Effect of slip flow on pressure drop of nanofiber filters

Abstract

The slip flow effect is often brought out to explain the reduction in pressure drop for nanofiber filters. Kirsch et al. (1973) studied the slip flow effect on the pressure drop of fibrous filters consisting of micron fibers, and proposed an empirical equation to predict the dependence of the dimensionless drag, F, on the Knudsen number, Kn, with considering non-uniformity of fiber packing. However, their empirical equation was derived based on the experiments with filters consisting of micron fibers so that the empirical equation is not yet verified for nanofiber filters. In the present work, we used various commercially available nanofiber filters with various physical properties, and the pressure drop was measured at low pressures in order to examine the validity of the empirical equation. As a result, we found that the empirical equation is valid even for nanofiber filters with a large inhomogeneity factor at a large Kn up to 20.

Nanofiber Filter Performance Improvement: Nanofiber Layer Uniformity and Branched Nanofiber

“…6(b) shows SEM images of branched nanofiber fabricated from polymer solution with a viscosity of 45 mPa•s (Sample #5), which has additional nanofibers spun out from each bead. These third nanofibers can improve the filtration efficiency dramatically as this filter has a much higher chance to capture particles while minimizing filter pressure drop owing to (1) the slip flow effect by the smaller fiber size (average diameter: 45 nm) (Choi et al, 2017) and ( 2) the longer streamwise inter-fiber distance by the micron-sized beads.…”

“…However, as the concerned particle size gets smaller, air filter media needs to be denser and thicker to improve filtration efficiency, which results in increasing filter pressure drop and thus increasing total operation cost. In order to solve this problem, many researchers have investigated nanofiber filter media, which consists of nanometer-scale fibers layered on top of a regular micrometer-scale filter media as a substrate, and these nanofiber filters improved filtration performance dramatically in terms of both filtration efficiency and pressure drop (Leung et al, 2009;Choi et al, 2017). Recently, beaded nanofiber filters have been proposed to decrease pressure drop because of larger streamwise inter-fiber distance and similar to the regular nanofiber filters performed with high filtration efficiency (Fong et al, 1999;Yun et al, 2010;Wang et al, 2015;Gao et al, 2017).…”

Exploration of Ultralight Nanofiber Aerogels as Particle Filters: Capacity and Efficiency

“…16,17 A similar dependence of the filtration efficiency and pressure drop is found when investigating the nanofiber packing density and mat thickness. 6,24 Such NFM filters are restricting the gas flow. Furthermore, they still suffer from further significant drawbacks: inadequate filtration performance, weak mechanical properties, and short service life caused by accumulated particles on the filtration surface.…”

Synthesis of CTAB-Functionalized Large-Scale Nanofibers Air Filter Media for Efficient PM2.5 Capture Capacity with Low Airflow Resistance

“…The pressure drop across the filter medium increases because of the resistance of individual fibers to air flow. The pattern of gas flow structure around fibers depends on the flow rate, nanofiber packing density, porosity, and nanofiber 46 The corresponding values of K n are 0.48, 0.68, 0.76, and 0.81 for PAN, PATAB-1, PATAB-1.5, PATAB-2, nanofibers, respectively, which appropriated for the transition flow regime. In this regime, the air velocity at the nanofiber surface is non-zero, so the air molecules could easily be passed around the fibers, and the drag force of airstream decreases.…”

“…The pattern of air flow around fibers depends on thermodynamic conditions. The Knudsen number, K n = 2λ/ d f , is used to characterize the different flow regimes of the air around the fibers, where λ is the mean free path of air molecules and d f is the diameter of the fibers . The corresponding values of K n are 0.48, 0.68, 0.76, and 0.81 for PAN, PATAB-1, PATAB-1.5, PATAB-2, nanofibers, respectively, which appropriated for the transition flow regime.…”