1、298 2010 ASHRAEABSTRACTHuman respiratory exposure to indoor airborne contam-inants is of great concern as in modern society human spendmost of their time indoor. To evaluate the risk of such exposure,computer simulations examined the transport of inhaled par-ticles. The human upper respiratory syste
2、m containing the tra-chea and the main bronchi is modeled. The effects of thebreathing pattern, particle to flow density ratio, human airwaygeometry toward particle motion and particle fate in the upperrespiratory tree are evaluated. Breathing patterns from mild tointense are considered. It is found
3、 that the airflow patterns inthe selected domain are more affected by the geometry than thebreathing intensity. Particles of size 0.01 m, 1m and 30 mwere studied. Particle dispersion, deposition pattern and pen-etration rates are calculated. Human exposure risk to thesepollutants can be derived.HUMA
4、N LUNG ANATOMY AND COMPUTATIONAL DOMAINThe upper conducting airways of human lung (Figure 1a)contains the trachea and bronchi that form a tree structure withmillions of branching tubes. The computational domain of thisstudy contains the trachea and the first bronchial bifurcation(Figure 1b) followin
5、g the description of a physiologically real-istic bifurcation lung model proposed by Heistracher andHofmann (1995), and Phillips (1997).COMPUTATIONAL GRIDFigure 2 displays the computational grid. Unstructuredtetrahedral elements of dimension 1mm (0.039 in.) are used inthe fully developed core region
6、. Near the wall, a highly refinedboundary layer of hexahedron elements is employed, whichFigure 1 (a) Human lung image (Anatomical Institute, Bern); (b) Computational domain.CFD Study of Human Respiratory Dose to Indoor Particular ContaminantsLin Tian, PhD Goodarz Ahmadi, PhDMember ASHRAEPhilip K. H
7、opke, PhD Yung-Sung Cheng, PhDLin Tian is assistant professor in the Department of Science and Engineering Technology, State University of New York, Canton, NY. GoodarzAhmadi is professor in the Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, NY. Philip K. Hopkei
8、s professor in the Department of Chemical Engineering, Clarkson University, Potsdam, NY. Yung-Sung Cheng is senior scientist and researchdirector at Lovelace Respiratory Research Institute, Albuqerque, NM.OR-10-032 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
9、 (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 299evolves from a cell dimension of
10、 0.05 mm (0.002 in.) to theairflow core with a growth factor of 1.2. Grid independency isconducted and verified.AIR FLOW FIELDMild, moderate and intensive breathing conditions areconsidered in this study. Table 1 lists the flow field propertiescorresponding to different breathing conditions.For the
11、flow field, the continuity equation is given as:(1)Here uiis the instantaneous velocity and xiis the positionvector. The momentum equation for flow in laminar regime isgiven as:(2)Here p is the pressure, and is the kinetic viscosity of thefluid. For turbulence flow the Reynolds-averaged Navier-Stoke
12、s equation is given as:(3)where Rijis the Reynolds stress tensor, and it is directly solvedwith Reynolds stress transport model that is of the followingform:(4)In Equation (4), Pijis the turbulence production, P = Pii/2. Thevalues of the standard values of constants are: k =1.0, C1 =1.8, C2= 0.6 (La
13、under 1975). These values of constant leadsto the proper values of , but over estimates . He andAhmadi (1999) suggested using C1 = 1.5, C2= 0.1 for properestimation of .In addition to the Reynolds stress transportequation, Equation (5) is used to model turbulence dissipationrate :(5)Here, T= ck2/, i
14、s the eddy viscosity, and isthe turbulence kinetic energy. is the mean velocity. Thestandard values of the constants are: c = 0.09, c1 = 1.45, c2= 1.9, k = 1, = 1.3 (Jones and Launder 1973).PARTICLE TRANSPORT MODELIn this study, it is assumed that concentration is suffi-ciently dilute that the airfl
15、ow field is not affected by the pres-ence of particles. The governing equation of particle motion isgiven by:(6)Here = dxi/dt is the particle velocity, FiLis the liftforce, giis acceleration of gravity, ni(t) is the Brownian forceper unit mass, and is the particle relaxation time. In Equation(6) CDi
16、s the drag coefficient (Hinds 1982), and Repis the parti-cle Reynolds number.FLOW FIELD SIMULATIONBy nature human breathing contains periodic inhalationand exhalation, however, it is assumed that the particle depo-sition mainly occur during the inhalation process. Therefore,steady-state inhalation i
17、s considered in the simulation. Corre-Figure 2 Computational grid.uixi- 0=ujuixj-1-pxi- 2uixjxj-+=UjUixj-1-Pxi- 2Uixjxj-xj- Rij+=Table 1. Flow Field Properties under Different Breathing ConditionsBreathing PatternVolume RateL/min (cfm)ReynoldsNumberLight 15 (0.53) 1400Moderate 37 (1.31) 3450Intensiv
18、e 60 (2.12) 5600Ukxj- Rijxk-Tk-xk-RijuiukUjxk-uiukUixk-+=C1k- Rij23-ijk C2k- Pij23-ijP23-iju12u22u22ddt-xj-T-xj-c1Tk-uixj-ujxi-+uixj- c22k-+=ku1u12=uiduipdt-1-CDRep24- uiuipFiLginit()+=uip 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Publish
19、ed in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 300 ASHRAE Transactionssponding to the light, moderate and intense breathing cond
20、i-tions, uniform air velocity of 0.265 m/s (0.87 ft/s), 1.6 m/s(5.25 ft/s), and 3.2 m/s (10.5 ft/s) are specified at the tracheainlet. The flow is driven by pressure gradient in main flowdirection and a standard no slip boundary condition is appliedat the wall. Because of the uncertainty in the flow
21、 regime, lam-inar flow is considered for light and moderate breathing con-ditions, while turbulence modeling is performed for all of thebreathing conditions. A turbulent intensity of 2% at the tracheainlet to simulate the larynx jet is assumed for all turbulent con-dition. The air properties are: te
22、mperature = 288 K (518 R),dynamic viscosity = 1.84 105 Ns/m2 (3.84 107 lb-s/ft2),and density = 1.125 kg/m3 (2.18 103slug/ft3).PARTICLE SIMULATIONThe transport and deposition of spherical particles of size0.01 m, 1 m, and 30 m are numerical simulated in thetrachea and the main bronchus of human respi
23、ratory system.Typically 10,000 particles are released uniformly from thetrachea inlet. Statistical consistency on the deposition rate isobserved for lower ensemble of population. It is worth to notethat particles of size 30 m will not pass the nasal passage,however, they might be inhaled to human tr
24、achea bronchialtree via mouth breathing. Figure 3 displays the particle deposition pattern in thebifurcation model. Moderate cardiac load of 37 L/min (1.31cfm) is assumed in these simulations, and the laminar flowmodel is used. Diesel-oil-liquid droplets of diameter 0.01 m,1 m, and 30 m are included
25、 in these analyses. Figure 3shows that the deposition rates and deposition patterns varysignificantly with particle size. For example, Figures 3b showsthat no 1 m particles are deposited for the sample size used,while highly localized deposition is observed for the 30 mparticles. Figure 3 also shows
26、 that there is no particle deposi-tion in the trachea region for all of the cases studied with thelaminar flow assumption. For the ultrafine particles with d =0.01 m, deposition occurs uniformly.Figure 4 displays the results for the moderate breathingcondition with the turbulence assumption. A Reyno
27、lds stresstransport model was used for the airflow simulation and theturbulence fluctuations are included by incorporating stochas-tic “discrete random walk” (DRW) model. Comparing Figures4 with 3, it shows significant enhancement of the particledeposition because of the effect of turbulence dispers
28、ion. Thedeposition patterns are also altered. Figures 4a shows that the30 m particles have highest deposition rate, and the deposi-tion sites are more uniformly distributed across the entirebifurcation model. The deposition on the trachea is markedlyincreased. All of these changes are due to the add
29、itionaldispersion effects introduced by the turbulence fluctuatingvelocity field. For the 0.01 m particles, the turbulence effectincreases the trachea and overall deposition. The depositionpattern is roughly uniformly across the entire bifurcationmodel. Figures 4b shows that some 1m particles are de
30、pos-ited under the turbulent flow condition. The deposition rate ofthis particle size, however, is still the lowest.From the results shown in Figure 3 and 4, it is deducedthat the turbulence dispersion mechanism plays an importantrole in the transport and deposition processes of the inhaledparticles
31、 in the upper human respiratory system. Turbulencefluctuation velocity enhances mixing and provides randomdispersion of particles.Figure 5 and 6 displays the comparison of the simulatedparticle deposition efficiency in the trachea and the bronchusat moderate and light breathing conditions with that
32、of theexperiment measurements. Of the experimental studies, iner-tial range of 0.9 m d 30 m polystyrene latex particleswere used by Zhou and Cheng (2005). Ultrafine 212Pb parti-cles attached to the silver particles in the range of 1.7 nm d 200 nm were used by Smith et al. (2001). In addition, theexp
33、erimental data of Chan and Lippmann (1980), andSchlesinger et al. (1977) by using ferric oxide particles areincluded as well. Of all these experiments, human tracheo-bronchial casts were used and steady-state inhalation airflowrate of 15 (0.53), 30 (1.06), and 60 (2.12) L/min (cfm) wereconsidered.Bo
34、th turbulent and laminar airflow are considered in thesimulation. Particles in the range of 0.01 m to 30 m areincluded in this study. It is shown that the predicted depositionFigure 3 Particle deposition pattern with laminar flow assumption at moderate breathing condition: (a) 30 m; (b) 1 m;(c) 0.01
35、 m. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without
36、ASHRAEs prior written permission. ASHRAE Transactions 301efficiency by using CFD with turbulence assumption agreeswell with the experiments. However, the simulation resultswith laminar airflow assumption underestimates the particledeposition efficiency, especially in the trachea and for fineparticle
37、s in the range from 0.1 to 10 m. Three distinctiveparticle deposition characteristics are displayed in Figure 5and 6. For particle smaller than 0.1 m the deposition ratedecreases with the size. This implies that the Brownian motionis an important factor governing the motion of the particle.However f
38、or particle larger than 10 m the deposition rateincreases with the size, implying a different driving transportmechanismparticle inertia. For particles in between 0.1 and10 m, the deposition rate is the minimum and no distinctdeposition characteristics can be identified. It is also shown inFigure 5
39、that turbulence diffusion affects particle depositionsignificantly in the trachea over the entire range. Withoutturbulence diffusion, as indicated by the laminar simulation,the deposition rate is lower than the experiments. For particlearound 1 m, no deposition is detected. It should also bepointed
40、out that the simulation result is slightly higher than theexperimental measurement in this region due to the solutionsensitivity to the computational grid. Similar results areobtained in the first bifurcation (Figure 6) though turbulencediffusion have less effect in the Brownian and ineria regions.E
41、valuating the fate of particles of size 0.01 m, 1 m, and30 m, 1m particles have the lowest deposition in upperrespiratory systems and pose the highest risk. For 30 m parti-cles, they are more likely to be caught by the airway surfacesbefore they penetrate to deeper airways. The risk of dose for0.01
42、m particles is in between that of the 1 m and 30 mparticles.CONCLUSIONHuman exposure risk to particles of size 0.01 m, 1 m,and 30 m is evaluated by performing CFD study of the trans-port of these particles once inhaled. Based on the study,following observations are made:Particle of 1 m poses the hig
43、hest risk to human respira-tory health as it is more likely to penetrate into deeperlung.Figure 4 Particle deposition pattern with turbulent flow assumption at moderate breathing condition: (a) 30 m; (b) 1 m;(c) 0.01 m.Figure 5 Comparison of particle deposition efficiency withexperiment in the trach
44、ea.Figure 6 Comparison of particle deposition efficiency withexperiment in the first bronchi bifurcation. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional
45、 reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 302 ASHRAE TransactionsOf the three sizes, particle of 30 m are more likely tobe caught by upper respiratory airways.Airflow patterns in the trachea and bronchi wer
46、eaffected more by the airway geometry (larynx) than thebreathing intensity. Turbulence occurs at mild breathingcondition.Turbulence significantly affects the particle transportand dispersion in the human respiratory system.NONMECLATURECD= particle drag coefficientd = particle diameterFiL= lift force
47、gi= acceleration of gravityk = turbulence kinetic energyni= Brownian force vectorp = pressure of the fluidP = Turbulence productionRep= Particle Reynolds numberRij= Reynolds stress tensor of the fluidt = time, ui,ui = mean, instantaneous and fluctuation velocity vector of the fluiduip= velocity vect
48、or of particlexi= position vector, i = 1, 2, 3 for stream wise, lateral and span wise direction = mass density of the fluidT= eddy viscosity of the fluid = turbulence dissipation rate = particle relaxation timeREFERENCESChan, T.L. and M. Lippmann. 1980. Experimental measure-ments and empirical modeling of the regional depositionof inhaled particles in humans. Am. Ind. Hyd. Assoc. J.,41, pp. 399-409.Heistracher, T. and W. Hofmann. 1995. Physiological realisticmodels of bronchial airway bifurcations, Journal
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