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Nano-aerosols are airborne aerosols in the range of 100 nm. They are present abundantly in both pollutants and viruses, both of which affect human health. Filtering these nano-aerosols leads to high-pressure drop in cabin filters due to their small sizes forming impermeable dust cake, and prior to cake formation the capture efficiency is rather poor with conventional microfiber filters. We have investigated a composite filter with a microfiber layer positioned immediately upstream of a nanofiber layer. Such composite filter provides high efficiency and low-pressure drop during all phases of filtration of nano-aerosols: initial depth filtration, transition, and final cake filtration. The behavior of the composite filter together with various filter benchmarks (single and composite filters of microfiber/nanofiber filters) has been investigated by monitoring the pressure drop, especially during cake filtration. Using the measurements together with Ergun/Kozeny-Carman equation and material balance we come up with a new method to determine the solid volume fraction and permeability of the cake. The grade efficiency for the most penetrating particle is also measured concurrently to define the incipient point to cake formation and provide material balance. Most importantly, for the first time we have confirmed that the cake ultimately forms on the composite filter is due to the cake that originates from the upstream microfiber layer and not from the downstream nanofiber layer. Therefore, the pressure drop for the composite filter can be sustained relatively low throughout the filtration. On the other hand, the aerosol capture efficiency stays very high from start-to-end of filtration, with high initial filtration efficiency courtesy of the nanofiber filter installed downstream of the composite filter. The price to pay is a slightly higher pressure drop for the composite filter in the clean unloaded state, but this is insignificant during aerosol loading as most pressure drop is attributed to that of the cake. For the first time we have introduced, also independently using Buckingham-Pi analysis, a brand new dimensionless parameter, Beta, which measures the equivalent “thickness” of additional cake deposition (in molten form) to the viscous flow resistance “path length” due to the additional cake deposit. A high-capacity filter aimed for heavy aerosol loading should have large Beta that results in low solidosity, high porosity, and high permeability for the cake. In our study, the microfiber-nanofiber composite filter has the highest Beta of nearly 4 and also highest efficiency. Despite high efficiency the single nanofiber filter has much lower Beta of 2, while the single microfiber filter has Beta of 3, yet much lower efficiency, especially at initial filtration. Using the microfiber-nanofiber composite filter design, we can “engineer the cake” deposited on the filter to be more permeable and with least flow resistance during aerosol loading to attain high Beta values. This is an innovative approach to increase the badly needed aerosol storage capacity with low pressure drop for the nanofiber filter while maintaining high efficiency throughout the aerosol loading for extended filtration applications. The highly permeable aerosol cake can also be cleaned by backpulsed-and-backblow readily for filter reuse.

Highlights • Cyclic loading/cleaning nanofiber filter with nano-aerosols. • Thin skin layer upstream of filter controls loading behavior, permeability drops to 10–20% of initial value. • Capillary model for aerosol deposit, pore bridging, cake model match 6-cycle loading tests. • Transit from depth-to-surface-filtration – tests and modelling., Nano-aerosols from viruses to virgin pollutant particulates from combustion, 100 nm or smaller, are harmful to our health as they penetrate readily into our body causing various diseases. Nanofiber filter can capture effectively these nano-aerosols. However, over time the pressure drop increases dramatically and cleaning of the filter by backpulse/backblow is essential for filter reuse. The cyclic loading-and-cleaning of a nanofiber filter has been investigated for the first time experimentally and theoretically. The “skin” layer, a thin region upstream of the nanofiber filter, plays a pivoting role in controlling the pressure drop excursion of the filter. We model the skin layer to be made up of numerous fine capillaries and examine how continuous aerosols deposited in the capillaries affect rapid rise in pressure drop followed by bridging of aerosols across the capillary openings leading to more bridging and ultimately formation of cake on top of the bridges and filter surface. We have been able to describe the deposition of aerosols in the capillary pores for depth filtration, the deposition of aerosols in the cake (surface filtration), and the intermediate bridging regime between these two. We can depict the complete pressure drop excursion including the S-shaped curve behavior from depth filtration transiting to surface filtration for a filter with severe skin effect. Our prediction matches extremely well with the 6 cycles of loading/cleaning on a 280-nm nanofiber filter subject to challenging nano-aerosols, 50–400 nm. During cyclic loading and cleaning, the porosity and permeability in the skin layer for our experiment drop to 68% and 11–21% of their original values, respectively, and the effective pore diameter also drops from 1.2 to 0.6 μm.

A two-parameter model has been developed to interpret the pressure excursion during cleaning of a nanofiber filter preloaded with nano-aerosols. The model can well predict the three stages of cleaning – an initial rapid cake discharge phase, and a transition and a final phase, both of which are related to cleaning aerosols trapped in the filter. One of the two parameters of the model affects the residual pressure drop while the other parameter affects the cake discharge. The filter cleaning test data based on variations from the five operating and geometric parameters have been well explained by the new model – the backpulse, backblow, and combined mode; pulse duration; applied over-pressure; diameter of nanofiber in filter; and the filter thickness. Furthermore, the model can project the residual pressure, which provides an indication of residual aerosols trapped in the nanofiber filter, using individual or combination of these operating and geometric parameters.

Highlights • Backpulse (BP) and backblow (BB) remove nano-aerosols in cake and in filter, BP more effective than BB. • BB following BP can further remove the detached aerosols preventing aerosols recaptured by the filter. • Higher tank pressure up to 6.5 bars can be used for air jets., Nanofibrous filter have been proven effective to remove nano-aerosols with size less than 100 nm. Cleaning is required after long-term use; however, very little has been published on the subject. An experimental investigation has been launched to determine backpulse, backblow and combined backpulse–backblow on cleaning of a loaded nanofiber filter. Nylon 6 nanofiber filters were loaded with polydispersed NaCl particles, 60%