EXPERIMENTAL INVESTIGATIONS OF GASDYNAMIC N2/CO2 MIXING LASERS

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EXPERIMENTAL INVESTIGATIONS OF

GASDYNAMIC N2/CO2 MIXING LASERS

H. Hügel, W. Schall, P. Hoffmann

To cite this version:

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JOURNAL DE PHYSIQUE CoZZoque C9, suppZ6ment a u n O 1 l , Tome 41, novembre 1980, page C9-335

.EXPERIMENTAL I N V E S T I G A T I O N S OF GASDYNAMIC N ~ / C O ~ M I X I N G LASERS H. Hiigel, W. Schall and P. Hoffmann.

I n s t i t u t fLir T e c h i s c h e Physik, DFVLR, 0-7000 S t u t t g a r t 80, Fed. Rep. Germany.

Abstract.- N2/C02 mixing lasers with thermal excitation of the N2 in arc-heaters and with screen nozzles for the expansion and mixing process are investigated. Results obtained with two devices having different cross-sections of the laser channel are compared. For equal operating conditions the small signal gain is well correlated by a normalized distance downstream of the nozzle exit plane. The dis- tribution of gain along the flow direction is mainly determined by the stagnation temperature, the ratio of N2/C02 and the presence of He. Values of small signal gain up to 3 % CV:' are achieved. The effect of replacing N2 by CO is studied in detail. The observed reduction of laser performance depends on the ratio of N2/CO/C02 but is almost independent of stagnation temperature. In addition, prelimiriary experiments aiming at the substitution of Np by air yield a specific laser energy of 5 Jlg.

1. Introduction

Over the past decade the gasdynamic N2/C02 mixing laser with thermal excitation has gained considerable importance. Its high performance potential originates from the

facts that reservoir temperatures above the dissociation limit of the CO2 can be employed and more vibrational energy may be preserved during the expansion of the pure N2-flow. The extensive experimental and theoretical research conducted up to 1978 is summarized in Ref. 1.

The crucial point of the mixing laser con- cept is the transfer of vibrational energy from N2 to COZ. Therefore, in the course of the investigations the search for an

' efficient mixing technique became of par-

ticular interest. O f the various methods suggested, the screen mixing nozzle has proved to yield the best performance da- ta2' 3 . This technique is based upon the screen nozzles applied in premixed gasdynamic lasers4 and was introduced in the

. mixing laser by two groups independently

of each

A main goal of the experimental work re-

ported here was to investigate the effect of geometrical up-scaling of the screen nozzle and the laser channel. For this pur- pose a laser was constructed similar to the small-scale apparatus used in earlier experiments2' 3' 6 . Experimental data of

small signal gain and laser power will be presented and discussed.

Considering some applications of the mixing lasers the substitution of N2 by other gases or gas mixtures is of interest. With respect to this, the effects on the laser

performance of a partia1,substitution of

N2 by CO and of a complete replacement of

N2 by air have been investigated and will. be discussed.

The presented experimental results are se- lected from the compilation of data in Ref. 7.

2. Experimental Apparatus

The configuration of the screen mixing

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C9-336 JOURNAL DE PHYSIQUE

n o z z l e i s shown i n F i g . 1. The N2 i s ex- panded t h r o u g h an a r r a y o f 4 X 33 c o n i c a l n o z z l e s w i t h 15O h a l f a n g l e , 0.75 mm t h r o a t d i a m e t e r and d = 4 mm e x i t d i a m e t e r . Am- N 2 b i e n t t e m p e r a t u r e CO2 o r CO /He-mixtures 2 a r e i n j e c t e d t h r o u g h s o n i c o r i f i c e s o f d = 0.7 mm d i a m e t e r . The s t r e a m s e n t e r c02 a c h a n n e l w i t h 16 X 150 mm2 c r o s s s e c t i o n which i n downstream d i r e c t i o n s l i g h t l y i n - c r e a s e s t o a c c o u n t f o r t h e growth of t h e boundary l a y e r s . A t d i s t a n c e s 22, 60, 110 and 160 mm from t h e n o z z l e e x i t p l a n e op- t i c a l p o r t s o f 15 mm d i a m e t e r p e r m i t measurements o f s m a l l s i g n a l g a i n and l a s e r power. N i t r o g e n i s h e a t e d i n a d c a r c - h e a t e r . A d i v e r g i n g two-dimensional s t a g n a t i o n and s e t t l i n g chamber c o n n e c t s t h e a r c - h e a t e r w i t h t h e s c r e e n n o z z l e b l o c k . The d e v i c e i s o p e r a t e d i n a cw manner. The s t a g n a t i o n t e m p e r a t u r e i s determined from t h e measured e l e c t r i c a l power i n p u t , t h e h e a t l o s s e s t o t h e a r c - h e a t e r , s t a g n a t i o n chamber and n o z z l e b l o c k , and t h e N2-flow r a t e t h r o u g h t h e h e a t e r . T e s t r u n s were conducted w i t h f l o w r a t e s o f 5, 7.5 and 12.5 g / s N 2 , 9.4 and 18.8 g / s C O 2 , and w i t h z e r o o r 1.9 g / s He. The s t a g - n a t i o n t e m p e r a t u r e was v a r i e d i n t h e r a n g e between 1400 and 4 0 0 0 K a t s t a g n a t i o n p r e s - s u r e s of 2 t o 5 b a r . Accordingly t h e p r e s - s u r e i n t h e l a s e r c h a n n e l v a r i e d from 3 t o . 2 5 mbar. 3.

Small

S i g n a l Gain Measurements o f t h e s m a l l s i g n a l g a i n a r e performed w i t h a P20 s t a b i l i z e d TEMoo probe l a s e r beam o f 100 mW. The g a i n d i s - t r i b u t i o n a l o n g t h e l a s e r c h a n n e l depends e x t r e m e l y upon t h e o p e r a t i n g c o n d i t i o n s o f t h e laser d e v i c e , i . e . on t h e g a s composi- t i o n and t h e s t a g n a t i o n t e m p e r a t u r e . I n p a r t i c u l a r , t h e p r e s e n c e o f helium p l a y s an i m p o r t a n t r o l e , a s i s shown i n F i g s . 2 t h r o u g h 4. A normalized d i s t a n c e z/h i s chosen a s a b s c i s s a where z i s t h e d i s t a n c e downstream o f t h e n o z z l e e x i t p l a n e and h i s t h e d i s t a n c e between t h e a x e s o f N2- n o z z l e and C O 2 - i n j e c t i o n b o r e . I n F i g . 2 t h e g a i n o b t a i n e d w i t h and with- o u t He i s p l o t t e d f o r a c o n s t a n t s t a g n a t i o n t e m p e r a t u r e o f 2000 K , a c o n s t a n t f l o w r a t e o f N 2 and two d i f f e r e n t v a l u e s o f CO2 f l o w r a t e . The maximum v a l u e o f g i s n o t much

0 a f f e c t e d by t h e p r e s e n c e o f He, a p a r t from t h e f a c t , t h a t t h e l o c a t i o n where it o c c u r s moves s l i g h t l y c l o s e r towards t h e n o z z l e e x i t p l a n e i n t h e a b s e n c e o f He. The c u r v e s a l s o i n d i c a t e t h a t i n t h i s c a s e t h e g a i n grows f a s t e r . On t h e o t h e r hand, t h e g a i n i n downstream d i r e c t i o n d e c r e a s e s f a r more F i g . 1: Schematic o f s c r e e n mixing n o z z l e . r a p i d l y w i t h o u t Be a s compared t o t h e c a s e

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These e f f e c t s a r e enhanced w i t h i n c r e a s i n g f l o w r a t e o f CO2. The d o u b l i n g o f t h e CO2

f l o w r a t e y i e l d s an i n c r e a s e i n s m a l l s i g - n a l g a i n o f merely a b o u t 20 % a t t h e s e h i g h mole f r a c t i o n s o f CO2. For lower v a l u e s of

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JOURNAL DE PHYSIQUE n o z z l e a r r a y i n t h e up-scaled d e v i c e . While i n t h e s m a l l l a s e r t h e w i d t h o f t h e n o z z l e a r r a y was comparable t o t h e 2 0 mm d i a - m e t e r o f t h e anode o f t h e a r c - h e a t e r , i t i s now a f a c t o r o f 7 . 5 l a r g e r t h a n t h e anode d i a m e t e r . e x p e r i m e n t a l v a l u e s a r e compared w i t h t h e o r e t i c a l p r e d i c t i o n s o f l a m i n a r mixing f lows8 and o f h y p o t h e t i c a l i n s t a n t a n e o u s mixing. The l a t t e r a r e r e s u l t s o f own c a l - c u l a t i o n s u s i n g t h e k i n e t i c model d e v e l - oped i n Ref. 9. The p r e s e n t e d d a t a o f s m a l l s i g n a l g a i n i n d i c a t e t h a t t h e s t r o n g dependence o f i t s d i s t r i b u t i o n a l o n g t h e c h a n n e l on t h e op- e r a t i n g p a r a m e t e r s r e s u l t s from t h e r m a l and gasdynamic i n t e r f e r e n c e s : So t h e s t a t i c t e m p e r a t u r e i n t h e c h a n n e l i s c o n s i d e r a b l y r e d u c e d w i t h t h e a d d i t i o n o f H e . F u r t h e r , it a p p e a r s p l a u s i b l e , t h a t t h e e f f e c t of wakes and shocks becomes more s e r i o u s a s t h e v a r i o u s g a s f l o w r a t e s a r e i n c r e a s e d . I n t h i s c o n n e c t i o n t h e q u e s t i o n a r i s e s a s t o t h e c h a r a c t e r o f t h e mixing p r o c e s s o f t h e Mw5 s t r e a m s o f N 2 and t h e h i g h l y un- derexpanded C02/He s t r e a m s . Because o f t h e p r e s e n t l y n o t a v a i l a b l e d a t a o f p r e s s u r e s and t e m p e r a t u r e s i n t h e p l a n e o f t h e noz- z l e e x i t no q u a n t i t a t i v e answer c a n b e given. However, t o p r o v i d e some q u a l i t a - t i v e i d e a of how t h e mixing might o c c u r , t h e e x p e r i m e n t a l r e s u l t s a r e compared w i t h t h o s e o f t h e o r e t i c a l models. . For t h i s p u r p o s e e ? p e r i m e n t a l d a t a i s cho- s e n t h a t i s o b t a i n e d w i t h a g a s composi- t i o n c l o s e s t t o t h a t of t h e e x p e r i m e n t a l r e s u l t s o f Ref. 5 and t o t h e t h e o r e t i c a l o n e s o f Ref. 8. F i g . 5 shows a r e m a r k a b l e agreement between o u r r e s u l t s and t h a t o f

i

Ref. 5, a l t h o u g h t h o s e were o b t a i n e d w i t h a d i f f e r e n t s c r e e n mixing n o z z l e ( s u p e r - s o n i c i n j e c t i o n o f C O 2 ) , a l a r g e r c h a n n e l w i d t h and N 2 h e a t i n g by a shock t u b e . The

To = 2000 K = TVlb theor : N 2 /CO2 = 1279 j0.21 instantoneou_s present inv

I

y ~ c o ,

= a6qo.32

I

Cossady et. 01. N2/Cq/H20 =0.89/03/0Dl 0 exp.: To U 2000 K

Fig. 5: Comparison o f e x p e r i m e n t a l and t h e - o r e t i c a l s m a l l s i g n a l g a i n a l o n g t h e l a s e r c h a n n e l . Two f e a t u r e s a r e a p p a r e n t . The o n e i s t h e growth o f g a i n which i n t h e e x p e r i m e n t s i s f a s t e r t h a n t h a t p r e d i c t e d by d i f f u s i v e l a m i n a r mixing. The o t h e r i s t h e l e v e l l i n g o f f and d e c r e a s e o f g a i n a t d i s t a n c e s r e l a - t i v e l y c l o s e t o t h e n o z z l e e x i t p l a n e . Of i n t e r e s t i s f u r t h e r t h e a l m o s t c o i n c i d e n t l o c a t i o n o f t h e maximum s m a l j s i g n a l g a i n a s measured and a s p r e d i c t e d by t h e i n - s t a n t a n e o u s mixing. From t h e s e e v i d e n c e s t h e q u a l i t a t i v e c o n c l u s i o n may b e drawn t h a t t u r b u l e n t phenomena induced by gasdynamic d i s t u r b a n c e s a r e o f i m p o r t a n c e f o r t h e mixing p r o c e s s .

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To = 20M1 K

-

N2/C02/~e = 0.28/0.22/0.5 0 small-scale device

fl,

present investigations

-

F i g . 6: C o r r e l a t i o n o f s m a l l s i g n a l g a i n o b t a i n e d w i t h d i f f e r e n t d e v i c e s . Such an e x c e l l e n t c o r r e l a t i o n F i g . 7: L a s e r power a l o n g t h e l a s e r cFjan- n e l a t a s t a g n a t i o n t e m p e r a t u r e o f 3000 K.

a s shown i n t h i s p l o t i s found o n l y f o r a r e p r e s e n t e d i n F i g s . 7 and 8. The bane- v a l u e s o f t h e s t a g n a t i o n t e m p e r a t u r e below f i c i a l e f f e c t o f , H e i s o b v i o u s . A t 3000 K 2300 K. T h i s i s a t t r i b u t e d t o t h e d i f f e r e n t and i n t h e a b s e n c e o f He no l a s e r o s c i ' l l a - dependence o f s m a l l s i g n a l g a i n measured t i o n i s o b s e r v e d a t t h e l a s t p o r t i n alc- i n t h e two d e v i c e s . I n s p i t e o f t h i s d e f i - c o r d a n c e w i t h t h e r e s u l t s o f F i g . 3. The c i e n c y , however, i t i s f e l t t h a t t h e s m a l l s h i f t o f t h e power maximum towards h i q h e r s i g n a l g a i n a c h i e v e d by s c r e e n n o z z l e t e m p e r a t u r e s i n t h e p r e s e n c e o f He i s i n mixing can b e c o r r e l a t e d by z / h and i s agreement w i t h r e s u l t s of Ref. 10.

l a r g e l y i n d e p e n d e n t o f t h e c h a n n e l dimen-

t i o n , some measurements were performed w i t h 100

r e s o n a t o r s o p e r a t i n g s i n u l t a n e o u s l y a t t h e s i o n s .

300

4 . L a s e r Power '=L

[W

l

Laser power was e x t r a c t e d a t t h e f i r s t , s e - 200 cond and f o u r t h measuring p o r t . I n a d d i -

second ( z = 6 cm) and t h e f o u r t h ( z = 16 cm) 0

l

I I I

,

,A

1000 l500 2000 2500 3000 3500 l o c a t i o n . The r e s o n a t o r s o f t h e s t a b l e t y p e To

[K]

,S---

*-

i - -

were p r o v i d e d w i t h f l a t and 4 m r a d i u s 1 " F i g . 8: V a r i a t i o n of l a s e r power w i t h

a s t a g n a t i o n t e m p e r a t u r e a t p o s i -

d i a m e t e r m i r f o r s . The r e f l e c t i v i t y o f t h e t i o n z = 6 cm. c o u p l i n g m i r r o r was 90 % o r 9 6 8.

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JOURNAL DE PHYSIQUE Fig. 9: V a r i a t i o n o f l a s e r power w i t h s t a g n a t i o n t e m p e r a t u r e a t p o s i - t i o n z = 16 cm w i t h o u t and w i t h s i m u l t a n e o u s power e x t r a c t i o n a t p o s i t i o n z = 6 cm. formance p o t e n t i a l o f t h i s l a s e r t y p e . An i n d i r e c t l y i n f e r r e d ' v a l u e f o r t h e s p e c i f i c l a s e r power i s 20 J o u l e p e r gram o f t o t a l mass f l o w r a t e . T h i s i s e s t i m a t e d from t h e l a s e r power measured a t z = 16 cm w i t h o u t and w i t h s i m u l t a n e o u s power e x t r a c t i o n a t z = 6 cm, s e e F i g . 9 , t o g e t h e r w i t h t h e r e s u l t s o b t a i n e d a t t h a t l o c a t i o n and p r e - s e n t e d i n F i g . 8. 5. S u b s t i t u t i o n o f N 2 For p r a c t i c a l a p p l i c a t i o n s o f t h e mixing l a s e r i t might b e d e s i r a b l e t o r e p l a c e t h e h o t N 2 flow by a s t r e a m o f combustion pro- d u c t s . With r e s p e c t t o t h i s , t h e p a r t i a l s u b s t i t u t i o n o f N2:by C O i s o f p a r t i c u l a r i n t e r e s t . Using t h e s m a l l - s c a l e d e v i c e , a s e r i e s o f t e s t s h a s been conducted t o s t u d y t h e e f f e c t o f l a r g e amounts o f CO on t h e l a s e r performance. These measurements com- p l e t e t h e p r e v i o u s l y r e p o r t e d r e s u l t s t h a t l 6 were o b t a i n e d f o r v e r y s m a l l CO f r a c t i o n s

.

T y p i c a l r e s u l t s o f t h e s m a l l s i g n a l g a i n a r e p r e s e n t e d i n F i g . 10. With and w i t h o u t

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C o n c e r n i n g c o s t - e f f e c t i v e o p e r a t i o n o f t h e m i x i n g l a s e r t h e c o m p l e t e s u b s t i t u t i o n o f N by a i r m i g h t g a i n i m p o r t a n c e . Some ex- 2 p e r i m e n t s w i t h t h e l a r g e r d e v i c e were p e r - formed and y i e l d e d i n t e r e s t i n g r e s u l t s . F i g . 11 shows t h e d e p e n d e n c e of l a s e r power o n s t a g n a t i o n t e m p e r a t u r e and f l o w r a t e s o f CO2 and H e . A s compared t o t h e perform- a n c e w i t h p u r e N2 a r e d u c t i o n o f 25 t o 40 % i s found f o r s t a g n a t i o n t e m p e r a t u r e s o f 2000 K t o 2500 K. T h i s i s more t h a n what would c o r r e s p o n d t o t h e s t o i c h i o m e t r i c re- d u c t i o n o f t h e N 2 f l o w r a t e . However, t h e p e r c e n t u a l d e c r e a s e i n measured l a s e r power a g r e e s w e l l w i t h t h e measured p e r c e n t u a l d e c r e a s e i n a v a i l a b l e e n e r g y a t a l m o s t 12 i d e n t i c a l o p e r a t i n g c o n d i t i o n s

.

F i g . 11: V a r i a t i o n o f l a s e r power w i t h s t a g n a t i o n t e m p e r a t u r e a t c o m p l e t e r e p l a c e m e n t o f N2 by a i r . I n t h e t e m p e r a t u r e r a n g e between 2000 K a n d 2500 K H e h a s a n e g l i g i b l e s m a l l e f f e c t o n t h e l a s e r power. Hence, t h e a d m i x t u r e o f t h e e x p e n s i v e H e c a n be abandoned. Without a n y o p t i m i z a t i o n , s p e c i f i c l a s e r e n e r g i e s o f up t o 5 J / g were a c h i e v e d a t t h e s e o p e r - a t i n g c o n d i t i o n s . R e f e r e n c e s 1 C a s s a d y , P . E . , P r o c . I n t . Symp. o n Gas- Flow and Chemical L a s e r s , e d . J.E.Wendt, Hemisphere P u b l i s h i n g C o r p . , 95 ( 1 9 7 9 ) . 2 Hoffmann, P . , Hugel, H . , M a i s e n h a l d e r ,

F . , S c h a l l , W., F r u h j a h r s t a g u n g DPG, Hannover, P a p e r No. P74, Verhandlungen DPG

2,

150 ( 1 9 7 6 ) .

3 Hoffmann, P - , Hugel, H . , S c h a l l , W . ,

P r o c . I n t . Symp. on Gasdynamic and Chem- i c a l L a s e r s , e d . M. F i e b i g , H. Hugel, DFVLR P r e s s , 171 ( 1 9 7 6 ) .

4 Russell,D.A., N e i c e , S.E., Rose, P.H.,

AIAA J.

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5 C a s s a d y , P.E., Newton, J . F . , Rose, P . H . , AIAA P a p e r No. 76-343 ( 1 9 7 6 ) . 6 Hoffmann, P., Hugel, H . , S c h a l l , W . , A I A A J.

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1527 ( 1 9 7 7 ) . 7 ~ e n n i g , W . , I B 452 432/80, ( 1 9 8 0 ) , un- p u b l i s h e d . 8 P a r t h a s a r a t h y , K . N . , Anderson J r . , J . D . , Jones, E., AIAA J.

17,

1208 ( 1 9 7 9 ) . 9 H a r r a c h , R.J., Einwohner, T.H., UCRL-

51399, 1973.

10 K r a u k l i s , A.V., Croshko, V.N., S o l o - u k h i n , R . I . , Fomin, N.A., P h y s i c s o f Combustion, E x p l o s i o n s a n d Shock Waves

'

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12, 792 ( 1 9 7 6 ) .

11 B a i l l y , R . , P e a l a t , M . , T a r a n , J . P . E . , Rev. Phys. Appl. 1705 ( 1 9 7 7 ) .

12 W a t t e r s o n , J.B., Knoke, G.S., P r o c . 1 1 t h I n t . Symp. Shock Tubes and Waves, ed.