褶型空气过滤介质微观结构三维建模及其含尘性能模拟

基于实际熔喷纤维电镜扫描(SEM)图像,建立了不同褶间角、层数、直径、纤维个数、曲率的褶型空气过滤介质微观结构模型,实现了对褶型空气过滤介质微观结构的动态控制.通过计算流体力学和离散单元法(CFD-DEM)耦合的方法对褶型过滤介质含尘过滤阶段的过滤特性进行模拟,将模拟结果与相关经验关联式比较.结果表明,所建模型与实际过滤介质电镜图像基本相似,压力损失模拟值与计算值误差在10%以内,表明CFD-DEM耦合计算方法可行;在含尘过滤阶段,同一进口风速(v)下随颗粒沉积量增加,压力损失非线性增大;不同进口风速下的压力损失增加均随过滤时间增加而增大,v=0.6 m/s时压力损失增加最大,为0.214 Pa,v=0.4 m/s时压力损失增加最少,为0.133 Pa.当3.216 ms时,颗粒出现明显沉积与团聚,纤维彼此贯穿的区域沉积更明显,表明纤维的排列方式对颗粒的沉积影响显著.     

基于阻力的V型褶式滤芯结构参数的响应面法优化

应用计算流体力学方法建立褶式滤芯阻力的二维计算模型,对其气相流场进行了数值模拟,并与文献结果比较. 在此模型基础上,利用响应面法研究了褶高(h)、褶间距(W)和过滤风速(Vf)对V型褶式滤芯阻力的影响,得出了预测模型,并对滤芯结构参数和运行条件进行了优化. 结果表明,滤芯阻力的模拟值和文献计算值变化趋势一致;h, W及Vf对滤芯阻力均有影响,但影响显著性Vf>W>h;滤芯阻力随Vf近似呈线性增大,当Vf一定时,存在最优W值使滤芯阻力最小,且最优W值随h增大而增大;在本研究的各参数范围内,最佳结构参数h=25 mm, W=10.899 mm,最佳运行参数Vf=0.005 m/s.     

移动式颗粒床气固分离性能冷态试验

以热解为首要步骤的煤炭分质转化技术可在较温和的条件下实现煤炭高效利用,其中含尘热解气净化是该技术规模化应用需要解决的关键问题之一。颗粒床过滤技术具有过滤效率高、滤料成本低等优势。为考察移动式颗粒床对粉煤热解气除尘过滤的有效性,选用小麦为颗粒床冷态试验的滤料介质,硅胶粉为试验粉尘,在自行设计的冷态试验平台上考察各操作条件对颗粒层过滤除尘效率的影响规律。结果表明:过滤效率的影响因素顺序为:表观风速过滤层厚度滤料下料速度;过滤效率随表观风速的增大而减小,随过滤层厚度的增加而增大。过滤层厚度增至200 mm以上时,过滤效率变化较小;而滤料下料速度增大,过滤效率减小。粉尘粒径达10μm后,过滤效率基本可维持在98%。在优化的过滤条件下(过滤层厚度为300 mm,滤料下料速度为0.002 m/s,过滤时间为10 min),最佳过滤效率可达98.1%。试验结果不仅实现了对常温下移动颗粒层过滤性能的预测,也为粉煤热解含尘煤气颗粒床热态除尘装置的设计提供了依据。     

旋流气浮中气泡-颗粒碰撞效率影响因素理论分析

在旋流气浮接触区碰撞模型基础上,通过理论计算考察了物性、运行和结构参数对分散相颗粒/油滴与气泡碰撞效率的影响. 结果表明,物性参数中的分散相粒径与密度、运行参数中气泡直径与切向速度和结构参数中等效旋流直径对碰撞效率影响较大. 在旋流气浮工艺中,碰撞效率随分散相粒径增大而增大,但随气泡直径和分散相颗粒/油滴密度增大而减小;分散相粒径小于0.02 mm时,碰撞效率随切向速度增大而减小、随等效旋流直径增大而增大;分散相颗粒/油滴大于0.02 mm时,碰撞效率随切向速度增大而增大、随等效旋流直径增大而减小. 旋流气浮去除的主要是油滴大于0.02 mm的非溶解性油,因此,设备紧凑可提高气泡与分散相颗粒的碰撞效率,达到高效分离目的.     

折叠滤芯气液过滤性能测试及优化

采用聚结型滤芯气液过滤性能实验装置,研究了油雾加载率和表观过滤速度对折叠滤芯过滤性能的影响及涂覆粘合剂对折叠滤芯过滤性能的优化作用。结果表明,涂覆粘合剂后,滤材抗张力强度明显增大,滤材孔径减小。随油雾加载率增大,滤芯过滤层液体运移通道数增加,通道压降升高。初始压降随表观过滤速度增加而升高。粘合剂主要凝固在渗透性低的区域,压降变化较小。表观过滤速度增加抑制了二次夹带,折叠滤芯过滤效率升高,而由于粘合剂脱落,涂覆粘合剂的滤芯过滤效率下降。表观过滤速度为0.10 m/s时,随油雾加载率增大,聚结在滤材表面的粘合剂抑制夹带,滤芯过滤效率升高。     

旋风-颗粒床过滤器两级除尘的分析与优化

为解决粉煤热解工艺中焦油气除尘问题,采用单因素和正交试验法研究了滤料粒径、滤料厚度和氮气流量对旋风-颗粒床过滤器两级除尘性能的影响。结果表明:在实验参数范围内旋风分离器平均除尘效率为98.43%。相比石英砂滤料,以半焦为过滤介质的颗粒床过滤器具有较高的过滤性能。滤料粒径由1.25—2.5 mm减小到0.38—0.83 mm时,颗粒床过滤器平均除尘效率由93.88%增大到97.18%,而最大床层压降也随之增加了59.99%。滤料厚度由100 mm增加到200 mm时,颗粒床过滤器平均除尘效率由90.99%增大到95.91%。随氮气流量的增大,颗粒床过滤器除尘效率降低,而两级除尘系统总效率略有降低。正交试验结果表明:滤料厚度是影响两级除尘效率的主要因素,其次是滤料粒径,氮气流量对系统总除尘效率影响最小。     

磁性纤维直径对捕集Fe基细颗粒影响数值模拟

为了实现对钢铁行业微细颗粒的超低排放,提出磁性纤维提高对 Fe基细颗粒物的捕集。 基于计算流体力学?离散相模型CFD-DPM对比研究了传统纤维、磁性纤维直径对Fe基细颗粒捕集效率以及过滤阻力的影响。结果表明：当风速为0.10 m/s时,对于直径为35~45 μm范围的纤维,直径的增大能够明显增加过滤阻力。对于粒径小于2.5 μm的颗粒,磁性纤维直径的增加对捕集效率提高的影响相对较小,当颗粒粒径大于2.5 μm时,增大纤维直径能够显著提高捕集效率。风速处于0.01~0.05 m/s范围时,增大纤维直径对提高磁性纤维捕集效率作用明显;当风速为0.08~0.10 m/s时,纤维直径变化对捕集效率的影响较小。磁性纤维质量因子随纤维直径增大而下降。     

内联式脱液器分离性能的实验研究

以空气-纯净水为实验介质,采用称重法和控制变量法对自制的内联式脱液器的脱液性能进行了实验研究,分析了入口流速和含液量对分离效率及压力损失的影响. 结果表明,在入口流速不变的条件下,分离效率随含液量增加而增大,含液量超过一定值后,随含液量增加而下降,该定值随速度增加而增大;含液量对压力损失影响不大. 分离效率和压力损失均随入口流速增大而增加,液滴粒径为50~80 mm、入口流速从14 m/s增加到22 m/s时,气液分离效率从39.17%增加到77.85%,压力损失从2200 Pa增加到3400 Pa. 所设计的内联式脱液器脱液效果良好.     

不同滤料固定颗粒床过滤性能对比

在固定床冷态实验装置上研究了平均粒径和稳态过滤压降相近、颗粒形状和表面状况不同的两种滤料(陶瓷球和石英砂)的过滤性能. 结果表明,两种滤料过滤性能变化规律基本一致. 粉尘沉积量(?m)增加,过滤效率先增大后减小,过滤压降偏离稳态过滤压降程度(G)增大. ?m相同时,过滤效率和G均随过滤气速增大而减小,但过滤后期高气速下G与低气速下接近. 增大入口粉尘浓度,总体过滤效率无明显改变,粒径大于0.7 ?m的粉尘过滤效率提高,G更显著. 两种滤料难过滤粉尘粒径均为0.35~0.6 μm. 因颗粒形状和表面状况不同,两种滤料过滤性能存在差异,其它条件相同时,石英砂总体和分级过滤效率均高于陶瓷球,G也较大;增大入口粉尘浓度,石英砂过滤效率随粉尘沉积量变化程度相对较小.     

有限截面通道内喷雾顺流掺混动力学特征的实验研究

设计搭建了有限截面通道顺流喷雾掺混实验台,在横截面为70 mm×70 mm的透明方形掺混段内,将室温水经喷嘴雾化后顺流掺入不同流速的室温空气。实验中,喷水压力为0.1～1.5 MPa,风速为14.6～46.2 m?s^-1。分别采用高速摄影和马尔文粒度仪对该雾羽的速度场和初始粒径等动力学特征开展了实验研究。结果指出：掺混雾羽的径向速度及喷射轴线附近的轴向速度主要受制于喷水压力;而雾羽两翼处的轴向速度主要受风速影响。定义轴向平均速度为雾羽轴向特征速度,该平均速度随喷水压力或喷射距离的增大而增大;在喷水压力小时,风速的增大可使轴向平均速度随喷射距离增大的速率提高;在喷水压力高时则反之。掺混雾羽的初始粒径随喷水压力的减小或喷嘴出口处气液相对速度的增大而减小。最后,根据实验结果拟合了轴向平均速度和初始粒径的实验关联式,其计算值与实验值吻合良好。     

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.     

Characteristics of Nylon 6 nanofilter for removing ultra fine particles

Electrospinning is a fabrication process that uses an electric field to make polymer nanofibers. Nanofibers have a large specific surface area and a small pore size; these are good properties for filtration applications. In this paper, the filtration characteristics of a Nylon 6 nanofilter made by electrospun nanofibers are tested as a function of the fiber diameter. Nanofilter media with diameters in the range of 100–730 nm can be produced in optimized conditions. The pressure drop of a Nylon 6 nanofilter linearly increases with the increasing face velocity. An electrospun Nylon 6 filter (mean fiber diameter: 100 nm) shows a much lower pressure drop performance relative to the commercial HEPA filter media when the filtration efficiency of the Nylon 6 nanofilter and the HEPA filter are over 99.98% with test particles of 0.02–1.0 μm in diameter. The pressure drop at 5 cm/s of the face velocity is measured as 27 mmAq for the Nylon 6 nanofilter media, and 37.1 mmAq for the HEPA filter media. The particle size with minimum efficiency decreases with the decreasing fiber diameter. And the minimum efficiency becomes greater as the fiber diameter is decreased.     

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.     

Application of nanofibers to improve the filtration efficiency of the most penetrating aerosol particles in fibrous filters

Conventional, mechanical fibrous filters made of microfibers exhibit a local minimum of fractional collection efficiency in the aerosol particle size-range between 100 and 500 nm, which is called the most penetrating particle size (MPPS). Simple theoretical calculations predict that this efficiency may be significantly increased using nanofibrous media. The main objective of this paper is an experimental verification of these expectations and simultaneously checking whether this anticipated gain in the filtration efficiency is not overpaid with an excessive pressure drop. For this purpose we developed a modified melt-blown technology, which allowed us to produce filters composed of micrometer as well as nanometer sized fibers. One conventional microfibrous filter and five nanofibrous filters were examined. The complete structural characteristics, pressure drop and efficiency of removal of aerosol particles with diameters 10-500 nm were determined for all media. The results of the experiments confirmed that using nanofibrous filters a significant growth of filtration efficiency for the MPPS range can be achieved and the pressure drop rises moderately. Simultaneously, we noticed a shift of the MPPS towards smaller particles. Consequently, the quality factor for bilayer systems composed of a microfibrous support and a nanofibrous facial layer was considerably higher than this one for a conventional microfibrous filter alone. Additionally, it was found that utilization of many-layer nanofibrous filters combined with a single microfibrous backing layer is even more profitable from the quality factor standpoint. Comparing experimental results with theoretical calculations based on the single-fiber theory we concluded that for microfibrous filters a fairly good agreement can be obtained if the resistance-equivalent fiber diameter is used in calculations instead of the mean count diameter determined from the SEM images analysis; in the latter case, filtration efficiency computed theoretically is slightly overestimated. This is even more evident for nanofibrous media, suggesting that in such case a structural filter inhomogeneity has a strong influence on the filter efficiency and its resistance and one should strive for minimization of this effect manufacturing nanofibrous filters as homogeneous as possible. We can finally conclude that fibrous filters containing nanofibers, which are produced using the melt-blown technique, are very promising and economic tools to enhance filtration of the most penetrating aerosol particles.     

三维上流式反应器床层流动和返混特性

采用内径为280 mm的上流式反应器,以空气模拟气相、甘油和水混合溶液模拟渣油。用3种不同粒径的氧化铝球形工业催化剂颗粒为填充颗粒,考察了不同模拟物系的颗粒粒径、颗粒密度、液相黏度、不同床层的高径比和不同操作条件对上流式反应器内床层压降及其波动、床层轴向返混的影响规律。得到模拟工业运行物系和操作条件的上流式反应器床层总压降关联式,相对误差在12%以内。床层总压降均随床层高径比、颗粒密度和液相黏度增加而增大,但随颗粒粒径的增大而减小,床层压降波动随表观气速增加而增大。填充颗粒粒径越小、颗粒密度越小、高径比越大,床层内轴向返混越严重;床层内压降和轴向返混均随表观气速的增加而增大。     

Dust Collection by a Fiber Bundle Electret Filter in an MVAC System

Electret filters are composed of thin, electrically charged fibers that are often utilized in industrial fields that require high collection efficiency with low flow resistance. A bundle-type electret filter in the Mechanical Ventilation and Air-Conditioning (MVAC) system of a Metro-subway was characterized in this study. The particle penetration and pressure drop parameters were examined under a filtration velocity ranging from 0.5 to 2.5 m/s. Particle penetration increased significantly in the early stages of filtration, but then became steady. The filter quality, which is a useful index of the filtration performance incorporating pressure drop and filtration efficiency, was evaluated for the test filters. The fiber bundle filter demonstrated a higher filter quality than the mechanical filter or the general panel-type electret filter with a small drop in pressure even at a high filtration velocity. In addition, the three dimensional structure and high electrostatic charge of the fiber bundle filter would enable a long retention time and constant level of pressure drop throughout the filtration.     

Centrifugal Filter for Aerosol Collection

Air filters collect particles by the mechanical collection mechanisms, namely, inertia, interception, gravitational settling, and Brownian diffusion. There exists the most penetrating particle size (MPPS) in submicron size range for which none of the collection mechanisms work effectively. In this study, we propose a new type of filter named as “centrifugal filter,” which collects aerosol particles by centrifugal force together with the conventional mechanical collection mechanisms. The centrifugal filter proposed in the present work may be rotated by a motor or compressed air. Air passes through the filter in the axial direction of filter rotation. The filter rotates so does the air embedded in the filter, and therefore centrifugal force exerts on particles. In addition to the mechanical collection mechanisms, small migration of particles due to the centrifugal force enhanced the collection efficiency of submicron particles significantly without increasing the pressure drop. The performance tests of centrifugal filter were conducted by changing the fiber diameter, the air flow velocity and the rotation speed. We found that the collection efficiency of filter is enhanced significantly by rotating the filter without increasing the pressure drop and that the filter efficiency is well predicted by the conventional filtration theory accounting for the centrifugal force.

Copyright 2015 American Association for Aerosol Research     

Filtration Efficiency and Pressure Drop of Miniature Diesel Particulate Filters

A method was developed to evaluate miniature diesel particulate filters (DPFs). To validate the performance of the instrumentation and test apparatus, measurements were made using silicon carbide (SiC) and cordierite miniature filters with representative microstructures. Filtration efficiency (FE), the most penetrating particle size (MPPS), and pressure drop were measured for catalyzed and uncatalyzed advanced ceramic material (ACM) acicular mullite and representative commercial filters to determine the impact of substrate morphology, the formation of a soot cake, and the presence of a catalyst coating on filtration properties. FE measurements demonstrated that filter geometry and microstructure significantly influence initial filtration performance. ACM filters had high initial FE and the MPPS near ～200 nm. Reduction of the ACM pore size in the absence of a reduction in porosity increased initial FE even more, but its influence on MPPS was not resolvable. The presence of a catalyst and washcoat on the ACM increased the pressure drop but increased initial FE and reduced MPPS to <100 nm. The addition of a washcoat allowed the rapid buildup of a soot cake, which resulted in a more rapid rate of increase in FE compared to uncatalyzed ACM. The similarity in the ACM and cordierite soot cakes after a long loading time is consistent with theory that suggests the formation of the soot cake depends primarily on the Péclet (Pe) number, which is influenced only by macroscopic filter geometry and prevailing test conditions.

Copyright 2013 American Association for Aerosol Research     

操作温度对褶皱型不锈钢编织型过滤器阻力特性的影响

Effect of working temperature on the resistance characteristic including the permeability coefficient and the pressure drop evolution of a pleated stainless steel woven filter with a nominal pore size of 0.5 μm has been studied. The permeability coefficient was obtained based on the pressure drop data and the Darcy’s law. In three filtration experiments, pure carbon dioxide at 283 K, nitrogen at 85 K and liquid helium at 18 K are adopted, respectively. It is found that the permeability coefficient decreases at the working temperature due to the cold shrink of the filter element at cryogenic temperature. Then, two kinds of feed slurries, mixture of liquid nitrogen and solid carbon dioxide at 85 K, and mixture of liquid helium and solid nitrogen at 18 K, flow into the filter cell. The solid particles are deposited on the filter surface to form a filter cake and the purified liquid flows through the filter. It is found that the pressure drop evolution shows the same trend on these two temperatures, which can be divided into three stages with high filtration efficiency, indicating the feasibility of the filter for cryogenic application. However, variant cake resistances are obtained, which is resulted from the different interactions between solid particles in the feed slurry at lower working temperature.     

A semi-analytical model for gas flow in pleated filters

A semi-analytical model of gas flow in pleated fibrous filters is developed for large filtration velocities. This case presents two main new and distinguishing features compared to the low filtration velocity situations studied in previous works: the velocity profiles are not parabolic within the pleat channels and the filtration velocity is not uniform along the pleated filter element and this has a great impact on the filter loading. The model relies on similarity solutions to the Navier–Stokes equations in the channels formed by pleating the filter medium. After validation by comparison with direct CFD simulations and experimental data, the model is used to determine the optimal pleat density, i.e. the pleat density minimizing the overall pressure drop across the filter for given flow rate, pleat length and given filter medium properties. As illustrated in the paper, this model greatly facilitates the study of flow within the pleated filter compared to a standard CFD approach. It represents an excellent basis for the more involved problem of filter loading computation. In particular, no remeshing across the width of the pleated filter entrance channels is needed when a filtration cake forms at the channel walls.