STUDY ON N2/CO2 MIXING GASDYNAMIC LASER BY MEANS OF SYNCHRONIZED OPERATION OF TWO SHOCK TUBES

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STUDY ON N2/CO2 MIXING GASDYNAMIC LASER

BY MEANS OF SYNCHRONIZED OPERATION OF

TWO SHOCK TUBES

K. Maeno, H. Oguchi

To cite this version:

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JOURNAL DE PHYSIQUE Col2oqv.e C3, suppl6ment au n O 1 l , Tome 41, novembre 1980, page C9-209

STUDY ON N 2 / c o 2 MIXING GASDYNAMIC LASER BY MEANS OF SYNCHRONIZED OPERATION OF TWO SHOCK TUBES

K. Maeno and H. oguchi*

+Tokyo Metropolitan College o f Aeronautical Engineering, Tokyo, Japan,

I n s t i t u t e o f Space and Aeronautical Science, U n i v e r s i t y o f Tokyo, Tokyo, Japan.

R6sumS.- On exp6rimente l'obtention de GDL par mglange de N2 et de CO B l'aide d'un bec B 6cran con- 2

qu pour u n m6lange supersonique aval, par utilisation synchronis6e de deux tubes de choc s6par6s. Les distributions de gain mesurdes sont compar6es B une estimation fond6e sur une analyse simplifi6e quasi uni-dimensionnelle.

Abstract.- The experiment o n N2/C02 mixing GDL is conducted using a screen nozzle, contrived to make supersonic downstream mixing, by means of a synchronized operation of two separate shock tubes. The measured gain distributions are compared with a n estimate based on simplified, quasi one-dimensional analysis.

I n t r o d u c t i o n

I n t h i s paper we concern a N2/C02 mixing GDL experiment. I t has a l r e a d y been demonstrated by p r e v i o u s i n s e ~ t i ~ a t o r s ~ - ~ l t h a t a m i x i n g GDL i s one o f the promising and powerful methods transcending a gain l i m i t p e r t i n e n t t o a conventional premixed GDL; notably, comprehensive reviews should be r e f - e r r e d t o Refs. 9 and 11. So f a r as thermal e x c i t a - t i o n o f t h e donor gas N2 i s concerned, t h e shock tube i s one o f t h e most u s e f u l means, because o f t h e f e a s i b i l i t y t o cover a wide range o f f l o w con- d i ti o n s .

Regarding t h e m i x i n g o f C02 (+ He) w i t h a do- nor gas

N2,

various schemes f o r t h e i n j e c t i o n o f C02 i n t o a supersonic nozzle o f N2 have been n o t i - ced and/or. contrived; f o r example, a normal t h - r o a t i n j e c t i o n from w a l l o r s l o t t e d tube v e r t i c a l l y mounted a t t h e nozzle a downstream s c r - een nozzle i n j e c t i o n w i t h sonic nozzle o f

and

f u r t h e r a supersonic downstream i n j e c t i o n by screen n o ~ z l e . ~ I n any scheme, however, t h e gasdynamics p e r t i n e n t €0 m i x i n g processes above mentioned i s so i n t r i c a t e t h a t s i m p l i f i e d assumptions c o u l d be i n v o l v e d i n any analysis; otherwise, t h e descrip-

t i o n o f t h e r e l e v a n t f l o w phenomena must l a r g e l y r e l y on t h e experimental evidences.

As a m i x i n g scheme o f our experiment i s chosen a screen nozzle which proved t o y i e l d h i g h gain. This screen nozzle enabled us t o make a supersonic downstream m i x i n g o f a donor gas w i t h C02 w i t h o r w i t h o u t c a t a l y s t He. The screen nozzle i s r a t h e r f i x e d i n c o n f i g u r a t i o n and designed t o comprise o f simple c o n i c a l nozzles, because o p t i m i z a t i o n o f t h e nozzle c o n f i g u r a t i o n i s n o t our primary purpose. This paper aimes t o c l a r i f y some o f fundamental c h a r a c t e r i s t i c s o f N2/C02 m i x i n g GDL, associated w i t h supersonic downstream m i x i n g over a wide range o f stagnation c o n d i t i o n s o f t h e component gases.

Experimental Apparatus

This experiment was conducted by a synchronized o p e r a t i o n o f two shock tubes, which produce separate stagnant c o n d i t i o n s f o r each gas. These two tubes have q u i t e t h e same d r i v i n g mechanism, which i s i l l u s t r a t e d i n Fig. 1. The diaphragm o f an o r d i n a r y shock tube i s replaced by a f r e e p i s t o n which i s c o n t r i v e d t o q u i c k l y move back and f o r t h

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C9-210 JOURNAL DE PHYSIQUE

f o l l o w i n g t h e movement o f an a u x i l i a r y f r e e p i s t o n The a u x i l i a r y p i s t o n can be moved by o n - o f f s w i t c h i - ng o f small magnetic valve, equipped o u t s i d e o f t h e compression chambers. Consequently, t h e o p e r a t i o n o f each tube can be made simply by a snap a c t i o n . Both f e a s i b i l i t y and r e p r o d u c i b i l i t y o f t h e opera- t i o n i s q u i t e s a t i s f a c t o r y , as p r e v i o u s l y r e p o r t e d i n Refs 12, 13. Thus, t h e synchronized o p e r a t i o n of these two tubes was e a s i l y achieved by e l e c t r i c c o n t r o l o f t h e r e s p e c t i v e magnetic valves.

Start S w ~ t c h Delay

C02(or+ He)

Fig. 1 Schematic diagram o f d r i v e r sections. The shock tubes are d i f f e r e n t from each o t h e r i n dimension; t h e tube f o r a donor gas N2 has a

d r i v e n s e c t i o n o f 50 mm i n i n n e r diam. and 5,500 mm i n l e n g t h w i t h t h e f o l l o w i n g 20x80 mm square cross section, and t h e o t h e r f o r C02 has a d r i v e n s e c t i o n o f 20 m i n i n n e r diam. and 5,000

mm

i n length. The t e r m i n a l s e c t i o n o f the C02 tube i s connected w i t h t h e screen nozzle through a s t r a i g h t tube, and i n i t i a l l y separated by i n s e r t i o n o f a polyethy- l e n diaphragm, t h i n enough t o break a t a r r i v a l o f t h e shock wave, i n a p o r t o f t h e connection tube ( F i g . 2).

The screen nozzle i s i n s t a l l e d w i t h i n t h e squa- r e cross s e c t i o n (20x80

nun)

and t h e l a s e r c a v i t y i s f o l l o w e d w i t h t h e same cross s e c t i o n downstream o f t h e nozzle e x i t . The windows f o r o p t i c a l observa- t i o n as w e l l as g a i n measurement a r e mounted f l u s h w i t h t h e s i d e w a l l s o f the c a v i t y . I n Fig. 3 i s

screen nozzle Windows Fig. 2. Test s e c t i o n .

shown t h e schematics o f t h e screen nozzle. Since

,

i n t h e experiment, t h e o p t i m i z a t i o n o f the nozzle c o n f i g u r a t i o n was n o t aimed, a simple geometry which c o n s i s t s of c o n i c a l supersonic nozzles was chosen; t h a t i s , one row o f 6 nozzles f o r C02 sand- wiched w i t h tworrows of 8 nozzles f o r N2. NO sepa- r a t i o n diaphragm was used, because o f t h e small area r a t i o o f t h r o a t t o cross s e c t i o n 5 %; t h i s assured f u r t h e r f e a s i b i l i t y o f t h e operation, though a s l i g h t l o s s i n f l o w d u r a t i o n was incurred.

C02(+ He)

1

CO,(+He)

(+H" e ( o r 4.7) Half Angle : 6.1' Fig. 3. Screen nozzle.

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a pressure from 20 t o 50 Torr, while the C02 was f i l l e d a t a pressure from 200 t o 400 Torr.

The pressure was measured by means of piezo- e l e c t r i c pressure gauges mounted a t appropriate locations close t o the end wall ( o r the f r o n t face of screen nozzle). The incident shock speed was recorded on the time counters detecting shock a r r i - val by pressure gauges. In Fig. 4 i s shown an example of pressure records, e i t h e r of which i s t h a t f o r a donor gas j u s t ahead of the f r o n t face of screen nozzle or t h a t f o r C02 j u s t ahead of the end wall. We can see from t h i s figure t h a t a s a t i - sfactory synchronization of two shock tubes i s achieved. In f a c t , the operation was capable of making the difference in a r r i v a l times of both in-

2 cident shock waves l e s s than about 2 ~ 3 x 1 0 psec. Since the flow duration estimated from the pressure history were about 2 msec f o r a donor gas N2 and about 4 msec f o r C O Z Y the achieved synchronization was t o l e r a b l e f o r the measurement of any l a s - ing performance. Apparently, t h i s must be further improved i f longer driven tubes a r e available t o use.

Fig. 4. Pressure record a t shock tube ends. The stagnation temperatures were estimated from the ideal reflected shock r e l a t i o n using the measured incident shock speeds. The experimental conditions regarding the stagnation pressure and temperature a r e summarized as follows:

2 atrn < p? < 6 atm, 500°K < TE < 2400°K 2 atrn < pzH< 5 atm, 700°K < T&< 2300°K 5 atrn < p i <10 atm, 1800°K <

~i

< 3500°K where pS and

T'

a r e pressure and temperature a t stagnation, respectively, and the subscripts N , C , and CH r e f e r t o the t e s t gases

N2,

Cop,

and mixture of C02/He, respectively. I t i s noted t h a t no dissociation occurs f o r e i t h e r C02 or N2 over t h i s experimental condition.

Measurement of Small Signal Gain Coefficient The small signal gain G was determined from the i n t e n s i t y increment of the beam which was pro- duced by an e l e c t r i c a l l y pumped CW C02 l a s e r . The beam homogeneously scattered by reflection upon a diffusive alminium mirror was s e t t o pass through Ge windows, so t h a t the beam i n t e n s i t y was measured by a dewer-type Hg-Cd-Te photoconductive infrared detector. The measurement scheme f o r small signal gain i s i l l u s t r a t e d i n Fig. 5. Care was

Hg-a-Te I R Detector Partial Transmitter

Test Sect ion Nozzle

CE

CW

Probe

7

p l e d o r Fig. 5. Arrangement of gain measurement.

taken f o r protection of the Hg-Cd-Te infrared dete- c t o r ; f o r example, a shutter was inserted in a beam path t o avoid unnecessary exposer of the dete- c t o r t o the continuous beam, and also

a

Ge transmitter was inserted in front of the detector i n order t o attenuate the beam i n t e n s i t y .

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C9-212 JOURNAL DE PHYSIQUE

t h e beam i n t e n s i t y p r i o r t o t h e f l o w s t a r t o r non- a r r i v a l o f t h e l a s i n g media i n d i c a t e d no appreci- a b l e change d u r i n g t h e t e s t time. Therefore, t h i s i n i t i a l i n t e n s i t y was a b l e t o regard as t h e undis- t u r b e d beam i n t e n s i t y IO. I n F i g . 6 an example o f t h e beam s i g n a l i s shown together w i t h t h e pressure h i s t o r y a t the end o f the C02 tube. The f l o w i n a c a v i t y i s estimated t o s t a r t a f t e r a t most a few hundreds microseconds from a r r i v a l o f t h e i n c i d e n t shock wave a t t h e end o f t h e C02 tube ( t h e i n i t i a l r i s e o f t h e pressure i n Fig. 6). I t i s noted t h a t t h e maximum o f g a i n i s achieved w i t h some delay from the f l o w s t a r t . This was a i s o confirmed n o t t o be caused by mismatch i n o p e r a t i o n o f t h e two tubes. The g a i n d i s t r i b u t i o n s along the c e n t e r ax- i s as w e l l as l i n e s normal t o t h e a x i s were measured a t window l o c a t i o n s downstream o f t h e nozzle e x i t .

Fig. 6. Gain s i g n a l and pressure o f C02 tube.

Simple Estimate by Quasi One-Dimensional Analysis

-

f o r Comparison w i t h Experiment

-

So f a r t h e r e have been worked o u t many i n v e s t i - g a t i o n s on t h e screen nozzle o f a s i n g l e gas from a viewpoint o f t h e a p p l i c a t i o n t o wind tunnel o r o t h e r f l u i d machinery. Recently, f o r gasdynamic l a s e r a p p l i c a t i o n Russel e t a l l 4 developed an e l a -

b o r a t e a n a l y s i s o f t h e screen nozzle f l o w , t a k i n g i n t o account o f t h e viscous e f f e c t s . Cassady e t a l l 1 e x t e n s i v e l y a p p l i e d i t t o an estimate o f the l a s e r performance f o r the m i x i n g GDL u s i n g a screen nozzle s i m i l a r t o t h e present one, and showed a reasonable agreement o f the estimate w i t h t h e i r experiment. Even i n these elaborate analyses, t h e f l o w behavior i n t h e m i x i n g r e g i o n i s n o t d e a l t w i t h . This i s mainly because the m i x i n g processes o f viscous f l o w s mismatched i n v e l o c i t y and tempe- r a t u r e are much i n t r i c a t e f o r the s t r a i g h t f o r w a r d analysis. l5 I n a d d i t i o n , t h e outgoing disturbances from the nozzle e x i t a r e a l s o u n t r a c t a b l e w i t h o u t any semi-empirical considerations.

Regarding the mixing GDL f l o w a n a l y s i s , more 9 simple a n a l y s i s was done by Soloukhin e t a1 f o r t h e mixing nozzle scheme, i n which the secondary sonic j e t i s i n j e c t e d a t t h e t h r o a t o r from t h e s i d e w a l l o f a primary nozzle f l o w o f a donor gas. I n t h e i r a n a l y s i s , one o f t h e most s i m p l i f i n g assumptions i s t h a t o f an instataneous mixing. Despite such a simple analysis, t h e estimate was shown t o be i n a reasonable agreement w i t h t h e i r own e x p e r i - ment.

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Once t h e s t a g n a t i o n c o n d i t i o n o f each compone- n t gas N2 o r C02 (+ He) and t h e geometry o f nozzles a r e prescribed, a s e t o f governing equations can be i n t e g r a t e d from t h e t h r o a t t o the e x i t , separately f o r each nozzle. Since t h e instantaneous m i x i n g j u s t a t t h e nozzle e x i t i s assumed, t h e f l o w p r o - p e r t i e s a t t h e nozzle e x i t a f t e r m i x i n g a r e d e t e r - mined from t h e a l g e b r a i c conservation equations, i n which a l l t h e v i b r a t i o n a l temperatures p e r t i n e n t t o each mode a r e assumed t o remain unchanged across t h e m i x i n g l a y e r . With t h e f l o w p r o p e r t i e s a f t e r mixing, thus obtained a t t h e e x i t , t h e governing equatipns a r e a l s o i n t e g r a t e d through t h e c a v i t y from the e x i t toward downstream. The estimate o f small s i g n a l g a i n obtained by the above a n a l y s i s wi 11 be compared w i t h t h e present experiment.

Results and Discussion

I n F i g . 7 i s shown the d i s t r i b u t i o n o f small s i g n a l g a i n c o e f f i c i e n t G (l/m), measured a t seve- r a l l o c a t i o n s along t h e c e n t e r a x i s from t h e nozzle e x i t toward downstream. Two kinds o f measurement data are p l o t t e d f o r a f i x e d stagnation c o n d i t i o n o f a donor gas N2; namely, one i s f o r t h e case o f m i x i n g w i t h pure C02 and t h e o t h e r f o r t h e case o f m i x i n g w i t h a m i x t u r e C02/He. For comparison, the estimates, obtained by t h e simple a n a l y s i s i n t h e previous section, a r e a l s o p l o t t e d i n t h e same f i g u r e . It f o l l o w s from t h e f i g u r e t h a t the e f f e c t o f c a t a l y s t He i s n o t so appreciable on an achieved maximum g a i n b u t the decrease i n g a i n toward down-

stream i s suppressed by a d d i t i o n o f t h e c a t a l y s t . F u r t h e r , f o r cases o f a m i x t u r e C02/He, the simple estimate o f G i s r a t h e r lower than t h e measured data. T h i s i s l i k e l y t o be m a i n l y due t o the f a c t t h a t , as shown l a t e r , t h e g a i n G on t h e a x i s takes t h e maximum i n t h e d i s t r i b u t i o n along a l i n e normal t o t h e axis. I t a l s o f o l l o w s from t h e f i g u r e t h a t 0 , ---; T:=65O0K, pt=6.4 ATM A , -; ~!=720'K. @:4.9 ATM (

l/m

(T:=2000% , p ~ z 5 . 6 ~ ~ ~ ) F i g . 7. Gain d i s t r i b u t i o n along c e n t e r o f c a v i t y . the g a i n d i s t r i b u t i o n o f a q u a l i t a t i v e behavior i s reasonably s i m i l a r t o t h a t from t h e simple analysis.

For t h e case o f pure

C02, however t h e r e appears

a g r e a t discrepancy o f t h e estimate from the data. As p r e v i o u s l y mentioned, the present estimate i s based upon the n e g l e c t i o n o f l o s s due t o the v i s c o - s i t y as w e l l as disturbances, along w i t h an i d e a l mixing. Therefore, t h e estimate thus obtained must l e a d an overestimate o f the gain. Nevertheless, f o r the case o f pure C02, t h e data i n d i c a t e t h e g a i n much g r e a t e r than t h e estimate. Physical explana- t i o n on t h i s discrepancy must remain i n f u t u r e study.

The g a i n d i s t r i b u t i o n s over l i n e s normal t o the a x i s were measured a t several s t a t i o n s from t h e nozzle e x i t toward downstream. The data are shown i n Fig. 8, where x i s measured along the nozzle e x i t toward downstream and y normal t o it. As ex-

Y l 0 1 FLOW

Fig. 8. Gain d i s t r i b u t i o n s normal t o flow.

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C9-214 JOURNAL DE PHYSIQUE

pected, t h e maximum gains occur on the a x i s y = 0 and a steeper p r o f i l e a t t h e s t a t i o n c l o s e r t o the nozzle e x i t becomes p l a t e a u toward downstream. The experimental c o n d i t i o n f o r t h e data shown i n Fig. 8 was the same as t h a t shown i n F i g . 7. I n these measurements, t h e area r a t i o o f C02 nozzle was 1.8 w i t h the t h r o a t o f 3 mm i n diam. (see Fig. 3), so t h a t the mole f r a c t i o n o f C02 was comparatively l a r g e r ; e q u i v a l e n t mole f r a c t i o n was 56N2/44C02 o r 5;N2/19C02/28He, r e s p e c t i v e l y , f o r pure C02 o r C02/ He i n j e c t i o n .

I n o r d e r t o e l u c i d a t e t h e e f f e c t o f t h e stag- n a t i o n c o n d i t i o n s on t h e achieved gain, t h e g a i n measurement was conducted a t a f i x e d reference s t a t i o n which i s l o c a t e d on t h e c e n t e r a x i s o f 97 mm downstream from t h e nozzle e x i t , i n v a r i a t i o n o f N2 s t a g n a t i o n c o n d i t i o n f o r a f i x e d C02/He stagna- t i o n c o n d i t i o n , and v i c e versa. The r e s u l t s a r e summarized i n Figs. 9 and 10, t o g e t h e r w i t h t h e estimate obtained by t h e simple a n a l y s i s p r e v i o u s l y mentioned. A s t a g n a t i o n c o n d i t i o n i s s p e c i f i e d by a s e t o f temperature and pressure. I n v a r i a t i o n o f s t a g n a t i o n c o n d i t i o n o f N2 ( o r CO*), n o t o n l y temperature b u t pressure varied, so t h a t t h e equi- v a l e n t mole f r a c t i o n a f t e r the mixing a l s o varied. Thus t h e e q u i v a l e n t mole f r a c t i o n YN o f N2 i s p l o t - t e d vs t h e s t a g n a t i o n temperature TS. The optimum g a i n appears t o depend s e n s i t i v e l y on t h e C02/He s t a g n a t i o n c o n d i t i o n than on t h e N2 s t a g n a t i o n con- d i t i o n , except h i g h e r temperature r e g i o n

(T;

> 2.5

3

xlo3, T t H > 1 . 5 ~ 1 0 ). I t can a l s o be seen t h a t t h e present estimate provides a good agreement w i t h the experiment.

F i n a l l y , i n t h i s experiment an advantage o f

this t y p e o f m i x i n g

GDL was confirmed i n comparison w i t h a conventional premixed

GDL,

though t h e geometry o f the nozzle was n o t n e c e s s a r i l y an o p t i m i - zed one. The simple estimate based on t h e assump-

t i o n s o f quasi one-dimensionality, instantaneous mixing, and n e g l e c t i o n o f v i s c o s i t y as w e l l as disturbances i s shown t o simulate a t l e a s t q u a l i t a - t i v e behavior o f small s i g n a l gain, so f a r as t h e c a t a l y s t He i s contained i n l a s i n g media C02- For cases o f no c a t a l y s t , however t h e r e appears a g r e a t discrepancy o f t h e simple estimate from t h e data. Physical e x p l a n a t i o n on t h i s discrepancy remains i n f u t u r e study.

F i g . 9. E f f e c t o f N2 s t a g n a t i o n c o n d i t i o n on g a i n f o r f i x e d CO2

~ta~nation;.=720.

ptH=4.8 atm, C02:He=2:3, Ac/AI = 4.

T&

(X 1

o3

--K )

Fig. 10. E f f e c t o f C02 s t a g n a t i o n c o n d i t i o n on

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Acknowledgement thanks t o Mr. K. FunabS k i o f ISAS, U n i v e r s i t y of The authors are g r a t e f u l t o express t h e i r Tokyo f o r h i s h e l p f u l t e c h n i c a l assistances.

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