Numerical modeling of aerosol filtration using a nanofiber filter

Numerical modeling is used to complement analytical and experimental results on the nanofiber filter for filtering submicron- and nanoaerosols. Deeper insights can be gained from results of the model that might be difficult to obtain with analytical approach and experiments. First, using a simplified two-dimensional (2D) model, loading of a nanofiber filter demonstrates the formation of dendritic nanoaerosol deposit on fibers by diffusion and interception mechanisms. Subsequently, the dendrites formed from fibers interact with each other blocking flow passages between the fibers in a thin region upstream of the filter (referred to as skin layer). Further, when the passages in the skin layer get blocked, captured aerosols start to deposit on the filter surface. Dendritic structure of aerosol deposit also evolves above the filter. Upon interaction, they form continuous cake layer with properties that can be characterized by permeability and porosity. The cake layer continues to grow with all challenging aerosols regardless of size being captured by the cake. The results from the numerical model match the behavior as obtained previously from analytical and experimental results. The additional aerosols captured and deposited on the cake surface by monodispersed challenging aerosols result in a sharp distinct, stratified cake front, whereas those by polydispersed aerosols result in a rather dispersed cake front with small aerosols penetrating through the pores of the cake formed by the larger aerosols during initial cake filtration. Also, simulation demonstrates that by reducing feed concentration challenging the filter it can mitigate the skin effect of the filter with result of more aerosols being captured in depth filtration, and a more porous and permeable cake that forms ultimately on the filter surface. The 2D model is further used to simulate a composite filter with a microfiber layer positioned upstream of a nanofiber layer. Two cake layers form simultaneously, respectively, on the microfiber layer and the nanofiber layer. The growth of the cake in each fiber layer depends on the skin effect of the particular layer, which in turn is controlled by the fiber packing density of the layer. The growth of the cakes also depends on the aerosol loading on the individual filter layers. The downstream nanofiber layer would experience a lesser loading as part of the total aerosol load challenging the composite filter has been filtered by the upstream microfiber layer, hence the growth of the nanofiber cake is deliberately delayed or slow-down. With optimally fiber packing density for the microfiber layer, a permeable and porous cake can be formed on the layer first, outpacing that of the downstream nanofiber layer. Further, upon a cake being formed on the upstream microfiber layer, the cake growth in the downstream nanofiber layer stops as all incoming aerosols are captured by the cake of the upstream microfiber layer. The composite filter would have lower pressure drop during continuous cake formation on the microfiber layer while a high efficiency for such composite filter can be achieved during initial filtration period prior to cake formation, courtesy of the downstream high-efficiency nanofiber layer. For a clean unloaded filter, a three-dimensional model with randomly generated fiber mesh can be used to study the effect of mechanical capture by diffusion for small aerosol size (<200 nm), and by respectively, interception, van der Waals force, and electrostatic force for larger aerosols (>200 nm). The model exhibited the U-shape characteristic typical of diffusion and interception dominating aerosol capture. With an addition of van der Waals force, this generally raises the capture efficiency without changing the shape of the efficiency curve. With electrostatic force added for as-spun nanofibers with residual electrostatic force from electrospinning, this benefits more especially with larger submicron aerosols.