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Modelling of microparticle transport and deposition

in respiratory airway via nasal inhalation

Kiao Inthavong, Qinjiang Ge, Jiyuan Tu

School of Aerospace Mechanical Manufacturing Engineering, RMIT University

Bundoora, Australia

Abstract Modelling of the respiratory airways has been limited

to local regions only such as the nasal cavity or the lung airways.

This paper integrates the different respiratory organs from thenasal cavity through the pharynx, larynx, trachea, and the first

three generations of the lung airways. Computational Fluid

Dynamics (CFD) simulation of steady inhalation at 15L/min wasperformed on the model and a range of micron particles were

tracked through the domain. It was found that particles largerthan 20m deposit mainly in the nasal cavity and in fact a small

percentage of particles reach the pharynx and larynx. The larynx

region was identified as a region that produces complex flowpatterns and enhances mixing of any particles that pass through

it. The deposition fraction and patterns within the upper andlower respiratory regions is presented which will help put into

perspective current deposition efficiencies reported for local

respiratory organ

Keywords: micronparticle; respiratory airway; deposition; CFD

I. INTRODUCTION

Numerical simulation through the use of ComputationalFluid Dynamics (CFD) is a powerful technique for particledeposition, transport and airflow pattern analysis andvisualization in the respiratory system. Recently there havebeen a number of simulations of aerosol dynamics in therespiratory system using CFD. This includes airflow patterns

and particle deposition in the nasal cavity [1-3]; in the tracheaand larynx [4, 5], and in the tracheobronchial/lung airways [6,7]. These past studies among others in the literature havemainly focused on one region only of the respiratory system.By doing so the upstream flow conditions are often assumed byapplying a suitable boundary condition.

The primary function of the respiratory system is gasexchange whereby oxygen from the external environment ispassed into bloodstream to replace the carbon dioxide which isexpelled during exhalation. The respiratory system can beseparated into regions based on the upper respiratory tract(consisting of the oral and nasal cavity, pharynx, and larynx)and the lower respiratory tract (trachea, bronchi, andbronchioles). The inhaled air passes through each of the

conducting passageways, traversing through the highlycomplex geometry which exhibits many irregular shapes andsharp curves. Consequently complex airflow patterns areformed in the different regions of the airway. Furthermoreinhaled particles, whether they are pollutants found in thesurrounding environment or drug particles injected into theinhaled air, are transported by the airflow.

It has been found that larger particles demonstrate highinertial properties which lead to deposition by impaction whenthe air flow streamlines curve and the particle no longer canfollow the stream pathlines [8]. However smaller particles haveless mass and hence lower inertial properties which allowsthese particles to behave similar to tracer-gases. Despite this asmall percentage of these particles will deposit in the nasalcavity. Because of the early deposition within the nasal cavity,the remaining deposition fraction will travel furtherdownstream. Determining this value can be valuable for CFDsimulations that focus on individual organs or components of

respiratory system, since the fraction represents the likelihoodof further deposition of particles of a given size that has beeninhaled via the nasal cavity.

This paper presents a study of the deposition of differentsize micron particles (1-50m) under a steady laminar flow rateof 15L/min throughout the respiratory airway that extends fromthe nasal cavity through the conducting airways down to thefirst three generations of the tracheobronchial airway tree.Airflow patterns and velocity profiles are shown to highlightsome significant flow regions. Furthermore the depositionpatterns and fraction within the upper and lower respiratoryregions is presented which will help put into perspectivecurrent deposition efficiencies reported for local respiratoryorgans (e.g. trachea, lung airways).

II. METHOD

A. Model reconstruction from CT scansCT scanning requires the use of x-rays and which means

ionizing radiation exposure, which limits the scanning time.This means that current CT scan protocols will provide enoughscanned images for one region of the respiratory airway alone.This means that in order to retain the high resolution of the CTscans the respiratory airway model in this study was obtainedthrough three different sets of CT scans which representedthree different regions of the respiratory airway. Typically ascan protocol will involve 1-mm collimation, a 40-cm field ofview, 120 kV peak and 200 mA producing 1-5mm thicknesswith voxel size of 0.625 x 0.625. Each set of scans produced

DICOM images that were converted into 3D models andstitched together within the computer software Geomagics.ICEM-CFD (ANSYS Inc.) was used to create the datastructures and the mesh for it to be ready for ComputationalFluid Dynamics (CFD) analysis using Fluentv12.1. An initialmodel with 100,000 unstructured tetrahedral cells was initiallyused to solve the air flow field at a flow rate of 15L/min. The

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model was then improved by cell adaptation techniques torefine the grid in order to achieve grid independence. The finalgrid consisted of 2.5 million cells which is shown in Figure 1.The meshing scheme used a hybrid mesh that included five-prismatic layers with inner tetrahedral core. It is noted that thenasal cavity used in this study is the same model that has beenapplied in earlier gas-particle studies within the nasal cavity bythe authors [8-11].

Figure 1. The nasal-respiratory airway model used in divided into three

regions for analysis of local deposition: i) nasal cavity; ii) pharynx and larynx

and, iii) bronchial airway tree. Cross-sectional areas taken at the nasopharynxand hypopharynx are shown with the computational mesh.

B. Numerical procedureThe commercial CFD code, Fluentv12.1 is used to simulate

the steady inhalation at 15L/min. The steady-state continuityand momentum equations for the gas phase (air) in Cartesiantensor notation can be cast as:

( ) 0=

gig

i

ux

(1)

+

=

j

g

ig

ji

g

j

g

igj

xu

xxp

xuu

1 (2)

The equations were discretised with the QUICK schemewhile the pressure-velocity coupling was resolved through theSIMPLE method. For a low volume fraction of dispersed phase(particles), the Lagrangian approach with one-way coupling isused, i.e. the airflow transports the particles, but the effect ofparticle movements on the flow is neglected. In this approach,the airflow field is first simulated, and then the trajectories ofindividual particles are tracked by integrating a force balanceequation on the particle which can be written as:

)

p

px

pg

pD

pp

gig

uuC

ddt

du

+= )(

24

Re18

2

(3)

where the first tem is the particle acceleration, the secondterm is the drag force [12], and the third term is the gravityterm taken at per unit particle mass. Additional forces such asSaffman's lift force, thermophoretic force, and Brownian forceare neglected for the micron sized particles. Particlerebounding from the surfaces was ignored and particledeposition was determined when the distance between the

particle centre and a surface was less than or equal to theparticle radius. The particle tracking is then terminated.

Inhalation through the nasal cavity is induced through apressure difference between the nostril inlets (Pin = 0Pa) andthe nasopharynx outlet (Pout) that is set to a negative pressurerelative to atmospheric pressure that is caused by themovement of the diaphragm. Boundary conditions for theparticles are set up as a circular particle release entrained in theflow field. Particles were released from 0.01m from the inlet to

prevent any spurious data exiting the inlet upon immediaterelease.

III. RESULT

A. Airflow patternsFigure 3 shows the streamlines coloured by velocity as they

pass through the respiratory airway. In general the averagevelocity is around 1-2m/s in regions where there are no suddenchanges in geometry. Peak velocities of up to 6m/s are found inthe larynx area. In this region the cross-sectional area reducesto a small diameter, leading an acceleration of the inhaled air.Just after the larynx, the airway suddenly expands producing apositive pressure gradient. The flow velocity decreases

significantly and complex flow patterns arise as the flowstreamlines recover from the initial acceleration. This can beseen in Figure 2b which is a side view of the flow structureswithin the laryngeal region. It can be seen that the flow isdirected anteriorly, and flow separation occurs at the larynxexit. Posterior to the bulk flow a complex recirculating regionis found. This will have a significant effect on the transport ofparticles through this region.

(a) (b) (c)

Figure 2. Flow streamlines in the (a) entire nasal-respiratory airway model.

(b) Magnified side on view of flow streamlines passing through the larynx and(c) frontal view of the flow streamlines in the larynx.

B. Particle deposition patternsThe Lagrangian particle tracking model is verified by

checking the deposition efficiency in the nasal cavity region

with existing data in the literature (Figure 3). It can be seen thatthe current model provides a similar general trend butunderpredicts the deposition. The recent work by Shroeter et al.[13] has shown that the discrepancy may be caused bydifferences in the surface smoothness of the computationalmodel.

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Figure 3. Particle deposition efficiency vs Inertial parameter in the nasal

cavity comparisons with other experimental data.

The deposition fraction in each region (nasal cavity;pharynx and larynx; tracheobronchial airways) in Figure 4shows that only particles 10m persist deep into therespiratory airways from the pharynx and larynx region andfurther downstream. For all particles sizes tested, the highest

regional deposition is found in the nasal cavity. However for10m particles where the total deposition efficiency for theentire model reaches 42.3%, a majority of particles are found todeposit in the pharynx and larynx region.

0%

20%

40%

60%

80%

100%

0 10 20 30 40 50 60

micron particle (m)

Deposition

fraction(%)

Entire model

Nasal cavity

Pharynx & Larynx

Tracheobronchial airway

Figure 4. Deposition fraction in the different regions of the respiratory

airway and also the entire respiratory model.

Visualisation of particle deposition patterns are performedfor two distinctly different particle sizes, (1m and 20m) interms of their inertial property. 1m particles exhibit lowinertia which means that they are able to follow the flowstreamlines. This is evident in Figure 5a where the totaldeposition efficiency for the whole respiratory model is 7%.

For the larger particle of 20m there is a cluster of particlesthat deposit early, impacting at the roof immediately after thenostril opening (Figure 5b). Another cluster of particles arefound at the posterior end of the nasal cavity, and a third clusterdownstream just before the larynx. These locations appear inregions where the inhaled air changes direction as it is

transported through the nasal cavity. For example, the flowbegins at the nostrils, vertically aligned.

Side View Frontal Viewa) 1m particle deposition - Total deposition efficiency is 7%.

Side View Frontal Viewb) 20m particle deposition pattern. Total deposition efficiency is 99%.

Figure 5. Particle deposition patterns for (a) 1m particle and (b) 20m

particle for a flow rate of 15 L/min. Particles are coloured by velocity

magnitude [m/s].

The flow changes direction to a horizontal flow turningapproximately 90

o. This occurs again at the posterior end of the

nasal cavity where the flow changes from horizontal tovertically down as it enters the pharyngeal region. Finally justbefore the larynx, where the airway decreases in cross-sectionalarea, the flow is accelerated which enhances the inertialimpaction of the transported particle. The larynx is well knownfor its functionality in sound production which produced by thegeometry which changes dramatically by first decreasing in

cross sectional area, reaching a minimum before diverging andexpanding in its cross-sectional area. This is typical of aconverging-diverging nozzle which exhibits sharp pressuregradients, and rapid changes in velocities leading to complexflow patterns. Slice CC in Figure 6 is upstream to the larynx asthe flow is about to accelerate due to the converging geometrywhich shows the streamlines directed from the posterior C tothe anterior side C of the airway.

Figure 6. Velocity magnitude contours in [m/s] for slice CC superior orupstream to the larynx, and slice DD which is inferior or downstream of the

larynx.

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The bulk flow is centrally located on the slice with peakvelocity values of approximately 4m/s. As the flow movesdownstream, it accelerates and is directed anteriorly. Slice DDis located downstream of the larynx which shows thatgeometry has changed shape. The bulk flow is much morenarrow and concentrated closer to the anterior side of the slicewith higher peak velocities of up to 6m/s. These flow patternstransport the micron particles through the airway and this canbe observed by introducing 1m particles into the airway from

the left nostril and right nostrils and comparing its relativepositions.

Figure 7 shows the particle locations as they pass throughboth slice CC and DD. Particles released from the left nostrilare concentrated towards the left side and similarly particles areconcentrated on the right side when released from the rightnostrils. After passing through the larynx the particles becomemore evenly distributed and towards the anterior side of sliceDD.

(a) 1m particle released from the left nostril

(b) 1m particle released from the right nostril

Figure 7. Deposition patterns of 1m and 20 m at flow rate of 15 L/min.(density=1000 kg/m3)

IV. CONCLUSION

Airflow streamlines through the respiratory airway showedthat in general the average velocity was around 1-2m/s. Peakvelocities of up to 6m/s were found in the larynx area. Micronparticles were analysed to determine the level of particledeposition, and in particular the deposition fraction withinregion. Larger particles (i.e. 20m) were not present in thetracheobronchial airway since a large percentage of deposition

occurred in the nasal cavity. For 10m particles a total of 42.3

% of particles deposited in the airway, which means that alarge percentage of particles still remain entrained in the flowfield. This is significant as these particles persist in the deeperlung region. Finally the larynx was found to be a major sourceof complex flow phenomena which includes, reverseflow/recirculations, and increased mixing. This is attributed tothe sharp pressure gradients produced by the converging-diverging change in the flow geometry.

ACKNOWLEDGMENTThe financial support provided by the Australian Research

Council (project ID LP0989452) and by the RMIT Universitythrough an Emerging Research Grant is gratefullyacknowledged.

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