Fluid Mechanics of Nano-textured Coalescing Filters
Sahu, Rakesh P.
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The work deals with the different stages of filtration within the coalescing filters on the physical level. The main stages involved in separation of airborne liquid drops brought by a gas flux onto a coalescing filter were separated into several fundamental problems studied experimentally and theoretically. Novel experimental setups were designed for conducting the experiments and theoretical modeling was used to explain, rationalize and predict the observations. The effect of static wettability under dynamic drop impact was found to be negligible after the drop crosses a certain threshold velocity. The hydrodynamic focusing of the liquid drop into the pores of the membrane overcomes the static wettability. It has been shown that the superhydrophobicity does not prevent water from being delivered into a filter if the impact velocity surpasses a threshold value of the order of 3 m/s, and the hydrodynamic focusing becomes important. As the liquid drops get inside the filter, it gets intercepted with the fiber matrix inside. As the air keeps blowing the intercepted liquid drops move over the fibers in different directions depending on the direction of air blowing with respect to the fiber axis. The different breakup modes observed during cross blowing was mapped onto the We-Oh plane. Drop hopping between the fibers was also observed. Liquid drops were observed to reach a significant depth of its thickness due to the hydrodynamic focusing. In addition, propagation of liquid fronts associated with drop boundaries inside non-wettable nonwovens was studied experimentally and theoretically, as well as the entrainment of nanoparticles by such fronts. As the usage of nanofibers is steadily increasing in the filtration industry, the mechanically-driven rearrangement of nanofiber membranes under the action of drop evaporation at the front surface becomes very important. The formation of big holes on the suspended nanofiber mat is addressed in the experiments of the present work. Void formation was uncovered and quantitatively described. The findings of the present work reported are useful for development of novel coalescing filters and proper understanding of their operation.