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Abstract
Air filtration mechanisms in the composite filter media used in practical applications are important and challenging to understand because the component fibers could have various size scales and morphologies. In this work, a three-dimensional digital model of nanofiber-based filter media was reconstructed for the first time based on the X-ray tomography data for the cellulose substrate and the Focused Ion Beam-Scanning Electron Microscope (FIB-SEM) image analysis for the several micrometers thick (3.82–7.90 μm) electrospun polyvinylidene fluoride (PVDF) nanofiber membrane. Besides the high-resolution model where the details of the fibrous structures were fully resolved, another low-resolution model with approximated unresolved structures was also established. Filtration simulations utilizing these models were conducted considering the drag force, Brownian diffusion and aerodynamic slip. The simulated filtration efficiencies agreed well with the experiments for particles of 70–400 nm, including the most penetrating particle size (MPPS, 100–200 nm). Moreover, the structure-resolved models had higher accuracy but higher computational costs, while the unresolved simulations saved much running time but over-predicted the filtration efficiency, especially for smaller particles (<100 nm). Our study presents a comprehensive strategy for investigating the composite filter media with multiscale complex structures using a combination of advanced characterization technologies and modular simulation models.
Highlights
The safety and environmental issues have recently attracted broad attention due to the continuously deteriorating environment, air pollu tion and outbreaks of the respiratory infectious diseases [1], which drives the investigation of air filtration technology and application [2]
The pressure drop and the particle filtration character istics of dual layer filter media composed of a nanofiber membrane layered on the substrate were investigated with experimental and simulation methods
The composite filter media with three different thicknesses of nanofiber membrane were respectively prepared by fabricating polyvinylidene fluoride (PVDF) nanofibers on the substrate with electrospinning method, and the structure properties and particle filtration per formance for sodium chloride particles in the range of 30–500 nm were experimentally investigated
Summary
The safety and environmental issues have recently attracted broad attention due to the continuously deteriorating environment, air pollu tion and outbreaks of the respiratory infectious diseases [1], which drives the investigation of air filtration technology and application [2]. The thick ness of the nanofiber membranes was often ignored or derived from empirical equations since the cross-section of the nanofiber membranes could be heavily deformed during the cutting process for SEM images [34] These idealized models used for nanofiber membranes were insufficient to represent the actual structure of the fibers in substrates, especially the cellulose fibers with hollow structure, where the inner diameter could be different due to the difference in sources and treat ment [35], and could compromise the quality of numerical calcu lations for the filter performance. The flow regimes in the substrate and nanofiber membrane were different due to the various fiber scales [36] It remains unclear whether the slip model used at the surface of the nanofiber in numerical simu lations could be suitable for composite filter media. The various composite filter media models proposed here can provide reference for further investigation of the dynamic evolution of the microstructure and the macroscopic filtration characteristics of composite filter media in the long-term particle loading process
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