Research On Gradient Structure Design And Filtration Performance Of Micro/Nano Fiber Filter With Bimodal Diameter Distribution

With the development of industrialization,the pollution caused by particulate matter（PM）in the air has become more and more serious,especially the micro-/nano-particulate matters can easily carry toxic substances and do serious harm to public health.Air filtration is one of the effective strategies to remove harmful particles in the air.Among the materials for air filtration,the electrospun micro-nano fibrous membrane have relatively small fiber diameter and good pore connectivity,which significantly improves the filtration efficiency against PM_2.5.Generally,the finer the fiber is,the denser the pile is,the smaller the particles which can be filtered are,and the higher the filtration efficiency is,but high fiber packing density also lead to high pressure drop.How to reduce the pressure drop under the premise of high filtration efficiency remains challenging and more insight efforts are in need.In order to solve this problem,this paper optimizes the structure of the fiber material through numerical simulation with the CFD module in COMSOL Multiphysics 6.0 finite element software,and produce the micro-nano fibrous membranes via free surface electrospinning.First,the fibrous membrane with a bimodal diameter distribution was designed to reduce the fiber packing density,and use the slip effect to reduce the filtration resistance,then design a three-layer fiber membrane with a gradient pore size distribution,finally obtain a high-efficiency and low-resistance three-layer filter material with a rough surface of the fiber.The main research contents are as follows:（1）The filtration performance of unimodal and bimodal fiber membranes with different structures was explored by numerical simulation,the results showed that the filtration performance of bimodal fibrous membranes was better than that of unimodal fibrous membranes,and the more uniform the fiber mixing,the better the filtration performance.The two concentrations of polyacrylonitrile（PAN）fibers were alternately fabricated by free surface electrospinning.The test results proved that the superiority bimodal fibrous membrane in the performance of filtration and strength.（2）By modelling and numerically simulating the air flow field around single fiber,the fiber with diameter of 70 nm was found to be most effective for slip flow to reduce air resistance.On this basis,bimodal structured fibrous membrane filters（fiber diameters:200nm and 70nm）were designed,and the optimized bimodal fibrous membrane was fabricated using polyacrylonitrile/poly（1,1-difluoroethylene）（PAN/PVDF）as raw materials.In order to further improve the filtration efficiency of fiber membranes to meet the needs of precision filtration,a three-layer gradient material Uni2/Bi/Uni1 with a gradually decreasing fiber diameter was designed,that is,from the windward side to the leeward side,there were 110 nm unimodal fiber layers,70+200 nm bimodal fiber layer,70 nm unimodal fiber layer,this structure filter material had the best filtration performance against PM_0.26,with a filtration efficiency of 99.984%and a pressure drop of 85.02 Pa.（3）Numerical simulation results showed that the filtration performance of single fiber increased with the fiber specific surface area.By adding nano-Ti O₂ particles to the spinning solution to increase the specific surface area of the fiber,a filter material with a rough surface of the fiber was fabricated on the basis of the aforementioned three-layer gradient structure,and its filtration performance was the best(quality factor=0.1073 Pa^-1),the filtration efficiency had reached 99.992%,the resistance pressure drop was 87.93 Pa.There are two causes for its excellent filtration performance:For one thing,the pressure drop was reduced by the lower packing density and the higher porosity of the fibrous membrane;for another,the rough structure of the fiber surface increased the contact area between the particles and the fiber,which was conducive to the capture of the particles.The fiber material applied the characteristics of slip effect,fiber diameter bimodal distribution,gradient air channel and fiber surface rough structure,which provided a new idea for the development of high-performance air filter materials.