不同排列结构的纤维层对捕集PM2.5性能影响的数值模拟

基于离散颗粒模型(Discrete Phase Model,DPM)研究了三种纤维排列结构捕集颗粒物规律.模拟了不同排列结构的纤维层在拦截和惯性碰撞两种捕集机制下捕集颗粒物的性能,考察了颗粒物粒径、入口风速和纤维层填充率对平行排列、单层垂直排列和双层垂直排列纤维层捕集颗粒物性能的影响.结果表明,当颗粒物粒径为0.5～2...     

R—S型旋风分离器的开发及在干燥过程中的应用

Ｒ－Ｓ型旋风分离器是在对旋风分离器出口管研究的基础上,充分利用气流的旋转能。开发出的一种由返流式旋风分离器在直流式旋风分离器优化组合的新结构。试验结果表明,该设备在压降相近的条件下,分离效率优于二级串联的旋风分离器,与三级串联旋风分离器的分离效率盯近,而压降仅为它的６０％。工业应用实践表明,在漂粉精生产装置中,该设备可取代脉冲布袋除尘器、用作振动流化干燥造粒装置尾气的粉尘捕集设备,捕集效率达９８％     

数值研究壁面参数对单纤维捕集效率的影响

基于开源OpenFOAM软件模拟单纤维过滤介质内气-固两相流运动,着重研究了单纤维过滤介质壁面参数与捕集效率之间的关系,即为纤维过滤介质的碰撞恢复系数和碰撞阻尼系数的变化对捕集效率的影响。将模拟结果与经验关联式进行对比验证,且具有很好的一致性。模拟结果表明:固体颗粒物的捕集效率随着碰撞恢复系数的增大呈现先减小后平稳的变化趋势,而碰撞恢复系数为0.5之后捕集效率基本保持稳定;颗粒物捕集效率随着碰撞阻尼系数的增加,固体颗粒物的捕集效率呈现先增大后平稳的趋势,且在碰撞阻尼系数在0.7以后,捕集效率基本趋于稳定。     

颗粒床-旋流耦合分离器不同操作模式下的性能分析

通过实验分别考察了满床/空床操作模式对内置颗粒床-旋流耦合分离器分离性能的影响,获得了两种操作模式下的设备压降和捕集效率。通过改变入口粉尘浓度、入口气速和粉尘颗粒种类,发现满床操作条件下的分离效率比空床操作条件下的分离效率高,且前者压降较低。通过对出口粉尘粒径的分析,含有捕集颗粒的内置颗粒床可有效提高5 μm以下的粉尘颗粒的捕集效率,弥补了离心分离的短板。引入性能指数对不同操作模式进行定量分析,验证了满床操作条件下的耦合分离设备具有更好的综合分离性能。     

方形截面纤维表面气溶胶粒子多机理过滤性能数值分析

采用数值方法求解绕方形截面纤维流场,考虑粒子布朗扩散、拦截效应和惯性碰撞捕集机理的联合作用,用布朗动力学方法研究方形截面纤维的过滤性能,考察了纤维迎风角(θ)、填充率(C)和过滤风速(u?)对捕集效率、质量因子及粒子沉积分布的影响。结果表明,小粒子的扩散捕集或大粒子的惯性捕集在方形纤维表面的粒子沉积行为均表现出显著的局部沉积特征,且与粒子捕集机理和迎风角有关。方形纤维质量因子的分析结果表明,在高填充率下,方形纤维的过滤压降虽高于圆截面纤维,但具有较高的捕集效率,综合过滤性能仍明显优于圆截面纤维,但在低填充率下,方形纤维综合过滤性能劣于圆截面纤维。     

三维丝网网胞特征参数的数值模拟

针对工业废气对环境造成巨大危害的问题,为提高脱白和VOCs废气处理效率,提出了三维网胞结构,并利用Fluent软件对该结构进行优化,研究了不同丝径、目数、层数的三维丝网结构对压降和水滴捕集效率产生的影响.结果表明:丝径和目数的增加会使压降增加,而水滴的捕集效率是先增加然后趋于稳定;随着层数的增加,压降减小,而水滴的捕集...     

气固顺流式移动床过滤器的除尘性能

为了探究气固顺流式移动床过滤器的除尘性能。通过在大型冷模实验中改变表观气速、颗粒循环强度、粉尘在过滤器中的比沉积率σ等操作参数,考察了过滤器的两个重要性能参数——操作压降和捕集效率的变化。实验发现,随着表观气速增大,设备的压降随之增大,设备的除尘效率呈下降趋势。随着粉尘的比沉积率σ增大,稳定后的操作压降也会有所增长,但操作压降的稳定性随着比沉积率的增大呈现"不稳定-趋于稳定-不稳定"的趋势,设备的过滤效率先逐渐增大,而后降低。当表观气速u_g为0.126 m×s^-1,比沉积率σ为0.000 735时,过滤器的操作压降可以达到相对稳定的状态,此时除尘效果最优,捕集效率可达97%以上。     

固定床颗粒层过滤性能分析及预测

使用两种滤料颗粒在一套冷态试验装置上考察了过滤气速和颗粒层厚度对颗粒层过滤性能的影响。结合颗粒层过滤宏观模型,分析了不同操作条件下粉尘比沉积率σ对颗粒层粉尘捕集能力偏离初始值程度F和过滤压降偏离初始值程度G的影响,然后预测了过滤效率和压降。结果表明,试验范围内,随着σ的增大,F呈现先增加后降低的变化趋势,而G逐渐增加。结合过滤气速u=0.2~0.6 m·s^-1、颗粒层厚度L=0.11~0.2 m条件下的试验数据拟合得到了F-σ和G-σ函数关系,过滤效率和压降的计算值与试验吻合较好,优于文献中相关公式,可为颗粒床过滤性能的预测提供参考。     

带液滴辅助捕集结构的折板除雾器分离性能研究

采用Fluent对折板除雾器内气液两相流场、分离效率和压降进行数值模拟。气相采用SST k-ω模型,液滴相采用离散相模型。为改善分离性能,引入液滴辅助捕集结构,对比前后除雾器内的流场和液滴轨迹的区别,结果表明,后者气相湍动能增强、出口液滴轨迹线减少。通过改变各级捕集结构高度并进行组合,研究效率和压降随着进口粒径的变化。然后改变进口粒径分布,模拟27种不同高度组合下的分离效率和压降,得到最优的组合方式,对除雾器设计有一定指导意义。     

固定床颗粒层过滤性能分析及预测

使用两种滤料颗粒在一套冷态试验装置上考察了过滤气速和颗粒层厚度对颗粒层过滤性能的影响。结合颗粒层过滤宏观模型,分析了不同操作条件下粉尘比沉积率?对颗粒层粉尘捕集能力偏离初始值程度F和过滤压降偏离初始值程度G的影响,然后预测了过滤效率和压降。结果表明,试验范围内,随着?的增大,F呈现先增加后降低的变化趋势,而G逐渐增加。结合过滤气速u=0.2~0.6 m·s~(-1)、颗粒层厚度L=0.11~0.2 m条件下的试验数据拟合得到了F-σ和G-σ函数关系,过滤效率和压降的计算值与试验吻合较好,优于文献中相关公式,可为颗粒床过滤性能的预测提供参考。     

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     

Numerical Study of an Electret Filter Composed of an Array of Staggered Parallel Rectangular Split-Type Fibers

The flow field through a staggered array of parallel, rectangular split-type electret fibers was numerically modeled. The particle trajectory and the collection efficiency were simulated by solving the equation of particle motion, taking into account the effects of diffusion, interception, inertial impaction, and electrostatic forces. The model was validated against results calculated from semiempirical expressions. The model was applied to investigate the role of the inertial impaction and the interception mechanisms in the particle collection by an electret fiber, the particle trajectories under various filtration conditions, the effect of the aspect ratio of the rectangular fiber on the filter penetration, and the distribution of the deposited particles on the surface of the fiber. The simulated results indicate that the inertial impaction and interception mechanisms account for a major portion of neutral particles collected by an electret fiber when the Stokes number is higher than 0.5. For neutral particles, fibers with an aspect ratio of 38/10 have almost the same penetration as fibers with an aspect ratio of 10/38; while for singly charged particles, fibers with an aspect ratio of 38/10 achieve a much lower penetration when the electrophoretic collection mechanism dominates. In addition, it is predicted that a filter composed of fibers with an aspect ratio of 38/10 will result in a lower flow resistance and thus a slower clogging process when the dielectrophoretic collection mechanism dominates.     

The Influence of Fiber Geometry and Orientation Angle on Filtration Performance

In this article, the particle filtration processes of five noncircular fibers with triangular, quatrefoil, trilobal, rectangular, and elliptical cross-sections were numerically investigated, and the pressure drop, capture efficiency, and quality factor due to three main capture mechanisms (diffusion, interception, and inertial impaction) were calculated. By comparing the results with circular fiber, which has the same volume fraction as the five noncircular fibers, the following results can be found. The diffusional capture efficiency, which is highly dependent on the superficial area of fibers, is almost independent of the orientation angle for all fibers. For the quatrefoil, trilobal, and elliptical fibers, as the aspect ratio of component ellipse increases, the capture range also increases, as does the capture efficiency due to three different mechanisms. When considering submicron particles with medium size, which are difficult to capture with circular fibers (especially when dominated by the interception mechanism), triangle and trilobal fibers have higher capture efficiency when the orientation angle is 0° and 60°, respectively. The quality factors of these fibers due to the three capture mechanisms were also investigated in this article. The triangular and rectangular fibers placed horizontally perform better for intermediate and high-inertia particles, and the elliptical fiber placed horizontally shows an advantage in capturing small particles with strong Brownian diffusion.

Copyright 2015 American Association for Aerosol Research     

Dendritic deposition of aerosol particles in fibrous media by inertial impaction and interception

When an aerosol of fine solid particles flows through a fibrous filter, particles deposit on fibers and form chainlike agglomerates known as dendrites (see for instance, Fig. 1). These dendritic structures grow relatively unhindered for an initial period of time, but eventually they begin to interfere with each other's growth and to intermesh (Fig. 2). This pattern of deposition has profound effects on the filtration efficiency and pressure drop, both of which increase rapidly with time. Therefore, rational design, optimization, operation, troubleshooting and innovation require intimate understanding and accurate analysis of the dendritic deposition process. A theoretical model of dendritic deposition for the period during which dendrites do not intermesh was developed by Payatakes [1], subject to the assumption that interception is the dominant capture mechanism. In the present work the model is extended to include deposition by inertial impaction and interception, mechanisms which are dominant for particles larger than about 1 μm. The “shadow effect” is also incorporated in the analysis.     

异形纤维阵列过滤特性的数值模拟

为研究异形纤维排布方式对其过滤性能的影响,以矩形及交错排布为基础,在保持纤维体积分数不变的条件下,通过改变纤维的列间距调整阵列结构,运用CFD-DEM耦合方法对含尘空气通过具有不同异形纤维阵列结构的简化过滤器模型过程进行数值模拟。结果表明：当纤维的体积分数保持不变,交错纤维阵列较矩形阵列过滤效率高出35%,且对于颗粒的吸附力更强;而在交错阵列的基础上,调整列间距得到的前密阵列和后密阵列均可保持80%左右的过滤效率,且不影响颗粒吸附力的大小,但前密阵列产生的压降更低,即具有更高的品质因数;在整个过滤过程中,纤维-颗粒间的黏附作用远远高于颗粒与颗粒间的相互作用,说明颗粒过滤主要来源于纤维-颗粒作用的贡献。     

Pressure Drop and Interception Efficiency of Multifiber Filters

ABSTRACT

The viscous flow fields around multifiber filters have been investigated in a previous paper. The results of the previous work show that the flow becomes periodic immediately after the first fiber array downstream from the entrance if the fibers are arranged uniformly along the flow direction. The characteristics of such flow fields enable the pressure drop and the particle interception efficiency of a multifiber filter to be represented by single-fiber models. The total filtration efficiency, however, cannot be so represented since fibers interact during filtration processes. In this study, the pressure drop and the interception efficiency were investigated by making use of the viscous flow fields modeled in the previous research. The fiber separation ratio was found to have significant effects on pressure drop and efficiency. At a given volume fraction, changes in the fiber separation ratio will result in changes to the patterns of fluid flow and aerosol particle motion. Therefore, the fiber separation ratio significantly affects pressure drop and interception efficiency.     

Finite element modeling of ultra fine particle filtration by a membrane filter

Theoretical work has been carried out to investigate the filtration of ultra fine aerosol particles in a membrane filter. The analysis was done using a finite element method with a Newtonian fluid model for the carrier medium. Both inertial filtration and diffusional filtration were considered. Prior to the main analysis, our numerical scheme was tested with the analytical results for the diffusion of particles in the cylinder and showed good agreement, which confirms the importance of axial diffusion occurring in a short cylinder like a very thin membrane filter. Particle size, porosity, pressure drop, and flow velocity are found to be main variables that determine the filter efficiency. Two important mechanisms of filtration have opposite effects on the efficiency, depending on the variables. Increases in particle size, pressure drop, and flow velocity cause increases in the efficiency for intertial deposition, while decreases in those variables cause increases in the diffusional efficiency. The existence of a minimum value of total filtration efficiency (sum of inertial efficiency and diffusional efficiency) was indicated for intermediate values of the variables. Lower porosity is found to favor inertial deposition more than diffusion. Some other effects of filtration conditions on the total efficiency are also discussed.     

Continuum Model Evaluation of the Effect of Saturation on Coalescence Filtration

Abstract

Coalescing filters are widely used throughout industry for removal of liquid aerosols from gases or the separation of liquid droplets from emulsions. Typical filters are constructed of non-woven fibers. Fibrous filters are capable of efficient removal of micron and submicron sized droplets and particles. The filtration process is highly complex due to variability in fiber sizes, particle sizes, mixtures of particles and droplets, mixture of types of droplets (oil, water, etc.), and effects of viscosity, surface tension, and chemical reactions between components or with the filter fibers. Prediction of filter performance under such complex conditions is difficult.

Performance of a filter depends on many factors like particle and fiber sizes, flow rate, surface properties of the fibers etc. One of those parameters is the saturation of the filter medium. Saturation is a measure of the amount of liquid present in the void space. Prior models assume that the saturation is uniform along the depth of the medium. In real media, the liquid holdup at steady state need not be uniform with position. Local velocity increases when the saturation is high.

In this paper, a steady state model for a coalescing filter is used to evaluate the effects of saturation on void fraction and its subsequent effect on filter performance. Single fiber mechanisms of direct interception and diffusion deposition are used to model droplet capture efficiencies and drag forces. These mechanisms are applied to volume averaged continuum equations in which the saturation is varied linearly with position in the filter. The results show the minimum pressure drop and largest quality factor occurs with a uniform saturation profile and that variation in average saturation has a greater effect on filter performance than does the slope of the linear saturation profile. The model predicts that uniform saturation profile performs better than the other profiles.     

Retracted: Effects of the Operating Conditions and Geometry Parameter on the Filtration Performance of the Fibrous Filter

The gas‐solid two‐phase flows in fibrous filters were simulated by computational fluid dynamics (CFD) technology. The pressure drops and filter efficiencies with different operating conditions and geometry parameter, including face velocity, particle size, and solid volume fraction (SVF) were calculated. The effects of the operating conditions and geometry parameter on the filter performance of the fibrous filter were obtained. The results indicate that the pressure drop increases linearly with the face velocity and the predicted values of the pressure drops are in excellent agreement with the experimental correlation. Filtration efficiency decreases with the face velocity for submicrometer particles (0.1 μm) and, for larger particles (1 μm) the tendency is just the opposite. The filtration mechanism is different for different particle sizes. For the filter in this paper, when the particle size is smaller than 0.2 μm, Brownian diffusion plays a significant role in the filtration process. When the particle size is greater than 0.5 μm, inertial impaction becomes an important capture mechanism. For particle sizes in the range of 0.2–0.5 μm, the Brownian diffusion and inertial impaction are both relatively weak and, therefore, the filtration efficiency has the least value in this range. Additionally, the SVF distribution is an important geometry parameter in the filter. The filtration efficiency of the filter with a decreased SVF (geometry B) along the thickness of the filter is higher than that of the filter with the even SVF (geometry A), while maintaining a low pressure drop.