|Author:||Hau, Wing Yi Curie|
|Title:||Backpulse and backblow cleaning of nanofiber filter loaded with nano-aerosols|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Mechanical Engineering|
|Pages:||98 pages : color illustrations|
|Abstract:||Electrospun nylon-6 nanofiber filter, with fiber diameters of 100 to 300nm, has been used for nano-aerosols filtration. Despite the effectiveness of filtration, the aerosol loading capacity of the filter is low and pressure drop across the filter increases rapidly with aerosol loading. This study examines the effectiveness of backpulse and backblow cleaning on nanofiber depth filter loaded with challenging polydispersed nano-aerosols with 50-60% less than 100 nm and maximum size less than 300-500nm. To investigate filter regeneration, a series of backpulses followed by backblow at constant air velocity was applied ona loaded nanofiber filter with maximum allowable pressure drop across the filter set at 600-1000Pa. The function of backpulse is to provide inertia to break-up the particle-particle locking as well as particle-fiber adhesion while backblow is to carry the loosened particles away from the filter. Due to the fragility of nanofiber, a tri-nozzle setup has been introduced in the cleaning system to distribute the cleaning air relatively uniform across the filter instead of a concentrated jet targeted at the center of the filter that can damage nanofibers there. To optimize the cleaning conditions, the effect of several key parameters has been investigated-pulse jet, flow durations, numbers of jet pulses, and applied pressure. Also, the filter properties, such as nanofiber diameteraffecting particle-fiber adhesion as well as particle capture, and nanofiber filter thickness affecting particle capture and filter capacity have been studied. Unfortunately, both have negative effects on filter cleaning by backpulse and backblow due to large adhesion force of particles to fibers for nanofiber filter with smaller fiber diameter and due to increasing recapture of loosened particles for a thick nanofiber filter, respectively.|
For multilayer nanofiber filter, two inhomogeneous filters have been tested. One filter was made from a combo filter with a thickermicrofiber layer upstream and a thinner nanofiber layer downstream. The second filter was made from a nanofiber layer with mean fiber diameter of 280nm located upstream and another nanofiber layer with mean fiber diameter of 180nm located downstream. The cleaning effectiveness of the multilayer nanofiber filter was compared with a filter with only single nanofiber layer. The multi-layering configurationcan reduce the skin effect during aerosolloading, however, during cleaning with reversed flowthe downstream microfiber layer provided damping to the cleaning jet undermining the cleaning effectiveness on the upstream nanofiberlayer when compared to the case with regenerating only single nanofiber layer filter. For the filter with two nanofiberlayers, the downstream nanofiber layer enhanced the recapture of loosen particles during cleaning. Further, the presence of the downstream nanofiber layer also compromised the cleaning effectiveness of the nanofiber upstream layerduring reverse flow cleaning. Cyclic filtration, involving both loading and cleaning, have been carried out in both single-layer and multilayer nanofiber filters. Both filter configurations have shown stable behavior in which the growth in residual pressure drop and the decrease in filtration cycle time were present primarily in the first filtration cycle (i.e. the conditioning phase) and subsequently both variables remained relatively constant thereafter. The pressure versus time during loading wasconvex upward for the first loading cycle (at times linear), but changed over to concave downward in subsequent cycles. This isdue to the dead pores of the filter being prefilledwith aerosols after the first loading and cleaning cycle.
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