Experimental and numerical analysesof air flow and particle diffusion inside buildings

J. Aerosol Sci., Vol. 22, Suppl. I, pp. S831-$834, 1991.

0021-8502191 $3.00+0.00 Pergamon Press plc

Printed in Great Britain.

EXPERImenTAL AND NIJ~ICAL ANALYSES OF AIR FIOW AND PARTICLEDIFFUSION INSIDE BUILDINGS

R. O ~ A a n d K .

O{G~BAY~HI

Mitsubishi Heavy Industries. Ltd. Akunoura~chi I - I , Na~waki. Japan

ABSTRACT Air flows and p a r t i c l e d i f f u s i o n i n s i d e a building were simulated numerically using a viscous flow model and e x p e r i m e n t a l l y using a wind tunnel. The purpose of t h i s study i s to estimate amount of t o x i c p a r t i c l e s emitted from a v e n t i l a t i o n p o r t on the roof of a f a c t o r y , where p a r t i c u l a t e m a t e r i a l could be r e l e a s e d from a contained v e s s e l in a p o t e n t i a l accident.

KEY~RDS Particle diffusion: Indoor pollution; N~erical analysis: Wind tunnel experiment

WIND TUNNEL EXPI~IENT

~perimen~lMeth~ Wind tunnel ~periment was performed ~ measure the ~ n c e n t ~ t i o n of p a r t i c l e s i ~ i d e a building in a windy c o n d i t i o n using a USER l i ~ t s c a t t e r i n g ~ t h M . A schematic v i m of the measuring system i s shown in Fig. 1, where a p a r t i c l e of ~ P ( D i o c t y l P h t a l a t e ) was used as a tracer. F i r s t , t r a c e r p ~ t i c l e s were exhausted from maw p i p e s on the f l o o r u n t i l they d i s p e r s e d u n i f o m l y i n s i d e t h e b u i l d i n g . Then, t h e e x h a u s t i o n of p a r t i c l e s was stopped and t h e i r

~ncentmtion was measured ~ntinuo~ly ~ 4 poin~ inside the building.

i I

;,l

i. Vmt. Port ',

i1 )

""

II ttttttt

T

Fig. l S~ematic vi~ of the measurlng ~stem

.Similarity rules P a r t i c l e s i n s i d e t h e b u i l d i n g move and diffuse by turbulent flow. then are depleted and deposited by the gravity force, as shown in Fig. 2. The f o l l o w i n g s i m i l a r i t y r u l e s were derived from nondimensioual fundm~ntal equation and they must be s a t i s f i e d in order to s i m u l a t e the behaviorof p a r t i c l e s using a scaled model.

. 7 Adveet i on Depletion ~ I)eposl tlon Fig.2 Mechanism of p a r t i c l e emission

S831

R. O ~ A and K. OKABAYASHI

$832 particles was calculated by solving the equation of particle diffusion with the Monte Carlo Method(gCg). (Ohha et al. 1985) Each particle was moved by resultant velocity Ur defined by ~l. (1).

~: ~+ ~+ ~

............

I Intlal Cood. ] [(~, ', P)

1

(1)

where ~ , ~a,~ , ~ are the resultant, advection, diffusion and inertia velocity of paricle. respectively. Advection and inertia velocities were calculated from Navier-Stokes equation and the equation of particle motion, respectively. Diffusion velocity was calculated from Eq. (2) using a random number. ~: ~l~t

............

(2)

J

½

Intlal Cond.

(~, ~, )

,

L

L=k

,..o

I

~ig. 5 Flow chart of calculation

where 6r is a random number of Gaussian distribution with a standard deviation of (2.Ki-6t) e.s axl 6 t is the time step Diffusivity of particles Ki was calculated from Eq. (3) (Deardorff. 1970).

ra_u,/rou, _ _ _ . ~ ,@ ou u, ,. O u , ~

OT]½+KO

where Cl:parameter of turbulent diffusivity, subscript i :(x.y.z) component, y:=0 when~T/gz~0 and =56 when~T/~z<0, K0:parametec of molecular diffusivity. Ai:width of finite difference grid. Parameters Cl and K0 were determined from a comparison with experimental results of concentration and deposition, respectively. Deposition velocity Vp was measured for sample platesof concrete in a closed box without air flow using the particle of pp=SE/c~ and Dp=I/~m. I t was found from measured results that the deposition velocity is almost equal to the value of depletion velocity Vg=0. 015cm/s for a floor plate, and 0.0 for side and upper plates. Therefore. the value of the parameter K0 was determined to be 0.0. neglecting the deposition by molecular diffusivity.

Calculated Results

Air flow and particle distribution were numerically calculated for the same building used in the wind tunnel experiment, as shown in Fig. 5 and 6. It was found from Fig. 5 that a standing vortex appeared below the ventilation poet, as well as the wind tunnel experiment. I t can be seen from Fig. 6 that particles inside the building were moved by the vortex and emitted in to the atmosphere through an upwind part of the ventilation port. These phenomena were recorded as an animation movie and compared with a video movie of the wind tunnel experiment.

3.0m/s

Fig. 5 Velocity vectors of air flow below the ventilation port in a vertical cross-section

FI ."

FI

,

,~;

Fig. 6 Distribution of I~rticles

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Analyses of air flow and particle diffusion

(~)Reynolds number :Re:U-L/Km:[inertia force of air] / [viscous force of air] (~2)Richardson number:Ri=a-g-L-L~T/U2=[buoyancy force of air] / [inertia force of air] (~)Prandtl number :Pr=Km/Kh=[diffusivity of momentum]/[diffusivity of heat] :FR=Cd.pair.L/pp.Dp2=[visceus force of a i r ] / [inertia force of particle] ~)Force ratio ®Froude number :Fr=g-LAF=[depletion force of p a r t i c l e ] / [ i n e r t i a force of particle] ®Velocity r a t i o :VR=Vg/U=[deposition velocity of particle]/[reference velocity of air] where a :volume expansion r a t i o of air, AT:temperature difference of air, Cd:drag coefficient of particle. Dp:diameter of particle. Velocity acale Sv=l/lO was determined from the above similarity rules of (~) and (~) for length s c a l e Sl=l/lO0. The s i m i l a r i t y rules of (~) and (~) are s a t i s f i e d automatically in the turbulent flow. In the present study, a proto-type particle of pp=Sg/om3 and Dp=lum to be simulated was transformed to a model p a r t i c l e of p p=lg/c~ and Dp=I um used in the wind tunnel experiment. in order to s a t i s f y the similarity rules of (~) and (~).

Experimental results Photos of the situation inside the building are shown in Fig.3, which indicate that the influence of the air flow outside is limitted near the ventilation port. (a) Vertical view (b}Horizontal view

Fig. 3 Photos of particle distribution inside the building The concentration of p a r t i c l e s inside the building was measured for 9 experimental conditions. Results of the concentration changing with time are shown in Fig. 4, where the concentration is divided by that at the starting time of particle emission. We can find from Fig.4 that the decay of the concentration at downwind part is slightly faster than that at upwind part, which i s due to the influence of a i r flow in the atmosphere. I t can be seen frum Fig.4(b) and (c) that the decay of the concentration depends on a wind velocity. The i n i t i a l condition of the concentration shown in Fig. 3 is uniform inside the building. Next, particles were i n i t i a l l y stocked inside a small box below the floor and then measured a f t e r the floor panel was removed, in order to investigate whether the particles could be emitted from the ventilation port, or not. In the case, i t was found that p a r t i c l e s could not be emitted from the v e n t i l a t i o n port since the concentration of particles was not measured near the port. T i ~ (nin)

I0

5

1.0 .0 %

15

20

",

_

-: 3 m / s

x

. . . . . . :lOu/s

0.9 % % % % % %

0.8

Fig. 4 Decay of m r t i c l e concentration with time

NUMERICALCALCULATAION

Numerical model Flow chart of numerical calculation is shown in Fig. 5. Air flow inside and outside the building were numerically calculated by solving Navier-Stokes equation with the Finite Difference Method (FDM). After steady s t a t e v e l o c i t y , temperature and pressure were obtained, the behavior of

R. O ~ m A and K. O Z A B A Y A S m

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Number of particles were recorded for the calculated results inside a certain volume around the same point as the measurement in the wind tunnel experiment. The decay of the number was compared with the experimental results, as shown in Fig. 7. It was found from Fig. 7 that the p~meter of Deardocf-f model Cl=O.05 fitted well with the experimental results.

Experlmemt

Point

"~-"~" ""~'"~---A""

Cal.

~) -'-O

o ~ ,do T ~ = t(s~),~o ~ ~ ---Z~Fig. 7 Comparisonof calculated and experimental results for decay of particle concentration Next, leakage ratio of particles emitted to the atmosphere and initial number of particle was countted from calculated results for uniform and partial distributions inside the building as shown in Fig. 8. Calculated results of leakage ratio are shown in Fig. 9, which indicate that the leakage ratio increases and satulates with time. (a)Uniform distribution

[......... ~.'.':"

:.' ,';

0 : A=30m=

o o A o

......... :.......~.:..:'. ~... [ " "

i'."

: U=10m/s :6'1---0.1 : D,=0.Sum : U:3an/s,A=TnP CI--0.5,1},= l/~m

10 o

(b)Partial distribution e~ t~

¢

. t~ • .

.

,

". :%; : ~"

O

¢

0

fi,

g,

o

..

",

~ 0

Fig.8 Initial distribution of particles

6

~

~

o 50

~

A

A

o

o

o

I00

Time t (min) Fig.9 Change of leakage ratio of particles with a time

CONCLUSION

It was found from the present study that: (1)6 kinds of similarity rules were derived from nondimensional foundamental equation of air flow and particle motion for the scale modeling of particle diffusion, (2)numerical model of particle diffusion using the ranchn number can simulate a ventilation process through the ventilation port. (3)leakage ratio of particles released inside the building is lower than 10% for wind velocity U=3m/s, (4)leakage ratio is in almost proportional to wind velocity and area of ventilation port. References

(I)Deardorff, J. W. (1970). A numerical study of three-dimetsinal turbulent channel flow at large Reynoldsnumbers, J Fluid l~ch., Vol.~l (2)0hba, R. et al. (1985). Development of numerical simulation models on gas diffusion, Technical Review of Hitsubishi Heavy Industries