The Effect of the Layered Internal Structure of Fibrous Beds on the Hydrodynamic Diffusive Behavior of Microparticles

To separate and collect microparticles such as cells, the behavior of particles in fibrous filters was investigated. It is essential to understand, in detail, the motion of particles in microscale flows, because Re is often small, and particles exhibit complex behaviors such as changes in relative position and spreading owing to hydrodynamic interactions. We calculated the motion of microparticles passing through the fibrous bed using the Stokesian dynamics method, in which hydrodynamic interaction is considered, theoretically. The fibrous bed was modeled by particles and five types of structures (a monolayer with fiber volume fractions ϕ of 3%, 4%, and 5%, and a bilayer with ϕ = 3−5% and 5−3%) were considered. Our numerical results showed that the particles moved in a complicated manner, and spread throughout the fibrous bed. It was found that the behavior of individual microparticles varied depending on the internal structure, although the average permeation velocity was primarily determined by the fiber volume fraction. This great dependence of the behavior of particle assemblage on the internal structure of the fibrous bed was caused by the individual particle motion under the influence of the layers in front of and behind them, owing to the hydrodynamic interaction.

Self-Supporting Three-dimensional Electrospun Nanofibrous Membrane for Highly Efficient Air Filtration

High-performance air filtration was the key to health protection from biological and ultrafine dust pollution. A self-supporting, three-dimensional (3D) nanofibrous membrane with curled pattern was electrospun for the filtration, of which the micro-fluffy structure displayed high-filtration efficiency and low-pressure drop. The flow field in the 3D filtration membrane was simulated to optimize the process parameters to increase the filtration performance. The qualification factor increased from 0.0274 Pa−1 to 0.0309 Pa−1 by 12.77% after the optimization of the electrospinning parameters. The best filtration efficiency and pressure drop were 93.6% and 89.0 Pa, separately. This work provides a new strategy to fabricate 3D structures through the construction of fiber morphology and promotes further improvement of air filtration performance of fibrous filters.

Evaluation of Antimicrobial Effect of Zinc Pyrithione against Airborne Fungi and Bacteria Growth Collected onto New and Loaded HVAC Fibrous Filters

Microbial growth onto HVAC filters was observed in real conditions with possible degradation of the indoor air quality. The filtration performance of marketed antimicrobial filters containing zinc pyrithione was tested under laboratory conditions and compared to that of similar filters with the same classification, F7 (EN779:2002). The filtration performance of the two tested filters during loading with PM10 particles was quantified in an experimental setup with filter pressure drop measurement and particle counting upstream and downstream of the filters. The microbial growth on the new and loaded filters, both contaminated with a microbial airborne consortium composed of two bacteria (Gram-positive and -negative) and fungi, was quantified by colony-forming units after conditioning the filters for a few days under controlled temperature (25 °C) and humidity (50% or 90% relative humidity). The results reveal that there was no degradation of the filtration performance of the filters treated with the antimicrobial agent. The efficiency of the antimicrobial treatment, i.e., the ability to inhibit the growth of microorganisms during the incubation period, was significant with the new filters regarding the fungal growth, but the results demonstrate that the antimicrobial treatment became inefficient with the loaded filters.

Evaluation of the Bioaerosol Inactivation Ability of Chitosan-Coated Antimicrobial Filters

This work considers the enhancement of indoor bioaerosol removal efficiency by liquid coating of the antimicrobial agent chitosan onto polypropylene fibrous filters (CCFs). Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) were chosen as the tested bioaerosols. The results revealed that 2.5% (w/w) of CCFs have significantly higher bioaerosol survival capability (23% and 34% of E. coli and B. subtilis, respectively), compared to an untreated filter (65% and 64% for E. coli and B. subtilis, respectively). Increasing face velocity and relative humidity during operating CCFs could reduce the bioaerosol removal capability. The regression analysis of the experimental findings demonstrated that the higher coating concentration of chitosan had the most positive influence on bioaerosol removal, while the face velocity and relative humidity had a negative influence, but a milder effect was observed (R2 = 0.83 and 0.81 for E. coli and B. subtilis bioaerosols, respectively). A CCF-loaded air-cleaning device was tested in a real indoor environment and resulted in 80.1% bioaerosol removal within 3 h of operating, which suggests that the chitosan-coated filter has the potential for further application in improving indoor air quality in the future.

Mass production of polyacrylonitrile sub-micron fibrous webs with different aligned degrees via free surface electrospinning for air purification

Atmospheric pollution has emerged as causing irreversible harm to the ecosystem and people. Sub-micron fibrous filters play an incomparable role in effective air purification, owing to their excellent internal connectivity. Herein, three-dimensional sub-micron fibrous webs with various aligned degrees were conveniently fabricated via free surface electrospinning with different rotation speeds of the roller with a large diameter in large quantity, applied in air filtration. The influence of the orientation degrees of fibers on the performances of the fibrous filter was analyzed systematically. Results showed that the filtration performance of fibrous filters was inversely proportional to the orientation degree of the sub-micron fibers. Random fibrous webs with areal densities of ≤2.0 g m−2 exhibited high porosity (∼90%), ensuring qualified air permeability and outstanding filtration efficiency from 92% to 99.5% for ultra-fine aerosol particles (∼0.26 µm) under a higher air velocity of 14.1 cm s−1. The internal aperture channels were twists and turns with irregular polygon shape for random fibrous webs, while they were a narrow strip in the horizontal and straight in the longitudinal for aligned ones, which influenced the filter’s performances. Fibrous webs with better orientation of fibers and larger pore size are beneficial for energy efficiency and exhibited good filtration performance, better air permeability, and an improved mechanical property along the longitudinal direction. A cost-effective uniform sub-micron fibrous filter with different aligned degrees could be produced rapidly via free surface electrospinning with a mass production rate, which is beneficial for industrial production and commercial applications in respiratory protection and indoor air purification for precise purification of air pollution.