| With the rapid development of Chinese industry and transportation,fine particulate matter(PM2.5)pollution is becoming more and more serious,which is harmful to our health,atmospheric and industrial production.Fibrous filtration is the most widely used air filtration technique for removal of PM2.5 from gas stream,and is applied in a variety of residential,medical,industrial air conditioning systems and personal respirators(e.g.,N95/N99).The main purpose of the research on fibrous filtration is to improve the filtration performance of PM2.5,and reduce its negative effects on human health and the environment.In this work,the comprehensive filtration performances of single non-circular fiber,single nanofiber,gradient fibrous media,composite fibrous media with nanofibers and microfibers and pleated fibrous filter were investigated from microscopic,mesoscopic and macroscopic levels in order to provide theoretical knowledge for detailed understanding of efficient removal mechanism of aerosol particles and for developing high efficient and energy-saving removal and purification technology of PM2.5.Euler-Lagrange method was employed to investigate particle multi-mechanisms filtration process of square,elliptical and slit-crescent-shaped fibers for particle sizes ranging from 0.1?m to 2.5?m.The results indicate that the quality factor of the elliptical fiber is closely related to orientation angle?,cross-section aspect ratio ? and fiber packing density C.When ? = 15° and 30°,the elliptical fiber does much better than the circular fiber for particles ranging from 0.1 mm to 1 mm,while comprehensive filtration performance of the elliptical fiber for 2.5 ?m particles is equivalent to or slightly lower than that of the circular fibers.When dimensionless slit width ranges from 0.2 to 0.4,the quality factor of the slit-crescent-shaped fiber is much higher than the circular fiber for particles ranging from 0.1mm to 2.5mm.The average collection efficiency and average quality factor were defined to evaluate the filtration performance of different cross-sectional fibers at the intermediate fiber packing density(C = 0.05).The average collection efficiency and average quality factor of the three non-circular fibers are higher than those of traditional circular fibers.For small particles with strong diffusion effect,the average collection efficiency of the elliptical fiber is higher than that of the square and slit-crescentshaped fibers,and the average quality factor of the elliptical fiber is equivalent to that of the slitcrescent-shaped fiber but higher than that of the square fibers.For sub-micron and micron particles dominated by inertia and interception,the average collection efficiency of the square fiber is higher than that of the slit-crescent-shaped and elliptical fibers,and the average quality factor of the slitcrescent-shaped fiber is higher than that of the square and elliptical fibers.The Brownian particle and inertial particle filtration process by nanofibers in slip/transitional flow regime was simulated by numerical methods.Significant differences can be found between flow field around the fiber in slip flow regime and in continuous flow,and slip velocity on the fiber surface increases with the increase of the Knudsen number(except at the front and rear stagnation points of the fiber).Based on the simulation results,a reliable expression for the nanofiber/microfiber filtration drag coefficient was developed.In the diffusion-interceptiondominated regime,the sum of inertial efficiency and interception efficiency seriously underestimated the single fiber efficiency,especially for the filtration situation with small Peclet number and large interception parameters.A reliable prediction expression for single fiber collection efficiency by combination of Brownian diffusion and interception was derived.In addition,in the inertia-interception-dominated regime,the additivity of diffusional collection efficiency and interception efficiency also underestimated the single fiber efficiency,especially for the case of small Stoke number filtration.Based on the simulation results,a reliable prediction expression for single fiber collection efficiency due to the combination effects of inertial impaction and interception was developed.Moment method was used to study transport and deposition characteristics of Aitken mode and accumulation mode particles in fibrous media with three different fiber packing density distributions(homogeneous,linear and exponential distribution)along the flow direction.The deposition mass distribution of the two mode particles in homogeneous fibrous media gradually decreases along the airflow direction,and Aitken mode particles deposition mass distribution inhomogeneity is larger than accumulation mode.Uniform deposition distribution of aerosol particles in fibrous media along the flow direction can be generally achieved by optimizing the geometric parameters of the gradient fibrous media(fiber packing density distribution ratio),and the exponential distribution fibrous media is slightly better than the linear distribution one,but not obvious.Based on the prediction model of nanofiber coupling efficiency established above,moment method was adopted to calculate the filtration performance of polydisperse particles by composite fibrous media.Results indicate that for sub-micron particles(0.1-1 ?m),the composite fibrous media has a higher quality factor than conventional microfiber media,while for nanoparticles(approximately below 0.1 ?m),the composite fibrous media has a worse compressive filtration performance.Quality factor of the composite fibrous media for treating Aitken mode particle increases with increasing coated nanofiber diameter,and decreases as increasing of the fiber packing density and thickness of the coated nanolayers.For filtration of accumulation mode particles,in order to enhance the comprehensive filtration performance of the composite fibrous media,the thickness,fiber packing density and diameter of the coated nanolayer should be controlled in a reasonable range.When nanofiber size is small(0.1-0.3 ?m),the fiber packing density and thickness of the coated nanolayer should be appropriately reduced,however,in the case of large nanofiber diameter(? 0.5 ?m),the reverse is true.For filtration of Aitken and accumulation mode particles,residual particle geometric mean diameter increases and decreases separately,and residual particle geometric standard deviation decreases.The size distribution of particle obeys the lognormal law as filtration goes on.Also based on the nanofiber coupling efficiency prediction model given above,the Euler-Euler two-phase flow model was employed to numerically analyze the filtration performance of conventional pleated filter and pleated composite filter for particles in the range of 0.1-1 ?m.The quality factor of the two types of pleated filter with various pleat numbers increases slightly as the permeability of the fibrous media decreases(fiber packing density increases or the nanofiber diameter decreases),except for small pleat numbers.When pleat number is less than 8~12 pleats/in,increasing the pleat height has a beneficial effect on the comprehensive filtration performance of the pleated filter.The quality factor of the two types of pleated filter first increases and then decreases as increasing pleat number.There is an optimal pleat number to maximize the quality factor.The optimal quality factor of the pleated composite filter increases with increasing pleat height,and increases as the diameter of the coated nanofiber decreases.The quality factor of the V-shaped pleated composite filter is slightly higher than that of the U-shaped pleated.The comprehensive filtration performance of the pleated composite filter is far better than the conventional pleated filter.The optimal pleat number based on the quality factor of the two types of pleated filter is generally about 1 to 2 pleats/in higher than the optimal pleat number based on the pressure drop.The pressure drop of six commercial fibrous media was measured at the normal-pressure and low-pressure conditions.Effects of filtration velocity,absolute pressure and Knudsen number on pressure drop,dimensionless drag per unit length of fiber and fiber filtration drag coefficient were analyzed.Based on the SEM image,fiber packing density and fiber diameter distribution of two representative commercial fibrous media,the 3D models of the virtual fibrous media were reconstructed using VBA and Auto CAD software.The flow field across the reconstructed 3D virtual fiber media was solved by the present slip velocity subroutine.The pressure drop predicted by our numerical model are coincided with our experimental data.Therefore,our slip velocity subroutine is accurate and reliable.Comparisons between fiber filtration drag coefficient of our experimental results and those expressions in the literature indicates that in the case of continuum flow regime(no-slip flow)tested at normal pressure condition,Davies' empirical correlation and Jackson and James theoretical model are consistent with our experimental results,while the theoretical models of Happel's and Kuwabara's overestimate the filtration drag of actual fibrous media.In the slip/transition flow regime tested at low pressure condition,Kirsch model and Ogorodnikov correlation are close to our experimental data,however,for the case of high Knudsen number,Kirsch model and Ogorodnikov correlation tend to slightly underestimate and overestimate the dimensionless drag coefficient respectively.Shou's model and Pich's theoretical expression overestimate the dimensionless drag coefficient to some extent,especially for the case of high Knudsen number.Davies' empirical correlation without considering slip effect is only applicable to the prediction of fiber dimensionless drag coefficient in the weak-slip regime(small Knudsen number). |
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