Single-Fiber Interception Efficiency for Elliptical Fibers

Synthetic fibers with non-circular cross-sections are used in air filters. These fibers may offer performance advantages over traditional fibers with circular cross sections because they have more surface per unit volume of fiber upon which particles can collect. Starting with a solution for the velocity field around fibers with elliptical cross-sections, expressions for predicting the single-fiber efficiency for particle collection by the interception mechanism were developed for elliptical fibers. The interception efficiency predictions depend on filter solidity, fiber properties such as size, aspect ratio, and orientation of the cross section relative to the incoming flow, and particle diameter. The expressions demonstrate that single fiber interception efficiency for elliptical fibers generally increases with increasing particle diameter, increasing solidity, increasing aspect ratio, and as the major axis of the ellipse becomes more perpendicular to the incoming air flow.     

Flow Field and Drag for Elliptical Filter Fibers

Filter fibers are now available in many cross-sectional shapes, including elliptical. Assuming viscous flow through a filter, analytical expressions are developed to predict airflow around elliptical fibers and the aerodynamic drag and lift on the fibers. Important variables influencing the velocity field and the drag are the length-to-width ratio of the elliptical cross section, the filter solidity, and the orientation of the fiber's cross section to the incoming airflow. Plots of the streamlines for flow around elliptical fibers show that pressure drop through the filter and particle collection are likely to increase as the length-to-width ratio and filter solidity increase and as more of the fibers are aligned with the cross section's major axis perpendicular to the flow. The expressions developed indicate that drag can vary by more than 4 orders of magnitude as filter properties change.     

Simulating and Modeling Particulate Removal Processes by Elliptical Fibers

A lattice Boltzmann-cellular automata (LB-CA) probabilistic model for two-phase flows was used to simulate the particle capture process of elliptical fiber. The pressure drop and capture efficiency due to various capture mechanisms (Brownian diffusion, interception, and inertial impaction) were investigated. It is found that the diffusional capture efficiency of the elliptical fiber is greater than that of the circular fiber because of its larger capture area, which is proportional to the aspect ratio. When the interception or inertial impaction is dominated, aspect ratio, orientation angle, and the ratio of particle diameter to the fiber diameter affect the capture efficiency of the elliptical fiber, which is usually higher than that of the circular fiber except that the major axis is parallel to the incoming flow. The correction factors for the pressure drop and capture efficiency of elliptical fiber from those of circular fiber were attained through the Levenberg–Marquardt algorithm, which is used to fit some well-organized LB-CA simulations. These empirical correction factors can combine the classical models for circular fiber to calculate the pressure drop and capture efficiency for elliptical fiber in a simple way. Finally, the quality factors of elliptical fibers as a function of the aspect ratio and orientation angle were investigated, which is conducive to optimization configuration of elliptical fiber in different operation conditions.

Copyright 2014 American Association for Aerosol Research     

The Single-Fiber Collision Rate and Filtration Efficiency for Nanoparticles II: Extension to Arbitrary-Shaped Particles

We extend the equations for the dimensionless collision kernel and filtration efficiency, attained previously via mean first-passage time (MFPT) calculations, to particles of arbitrary shape. Specifically, we show that the regression equations for the dimensionless collision rate found considering particle-fiber collisions driven by simultaneous diffusion and interception remain valid for non-spherical particles, provided that an appropriate collision length scale for the non-spherical particle (L) is defined and incorporated into the definitions of the dimensionless collision rate (H) and the diffusive Knudsen number (Kn_D). Regression equations are provided to calculate this length scale for quasifractal aggregates of varying fractal dimension, as well as cylinders. MFPT calculations reveal that, over ～5 orders of magnitude in H, these regression equations for the collision length are valid. Furthermore, using the previously attained proportionality between the predicted dimensionless collision rate and the single-fiber efficiency, comparison is made between the equations presented here and measurements of the penetration of both multiwalled carbon nanotubes and quasifractal aggregates through fibrous filters. Reasonable agreement is found between measured and predicted single-fiber efficiencies in both circumstances, supporting the use of the single-fiber efficiency calculation approach we developed.

Copyright 2014 American Association for Aerosol Research     

The Single-Fiber Collision Rate and Filtration Efficiency for Nanoparticles I: The First-Passage Time Calculation Approach

We describe an approach to filtration-efficiency calculations as an alternative to the traditional depth filtration theory. The new approach involves linking the single-fiber efficiency to the collision rate coefficient/kernel between nanoparticles and fibers, and correspondingly inferring the collision kernel via dimensionless mean first-passage time (MFPT) calculations. This method has the advantage of easily incorporating the influences of particle diffusion, inertia, and particle size; therefore, all filtration mechanisms can be considered simultaneously. Through non-dimensionalization of the equation of motion for a particle in MFPT calculations (the Langevin equation), it is shown that both the single-fiber efficiency E_f and dimensionless particle-fiber collision kernel, H, are functions of the ratio of particle radius to filter-fiber radius, R, the solid volume fraction in the filter, V_f, the ratio of particle persistence distance to the particle-filter collision distance, Kn_D (the diffusive Knudsen number), and the ratio of the particle translational kinetic energy to the thermal energy χ_f. Using a Kuwabara flow-cell model to define the geometry and flow field, MFPT calculations are used to determine H and E_f for nanoparticles in atmospheric pressure systems, i.e., when particle inertia is negligible but when diffusion and interception act in tandem to collect particles. From MFPT results, regression equations for both H and E_f are developed. A comparison is made between MFPT results and commonly invoked depth-filtration single-fiber efficiency relationships, experimentally measured values, and H equations derived from Sherwood number correlations based upon measurements of heat transfer from a fluid flowing perpendicular to an array of cylinders. Good agreement is found with both measurements and previously developed equations over a wide range of parameter space.

Copyright 2014 American Association for Aerosol Research     

Diffusion of Fibers in a Tubular Flow

Based on an ellipsoidal particle model, the equivalent diameter for the slip correction, diffusion coefficient, and diffusion diameter of fibers were obtained from the adjusted sphere method of Dahneke. The diffusion coefficient calculated for polydisperse crocidolite fibers compared favorably with available experimental data. Deposition of fibers in a tubular flow was then calculated with the use of the derived diffusion coefficient and applied to the human lung airways. The effect of velocity shear on particle orientation was also considered. It was found that the velocity shear had only a small effect on deposition. For a given fiber size, deposition increased in the lung distally, but at the same fiber diameter, the efficiency decreased with increasing aspect ratio.     

Functionalized Cellulose Fibers for Dewatering and Energy Efficiency Improvement

Paper drying accounts for nearly 80% of the energy used in the papermaking process. This is due to the high energy requirements for the process of drying by vaporization. Because the cost-effectiveness of the various physical means of dewatering far exceeds that of thermal drying, significant energy savings can be expected if the physical dewatering effectiveness is improved. To that end, a novel method of enhancing the physical dewatering process that involves the addition of hydrophobic fibers to the pulp furnish is described and evaluated. Freeness and water retention measurements indicate that the addition of hydrophobic fibers at even a few weight percent may have a significant impact on the freeness and water retention properties of the furnish and therefore a significant improvement in the effectiveness of the physical dewatering of webs made using the hydrophobically tailored furnish material.     

Analysis of the Single-Fiber Pullout Test Using Raman Spectroscopy: Part III, Pullout of Nicalon Fibers from a Pyrex Matrix

Single-fiber pullout specimens consisting of Nicalon fibers and a Pyrex-glass matrix were fabricated using a specially designed, simple mold. Raman spectroscopy was used to map the distribution of fiber strain within the glass matrix following deformation of the fiber section in air, and the results were compared with those predicted by theoretical analyses. It was demonstrated that the behavior also could be modeled using a partial debonding theory and that the force balance equilibrium allowed the variation of inter-facial shear stress along the interface to be derived. The partial debonding theory showed that debonding occurred at an interfacial shear stress of 60 ± 20 MPa and that the friction shear stress following debonding was 23 ± 8 MPa for the specimen investigated. Furthermore, it was shown that the fracture toughness of the fiber-matrix interface was 1.8 ± 0.6 J·m^–2.     

An Exact Solution of Interception Efficiency Over an Elliptical Fiber Collector

An exact solution for the interception efficiency of a particle carried by potential flow over an elliptical fiber acting as the collector has been developed based on the Zhukovsky conversion. It is shown that the interception efficiency depends on fiber geometric properties such as size, aspect ratio, the orientation angle of the incoming flow, and the particle diameter. The results show that the noncircular collector shape can improve the interception efficiency significantly when compared to a circular collector, and the maximum efficiency occurs when the incoming flow is parallel to the major axis of the elliptical fiber.

Copyright 2012 American Association for Aerosol Research     

Determination of Interfacial Properties Using a Single-Fiber Microcomposite Test

A single-filament microcomposite consisting of 143-μm-diameter CVD SiC, a thin "interfacial" coating, and a thick (130–160 μm) CVD SiC matrix sheath was tested in tension to determine the interfacial shear properties of carbon and BN interfacial coatings. Load/displacement and acoustic emission were used to determine the mechanical behavior of the microcomposites and matrix cracking. Optical and electron microscopy were performed to confirm the extent and nature of matrix cracking and to analyze the micro-composite microstructure. The interfacial properties were determined from matrix cracking and load/displacement hysteresis techniques. Direct measurement of the interfacial sliding stress from push-out and pull-out experiments were in good agreement with two of the three microcomposite systems studied.     

Collection Efficiency of Rectangular Slit-Nozzle Inertial Impactors with Impaction Plates of Elliptical Concave Curvature

In this study, the impaction plate with an elliptical concave groove was employed in an effort to enhance the collection efficiency of the slit-nozzle inertial impactor. The major axis length (A) and the minor axis length (B) of the elliptical concave curvature were varied. The effect of the ratio of A to B on the collection efficiency of slit-nozzle inertial impactors was investigated both numerically and experimentally. The numerical results were in good agreement with the experimental data for different values of A/B ratio. The elliptical concave impaction plates were useful in not only reducing the particle bounce but also lowering the cut-off size. The optimum range of the A/B ratio was found to exist for minimizing the cut-off size. When the elliptical concave impaction plates with the A/B ratio of 2 were used for the slit-nozzle inertial impactors, the square-root of the Stokes number was estimated to be lowered from 0.77 to approximately 0.6, resulting in the decrease of the cut-off size by 22% compared with the typical flat impaction plates.

Copyright 2013 American Association for Aerosol Research     

球形催化剂颗粒内气体扩散及其有效因子

研究了三元催化器处理天然气汽车尾气过程中球形催化剂颗粒内的尾气扩散系数、浓度分布以及催化剂有效因子(η)等影响因素.结果表明,适当增大催化剂颗粒的平均孔半径或增大分子的平均自由程,可使扩散系数在整个催化剂颗粒内均保持较高的值.随Thiele模数(Φs)值的减小,催化剂颗粒内的相对浓度分布更为均匀,且呈整体增大的趋势,有...     

Surface Crystallization and Water Diffusion of Silica Glass Fibers: Causes of Mechanical Strength Degradation

Pristine silica glass fiber is well‐known to become mechanically weaker when heat‐treated in air but the cause of such weakening is not presently known. The time dependence of mechanical degradation of various silica glass fibers containing varying impurity contents were studied in the range from 500°C to 1000°C. Two possible sources of strength degradation were considered: surface crystallization and water diffusion. Surface crystallization kinetics of silica glass fibers were investigated in a wide temperature range, including nanoscale surface nucleation at low temperatures via scanning electron microscopy. From the comparison of the strength degradation, surface crystallization, and water diffusion data in literature, it was concluded that surface crystallization may be responsible for the mechanical weakening observed in silica glass fiber surface during heat‐treatment at temperatures above ~800°C, whereas water diffusion into the glass surface may be responsible for the strength degradation at lower temperatures.     

钢板冲压中的工艺润滑

本文论述了钢板加工中润滑机理及常用的润滑剂。     

发展中的光纤保护性涂料

本文对发展中的光纤保护性涂料进行了介绍，重点介绍了热固化和紫外光固化两大类光纤涂料的固化原理、组成特性、涂覆工艺及主要性能。     

Fiber—Matrix Interface Properties of Single-Fiber Microcomposites as Measured by Fiber Pushin Tests

Fiber pushin tests have been performed on single-fiber microcomposite specimens embedded in glass. The interface properties determined were in agreement with those previously determined by two methods from tensile tests on microcomposites. The uncertainties in the interface properties calculated from the microcomposite pushin tests were not found to benefit from the superior alignment of the microcomposite specimens, as compared to literature results from composite specimens.     

椭圆形内开自紧式人孔的强度计算方法探究

锅炉、汽包等高压设备中经常使用的椭圆形内开自紧式人孔,具有自紧密封、方便拆卸、节省材料及空间等特点;高压设备上该类人孔的计算无参照标准,需进行结构设计.对比中外相关规范,对该类型人孔平盖强度计算、密封设计计算、铰链强度计算、筒节的设计计算等主要零部件的设计提出了一套计算方法,供设计人员参考.     

高科技纤维用于高性能能源工业

综述了燃料电池、锂离子电池、钠硫电池及太阳能电池的结构和性能以及高科技纤维在这些电池中的应用。文献表明，高科技纤维特别是纳米纤维越来越多地应用于高性能能源装置，例如锂离子电池用于电动车已实用化，太阳能电池在开发中。这是利用纳米纤维的比表面积大及排列取向的效果。静电纺丝法是取得纳米纤维的捷径，各国正在竞相发展。     

腈纶的油剂科技

腈纶油剂的作用一般包含使纤维与金属润滑,以保护纤维和金属表面接触,及使纤维与纤维润滑,以确保纱的均匀与质量。良好的抗静电保护和对湿度变化的低敏感性亦极为重要。长链磷酸酯被广泛认为对湿度变化低敏感,并具有良好的纤维对金属、纤维对纤维的润滑。本文提供了不同腈纶工艺对油剂需求的详细检视。一般上油量不超过0.3％～0.5％,但在工艺上却扮演极重要角色。油剂厂针对各种不同用途提供特别设计的油剂,及与纤维厂建立密切的合作伙伴关系,才能达到最佳的纤维工艺特性。     

Alternative Prooftester for High-Strength Optical Fibers

Reproducible, nondestructive methods of prooftesting continuous, multikllometer lengths of high-strength (>1.4 GPa) optical fiber are important in high-performance applications of lightguide technology. An alternative prooftester based on the concept of continuously imparting a constant strain rather than a constant weight to the optical fiber has been built and tested. The constant-strain design is somewhat simpler and has certain advantages over a widely used prooftester. The constant-strain method is explained, and prooftest data that substantiate the advantages of the design are presented.