The Relationship between Secondary Flows and Particle Deposition Patterns in Airway Bifurcations

The relationship between localized fluid dynamics and localized particle deposition patterns within bronchial airway bifurcations upon inspiration and expiration was analyzed for different bifurcation geometries, flow conditions, and particle sizes. For the simulation of three-dimensional airflow patterns in airway bifurcation models, the Navier-Stokes and continuity equations were solved numerically by the finite volume Computational Fluid Dynamics (CFD) program package FIRE. Spatial particle deposition patterns were determined by the intersection of randomly selected particle trajectories with the surrounding wall surfaces. While three-dimensional flow patterns were characterized by their corresponding two-dimensional secondary flow fields, three-dimensional deposition patterns were represented by their related two-dimensional deposition density plots. Two particle sizes were selected to explore the relationship between secondary flows and localized particle deposition patterns: 0.01 w m, to illustrate the effects of Brownian motion, and 10 w m, to display the effects of impaction and sedimentation. Changes in bifurcation geometry (shape of bifurcation zone, branching angle) and flow conditions (flow rate, inlet flow profile, direction of flow) lead to variations in resulting secondary flow patterns, which were reflected by corresponding differences in related particle deposition patterns. In conclusion, a distinct relationship could be observed between secondary flow patterns and deposition density plots, demonstrating that particle deposition patterns in airway bifurcations are not only determined by physical forces acting upon individual particles, but also by convective transport processes of the carrier fluid.     

Transport and deposition of inertial aerosols in bifurcated tubes under oscillatory flow

The transport and deposition of aerosol particles in a single bifurcation tube under oscillatory airflow condition are investigated via experiments and numerical simulations. In the experimental study, a novel stabilized laser-photodiode measurement technique is used to quantify the effects of frequency of oscillatory airflow and airflow rate on surface deposition of particles within the bifurcation tube. Surface deposition in the parent and daughter tubes is measured by orientating the laser-photodiode device parallel and perpendicular to the bifurcation plane and calculated using a transmission loss model. In the numerical simulation study, the same bifurcation tube is constructed using a two- and three-dimensional computer mesh in COMSOL^® to model particle mobility and deposition characteristics, considering the simultaneous effects of inertial impaction, gravitational settling and interception. The effects of frequency of oscillatory airflow, airflow rate and particle size on the particle trajectory and spatial deposition pattern are examined.     

Particle Deposition in a Cast of Human Tracheobronchial Airways

Abstract

Regional particle deposition efficiency and deposition patterns were studied experimentally in a human airway replica made from an adult cadaver. The replica includes the oral cavity, pharynx, larynx, trachea, and four generations of bronchi. This study reports deposition results in the tracheobronchial (TB) region. Nine different sizes of monodispersed, polystyrene latex fluorescent particles in the size range of 0.93–30 μm were delivered into the lung cast with the flow rates of 15, 30, and 60 l min^{? 1}. Deposition in the TB region appeared to increase with the increasing flow rate and particle size. Comparison of deposition data obtained from physical casts showed agreement with results obtained from realistic airway replicas that included the larynx. Deposition data obtained from idealized airway models or replicas showed lower deposition efficiency. We also compared experimental data with theoretical models based on a simplified bend and bifurcation model. A deposition equation derived from these models was used in a lung dosimetry model for inhaled particles, and we demonstrated that there was general agreement with theoretical models. However, the agreement was not consistent over the large range of Stokes number. The deposition efficiency was found as a function of the Stokes number, bifurcation angle, and the diameters of parent and daughter tubes. An empirical model was developed for the particle deposition efficiency in the TB region based on the experimental data. This model, combined with the oral deposition model developed previously, can be used to predict the particle deposition for inertial effects with improved accuracy.     

Aerosol Deposition Efficiencies and Upstream Release Positions for Different Inhalation Modes in an Upper Bronchial Airway Model

Accurate predictions of micron-particle deposition patterns and surface concentrations in lung airways are most desirable for researchers assessing health effects of toxic particles or those concerned with inhalation delivery of therapeutic aerosols. Focusing on a rigid, symmetric triple bifurcation lung airway model, i.e., Weibel's generations G3-G6, a user-enhanced and experimentally validated finite volume program has been employed to simulate the airflow and particle transport under transient laminar three-dimensional flow conditions. Specifically, the effects of 3 inhalation modes, i.e., resting and light and moderate activities, were analyzed for typical ranges of Stokes numbers (St h 0.2) and Reynolds numbers (0 h Re h 2100). The detailed results show particle deposition patterns and efficiencies in the triple bifurcation under cyclic as well as steady-state inhalation conditions. Cyclic inhalation generates higher local and segmentally-averaged deposition rates when compared to steady mean Reynolds number inhalation; however, matching Stokes and Reynolds numbers, i.e., the average between mean and peak values, were found to provide fully equivalent results for all inhalation modes and bifurcations. In addition, particle maps were developed that show the release positions of deposited aerosols.     

Comparison of analytical and CFD models with regard to micron particle deposition in a human 16-generation tracheobronchial airway model

A representative human tracheobronchial tree has been geometrically represented with adjustable triple-bifurcation units (TBUs) in order to effectively simulate local and global micron particle depositions. It is the first comprehensive attempt to compute micron-particle transport in a (Weibel Type A) 16-generation model with realistic inlet conditions. The CFD modeling predictions are compared to experimental observations as well as analytical modeling results. Based on the findings with the validated computer simulation model, the following conclusions can be drawn:(i) Surprisingly, simulated inspiratory deposition fractions for the entire tracheobronchial region (say, G0–G15) with repeated TBUs in parallel and in series agree rather well with those calculated using analytical/semi-empirical expressions. However, the predicted particle-deposition fractions based on such analytical formulas differ greatly from the present simulation results for most local bifurcations, due to the effects of local geometry and resulting local flow features and particle distributions. Clearly, the effects of realistic geometries, flow structures and particle distributions in different individual bifurcations accidentally cancel each other so that the simulated deposition efficiencies during inspiration in a relatively large airway region may agree quite well with those obtained from analytical expressions. Furthermore, with the lack of local resolution, analytical models do not provide any physical insight to the air–particle dynamics in the tracheobronchial region.(ii) The maximum deposition enhancement factors (DEF) may be in the order of 10² to 10³ for micron particles in the tracheobronchial airways, implying potential health effects when the inhaled particles are toxic.(iii) The presence of sedimentation for micron particles in lower bronchial airways may change the local impaction-based deposition patterns seen for larger airways and hence reduces the maximum DEF values.(iv) Rotation of an airway bifurcation cause a significant impact on distal bifurcations rather than on the proximal ones. Such geometric effects are minor when compared to the effects of airflow and particle transport/deposition history, i.e., upstream effects.     

Deposition of Ultrafine Particles at Carinal Ridges of the Upper Bronchial Airways

Bifurcations of the upper bronchial airways are primary hot spots for deposition of inhaled particles and noxious gases. Deposition of coarse particles in the carinal ridges results from inertial impaction, and deposition distal to these sites is attributed to secondary flows. Diffusional deposition of ultrafine particles on carinae surfaces is studied here. Similarity solutions for both the flow and concentration fields at the respective boundary layers that develop near the surface of a wedge are presented, corresponding to a relatively high Re number. The expressions developed for the deposition efficiency compare favorably to those obtained by rigorous computational fluid dynamics simulations. Yet unlike simulation-derived expressions that pertain to the specific geometry and flow conditions studied, our expressions are robust and can account for different branching angles, airflow rates, and particle sizes. The average diffusive flux toward the carina walls is in good agreement with experimental deposition data, as well as with simulation results specifically designed to account for deposition hot spots at airway bifurcations. The expressions obtained can be easily implemented in algebraic inhalation dosimetry models to estimate deposition profiles along the whole respiratory system.     

Particle Transport and Deposition in a Hot-Gas Cleanup Pilot Plant

ABSTRACT

Development of hot-gas filtration systems for advanced clean coal technologies has attracted considerable attention in recent years. The Integrated Gasification and Cleanup Facility (IGCF), which is an experimental pilot plant for testing performance of ceramic candle filters for hot-gas cleaning, has been operational at the Federal Energy Technology Center (FETC) in Morgantown, West Virginia, for several years. The present work describes a computer simulation study of gas flow and particle transport and deposition in the IGCF filter vessel with four filters. The stress transport model of FLUENT? code is used for evaluating the gas mean velocity and the root mean-square fluctuation velocity fields in the IGCF filter vessel. The instantaneous fluctuation velocity vector field is simulated by a filtered Gaussian white-noise model. Ensembles of particle trajectories are evaluated using the recently developed PARTICLE code. The model equations of the code include the effects of lift and Brownian motion in addition to gravity. The particle deposition patterns on the ceramic filters are evaluated, and the effect of particle size is studied. The results show that, for a clean filter (just after the backpulse), the initial deposition rate of particles on the candle filters is highly nonuniform. Furthermore, particles of different sizes have somewhat different deposition patterns, which could lead to nonuniform cake compositions and thicknesses along the candle filters. The effects of variations in the filter permeability on the vessel gas flow patterns and the pressure drop, as well as on particle transport patterns, are also studied.     

Flow of concentrated suspension through oblique bifurcating channels

Suspensions of solid particles in viscous fluid flowing through bifurcating channels are encountered in various industrial processes and biological applications. This work reports the detailed numerical simulations of shear‐induced particle migration in oblique bifurcating channels. The effect of particle concentration, bifurcation angle, and flow rate on the partitioning of bulk flow and particles in the downstream branches is studied. It was observed that the particle distribution in the downstream branches does not follow the flow distribution due to shear‐induced particle migration. The velocity and concentration profile for suspension flow were observed to be symmetric in the inlet branch but asymmetric in the daughter branches. The degree of asymmetry and bluntness of velocity profile was observed to depend on the bulk particle concentration and bifurcation angle. The reported results could be useful in the design of flow devices handling suspension transport in bifurcating channels. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2692–2704, 2014     

Electrohydrodynamic Flow and Particle Transport Mechanism in Electrostatic Precipitators with Cavity Walls

The electrohydrodynamic (EHD) flow induced by the corona wind was observed in a model electrostatic precipitator (ESP) of the simple geometry composed of the plates with a cavity. And the influence of the EHD flow and the turbulence condition of inlet cross-flow on the particle behavior inside the ESP and its collection efficiency were elucidated through experimental and numerical analysis. The profiles of streamwise gas velocities and turbulence intensities were measured in the ESP with a laser Doppler anemometer.A laser beam sheet visualized particle trajectories. Collection efficiencies were measured with a particle counter. In addition, numerical computations were performed to compare with the experimental results. The numerical results showed good agreement with the experimental data. As the corona voltage increased, the gas velocities of the core flow and the circulating flow inside the cavity increased due to the corona wind and the turbulence intensity increased near the cavity region. As the corona voltage increased for the low bulk gas velocity, corona wind prevented the particle transport into the cavity. And the particle transport into the cavity by turbulent dispersion was observed as the bulk gas velocity increased. When the flow with high turbulence intensity entered the ESP, the turbulent dispersion enhanced the transport of particles into the cavity; hence, the collection efficiency was higher compared with the case of the relatively lower inlet turbulence intensity below a critical corona voltage. However, the collection efficiency was slightly lower for the high inlet turbulence than for the low inlet turbulence above the critical corona voltage due to the turbulent diffusion of particles toward the centerline downstream from the corona wire.     

Continuous Drying of Slurry in a Jet Spouted Bed

ABSTRACT

he performance of a laboratory scale jet spouted bed (JSB) for drying rice flour slurry was studied. The bed consisted of ceramic balls (5028 mm diameter) and the rice flour slurry was sprayed onto the moving particle surface near the inlet part. All the experiments were carried out at the jet spouting regime. This regime has high bed void fraction and violent movement and collision of bed particles. As a result, the dried product layer is attrited from particle surface as a fine powder and entrained from the bed by the spouting air. The experimental result were presented to show the effects of static bed height, inlet air flow rate and temperature, and feed concentration and flow rate on the outlet air temperature, thernal efficiency, and mean particle size and moisture content of the product. Asimple mathematical model, which is based on the conservation of mass and energy equations, was developed. Predicted results agreed well with those obtained from the experiment.     

Hydrodynamic Characterization of SPLITT Fractionation Cells

Abstract

The efficacy of SPLITT fractionation requires an absence of hydrodynamic mixing between laminae constituting the thin liquid film streaming through a SPLITT cell and it requires structural elements capable of splitting the film evenly along streamplanes. These requirements are examined here by both experimental tests and by a numerical analysis of flow properties near the inlet splitter. The experimental tests, involving dye injection and the injection of pulses of latex particles that may or may not be driven across flow laminae by gravity, show that SPLITT cell performance is close to that of ideal theory at low Reynolds numbers. The computer results verify an absence of mixing under these conditions, but when the Reynolds number and inlet flow asymmetry are both high, vortex motion is found near the inlet splitter edge, suggestive of mixing. The conditions leading to vortex formation are defined. It is shown that tapering the splitter edge suppresses vortex formation.     

A priori modelling of an adiabatic spouted bed catalytic reactor

An a priori reactor model for an adiabatic spouted bed reactor has been developed. This model uses first-principles mass and energy balances to predict the concentration and temperature profiles in the spout, annulus and fountain regions of the reactor. The particle circulation and voidage profiles in the spout are calculated using previously developed analytical techniques. Particle circulation patterns in the annulus are determined by a minimum path-length analysis. The spout and fountain are shown to contribute significantly to the overall conversion in the bed. Predicted and experimental conversions at flowrates up to 1.2U_ms show that extension of the fountain reaction zone and increased particle circulation with increasing inlet flow makes up for the higher average voidage in the spout and fountain. Experimental data confirm the calculated results for a stably spouting bed with CO oxidation over a Co₃O₄/αAl₂O₃ catalyst. The effects of flowrate and inlet reactant concentration are confirmed.     

Inspiratory Deposition Efficiency of Ultrafine Particles in a Human Airway Bifurcation Model

The inspiratory deposition efficiency of ultrafine particles in a physiologically realistic bronchial airway bifurcation model, approximating the airway generation 3-4 juncture, was computed for different particle sizes, ranging from 1 to 500 nm, under three different flow conditions, representing resting to heavy exercise breathing conditions. For the smallest particle sizes, say between 1 and 10 nm, molecular diffusion is the primary deposition mechanism, as indicated by the inverse relationship with flow rate, except for the highest flow rate where the additional effect of convective diffusion has to be considered as well. For the larger particle sizes, say above 20 nm, the independence from particle size and dependence on flow rate suggests that convective diffusion plays the major role for ultrafine particle deposition in bifurcations. A semiempirical equation for the inspiratory deposition efficiency, m (D, Q), as a function of diffusion coefficient D and flow rate Q, due to the combined effect of molecular and convective diffusion was derived by fitting the numerical data. The very existence of a mixed term demonstrates that molecular and convective diffusion are not statistically independent from each other.     

Gas-Solid Chromatography of Methane-Helium Mixtures: Transmission of a Step Increase in the Concentration of Methane through an Activated Carbon Adsorber Bed at 25°C

Abstract

Time-dependent transmission or “breakthrough” curves of methane in helium flowing through an activated carbon adsorber bed were measured for methane concentrations between 34 and 105 ppm, and for mixture flow rates between 0.69 and 6.64 cm/s. The transmission is the ratio of the outlet concentration to the inlet concentration. The experimental transmission curves for a step-function increase in the methane concentration are compared with the predictions from a model which assumes a linear adsorption isotherm and equilibrium between the gas and solid phases. These two basic assumptions are discussed in detail. The data show that the two assumptions hold within the concentration and flow rate regions of this study. Effective diffusion coefficients of methane were calculated from the transmission data and found to increase with increasing flow rates.     

Study on Simulation of Airflow within the Enclosure in the Semiconductor Factory Cleanroom

The application of mini-environment and SMIF (standard mechanical interface) enclosure in the cleanroom can efficiently reduce airborne particles and isolate the personnel from the product. The purpose of this article is to analyze the airflow field and pressure distribution of SMIF enclosure and to investigate the air cleanliness inside the mini-environment. Based on the continuity, momentum and energy equations of airflow motion, the simulation code CFX will be used to study the flow field of air movement corresponding to the associated boundary conditions. The results show that proper drilling holes or slots can improve the circulation zones of SMIF enclosure. The positive pressure of SMIF enclosure is mainly affected by inlet air flux, area of outlets, and leakage area. The calculated results will provide the design rules for SMIF Robot inside the SMIF enclosure and reduce the particle accumulation during robot moving.     

Numerical Simulation of Particle Dispersion in the Wake of a Circular Cylinder

In this article, numerical simulation of the Navier-Stokes equations was performed for the large-scale structures of a two-dimensional temporally developing cylinder flow and the associated dispersion patterns of particles were simulated. The time-dependent Navier-Stokes equations were integrated in time using a mixed explicit-implicit operator splitting rules. The spatial discretization was processed using spectral-element method. Non-reflecting conditions were employed at the outflow boundary. Particles with different Stokes numbers were traced by the Lagrangian approach based on one-way coupling between the continuous and the dispersed phases.

The simulation results of the flow field agree well with experimental data. Due to the effects of the coherent structures, the particles demonstrate a more organized dispersion process in the space and a periodic dispersion characteristic in the time. Particle dispersion increases with the flow Reynolds number and so does for particle concentration, which is independent of particle size. However, for particles at different Stokes numbers, the dispersion patterns are different. The particles at smaller Stokes number congregate mainly in the vortex core regions and the particles at larger Stokes number disperse much less along the lateral direction with the even distribution. The higher density distribution at the outer boundary of large-scale vortex structure characterizes the dispersion pattern of particles at the Stokes numbers of order of unity. Furthermore, these particles disperse largely along the lateral direction and show the nonuniform distribution of concentration.     

Boundary layer model for char combustion and gasification

A non-steady boundary layer model is developed for numerical simulation of combustion and gasification of a single shrinking char particle. The model considers mass and energy conservation coupled with heterogeneous char reactions producing CO and homogeneous oxidation of CO to CO₂ in the boundary layer surrounding the char particle. Mass conservation includes accumulation, molecular diffusion, Stefan flow and generation by chemical reaction. Energy conservation includes radiation transfer at the particle surface and heat accumulation within the particle. Simulation results predict experimentally measured conversion and temperature profiles of a burning Spherocarb particle in a laminar flow reactor. Effects of bulk oxygen concentration and particle size on the combustion process are addressed. Predicted particle temperature is significantly affected by boundary layer combustion of CO to CO₂. With increasing particle size, char gasification to char combustion ratio increases, resulting in decreasing particle temperature and increasing peak boundary layer temperature.     

Experimental investigation of particle migration in suspension flow through bifurcating microchannels

Experimental measurements of velocity and concentration profiles were carried out to study transport of non‐colloidal suspension in bifurcating micro channels for both diverging and converging flow conditions using a combination of mirco‐particle image velocimetry and particle tracking velocimetry techniques. Migration of particles across the streamline was observed and symmetric velocity and concentration profile in the inlet branch becomes asymmetric in the daughter branches. Further migration of particles toward the center of the channel in the outlet branch make the profiles again symmetric. The evolution of velocity and concentration profiles was observed to be different in the symmetric and asymmetric bifurcation channels. The comparison of the streamlines for the fluid and the particles showed significant deviation near the bifurcation region. This may explain why there is unequal flow and particle partitioning during flow of suspension in asymmetric bifurcating channels as reported in many previous studies. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2293–2307, 2018     

Continuous Particle Separation in Split-Flow Thin (SPLITT) Cells Using Hydrodynamic Lift Forces

Abstract

In this paper we discuss the relationship between field-flow fractionation and split-flow thin (SPLITT) cell methodology, both of which utilize transverse driving forces to establish different transverse concentration profiles for various suspended particle populations carried by flow down a ribbonlike channel. It is shown that hydrodynamic lift forces can assume a particularly important role among the stable of forces available; when combined with certain other forces the lift forces lead to the formation of thin hyperlayers of particles distributed within the channel. The conditions necessary to split the channel flow into substreams containing different particle populations by SPLITT techniques are discussed. It is shown that the SPLITT system can be operated in either an equilibrium or a transport mode, both benefiting by the use of an inlet as well as an outlet flow splitter in the cell.     

Computational Fluid Dynamic Predictions and Experimental Results for Particle Deposition in an Airway Model

An area identified as having a high priority by the National Research Council (NRC 1998) relating to health effects of exposure to urban particulate matter is the investigation of particle deposition patterns in potentially-susceptible subpopulations. A key task for risk assessment is development and refinement of mathematical models that predict local deposition patterns of inhaled particles in airways. Recently, computational fluid dynamic modeling (CFD) has provided the ability to predict local airflows and particle deposition patterns in various structures of the human respiratory tract. Although CFD results generally agree with available data from human studies, there is a need for experimental particle deposition investigations that provide more detailed comparisons with computed local patterns of particle deposition. Idealized 3-generation hollow tracheo-bronchial models based on the Weibel symmetric morphometry for airway lengths and diameters (generations 3-5) were constructed with physiologically-realistic bifurcations. Monodisperse fluorescent polystyrene latex particles (1 and 10 mu m aerodynamic diameter) were deposited in these models at a steady inspiratory flow of 7.5 L /min (equivalent to heavy exertion with a tracheal flow of 60 L /min). The models were opened and the locations of deposited particles were mapped using fluorescence microscopy. The particle deposition predictions using CFD for 10 mu m particles correlated well with those found experimentally. CFD predictions were not available for the 1 mu m diameter case, but the experimental results for such particles are presented.