Inertial Particle Deposition Research Articles

Scalability of inertial particle deposition in bubbles with internal circulation

Inertial deposition of small (less than a few µm in diameter) aerosol particles in mm-scale bubbles is an old but unsettled issue in modeling of pool scrubbing phenomenon. Whereas existing practical models give no specific consideration to the bubble-internal transport, some studies have shown that inertial transport affects considerably the particle deposition rate. We show, on the basis of Lagrangian simulations of particles advected by steady internal circulation in a spherical bubble, that particle centrifugal velocity becomes scale invariant for low- Stokes numbers (St≤10-2) when the characteristic timescale is chosen to be that for transversal particle motion at the Stokes terminal velocity corresponding to the local fluid acceleration. Because a scaling law can be derived by running simulations with a small number of particles, it can provide a practical tool for considering the influence of inertial particle transport within the bubble on the decontamination factor.

Improving the efficiency of particle deposition on the filter fiber through its modification

In this work, we performed a numerical study of the parameters of a fiber filter that make the greatest contribution to the efficiency of particle deposition, using the example of a single fiber. The mechanism of inertial particle deposition is considered. The aerosol motion was numerically simulated in the ANSYS Fluent software package. The particle deposition efficiency was calculated for a single solid and porous fiber without a protrusion, with one protrusion and five protrusions. Based on the data obtained, the dependences of the particle deposition efficiency for a single fiber are graphically presented. A new method is proposed for increasing filter efficiency by creating modified fibers.

Detailed deposition analysis of inertial and diffusive particles in a rat nasal passage

Rats have been widely used as surrogates for evaluating the health effects of inhaled airborne particulate matter. To provide a thorough understanding of particle transport and deposition mechanisms in the rat nasal airway, this article presents a computational fluid dynamics (CFD) study of particle exposure in a realistic rat nasal passage under a resting flow condition. Particles covering a diameter range from 1 nm to 4 µm were passively released in front of the rat’s breathing zone, and the Lagrangian particle tracking approach was used to calculate individual particle trajectories. Detailed particle deposition analysis shows the deposition of inertial particles >2 µm is high in the rat nasal vestibule and more than 70% of all inhaled inertial particles were trapped in this region. While for diffusive nanoparticles, the vestibule filtration effect is reduced, only less than 60% of inhaled nanoparticles were blocked by the anterior nasal structures. The particle exposure in the olfactory region only shows notable deposition for diffusive nanoparticles, which peaks at 9.4% for 5 nm particles. Despite the olfactory deposition remains at a low level, the ratio between the olfactory and the main passage is kept around 30–40% for 10–800 nm particles, which indicates a particle-size-independent distribution pattern in the main nasal passage and olfactory. This study provides a deep understanding of particles deposition features in a rat nasal passage, and the research findings can aid toxicologist in inter-species exposure-response extrapolation study.

Lattice Boltzmann model for predicting the deposition of inertial particles transported by a turbulent flow

Deposition of inertial solid particles transported by turbulent flows is modelled in a framework of a statistical approach based on the particle velocity Probability Density Function (PDF). The particle-turbulence interaction term is closed in the kinetic equation by a model widely inspired from the famous BGK model of the kinetic theory of rarefied gases. A Gauss-Hermite Lattice Boltzmann model is used to solve the closed kinetic equation involving the turbulence effect. The Lattice Boltzmann model is used for the case of the deposition of inertial particles transported by a homogeneous isotropic turbulent flows. Even if the carrier phase is homogeneous and isotropic, the presence of the wall coupled with particle-turbulence interactions leads to inhomogeneous particle distribution and non-equilibrium particle fluctuating motion. Despite these complexities the predictions of the Lattice Boltzmann model are in very good accordance with random-walk simulations. More specifically the mean particle velocity, the r.m.s. particle velocity and the deposition rate are all well predicted by the proposed Lattice Boltzmann model.

Simulation of aerosol fibrous filters at Reynolds numbers of the order of unity

The deposition of aerosol particles from a flow in a model filter composed of parallel rows of parallel fibers oriented normal to the flow direction has been studied at Reynolds numbers of the order of unity, Re ∼ 1, which are inherent in analytical filtration. The Oseen flow field in a row has been determined by the method of fundamental solutions. It has been shown that the drag forces of the fibers to the flow and the efficiencies of inertial particle collection on the fibers calculated based on the obtained field insubstantially differ from those calculated by the Navier-Stokes equations. The higher the filter packing density, the larger the Re numbers at which the drag forces begin to differ. In the case of inertial particle deposition, the difference between the collection efficiencies increases with the interception parameter. For rows with a/h > 0.2 (a is the fiber radius, and 2h is the interfiber distance in a row), including rows with a nonuniform arrangement of fibers, the calculations performed by both the linearized and complete Navier-Stokes equations yield almost equal results up to Re ∼ 1.

A fully Eulerian approach to particle inertial deposition in a physiologically realistic bifurcation

The investigation of aerosol flows in bifurcations is significant in biomedical applications, because this geometry resembles the branching airways of the lung. Most of the existing computational studies regarding aerosol flow in branching airways use Lagrangian description for the particulate phase, when particle inertial effects are taken into consideration. In the present study a fully Eulerian model is used for studying the transport and deposition of inertial particles under steady state inspiratory flow in the single physiologically realistic bifurcation created by generations G3-G4 of the human lung. In-house codes are used to create the geometry and the computational grid, obtain the fluid flow field and calculate particle concentration. Deposition fractions are calculated, concentration profiles are shown and deposition sites are indicated under different flow conditions (Reynolds number and flow asymmetry in the branches) and particles of various sizes (1–10μm). It is found that total particle deposition fraction is loosely dependent on fluid flow Reynolds number and our results are in agreement with experimental findings. Moreover, total deposition fraction does not change significantly between symmetric Q1/Q2=1 and asymmetric Q1/Q2=2 flow conditions, but is considerably lower for the totally obstructed case, Q2=0. On the contrary, deposition sites and particle concentration profiles depend on the various flow conditions. Apart from deposition at the bifurcation, which is present at all cases, deposition sites downstream the bifurcation move towards the outer bifurcation wall and daughter tube exit as Reynolds number increases. For the Q2=0, it is also shown that particles are trapped and deposit even in the obstructed daughter tube. In all studied cases, it is found that there is a particle-free region at the outer wall, at the beginning of the daughter tubes opposite the bifurcation. The straightforward formulation of deposition modelling and derivation of detailed concentration profiles along the bifurcation are major advantages of the fully Eulerian methodology used in the study.

Deposition of inertial particles from turbulent flow in channels at high Reynolds numbers

On the basis of the previously developed asymptotic theory of turbulent particle-laden flow with particle deposition in channels coupled with the transport model for the particle Reynolds stress, an asymptotic solution to the problem on the deposition of particles in the limit of high Reynolds numbers was obtained. The numerical calculations confirmed the presence, in the region of the transition from the diffusion-impaction regime of particle sedimentation to the inertia-moderated regime, bifurcation phenomenon of a solution found previously in earlier studies. Features of particle accumulation in the viscous sublayer are analyzed. On the basis of the numerical solution, correlations for particle deposition velocity were obtained. Boundary conditions of the wall-function type for particle concentration whose use allows widening the applicability limits of the equilibrium Eulerian models in terms of particle inertia are proposed.

Efficiency of a gas screen in the supersonic dispersed flow under conditions of inertial deposition of particles on a protected surface

The results of numerical studying the influence of inertially deposited particles of the condensed phase on the efficiency of a gas screen in a supersonic dispersed flow are presented. The possibility of realizing the Leont’ev paradox on the adiabatic surface region streamlined by a supersonic dispersed flow, involved in attaining the temperature of the surface being protected smaller than the coolant temperature on the permeable section of screen formation is specified.

Effects of surface smoothness on inertial particle deposition in human nasal models

Effects of surface smoothness on inertial particle deposition in human nasal models

Effects of surface smoothness on inertial particle deposition in human nasal models

Computational fluid dynamics (CFD) predictions of inertial particle deposition have not compared well with data from nasal replicas due to effects of surface texture and the resolution of tomographic images. To study effects of geometric differences between CFD models and nasal replicas, nasal CFD models with different levels of surface smoothness were reconstructed from the same MRI data used to construct the nasal replica used by Kelly, Asgharian, Kimbell, and Wong (2004a) [ Aerosol Science and Technology, 38,1063–1071]. One CFD model in particular was reconstructed without any surface smoothing to preserve the detailed topology present in the nasal replica. Steady-state inspiratory airflow and Lagrangian particle tracking were simulated using Fluent software. Particle deposition estimates from the smoother models under-predicted nasal deposition from replica casts, which was consistent with previous findings. These discrepancies were overcome by including surface artifacts that were not present in the reduced models and by plotting deposition efficiency versus the Stokes number, where the characteristic diameter was defined in terms of the pressure-flow relationship to account for changes in airflow resistance due to wall roughness. These results indicate that even slight geometric differences have significant effects on nasal deposition and that this information should be taken into account when comparing particle deposition data from CFD models with experimental data from nasal replica casts.

Structure and density of deposits formed on filter fibers by inertial particle deposition and bounce

The morphology and packing density of particle deposits formed by accumulation on thin steel fibers suspended in an aerosol stream were studied by confocal microscopy. Measurements were made with electrically neutral polystyrene spheres ( d P=1.3, 2.0, 2.6 and 5.2 μm) as a function of flow velocity ( v=0.7–5 m/s) and fiber diameters ( d F=8 and 30 μm). Deposition under these conditions was dominated by inertia (Stokes number St=0.3–9), interception (interception parameter R=0.04–0.35) and particle bounce, with a negligible contribution from diffusion. The experiments show a systematic transition of deposit morphologies with a newly introduced particle bounce parameter β∼St/ R, where St and R are based on the diameter d F of the bare fiber. Compact, forward facing deposit structures dominate in case of significant particle bounce (i.e. for β> β⁎, where β* represents the critical conditions for the onset of bounce on the bare fiber). Dendritic structures with pronounced sideways branching are formed at β< β*. R is of relatively little influence as an independent parameter, probably because interception occurs mostly on preexisting deposit structures with dimensions in the order of d P. The mean porosity ε of the deposit structures was determined on the basis of contour measurements by confocal microscopy, in combination with data on the accumulated particle volume per unit fiber length (known accurately from a previous paper by Kasper, Schollmeier, Meyer, and Hoferer (2009). Once noticeable deposits had formed, ε was found to attain stable values between 0.80 at d P=1.3 μm and 0.55 at d p=5.2 μm.

Trapping and sedimentation of inertial particles in three-dimensional flows in a cylindrical container with exactly counter-rotating lids

Stirring and sedimentation of solid inertial particles in low-Reynolds-number flows has acquired great relevance in multiple environmental, industrial and microfluidic systems, but few detailed numerical studies have focused on chaotically advected experimentally realizable flows. We carry out one-way coupling simulations to study the dynamics of inertial particles in the steady three-dimensional flow in a cylindrical container with exactly counter-rotating lids, which was recently studied by Lackey &amp; Sotiropoulos (Phys. Fluids, vol. 18, 2006, paper no. 053601). We elucidate the rich Lagrangian dynamics of the flow in the vicinity of toroidal invariant regions and show that depending on the Stokes number inertial particles could get trapped for long times in different equilibrium positions inside integrable islands. In the chaotically advected region of the flow the balance between inertia and gravity forces (represented by the settling velocity) can produce a striking fractal sedimentation regime, characterized by a sequence of discrete deposition events of seemingly random number of particles separated by hiatuses of random duration. The resulting staircase-like distribution of the time series of the number of particles in suspension is shown to be a devil's staircase whose fractal dimension is equal to the 0.87 value found in multiple dissipative dynamical systems in nature. Our work sheds new light on the complex mechanisms governing the stirring and deposition of inertial particles and provides new information about the parameters that are relevant in the characterization of particle dynamics in different regions of chaotically advected flows.

Model for the combination of diffusional and inertial particle deposition on inverse surfaces at low pressure

A model was developed to estimate particle contamination probabilities on inverse surfaces at low pressure for inertial and diffusional particle deposition under the influence of gravity, drag, and thermophoresis. The model shows that contamination probabilities strongly depend on particle size, initial velocity, and initial distance between particle and surface. While thermophoresis effectively protects the surface against deposition of particles with 10mm (or more) initial distance and velocities below 1m∕s, the contamination probability of particles with an initial distance of 0.1mm is very high for small particles due to their high diffusivity and for fast particles due to inertia.

Modeling of inertial particle transport and deposition in human nasal cavities with wall roughness

Nasal inhalation helps to protect the lungs from detrimental effects of toxic particles which, however, may also place the nasal and adjacent tissues at risk. Alternatively, drug–aerosol deposition on pre-determined nasal airway surfaces can be a modern pathway for rapid medical treatment. The present study focuses on inertial particles in the range of 1 μ m ⩽ d p ⩽ 50 μ m , subject to steady laminar flow rates of 7.5 and 20 L/min. In contrast to ultrafine particles, for certain fine particle sizes deposition is strongly affected by wall roughness, which was incorporated with a selective micro-size airway-surface layer. The validated computer simulation results show that the inertial particle deposition in human nasal cavities increases with increasing impaction parameter, IP = d a 2 · Q . Most of the deposition occurs in the anterior part of the human nasal cavities, especially in the nasal valve region. Considering drug–aerosol targeting, an optimal impaction parameter value exists which generates for normal inlet conditions the largest deposition in desired areas, e.g., the middle meatus, inferior meatus and olfactory regions. However, the absolute deposition efficiencies, especially in the inferior meatus and olfactory region, are very small because particles hardly reach those regions due to the complex nasal geometric structures. The influence of gravity was also analyzed and an experimentally validated correlation for inertial particle deposition in human nasal cavities has been provided.

Stochastic modelling of inertial particle dispersion by subgrid motion for LES of high Reynolds number pipe flow

Aiming at the better prediction of inertial particle dispersion in LES of turbulent shear flow, a stochastic diffusion process of the Langevin type is adopted to model the time increments of the fluid velocity seen by inertial particles. This modelling is particularly crucial for dispersion and deposition of inertial particles with small relaxation times compared to the smallest LES-resolved turbulence time scales. Simple closures for drift and diffusion terms are described. The effects of inertia and external forces on particle trajectories are modelled. To numerically validate the proposed model, LES calculations are performed to track solid particles (glass beads in air with different Stokes' numbers) in a high Reynolds number, equilibrium turbulent pipe flow (Re = 50 000 based on the maximum velocity and the pipe diameter). LES predictions are compared to RANS results and experimental observations. Simulation findings demonstrate the superiority of LES compared to RANS in predicting particle dispersion statistics. More importantly, the use of a stochastic approach to model the subgrid scale fluctuations has proven crucial for results concerning the small-Stokes-number particles. The influence of the model on particle deposition needs to be assessed and additional validation for non-equilibrium turbulent flows is required.

Inertial Particle Deposition in a Monkey Nasal Mold Compared with that in Human Nasal Replicas

Information on nasal particle deposition is used in risk assessments for exposure to airborne particulate pollutants and for optimizing the delivery of therapeutic aerosols. Monkeys are commonly used to assess the therapeutic potential of inhaled substances and to a lesser extent the toxicity of inhaled xenobiotics. Yet no reliable measurements of the deposition efficiency of monkey nasal airways for particles >1 μm have been reported to date. The goals of this study were to measure the deposition efficiency (>1 μm) of a replica of monkey nasal airways and to investigate potential differences in nasal deposition between humans and monkeys by comparing results with similar measurements recently reported for human nasal replicas. The monkey nasal replica was an acrylic mold made from a postmortem cast of the nasal airways of a 12-kg, male rhesus monkey. Particle deposition in the monkey nasal mold was measured for monodisperse aerosols between 1 and 10 μm and constant inspiratory flow rates between 2 and 7 lpm. Total deposition efficiency increased from nearly 0 to 100% with increasing particle inertia and was uniquely determined by values of an inertial impaction parameter. The deposition efficiencies of the monkey replica agreed well with those of human nasal replicas when compared according to equivalent Stokes numbers based on minimum cross-sectional area. Results from this study could improve monkey-to-human extrapolation models and interpretations of data from particle toxicity and therapeutic aerosol studies using monkeys.

Particle Deposition in Human Nasal Airway Replicas Manufactured by Different Methods. Part I: Inertial Regime Particles

Information on the deposition efficiency of aerosol particles in the nasal airways is used for optimizing the delivery of therapeutic aerosols into the nose and for risk assessment of toxic airborne pollutants inhaled through the nose into the respiratory system. Nasal particle deposition is often studied using plastic replicas of nasal airways. Deposition efficiency in a nasal replica manufactured by stereolithography has not been reported to date. We determined the inertial particle deposition efficiency of two replicas of the same nasal airways manufactured by different stereolithography machines and compared results with deposition efficiencies reported for models manufactured by other techniques from the same magnetic resonance imaging scans. Deposition in the replicas was measured for particles of aerodynamic diameter between 1 and 10 μm and constant inspiratory flow rates ranging from 20–40 Ipm. Deposition efficiency of the replicas increased from nearly 0–100% with increasing particle inertia. For...

On model equations for particle dispersion in inhomogeneous turbulence

Comparisons are made between the Advection–Diffusion Equation (ADE) approach for particle transport and the two-fluid model approach based on the PDF method. In principle, the ADE approach offers a much simpler way of calculating the inertial deposition of particles in a turbulent boundary layer than that based on the PDF approach. However the ADE equations that have recently been used are only strictly valid for a simple Gaussian process when particle inertia is small. Using a prescribed, but in general non-Gaussian random particle velocity field, it is shown that the net particle mass flux contains a drift term in addition to that from the mean velocity of the particle velocity field, associated with the compressibility of the velocity field. Furthermore the diffusive flux in general depends not only upon the gradient of the mean concentration (true only for a Gaussian random flow field) but also upon higher order derivatives whose relative contribution depends on diffusion coefficients D ijk… etc. These coefficients depend upon the statistical moments associated with random displacements and compressibility of the particle flow field along particle trajectories which in turn depend upon particle inertia. In contrast the PDF approach offers the advantage of using a simple gradient (Gaussian) approximation in particle phase space which can lead to a non-Gaussian spatial dispersion process when particle inertia is important. Conditions based on the particle mean free path are derived for which a simple ADE is appropriate. Some of the features of particle transport in an inhomogeneous turbulent flow are illustrated by examining particle dispersion in a random flow field composed of pairs of counter rotating vortices which has an rms velocity which increase linearly from a stagnation point.

Supersonic dusty-gas flows with Knudsen effect in interphase momentum exchange

On the basis of the two-continuum model of dilute gas-solid suspensions, the dynamic behavior of inertial particles in supersonic dusty-gas flows past a blunt body is studied for moderate Reynolds numbers, when the Knudsen effect in the interphase momentum exchange is significant. The limits of the inertial particle deposition regime in the space of governing parameters are found numerically under the assumption of the slip and free-molecule flow regimes around particles. As a model problem, the flow structure is obtained for a supersonic dusty-gas point-source flow colliding with a hypersonic flow of pure gas. The calculations performed using the full Lagrangian approach for the near-symmetry-axis region and the free-molecular flow regime around the particles reveal a multi-layer structure of the dispersed-phase density with a sharp accumulation of the particles in some thin regions between the bow and termination shock waves.

Direct numerical simulation of particle wall transfer and deposition in upward turbulent pipe flow

Transfer and deposition of inertial particles or droplets in turbulent pipe flow are crucial processes in a number of industrial and environmental applications. In this work, we use direct numerical simulation (DNS) and Lagrangian tracking to study turbulent transfer and deposition of inertial particles in vertical upward circular pipe flow. Our objects are: (i) to quantify turbulent transfer of heavy particles to the wall and away from the wall; (ii) to examine the connection between particle transfer mechanisms and turbulence structure in the boundary layer. We use a finite difference DNS to compute the three-dimensional time dependent turbulent flow field ( Re τ =337) and Lagrangian tracking of a dilute dispersion of heavy particles––flyashes in air––to simulate the dynamics of particles. Drag, lift and gravity are used in the equation of motion for the particles, which are assumed to have no influence on the flow field. Particle interaction with the wall is fully elastic. Results on preferential distribution of particles in the boundary layer, particle fluxes to and off the wall and particle deposition mechanisms are shown. Our findings confirm: (i) the specific tendency of particles to segregate in the near-wall region; (ii) the crucial role of the instantaneous realizations of the Reynolds stresses in determining particle fluxes toward and away from the wall; (iii) the relative importance of free-flight and diffusion deposition mechanisms.