanofiber-based filter media were reconstructed based on the X-ray tomography data for substrate composed of fibers with dozens of micrometers diameter and FIB-SEM image analysis for PVDF nanofiber membrane.Considering the slip effect around the nanofibers,the initial filtration efficiencies of the filter media for particles of 10 nm～1000 nm were simulated,and compared with the test results by using a lad-made filtration efficiency test bench for monodispersed particles.For the optimization of the dust-holding performance of fibrous filter media,the particle loading process of an individual fiber and complete filter media were simulated respectively,and the corresponding variations of pressure drop and filtration efficiency against mass per unit area particle deposition were given.Based on the particle transmission model mentioned in Chapters 3 and 4,the simulation of particle loading process was conducted by updating the particle deposits and recalculating the flow field after loading each batch of particles.For the individual fiber,the effects of application conditions,cross-sectional shape and packing density of the fiber were studied.For the complete filter media,digital models with different thickness,solidities,fiber orientations,graded or layered structures were created.Then the effect of the microstructure on the dust holding performance of filter media were systematically investigated.This study showed:（1）The morphology and size of natural fibers could be described with a curved elliptical cylinder model.For single-constituent fibrous filter media,the orientation factors for x and y directions were close,while the orientation factor for the z direction t₃₃ was lower than 0.2.Mixing with other fibers could have little effect on the fiber orientation of the filter media.The permeabilities from the simulated results of the parametric agreed well with the results from CT models or experiments,and within the 10%deviation.（2）The simulated permeabilities and filtration efficiencies against 0.26μm Na Cl particles of the virtual single-layer composite filter media created based on the determined fiber models were in good agreements with the test values.（3）The solidity and permeability constant and average PET fiber size of virtual substrate composed of micron fibers from X-ray CT were 26.10%,1.57e-11 m² and 11.09μm respectively,which were in good agreement with the results from experiments.The determined thicknesses of the NFM of#1,#2 and 3#filters were 3.82μm,6.42μm and 7.90μm,respectively,and the corresponding solidities were 4.10%,6.25%and 7.00%,respectively.When the particle size of sodium chloride was 70～400 nm,including the most penetrating particle size（MPPS,100–200 nm）,the simulated initial filtration efficiencies were in good agreement with the experimental results.（4）The particle loading process on an individual fiber:a.As the face velocity increased,the pressured drop/mass per unit area particle deposits curve become steeper.However,when inertial impaction became the dominant capture mechanism with the further increase of face velocity,the mass per unit area particle deposits on the individual fiber increased.b.With the increase of particle size,the slope of the pressured drop/mass per unit area particle deposits curve declined,the formed particle dendrites had an impact and open structure.The particle density could affect the strength of inertial impaction capture mechanism.c.In the interception-dominant regime,particles tended to deposit near the vertices on the inflow side of the shaped fiber.The quatrefoil-shaped and trilobal-shaped fibers could have an improved filtration efficiency,but the pressure drop could increase rapidly during the loading process,leading to the decrease of the amount the particles deposited on the individual fiber.（5）Dust holding performance of fiber filter material:a.The dust holding capacity of the filter media increased with the increase in thickness,while the initial pressure increased.When the final pressure drop was fixed,the dust holding capacity of decreased as the thickness the filter media increased.b.The dust holding capacity of the filter media（β=30°）was about 13.67%higher than that of the filter media containing fibers isotropically arranged in the horizontal plane.As the orientation tensor for z direction decreased,the initial pressure drop of the filter media increased,while the dust holding capacity decreased.c.The distribution of fine fibers along the inflow direction of the filter media had little effect on the filtration efficiency.The open structure of the upper part of the depth filter media could lead to a smoother pressure drop/mass per unit area particle deposits curve in the depth filtration stage,more particles could enter the filter material,the utilization rate of the pore space of filter media was high.In the cake filtration stage,the pressure drop/mass per unit area particle deposits curves of filter media with different structures were close.