SECONDARY CONDENSATION AS CO2- LASER BEAM PROPAGATES THROUGH CLOUD MEDIUM

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SECONDARY CONDENSATION AS CO2- LASER

BEAM PROPAGATES THROUGH CLOUD MEDIUM

E. Ivanov, V. Korovin, Y. Tolstikov, O. Volkovitsky

To cite this version:

SECONDARY CONDENSATION AS COZ- LASER BEAM PROPAGATES THROUGH CLOUD MEDIUM

E.V. Ivanov, V.Y. Korovin, Y.V. Tolstikov and O.A. Volkovitsky. Institute of Experimentai! Meteorology, Obninsk, U, S. S. R.

R6sumg.- La condensation secondaire est un des phdnomsnes ddterminant l'attbnuation non lin6aire du

-.

rayonnement puissant du CO -laser par des milieux nuageux.

On prdsente les rdsultats $e l'expgrience ayant pour but d'examiner l'action du rayonnement continu du CO -laser d'intensitb % 10'' wt.cm-2 sur la condensation homogsne secondaire aux environs des

2

gouttes d'eau individuelles de rayons 15-60 pm se mouvant dans le flux d'air refroidi (k5'~). Le processus de la formation du nuage de particules secondairess'enregistre sous la forme de 16

photos avec un temps d'exposition de 30 ps chacune et un intervalle de temps de 100 ps entre les debuts d'exposition de photos subs6quentes. Un tableau typique du phdnomzne est donn6.

On discute les particularitds du processus de la condensation homogene secondaire. Le tableau que l'on observe est en bon accord avec le modPle thdorique examins avant.

Abstract.- Secondary condensation is a process that results in the non-linear high-power C02-laser radiation extinction by a cloud medium.

Experimental results in investigating the secondary homogeneous condensation in the vicinity of wa- ter drops 15-60 pm in radius moving in the cooled air flow (f 5'C) and being exposed to a continuous CO -laser radiation (intensity *1o4wt are presented in this report.

2

The secondary cloud formation process has been fixed as a sequence of 16 frames with an exposition

30 ps each and the 100 ps time interval between the beginnings of subsequent frame expositions. A pattern of the process is presented. specific features of the secondary homogeneous condensation are discussed. A good agreement between the observed pattern and the theoretical model studied before is established.

C02-laser r a d i a t i o n , when a p p l i e d t o l i q u i d w a t e r clouds, causes t h e h e a t i n g of d r o p l e t s and t h e i r i n t e n s i v e evapora- t i o n . A s a r e s u l t , t h e propagation condi- t i o n s f o r a C02-laser a c t i n g beam and f o r probing beams

a t

v a r i o u s wavelengths a r e improved ( t h e cloud " c l e a r i n g u e f f e c t )

.

However under some c o n d i t i o n s t h e r e i s

observed a n o p p o s i t e e f f e c t of cloud me- dium " b l u r r i n g N . A concept of secondary condensation i s involved i n t o considera- t i o n t o e x p l a i n t h e above phenomenon.

For t h e f i r s t time t h e secondary condensation process was observed i n t h e experiments when t h e continuous-wave COZ-

l a s e r r a d i a t i o n was a p p l i e d t o a n a r t i f i - c i a l cloud ( i c e c r y s t a l s ) a t n e g a t i v e tem- p e r a t u r e s

/

1

/.

The e f f e c t displayed as

Fig.1. Time dependence of transparen- cy,

TI

= 1/10, 09 cloud medium i r r a d i a t e d by GO2-laser beam:

1

-

i c e c r y s t a l s , 2

-

l i q u i d w a t e r d r o p l e t s . GO2-laser switching-on and -off a r e shown by arrows.

C9-294 _{JOURNAL DE PHYSIQUE} t r a n s p a r e n c y r e d u c e i o n of a c l o u d f o r a p r o b i n g l a s e r beam w i t h a wavelength of

A

= 0 . 6 3 ~ ~ i ~ t p r o p a g a t i n g w i t h i n t h e C02- l a s e r beam s p r e a d i n g zone. A t y p i c a l t r a n - s p a r e n c y b e h a v i o r w i t h time f o r a

3 m

p a t h a t t h e C02-laser switching-on and

-

o f f i s c o r r e s p o n d i n g l y p r e s e n t e d i n Fig-1. The v i s u a l o b s e r v a t i o n s performed r e v e a l e d

took p l a c e , under some c o n d i t i o n s , i n c a s e of l i q u i d d r o p l e t s a s well.

F u r t h e r experiments were aimed a t c l a r i f i c a t i o n of t h e c o n d i t i o n s when t h e r o l e of secondary c o n d e n s a t i o n became s i g n i f i c a n t a s w e l l as a t s p e c i f i c a t i o n of t h e secondary d r o p l e t f o r m a t i o n mecha- nism.

some f o g w r e a t h s a g a i n s t b r i g h t l i g h t pa- The e s t i m a t e s made and t h e plumes t c h e s from i c e c r y s t a l s which appeared w i - .ear t h e p a r t i c l e s have shown t h a t t h e ge- t h i n an i l l u m i n a t e d space- There were a l s o condapy condensation e f f e c t is i n a good observed t y p i c a l "plumes" n e a r s e p a r a t e agreement w i t h t h e homogeneous n u c l e a t i o n p a r t i c l e s

/

1-3

/

(Fig, 2,3). It h a s been

shown l a t e r

/

3

/

t h a t t h e same phenomenon

rn

Fig.3.

_-

T V - ~ i c t u r e a r e a of e v a ~ o r a t i n n

_-

d r o p l e t s i n CO - l a s e r beam

a

_{f i e l d w i t h pl$;es of secondary}

p a r t i c l e s .

concept of t h e secondary p a r t i c l e s . The n u c l e a t i o n r a t e of a new phase w i t h a s i z e exceeding some c r i t i c a l r a d i u s

i s known t o be c o n s i d e r a b l y dependent on p e n e t r a t i o n of t h e system c o n s i d e r e d i n t o t h e range of m e t a s t a b l e s t a t e s - If t h e se- condary d r o p l e t s a r e formed homogeneously due t o spontaneous n u c l e a t i o n i n a vapour- w a t e r system, then t h i s process must de- pend, p r a c t i c a l l y i n a u t h r e s h o l d n - l i k e manner, on s u p e r s a t u r a t i o n i n t h e v i c i n i t y

b

of a primary i n t e n s i v e l y e v a p o r a t i n g drop- l e t h e a t e d by r a d i a t i o n .

Fig.2. P i c t u r e s o f GO2-laser t r e a j -

ment zone (

I,=

800 wtvcro- ) : The s h a r p dependence mentioned e n a b l e s a , b

-

l i n e a r and t r a n s v e r s a l l i g h t - t o c o n s i d e r approximately t h e

range8

of

view of such parameters as the cloud medi-

um

temperature, radiation intensity and particle size. These values charactirize the droplet superheating and supersatura- tion. The approximate range can be found through analyzing the experiments perf or- rned when the C02-laser beam irradiates a cloud to cause its 'lblurringn. The analy- sis incorporates the specification (sepa- ration) of the test conditions according

to the fact whether the nblurring" is ob- served or not. Besides, an experimental investigation has been performed for the individual droplets

/

4

/.

Again the fact was registered whether the typical plumes of the secondary particles were observed or not. The observations were made at the cloud medium temperatures of -32O

+

+1Z0C, the droplet radius varying over the range

Fig.4. Limited values of Ik

R

(shaded) above whichn the secondary condensation was observed.

these experiments varied from 30 to 3000 wt. cm-2.

The estimates obtained at the test condition separation and observation of the plumes near the individual particles are rather approximate in their character (averaging over the path, considerable er- rors when determining both radiation in- tensifies and particle radii). As a re- sult, one can with assurance consider only a "transient region" (Fig. 4).

The measurements with the help of a high-speed photoregister were performed to further supplement the data on the secon- dary particle formation.

The following experimental procedure has been used

/

3

/.

Distilled water drops 15-60pm in radius are directed with the flow of cooled air

(+

5OC) into the objec- tive view of a high-speed photoregister. The drops are illuminated by a dark field method.

In

the view of the photoregister a focused beam of the continuous C02-laser radiation passes perpendicular to the opti- cal axis of the objective. In a beam's cross-section the radiation intensity is distributed by Gaussian law with the dis- persion of

6

= 250pm. In the objective view of the photoregister the radiation intensity is of an order of I 0%t. The C02-laser radiation beam is closed by a shutter which comes into action when a water drop occurs in a top part of the ob- jective view. Then, the same timing mecha- nism turna on a high-speed photoregister fixing a pattern of the developing process as a sequence of

16

frames with an expoei- tion 3 0 y each and the 100ps time inter-

C9-296 JOURNAL DE PHYSIQUE

v a l between two subsequent frames.

The o p e r a t i o n d e l a y o f t h e s h u t t e r , t h e c u r v e o f r a d i a t i o n i n t e n s i t y i n c r e a s e , and t h e a i r flow t e m p e r a t u r e a r e f i x e d and measured simul taneously.

Neasurement system h a s t h e f o l l o w i n g main parameters: o b j e c t i v e view s i z e of t h e high-speed p h o t o r e g i s t e r 380 x 3 8 0 ~ ~ - m ; s p a t i a l r e s o l u t i o n

+

3 y m ; e r r o r i n measu- r i n g t n e t e m p e r a t u r e of a i r flow w i t h drops ;t 3OC and t h e i n t e n s i t y of t h e expo-

s e d r a d i a t i o n ;t 15%; a c c u r a c y o f time mea- surements 2 1 0 ~ . A t y p i c a l p a t t e r n of t h e homogeneous condensation development n e a r ' a w a t e r drop h e a t e d by t h e COO-laser r a d i a t i o n i s L g i v e n i n Fig.5. Already i n t h e 6-th frame ( 6 0 0 ~ a f - t e r t h e beginning of o p e r a t i o n ) one can s e e a secondary d r o p l e t cloud h a v i n g t h e appearance o f a s p h e r i c a l l a y e r w i t h a s h a r p l y d e f i n e d e x t e r n a l boundary. I n most c a s e s t h e secondary d r o p l e t s a r e v e r y s m a l l and a r e n o t r e s o l v e d by t h e i n s t r u m e n t s . A cloud of secondary d r o p l e t s i s observed a s a l i g h t s c a t t e r i n g formation. However, i n d i v i d u a l p a r t i c l e s being d e t e c - t a b l e , i t may be found t h a t a n observed c l o u d expansion i s n o t due t o t h e f o r m a t i - on o f new d r o p l e t s i n t h e e x t e r n a l bounda- r y , but due t o t h e i r movement from t h e cloud c e n t e r t o i t s p e r i p h e r y (Pig.6).

Fig. 6. Same a s i n Pig.

5.

Seen a r e s e p a r a t e condensed d r o p l e t s .

A secondary cloud s t o p s t o grow approxima- 1 0 0 p m

-

t e l y 1 3 0 0 ~ s a f t e r t h e beginning of drop exposure t o t h e r a d i z t i o n .

Fig.5. S u c c e s s i v e s t a g e s of f i n e par- The d i s t a n c e from t h e drop s u r f a c e 'loud near where t h e secondary d r o p l e t c l o u d i s f o r - a d r o p l e t h e a t e d by r a d i a t i o n .

R a d i a t i o n t r e n d i s shown by t h e arrow. _{as well as a c l o u d form are in a good} Photoes sequence i s shown by t h e f i -

speed of homogeneous nucleation

/

2,6

/.

In this case, the non-stationarity of the process does not cause a considera- ble change in calculating the location of the maximum speed of nuclei formation /7/,

these calculations being carried out in quasi-stationary approximation /2/, The time period between the beginning of expo- sure and a cloud formation adequately coin- cides with the time estimated for the max- imum drop heating. Though, these experi- ments have been conducted under constant

ter understanding of a homogeneous conden- sation process development in space and time. It should be noted that the general pattern is in a good agreement with the theoretical part of the study

/

2

/.

References

1. Volkovitsky O.A. ,Ivanov

E.V.

,Kolo- meev

M.

P.

,Kraskovsky N.K. ,Semyonov L. P.

Izv. Akad. Nauk SSSR, ser. Pizika Atm.Okeana, 11, No8, 861-863, 1975.

-

2. Volkovitsky O.A. ,Ivanov E.V. ,Kolo- meev X.P.,Semyonov

L.P.

"Kvantovaya elek-

tronicav,

,

No.2, 404-416, 1976.

3.

Vo

1

kovitsky 0 . A . ,Denisova V.V., Ivanov E.V.,Kolomeev M.P. "Trudy IEM",

I

.

,

Gidrometeoizdat

,

vyp. 13 (58), 95-1 07,

1976. .. .

4.

Volkovitsky 0.

A.

,Kolomeev M. P.

conditions (in the range of experimental _{"Trudy -~-mn,}

_E.

_{,~idr~meteoizdat ,vyp.}

_I

₈₍₇₁₎ error), it should be noted that the size 290-2939 1978. _{5. Ivanov}

_E.V.

_{Korovin V.Ya,}

and the total nwnber of the secondary par- "Trudy IEM", vyp. 14[59), _{6. Kolomeev E.P. ,Semyonov} 67-79, 1976. _L.P. ticles great1 y varied (cf .Fig. 5 and Fig. 6) ;

this being indicative of the criticality of this process to the cloud medium tem- perature, radiation intensity and initial drop size.

One more interesting phenomenon can be seen in the photograph: motion of drop by a reactive force

/

8

/.

An

initial drop moves away through the spherical layer of the secondary droplet cloud: in spite of drop's being still present in the field of intensive radiation, no secondary droplets are formed near it. In the other photo- graphs similar droplets can move towards the radiation or remain motionless in rela- tion to the formed cloud; this movement be- ing dependent on the initial drop size that agrees well with the results of

/

8

/.

In the last case the intensity of light scat- tered by the secondary cloud greatly inc- reases from frame to frame.

"Trudy IEMV1,

M.

,~idrometeoizdat, vyp. 13

( 5 8 ) . . -

- .

3-20. 1976. -

7.

~ o i d i n - ~ ~ . ~ . ,Strelkov G.M. The 4-th All-Union symposium on laser radiation propagation in the atmosphere (Nonlinear effects). Abstracts volume. Tomsk, 1977.

8. Ivanov E.V. ,Korovin V.Ya,Sedunov Yu. S. "Kvantovaya

electronics"

,

4,

No,

9,