Turbulent deposition of particles on evaporating surfaces: Some experimental results

o045-6535/78/o4oi-o379~o2.oo/o

Chemosphere No. 4, PP 379 - 382, 1978. ©Pergamon Press L~d. Prin~ed in Great Bri~a/n.

TURBULENT

DEPOSITION OF PARTICLES ON EVAPORATING SOME EXPERIMENTAL M. Caporaloni

SURFACES:

RESULTS

and P. Silvestroni

Istituto di Fisica dell'Atmosfera, Via Castagnoli

C.N.R.,

I, Bologna,

Sezione Microfisica Italia

(Received in UK 31 March 1978; accepted for Imblica~ion 4 APril 1978) INTRODUCTION Dry deposition of particles on evaporatin~ the pollutants

from the atmosphere,

not much, at our knowledge,

because

surfaces

is an important ~echanism of remotion of

sea covers

the major part of the world. Rowever

is known about the actual mechanisms

governing

the transport of

particles

in turbulent flows on evaporatin~ surfaces. A great deal of work concerning . 1,2 transport on dry surfaces , on the other hand, has been carried out and evaporation

on particle

deposition

is well kno~m in laminar flows 3'4'5.

Some previous

influence

experimental

about the turbulent transport of particles on water surfaces has been done 6'7 8 A comparison between the particle deposited fluxes on dry and evaporating surfaces paper kept wet) in turbulent

turbulent

flows has shown a sharp decrease of deposition

~rk

(filter-

in presence of

evaporation. In the present velocity

V

paper we describe wind tunnel experimental

as a function of the vapour flux

F V

V

is defined as 9

particle

V = F/C

, where

tunnel mean concentration.

F Fv

is defined as

Cb

concentration

is computed

V

Reynolds numbers.

'

is the particle deposited

vapour bulk transfer velocity I0 , at the surface.

measures of the particle deposition

at different

flux and

Fv = Vv(C b - Cs)

is the vapour bulk concentration

C

, where and

Cs

in turbulent

flows;

•

this model

~s based on the stochastic

from the core of the turbulent region,

surfaces.

Different

the vapour

eddies that,

come in contact with the wall In.

surface gets rid of the wave effects and of the mixin B observed on water

evaporation

whose measure permits EXPERIMENTAL

-

renewal theory:

excllange between the surface and the fluid takes place because of the dissipative

A sol~d evaporating

V is the v is the vapour

in terms of a simple model well ex~lainin~

V

evaporation

is the

fluxes are obtained by raring

the computation

of

Cb

and

the tunnel relative humidity,

Cs

SET-UP

A mixing-room

(2 m.

3

In volume)

is tapered to a wind tunnel with square cross section of

IOxlO cm. 2 and 6 m. long. A fine mesh grid at the tunnel inlet ensures a fully developed turbulence Different

in the measure

section,

placed at 2/3 of the tunnel length.

turbulence degrees are obtained by a blowing pump with a variable rate of air

flow, while different

humidities

are obtained by vaporizing water in the ~ixing-room or

379

380

No. 4

warming the air at the inlet. The Reynolds number is obtained by computation of the diameter equivalent to the tunnel section and of the mean velocity. This latter is gathered from a Pitot tube measure of the maximum air velocity in the tunnel. Two thermocouples, placed in the tunnel axis over the measure region, record the relative humidity. This has been found constant along the normal to the evaporating surface except for a very thin layer near the wall. The tunnel floor consists of Agar gelatin prepared before each run dissolving 3 grams per litre of Agar Agar in boiling water. The hot solution fils completely t ~

tanks (2 cm. high,

iO cm. wide and 3 m. long each), then it becomes cold and solidifies. Agar gelatin is a good deposition wall for its smoothness and for its homogeneous evaporation. ~oreover the use of the same type of evaporation surface in every run ensures the same efficiency of capture of particles. A suspension of known particle concentration is continuously nebulized at the tunnel inlet. The tunnel particle mean concentration is then computed from the characteristics of the nebulizer and the tunnel rate of flow, being negligible the concentration decay along the tunnel itself II. Polystirene spheres of 0.8 and 5.7 ~m. in diameter are utilized. After each run, characterized by a particular Reynolds number and relative humidity, a sample of the measure region is analyzed at a microscope. The number of deposited particles is obtained from the arithmetic mean over one hundred optical fields stochastically chosen. (The particle number was almost constant in all the fields observed). The particles deposited fluxes are therefore measured under different evaporation rates and at various Reynolds numbers. The knowledge of the particle tunnel mean concentration allows the calculation of the deposition velocity. RESULTS AND CONCLUSIONS In Fig. i the deposition velocity is plotted versus the vapour flux at different Reynolds numbers. Experimental results show a particle deposition velocity enhancement both with the particle diameter and the tunnel Reynolds number. Further, whichever the Reynolds number is, Fig. I shows a particle deposition velocity decrease when the vapour flux increases, being unchanged the other transport mechanisms (Brownian diffusion and gravitational sedimentation). This trend can be explained in terms of the stochastic renewal theory by which the vapour transfer between the surface and the turbulent fluid takes place, within each eddy, by means of the same mechanisms of evaporation in laminar flows. If something like the Stefan flow (which keeps the particles away from the evaporating surfaces in laminar flows) sets up within each eddy in contact with the wall, this mechanism can be responsible of the observed particle deposition velocity decrease. On the other hand, when the Reynolds number increases, Fig. I shows a decayng slope of the deposition velocity versus the vapour flux (especially for 5.7 ~m. particles). If the dissipative eddies are responsible of vapour exchange between the surface and the fluid, at increasing turbulence degree, the characteristic length scale of these eddies diminishes.

~o.

4

Therefore

381

the above mechanism

is confined

in a more and more thin layer near the wall and its

influence on particle deposition decreases.

ACKNOWLEDGMENTS We thank F. Trombetti and F. Tampieri

and P. Mandrioli

for the useful advices

to prepare experimental

set-up

for the careful revision of the manuscript.

I

I

I

I

I I I llll

I I I 111

I

I

I

I

I llll

I

I I Ill m

(3

L~

ca SBC -I

10o

A

:

O O

O

Z~

E)

O

r-i

m

8 p

10 -1

O

_

Re=3xlO 4

Re= 2 x 104

m

Re: 5.4x!0"

4L

!o

41A

---,, q

b I

I

I

I I I Ill

I

I

i i I III

1O-'~

10J

I

I

i

Fig. I. Deposition velocity

I

I I Ill 10 .4

10-s

g Ca-2sec-t

V

versus

at various Reynolds numbers

Re

the vapour

flux

F

v

. Open symbols

refer to particles of 5.7 ~m. in diameter; symbols

IIIII

1O-5 Fv

I

closed

co 0.8 ~m. particles.

REFERENCES I.

G. A. $ e ~ e l ,

"Particle

and smooth surfaces",

eddy diffusivities

J. Aerosol

and deposition velocities

Bci. 4, 125 (1973).

for isothermal

flows

382

No. 4

2.

M. Caporaloni et al., "Transfer of particles in nonisotropic air turbulence", J. Atmosph.

3.

E. R. G. Eckert and R. M. Drake, "Heat and mass transfer", McGraw-llill,530 pp., (1959).

4.

O. Vittori, "Esperlenza dldactica sulla diffusione browniana", Giornale di Fisica, Vol. IX,

5.

L. Waldmann and K. H. Schmitt, "Thermophoresis and diffusiophoresis of aerosols", Aerosol

6.

U. M~ller and G. Shumann, "Mechanism of transport from the atmosphere to the earth's

Sci., 3, 565 (1975).

N. 4, 291 (1968).

Science, ed. by C. N. Davies, Academic Press, 137 (1966).

surface", J. Geoph. Res. 75, 3013 (1970). 7.

G. A. Semhel and S. L. Sutter, "Particle deposition rates on a water surface as a function

8.

~. Caporaloni et al., "Evidenza sperimentale dell'effetto ~tefan nella deposizione di

of particle diameter and air velocity", BNWL-SA-4755, 19 pp. (1973).

particelle in regime turbolento", R~vista Italiana di Geofisica e Scienze affinl, Vol. II, N. 2, 77 (1975). 9.

S. K. Friedlander and H. F. Johnstone, "Deposition of suspended particles from turbulent gas stream", Ind. Eng. Chem., 49, 1151 (1957).

iO. F. Trombetti et al., "Bulk transfer velocity to and from natural and artificial surfaces", Boundary-Layer Meteorology,

in press.

ii. C. N. Davies, "Deposition from moving aerosols", Aerosol Science, ed. by C. N. Davies, Academic Press, 393 (1966).