CFD Simulations of Contaminant Transport between two Breathing Persons Bjørn, Erik; Nielsen, Peter Vilhelm

Aalborg Universitet CFD Simulations of Contaminant Transport between two Breathing Persons Bjørn, Erik; Nielsen, Peter Vilhelm Publiation date: 1998 Doument Version Publisher's PDF, also known as Version of reord Link to publiation from Aalborg University Citation for published version (APA): Bjørn, E., & Nielsen, P. V. (1998). CFD Simulations of Contaminant Transport between two Breathing Persons. Aalborg: Dept. of Building Tehnology and Strutural Engineering, Aalborg University. Indoor Environmental Engineering, No. 96, Vol.. R989 General rights Copyright and moral rights for the publiations made aessible in the publi portal are retained by the authors and/or other opyright owners and it is a ondition of aessing publiations that users reognise and abide by the legal requirements assoiated with these rights.? Users may download and print one opy of any publiation from the publi portal for the purpose of private study or researh.? You may not further distribute the material or use it for any profitmaking ativity or ommerial gain? You may freely distribute the URL identifying the publiation in the publi portal? Take down poliy If you believe that this doument breahes opyright please ontat us at vbn@aub.aau.dk providing details, and we will remove aess to the work immediately and investigate your laim. Downloaded from vbn.aau.dk on: januar 5, 219

CFD Simulations of Contaminant Transport between Two Breathing Persons Erik B}tTI. Peter V. Nielsen.o m z Q).. ro (L Ol : QJ QJ Ol w : QJ E ' > w ' u E m r.. ' u 2 en.. ui x E (f) eo ::: Ol j: q, z w r :;::; C' > ::;,.D Q_ : Q_ u:; ::: i:5 N '+ ' <( (/) > Ol CO Ol u QJ Ol QJ u, QJ u QJ 2 QJ '+ u Q_ QJ s u (f)

The Indoor Environmental Engin eering papers are issued for early dissemination of researh results from the Indoor Environmental Engineering Group at the Department of Building Tehnology and Strutural Engineering, Aalborg University. These papers are generally submitted to sientifi meetings, onferenes or journals and should therefore not be widely distributed. Whenever possible, referene should be given to the final publiations (proeedings, journals, et.) and not to the Indoor Environmental Engineering papers. Pri nted at AaiiJorg University

CFD Simulations of Contaminant Transport between Two Breathing Persons Erik Bjrn, Peter V Nielsen

CFD SIMULATIONS OF CONTAMINANT TRANSPORT BETWEEN TWO BREATHING PERSONS Erik Bjm t) and Peter V. Nielsen Dept. of Building Tehnology and Strutural Engineering Aalborg University, DK9 Aalborg, Denmark I) email: eb@ivil.au.dk URL: http://iee.ivil.au.dk ABSTRACT Experiments have shown that exhalation from one person is able to penetrate the breathing zone of another person at a distane. Computational Fluid Dynamis (CFD) is used to investigate the dependeny of the personal exposure on some physial parameters, namely: Pulmonary ventilation rate, onvetive heat output, exhalation temperature, and rosssetional exhalation area. Fullsale experimental results are used to alibrate/validate the CFD model. Respiration, although an inherently transient phenomenon, is simulated by steadystate CFD with reasonably good results. Different geometries and grid distributions are tested to see what level of omplexity is neessary. To further evaluate the experimental results, the CFD simulations are then used to perform parameter variations. The simulations show that the simulated personal exposure is very sensitive to vanatwns in the onvetive heat output of both the exposed person and the exhaling person, and in the rosssetional exhalation area and the pulmonary ventilation rate of the exhaling person. KEYWORDS: Breathing zone, CFD, Contamination soures, Convetion flows, Respiration INTRODUCTION A subjet whih has not yet been studied thoroughly is the dispersion of ontaminants from the human exhalation, and the interation taking plae between respiration and the onvetive boundary layer flow indued by the human body. These phenomena are important when trying to understand and predit the flow fields and the resulting ontaminant distributions in displaement ventilated rooms, and may also be interesting with respet to ontaminant ontrol in hospitals and linis, in situations with passive smoking, and in the general improvement of indoor air quality. In the present study, fous is on situations where two persons are lose to eah other, so that the exhalation of one person an penetrate the breathing zone of another person. Aims The simulations presented in this paper are intended to omplement a series of full sale experiments made in a displaement ventilated test room (Bjm and Nielsen 1996). In these experiments, two breathing thermal manikins were used: manikin no.l ated as ontaminant soure, breathing diretly towards the fae of manikin no.2. Traer gas N2 was added to the exhalation, and manikin no.2 was used for measuring personal exposure. Manikin no. I exhaled through either nose or mouth, and different distanes between the two manikins were tested. The main results of the measurements are shown together with simulated results in figures 8, 1 and 12 in this paper. The prinipal aim of these simulations is to assess the sensitivity of the physial situation to variations of different parameters. This will assist in the evaluation of the experimental results.

METHODS The CFD ode A ommerial CFD ode named FLOVENT is used. The ode uses a standard finite volume method, the k turbulene model with logarithmi wall funtions, retangular strutured grid, the hybrid disretisation sheme, and the SIMPLE solving algorithm, see Patankar (198). Geometry The geometry of the test room and the experimental setup is symmetrial. For saving CPU time, only half of the room and half of the manikins are simulated, see figure 1. The symmetry plane is an adiabati, fritionless wall. To test the importane of the geometrial modelling of a human being, two different types of omputer simulated persons (CSPs) are ompared: 1) a simple box, and 2) a model with separate head, torso, and legs (see figure 2 and table 1). These two models have been used before by Brohus and Nielsen (1996), Brohus (1997). The surfae area is 1.62 m 2. This is the surfae area that is in ontat with the ambient air. The area in ontat with the floor is not inluded, and there is no heat ondution from the CSP to the floor. The present CSPs differ from those used by Brohus and Nielsen (1996) by having a "mouth", whih is used as an inlet for CSP no.l and as an exhaust for CSP no.2, thus simulating the effet of exhalation and inhalation, respetively. The entre of the mouth is plaed at the height of 1.5 m Symmetry.. lane.. EXHAUS!r <> I.27 x.27 j INLE Figure 1: Geometry of test room, loation of CSPs, inlet, and exhaust. D z Figure 2: Simple and detailed CSP. T a bl e 1 : n 1mens1ns o fc omputer 1mu ate dp ersons m. [ m ]. Simple CSP Detailed CSP Body part: X y z X y z Torso.16216.3 1.7.14429.3.67 Leg.14429.15.8 Head.18.13.23 Mouth.16.16.16.16 2

Grid Size The solution will hange with grid size. The disretisation error is of the first order of ell size. Theoretially, the solution will move asymptotially towards a "grid independent" solution (being the analytial solution of the differential equations). It is however not possible to say beforehand exatly just how fine the grid should be, or how large the deviations will be. A systemati investigation into this problem is very time onsuming, so it has only been dealt with for one situation. The results have then been used as a guideline for the rest of the situations. The distane to the first grid line is 1. m from all surfaes. This is an appropriate distane with regard to the alulation of the onvetive heat transfer oeffiient (Brohus (1997)). Convetive heat fluxes.temperatures T, mass flow rates m, and ontaminant onentrations are presribed (see table 2). Heat exhange by radiation is disregarded (sine the flow is dominated by onvetion), and the onvetive heat output of the CSPs is assumed to be 5% of the total heat output of the manikins, exluding the exhalation. The onstant flow rate and temperature of the extialation is a simplifiation ompared to reality, where the exhalation shows a pulsating, intermittent behaviour. The mass flow rate of 3.77 x 1 4 kg/s is the maximum instantaneous flow rate of a person breathing 6 liters pr. minute at a frequeny of 1 breaths pr minute, assuming a sinusoidal variation of the flow rate. T a bl e 2 : B oun d 1ary on d'ti 1 ons External boundaries CSPnol CSP no.2 floor eiling walls inlet exhaust inlet surfae exhaust surfae 25 [Wimz] 25 T [OC] 21.3 22. 21.5 19.5 32 m [kg/s] 5.33e2 5.33e2 3.77e4 3.77e4 [g/kg air] 7.9 Boundary onditions I I,.,.,,, oiut11ul I \ '...,.. lll't11111' \ \ \ 1\1.,,..,,,, 11R1ITtl I I I \ """.,,, llt\lnl 1.. Ref. vetor: 2. m/s ' ''"""'.. '',... m111f:, ' \ Tt..\'m',,,,.,.urHhi, I \ ".\\\." \ \ \ ' t o 1 llflfff1 1 \ "'""' \ \ \ \ \ I I lr.lltn I '''''\\\lttrtth i " i!ji::.: :: :... :. t: :: :... Figure 3: Veloity field, xz plane, y =. Veloities > 1. mls (lose to the exhalation opening) are omitted. 3

24.5 23.786 23.71 22.367 21.643 2.929 2.214 19.5 RESULTS Figure 4: Temperature field, xz plane, y =. Temperatures larger than 24.5 C appear as white. In all simulations, physially realisti results are obtained (see example in figures 3 and 4), i.e. the flow is quite similar to reality. The simulated personal exposure, however, is very sensitive to details. Grid independene An investigation was made to assess the optimum number of grid points for the simulations. The investigations were limited to a situation with horizontal exhalation through the mouth of one CSP and a mutual distane L!x =.4 m. The exposure Ce, i.e. the onentration inhaled by CSP no.2, is simulated. In the following the exposure is made dimensionless by dividing with the return onentration CR. In the equivalent fullsale experiment, elr = 6.9. Figure 5 shows that we an not be ompletely sure that grid independene is reahed, even for the rather large number of 4, grid points, but the results do seem to be reahing an asymptotial solution. However, when the number is below approx. 1, the results hange dramatially. '"2 12 1 Cl) 8 6 ::l m 4. 2 >< w.. 5, 1, 15, 2, 25, 3, 35, 4, Number of grid points.. + Simple CSP Detailed CSP Figure 5: Investigation of grid point independene. 4

The above mentioned gridpoint investigation inludes both types of CSPs. In fat, the result obtained with the simple CSP is very lose to the measured value, whereas the more detailed CSP has an overshoot of approx. 34%. It seems that the geometry is not the main reason for deviations. The retangular geometry of the detailed CSP is in fat not very preise ompared to the experiments. That the simulated exposures obtanined with the simple CSP are so preise must be onsidered a bit of a oinidene, though, sine there are a number of assumptions involved in the simulations, namely the lak of intermittent, pulsating breathing. The ode itself also inludes some inauraies (se the disussion later). Core region In the above alulations, grid points were onentrated in the area between the two CSPs, whereas they were more sare in the areas around and above the CSPs (beause of the strutured retangular grid distribution, however, there is a onsiderable waste of grid points). Apart from the density of gridpoints between the two CSPs, the grid density in the exhalation outlet proves to be importane, see figures 6 and 7. It is benefiial to use a high number of grid points in this region. For a given grid density, however, it seems that there exists a maximum number neessary of grid points in the ore region. Inreasing the number of points beyond this number does not hange the result muh. Based on this invesigation, it is assumed that the simple CSP, with a number of 1, 2, grid points, and with 8 ells evenly distributed in the exhalation outlet, will be appropriate for all the following simulations, sine this seems to produe both reasonably aurate solutions and at the same time reasonably eonomi alulation times. The number of 1, grid points is used for the distane of.4 m, at inreasing distanes the number of gridpoints in the xdiretion between the two manikins is inreased proportionally with the distane. Parameter variations 1. Inhalation Two simulations are made without air being extrated through the inhalation of CSP no. 2. The simulations are made for ili =.4 m and.6 m, with exhalation through the mouth of CSP no 1. The simulated exposures are 23 times as large as with air being inhaled, proving that this is an important detail, at least in this type of situation. Inhalation is used in all other simulations. 7 6 '"ii! 5... 4 Cl)... ::J 3 (/) e 2 w 5 1 15 2 ea. 3, G.P. ea. 5, G.P. ea. 1, G.P. Grid points in exhalation (yz plane) 5

Total. 3, In ore l 1 1, 8 Figure 7: Development of exhalation jet with different grid distributions. Contour lines represent equal onentrations in all ases. 2. Exess temperature of exhalation There is a slight differene ( < 1 %) between the density of exhalation air in the experiments and in real life, whih orresponds to a temperature differene of max. 3 C. The results are tested for sensitivity towards hanges in exhalation temperature, see figure 8. The results indiate that this parameter is of importane at distanes > approx..6 m. 3. Convetive heat output Sine the boundary layer flows around persons interat with the exhalation flow, different onvetive heat outputs are tested, see figure 9. The effet of variations of this parameter is onsiderable. Without any heat at all, the simulated personal exposure JR "" 15, i.e. more than twie the measured value. 4. Mouth area ofcsp no.l The mouth area was doubled (twie as wide) in a series of experiments. This has a dramati effet on the simulated exposure, whih drops to.7.3 for the distanes.4 1. m. When the flow rate is inreased, however, the differene diminishes, see figure 11. 7 6 a: 5 "' 4 ::l m 3 Cl. >< w 2 b.. CFD,T=35C Experiment CFD,T=32C D D.4.6.8 1. 1.2 1.4 Distane [m] Figure 8: Simulated exposure, sensitivity to hange in exalation temperature. 6

::l (J).. x w 9 8 7 6 5 4.6 I CSP no.1 CSP no.2 I I I.8 1. 1.2 1.4 Relative heat output[) Figure 9: Simulated exposure, sensitivity to hange in heat outputs, onstant distane.1x =.4 m. 5. Pulmonary ventilation rate When the flow rate of the air exhaled by CSP no.1 is ineased, a signifiant hange is seen in the simulated esposure, see figures 1 and 11. produes rather poor results for distanes larger than.4 m, see figure 12. The simulations m this situation had some onvergene problems, and no further simulations were attempted. 26 24 22 2 a: 18 Q) 16..::. 14 ::::> (J). X w 12 1 8 6 4 1 x area 2 2 x area 1. 1.2 1.4 1.6 1.8 2. Relative flow rate(] Figure 11: Sensitivity to hanges in flow rate, onstant distane.1x =.4 m. a: (,) 9 8 7 6 Q)..::. 5 D ::l 4 (J) 6.. 3 X w 2 D D. D Experiment Flow = 1% Flow= 11% D 3 (,)a: 2.,..::. ::::> (J). X w.4.6.8 1. 1.2 1.4 Distane [m] Figure 1: Sensitivity to hange in flow rate of exhaled air..4.6.8 Distane [m] Figure 12: Exhalation through nose 1. 6. Nose exhalation When simulating air exhaled through the nose, the mouth was made double as wide, and the exhalation was fored to leave the opening with a 45 downward inlination and a 15 horizontal inlination, orresponding to the experiments. This DISCUSSION Several aspets of the simulations do not orrespond well to reality. Logarithmi wall funtions are used, however, this will give poor preditions of the flow in areas with low loal Reynolds numbers, e.g. in 7

the lower parts of the boundary layer flow lose to the human body. It might be better to use a low Reynolds model, but this was not possible for pratial reasons (CPU time). The same problem arises with the steadystate assumption: It would be better to use transient alulations, but again, this would demand an inrease in CPU time beyond what was possible. Also, the ke turbulene model does not resolve the turbulent eddies in the flow, whih might be of importane when onsidering ontaminant exposure. The geometries in the simulations are muh simpler than reality. It might be better to have body fitted grids, but the FLOVENT ode does not support this feature. In spite of the prinipal limitations of the present simulations, it was possible to simulate results whih are physially realisti, although not ompletely aurate when ompared with fullsale measurements. For this reason, it is onluded that the simulations are adequate for testing sensitivity to ertain parameter variations, whih was the main objetive of this researh. The grid distribution is paramount; an inadequate number of grid points will have a severe influene on the auray of the results. This is a problem in all CFD work whih is sometimes negleted, probably beause it is so time onsuming. The simulations show that the simulated personal exposure is very sensitive to variations of the investigated parameters: onvetive heat outputs rosssetional exhalation area pulmonary ventilation rate. Furthermore, the results indiate that the simulated exposure is not very sensitive to variations of exhalation temperature at small mutual distanes, but more so as the distane inreases. However, sine only a narrow range of temperatures are relevant to this problem, this parameter (and other parameters influening the density of the exhaled air (as e.g. humidity and C 2 ontent) an be onsidered as less ritial. The results suggest that one should not fous so muh on the exat measured values when evaluating the pratial importane of the experiments, but rather look at the qualitative aspets, namely that the exhalation does not neessarily follow the onvetive flows lose to the body, but is able to penetrate the breathing zone of other persons loated nearby. It would have been interesting to look at the effet of breathing towards the side or bak of another person at different angles and with varying vertial and horizontal inlination of the exhalation jet, but it is deemed that the omputational model is not ideal for this type of problem, onsidering the problems enountered in simulating exhalation through the nose. REFERENCES E.Bjfjrn, P.V.Nielsen (1996). "Exposure due to Interating Air Flows between Persons." Pro. ROOMVENT '96, 5th lnt. Conf on Air Distribution in Rooms, Yokohama, Japan. Vol.l, pp.17114. H.Brohus, P.V.Nielsen (1996). "CFD Models of Persons Evaluated by FullSale Wind Channel Experiments." Pro. ROOMVENT '96, 5th lnt. Conf on Air Distribution in Rooms, Yokohama, Japan. Vol.2. pp 137144. H.Brohus (1997). "Personal Exposure to Contaminant Soures in Ventilated Rooms." Ph.D. Thesis, Aalborg University, Denmark. S.V.Patankar (198). "Numerial Heat Transfer and Fluid Flow." Hemisphere 8

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