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。     

Scale effects in fluidized multiphase reactors

It is shown that the two-phase model for bubbling gas—solid fluidized beds can be extended to bubble column slurry reactors operating in the heterogeneous flow regime by proper definition of the ‘dilute’ and ‘dense’ phases. The ‘dilute’ phase in a bubble column slurry reactor is to be identified with the fast-rising ‘large’ bubbles. The ‘dense’ phase consists of the slurry phase in which ‘small’ bubbles are finely dispersed. With the aid of extensive experimental data obtained in columns of 0.1, 0.19 and 0.38 m diameter it is shown that the rise velocity of the ‘dilute’ phase for gas—solid fluid beds and slurry reactors show analogous scale dependencies and can be modelled in a similar manner. It is also demonstrated that fluidized multiphase reactors can be modelled in a common manner using Computational Fluid Dynamics (CFD) within the Eulerian framework. It is concluded that CFD is an invaluable tool for scaling up of fluidized multiphase reactors.     

Correction for sampling errors due to coagulation and wall loss in laminar and turbulent tube flow: direct solution of canonical ‘inverse’ problem for log-normal size distributions

Aerosol sampling from industrial environments (e.g. combustion engines) or natural environments (e.g. the troposphere) frequently involves conveying the sample to a downstream (‘sheltered’) instrument via an upstream tube or duct. While the instrument may be capable of characterizing, say, the particle size distribution (PSD) of the aerosol actually presented to it, the investigator is, of course, usually more interested in the PSD of the aerosol entering the upstream sampling tube. Invariably, this differs from that measured because of several systematic phenomena—perhaps the two most obvious of which are particle size-dependent losses to the tube walls (i.e. incomplete ‘penetration’) and PSD distortion due to suspended particle-particle coagulation when the particle concentrations are sufficiently high. We show here how recent research on the use of ‘moment methods’ to predict the effects of size-dependent walls loss and/or Brownian coagulation in flow systems can now be brought to bear to conveniently solve this ‘inverse’ problem by numerically integrating the quasi-one-dimensional coupled moment equations in the upstream direction, using downstream (measured) aerosol properties in the definitions of all dimensionless dependent variables and parameters. Illustrative ‘universal’ graphs are presented here for the sampling of log-normally distributed ‘inertialess’ (Brownian) aerosols in long straight adiabatic ducts for both commonly encountered extremes of particle Knudsen number Kn_p 1(free molecule) or Kn_p 1 (continuum), as well as convenient rational approximations derived from the leading terms of a Taylor series expansion of the above-mentioned dimensionless moment equations.     

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     

Particle agglomeration and capture as a result of synergism between inertial impaction and eddy diffusion

Particle capture experiments conducted in turbulent cross flow with various aerosols involving liquid and/or solid particulates have resulted in collection efficiencies which are in excess of the values predicted by the various known models of particle capture, i.e., inertial impaction, interception and Brownian diffusion. In one type of experiment the turbulent air stream, carrying submicron dust particles, is flowing past a cylindrical collector (such as a piece of wire) with its axis orientated perpendicular to the direction of flow. Collecting efficiencies ranging up to about 20% have been found under conditions where the conventional models of particle capture predict practically zero collection efficiencies. In another type of experiment involving injecting a fog (2–80 μm diameter water droplets) into the dusty gas stream carrying submicron size dust particles which subsequently enters a slow turning fan. While passing through the fan, the fog agglomerates into raindrops while scavenging most of the dust particles. For example, a 0.8 μm median particle size aluminum silicate pigment was collected with 97–99.5% efficiency, the exact value depending on the operating conditions. Theoretical analysis of these phenomena may be based on the idea of synergism involving inertial impaction and eddy diffusion: the smaller dust particles/drops are captured by the larger drops in the fan and the dust particles are captured by the wires because (a) there is a significant relative velocity between them and (b) because the particles undergo eddy diffusion.     

天然气过滤器滤芯工艺计算

文章从工艺设计的角度通过过滤除尘机理,测算了滤料中过滤器的惯性捕捉效率、布朗扩散效率、直接拦截效率以及各种效应的联合效应得出所能达到的过滤总效率,根据压降测算出不锈钢烧结金属纤维毡的过滤精度和使用寿命,保证过滤器的使用提供了理论依据。     

Modeling Entry of Micron-Sized and Submicron-Sized Particles into the Indoor Environment

A theoretical approach, based on particle dynamics, was used to examine the outdoor-to-indoor penetration coefficient ( P ) of fine particles inside thin rectangular cracks. Parallel-plate flow theory indicates that crack infiltration flow can be assumed laminar for long, thin rectangular cracks. Considering laminar crack flow, three particle penetration models were used to estimate P . They are the Licht model, the Fuchs model, and the Taulbee model. The first two models consider gravitational sedimentation as the particle deposition mechanism, while the third model considers particle deposition induced from both gravitational sedimentation and Brownian diffusion. Modeling results indicate that gravitational sedimentation governs particle deposition behavior for micron-sized particles, and that all three models can be used to model penetration for these particles. For submicron-sized particles, Brownian diffusion becomes the major deposition mechanism, and only the Taulbee model is suitable to model particle penetration. The Taulbee model was validated using published experimental results of other researchers. Model validation indicated that the Taulbee model satisfactorily estimates particle penetration for micron-sized and submicron-sized particles. Application of the three models to actual building penetration is discussed.     

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     

Measurement of effective diffusivity in catalyst-coated monoliths

A review is provided about techniques that have been used to evaluate the effective diffusivity of gases in catalyst/washcoat layers, as used in catalytic monoliths.

The importance of making such measurements is described, in order to ensure that the choice of model for effective diffusivity can be verified, and if necessary an appropriate value of tortuosity can be back-calculated. Based on methods described in the literature, it is concluded that, where possible experiments should be performed on actual monolith structures, rather than those that have been reformed. The chromatographic technique is applied to a catalytic monolith and preliminary results of unpublished work are presented. A method of using a cut section from a catalytic monolith in a modified form of ‘Wicke–Kallenbach diffusion cell’ is also described.

Examples from the patent literature are provided showing, how interest in layered catalyst systems has started to grow, illustrating how diffusion in porous layers can be exploited to develop ‘designer catalyst systems’.     

Phoretic phenomena in tar vapor-particulate mixture separation from fuel gas streams

Analytical models were developed to predict the performance of a spray scrubber for separation of tars from gasifier off-gas streams. The models included heat transfer, mass transfer, condensation, nucleation, temperature and flow fields, and various phoretic phenomena involved in droplet-particle and droplet-vapor interaction and collection. The models indicate that for the tar particulates, both Brownian diffusion and inertial impaction, rather than diffusiophoresis and thermophoresis, are the more important phoretic forces causing the collection. For the limiting case of all tars being present only as vapor, the efficiency of their removal by condensation on water spray droplets could be extremely high. Scrubbing experiments were carried out on the hot gas/tar stream evolved from the simulated devolatilization section of a laboratory fixed bed gasifier. The combined overall collection efficiency for a mixed particulates and vapor stream compares satisfactorily with the model predictions.     

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.     

A novel system for self-validating adhesive joints

A self-validating adhesive system is proposed in which ‘zero volume unbonds’ or ‘kissing’ bonds, which are undetectable by NDE techniques, may be filled and bonded by a secondary component in the adhesive during cure. The system shows the principal criteria required for such a smart system, and has been shown to enhance the strength of a bonded joint without undue deterioration in durability or water uptake.     

Number of Trays in Gas–Liquid Columns in Case of Intensive Entrainment: Broadening of the Kremser Equation

The paper deals with the tray efficiency estimation in the presence of entrainment, using one-parametric diffusion model for the column as a whole. The case of mass transfer in the trayed column accompanied by intensive entrainment of liquid is analysed, when both equilibrium and ‘dry working regime’ operating lines are straight. Using proposed algorithm it is possible to determine the number of actual trays in the column based on the inlet and outlet concentrations in one phase, inlet concentration in the other phase and the characteristics of mass transfer at trays. The obtained equations present the broadening of well-known Kremser equations.     

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.     

Mechanisms of aerosol deposition in a nasal model

Total and regional aerosol deposition were investigated in a model of a normal human nasal airway. Contributions of fluid turbulence and particle inertia were evaluated using monodisperse aerosols. At fixed turbulent flow conditions, deposition percentage increased with particle size greater than 1 μm, suggesting that turbulent inertial deposition is a primary mechanism.

With same size aerosol, deposition increased with increasing fluid turbulence but its contribution was less with larger size aerosol. Turbulent diffusion was the dominant transport mechanism for particles less than 1 μm, where deposition decreased with particle size. Two major deposition sites were visualized with radio-aerosol in the anterior region of the nasal airway. One is close to the ostium internum where turbulent eddies are well developed, and the other is the anterior region of the middle turbinate where direction of airflow changes from upward to horizontal.     

Pore‐network modeling of particle retention in porous media

Transport and filtration of micron and submicron particles in porous media is important in applications such as water purification, contaminants dispersion, and drilling mud invasion. Existing macroscopic models often fail to be predictive without empirical adjustments and a more fundamental approach may be required. We develop a physically‐representative, 3D pore network model based on a particle tracking method to simulate particle retention and permeability impairment in polydisperse particle systems. The model includes the effect of hydraulic drag, gravity, electrostatic and van der Waals forces, as well as Brownian motion. A converging‐diverging pore throat geometry is used to capture the mechanism of interception. With the analytical solution of fluid velocity within a pore throat, the trajectory of each particle is calculated explicitly. We also incorporate surface roughness and particle–surface interaction to determine particle attachment and detachment. Pore throat structure and conductivity are updated dynamically to account for the effect of deposited particles. Predictions of effluent concentration and macroscopic filtration coefficient are in good agreement with published experimental data. We find that the filtration coefficient is dependent on the relative angle between fluid flow and gravity. Particle deposition by interception is significant for large particle/grain size ratios. Brownian diffusion is the primary cause of retention at low Peclet numbers, especially for small gravity numbers. Particle size distribution is found to be a cause of hyperexponential deposition often observed in experiments. Permeability reduction was small for strong repulsive forces because particles only deposited in paths of slow velocity. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3118–3131, 2017     

On shape factors for irregular particles-I. The steady-state problem. Diffusion and reaction : R. Aris, Chem. Engng Sci. 6: 262–268, 1957

The effectiveness of a catalyst particle with diffusion and first-order reaction is governed by a parameter proportional to the ‘size’ of the particle. If that size is taken to be the ratio of the volume to external surface area, the effectiveness factors for all particle shapes come together asymptotically and are reasonably close together over the whole range.     

Residence time distribution of sorbent particles in a circulating fluidised bed boiler

The residence time distribution of limestone sorbent particles has been studied in order to increase the understanding of the conditions for sulphur capture in fluidised bed boilers. Two methods were used. The ‘steady state method’ involves the study of residence time for various particle size fractions. The ‘transient method’ is based on the transient increase in the amount of sorbent carryover with the fly ash, following initial limestone addition to a fresh bed (i.e. a bed with little or no sorbent). For the boiler investigated both methods gave similar results, showing that the major fraction of the sorbent, 80–85%, had a residence time of one hour or more.     

The influence of an anisotropic Langevin dispersion model on the prediction of micro- and nanoparticle deposition in wall-bounded turbulent flows

This paper deals with the issues of stochastic dispersion models and associated best practice responses for the investigation of micro- and nanoparticle deposition in turbulent flows. For such applications, Reynolds averaged turbulence models are widely used in combination with particle Lagrangian tracking, due to their relative simplicity and computational efficiency. Such approaches imply to generate the instantaneous velocity of the fluid at particle location to reproduce the effect of turbulence on particle transport. The default dispersion model used in most CFD codes is an eddy lifetime model, which frequently overestimates the deposition rates. In this work, a simple method is proposed to implement a three-dimensional stochastic dispersion model based on the Langevin equation in the Fluent^® commercial code. Comparisons are provided between this model, complemented by the simulation of Brownian effects, and available numerical data obtained using either an eddy lifetime model or a simple Langevin model. Computations are carried out in horizontal and vertical channel flows and in circular pipe flows as well. The use of the proposed anisotropic Langevin model is shown to improve the accuracy of deposition prediction in the whole range of particle inertia.