COMBUSTION DRIVEN CHEMICAL LASER PERFORMANCES STUDY

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COMBUSTION DRIVEN CHEMICAL LASER

PERFORMANCES STUDY

F. Voignier, P. Regnier

To cite this version:

JOURNAL DE PHYSIQUE Colloque C9, supplément au n°ll, Tome 41, novembre 1980, page C9-45

F. Voignier and P. Régnier.

Laboratoires de Marooussis, Centre de Recherches de la Compagnie Générale d'Electricité, Division Lasers, Route de Nosay, 91460 Mœccoussis, France.

Résumé - Les performances d'un laser chimique DF continu à combustion ont été étudiées dans les neuf cas de combustion suivants : F_/H?/He F~/CH./He F„/C„H,/He F./H„5/He

-F2/H2/N2 - F2/H2/Ar - NF3/H2/He - N F y c ^ / H e et N F y ^ S / H e . Nous comparons ici les perfor-mances de chacun de ces systèmes. Dans chaque cas,, nous avons déterminé la puissance du laser et le rendement chimique en fonction de la proportion du diluant. Les résultats expé-rimentaux montrent que les performances du laser dépendent beaucoup de la concentration en diluant. Nous présentons ici une méthode permettant de prévoir dans un système quelconque, la concentration en diluant donnant le maximum de puissance.

COMBUSTION DRIVEN CHEMICAL LASER PERFORMANCES STUDY

Abstr4ct - The lasing performances of several reactant systems for use in combustion driven DF chemical lasers were investigated. Nine reactant systems, F„/H„/He - F7/CIL/He

-F2/C2H4/He - F2/H2S/He - F2/H2/N2 - F2/H2/Ar - NF3/H2/He - N F y t ^ / H e and NF3/H2S/He were

tested. The performances of each system are presented here. Laser power and chemical effi-ciency as a function of diluent concentration were determined for each system. Experimental results indicate that there is a strong influence of the diluent concentration on the la-sing performances. A method to predict the optimum diluent concentration providing the hi-ghest laser power in any reactant system is presented here.

IntAoduction - Since our first demonstration of the In addition, the characterization of nitrogen DF combustion driven CW chemical laser we impreved and argon substituted for helium as diluent is re-the performances achievable with combustion driven ported. A significant degradation (50 - 60 % with chemical laser by optimizing the fluorine atoms pro- H?, 20 - 30 % with Ar) in laser performances was duction an by minimizing the deactivator HF concen- observed.

tration produced in the combustor. HoJidwa/to. ducJilptA.on - To provide an unambiguous For this purpose we investigated several re- program of reactant evaluation all tests were con-actant combinations by substituing CH^, C„H. and ducted by using the same well-characterized hard-H„S for H_ as the combustor fuel. The resulting de- ware. This hardware was of modular construction crease in combustor produced HF (DF lasing) gave (fig. 1 ) .

substantial (up to approximately 30 %) increases in laser power.

In order to determine the theoretically pre-dicted degradation of the- performances when NF, is substituted for F? as the combustor oxidizer, we

tested the following reactant systems : NF,/H?/He, NF,/C„H./He, NF-,/H„S/He "and we compared the laser performances to the corresponding F„ systems. This characterization established that the use of NF,

reduces the laser power by 20 ?o. F-cg. 7 - Schematic, oi tke. IOAZA

JOURNAL DE PHYSIQUE

The modules used were :

-

a gas t r i p l e t i n j e c t o r s u i t a b l e f o r gas/gas opera- tion., Before e n t e r i n g t h e i n j e c t o r , d i l u e n t i s m i - xed both w i t h t h e o x i d i z e r and t h e f u e l .

-

a water-cooled combustion chamber

The l e n g t h o f t h i s modular combustion chamber was optimized i n order t o provide a r a p i d mixing, f a s t burning w i t h a minimum o f t h e a v a i l a b l e combus- t i o n energy l o s t t o t h e w a l l s . This l e n g t h used for a l l t e s t i n g was 6 cm (heat l o s s values corres- ponding t o 0.45 o f t h e energy a v a i l a b l e i n t h e .combustion processes were generaly observed i n water cooled hardware).

The pressure measured i n t h e combustor was ran- g i n g from 2.5 t o 3.5 atm. The c h a r a c t e r i z a t i o n o f t h i s combustor i s reported i n r e f . 1.

-

a c a v i t y i n j e c t o r water-cooled w i t h an axisymme- t r i c nozzle block c o n s i s t i n g o f a l t e r n a t i n g p r i - mary and secondary nozzles i n order t o freeze the F atoms and t o mix t h e deuterium w i t h t h e com- bustor gas. The Mach number i s about 5.5 f o r t h e primary nozzles' and 5 f o r t h e secondary nozzles.

-

an o p t i c a l c a v i t y

The s t a b l e resonator multimode l a s i n g c a v i t y was l o c a t e d j u s t downstream o f t h e nozzle e x i t plan. I t c o n s i s t e d o f a d i e l e c t r i c coated t o t a l l y r e f l e c - t i n g and a p a r t i a l l y r e f l e c t i n g m i r r o r . The range o f r e f l e c t i v i t i e s was lower when helium was used (about 70-80

L)

and 4igher when N p o r A r was subs- t i t u t e d f o r helium as d i l u e n t (about 90 %). The.distance between t h e o p t i c a l a x i s and t h e nozzle e x i t p l a n was continuously variable. I t ranged from 0 t o 5 cm. The shroud p o s i t i o n was v a r i a b l e over a range o f 4 t o 10 degrees. The

I

same 5 degrees half-angle value was used during a l l t e s t s .

-

a d i f f u s e r c o n s i s t i n g o f a simple rectangular channel.

Expenimental henida2

By u s i n g t h i s hardware, t h e n i n e r e a c t a n t combina- t i o n s shown below were t e s t e d f o r lasingperformances F2/H2/He

-

F2/CH4/He

-

F2/C2H4/He

-

F2/H2S/He - NF3/H2/He - NF3/C2H4/He

-

NF3/H2S/He

-

F2/H2/N2

-

F2/H2/Ar

I n each of t h e n i n e t e s t s e r i e s t h e e f f e c t s o f t h e d i l u e n t concentration on t h e c a v i t y pressure and t h e l a s e r performances were investigated. To p r o v i - de a comparison o f the r e a c t a n t systems tested, four key performances f i g u r e s o f m e r i t were used :

p

chemical e f f i c i e n c y

(?A)

W

s p e c i f i c power (watts/cm2 o f nozzle e x i t area) d s p e c i f i c power ( k i l o w a t t s / k g / s of F atoms)

7

s p e c i f i c power ( k i l o w a t t s / k g / s of t h e t o t a l l a -

ser f l o w ) .

I n each t e s t , f o r d i f f e r e n t d i l u e n t concentrations ranging from 60 t o 85 L, the r a t i o of the f u e l (Hz- CH4

-

C2H4 o r H S) t o t h e o x i d i z e r F2 o r NFJ was

2

adjusted i n order t o provide t h e maximum outcoupled l a s i n g power. I n a l l t e s t s (whatever the d i l u e n t concentration was), t h e maximum l a s i n g power was achieved f o r a plenum temperature o f about 1500 K. For example f i g u r e s 2, 3, 4 and 5 show t h e maximum l a s i n g power W versus t h e d i l u e n t concentration. Data presented i n fi g u r e s 2 t o 7 show the influence o f t h e d i l u e n t concentration on t h e c a v i t y pressu- re, t h e chemical e f f i c i e n c y p and t h e s p e c i f i c powers W,

6

and

_{7 .}

Figure 8 and t a b l e I summarize a l l t h e r e - s u l t s obtained.

D d c u ~ i o n

d e a c t i v a t o r (DF l a s i n g ) . R e s u l t s o f t h e t h e o r e t i - c a l t h e r m o c h e m i c a l a n a l y s i s , e v a l u a t i o n and r a n k i n g o f t h e r e a c t a n t s y s t e m s t e s t e d h a v e been t a b u l a t e d i n t a b l e 2. T h e o r e t i c a l r e s u l t s a r e i n good a g r e e - ment w i t h e x p e r i m e n t a l r e s u l t s c ~ ~ /C H tests ex-

3 2 4

c e p t e d ) . T h i s d i s c r e p a n c y is p r o b a b l y d u e t o abnor- mal c a r b o n f o r m a t i o n o b s e r v e d i n t h e combustor du- r i n g t h e NFJ/C2H4/He tests.

The c o m p a r i s o n o f t h e maximum power e x t r a c t e d from e a c h r e a c t a n t s y s t e m shows ( t a b l e 1 ) t h a t h i g h e r t h e F atorns/HF d e a c t i v a t o r c o n c e n t r a t i o n r a t i o is h i g h e r t h e d i l u e n t c o n c e n f r a t i o n a n d p e r f o r m a n c e s a r e .

C o n s i d e r i n g a l l n o n - d e a c t i v a t o r s p e c i e s (He,

Ar, N2, CF4 and SF6) a s d i l u e n t we found a n e m p i r i - c a l r e l a t i o n between t h e F/HF r a t i o and t h e d i l u e n t c o n c e n t r a t i o n c o r r e s p o n d i n g i n e a c h r e a c t a n t s y s t e m t o t h e maximum l a s e r power ( s e e s o l i d l i n e on f i g u r e 9 ) .

REACTANT F/w nib &tad

T - I S W K & P - 3 b s r s R e a c t a n t s y s t e m s w i t h Ar a s d i l u e n t o r H2S a s f u e l h a v e a t o o low m o l e c u l a r f l o w r a t e and a r e n o t i n good a g r e e m e n t w i t h t h e o t h e r s y s t e m s . By u s i n g t h i s empirical r e l a t i o n it i s p o s s i - b l e t o p r e d i c t t h e optimum d i l u e n t (He) c o n c e n t r a - t i o n p r o v i d i n g t h e h i g h e s t l a s e r power i n any g i v e n r e a c t a n t s y s t e m . F o r example we u s e d t h i s r e l a t i o n t o p r e d i c t t h e optimum He c o n c e n t r a t i o n i n t h e F

/

2 CH4/He s y s t e m . We u s e d t h e f o l l o w i n g method. I n t h e c o m b u s t o r , h e a t l o s s e s v a l u e s c o r r e s - p o n d i n g t o 0.45 o f t h e t o t a l e n e r g y a v a i l a b l e i n t h e c o m b u s t i o n p r o c e s s e s w e r e g e n e r a l l y o b s e r v e d . T h a t means t h a t i n o r d e r t o p r o v i d e f u l l y d i s s o c i a t e d f l u o r i n e i n t h e plenum c o n d i t i o n s ( T > , 1500 K) t h e t h e o r e t i c a l c o m b u s t i o n t e m p e r a t u r e v a l u e i n t h e F

/

2 CH /He s y s t e m must n o t b e l e s s t h a n 2500-2600 K . 4 By a s s u m i n g a t h e o r e t i c a l t e m p e r a t u r e o f 2500 K ( p r e s s u r e

=

3 b a r s ) , we d e t e r m i n e d t h e v a l u e o f t h e F/HF r a t i o f o r any g i v e n He c o n c e n t r a t i o n . The v a l u e s o f F/HF r a t i o v e r s u s t h e d i l u e n t (He

+

CF4) c o n c e n t r a t i o n a r e r e p r e s e n t e d by t h e d o t t e d l i n e ( f i g u r e 9 ) . The c r o s s i n g of t h i s l i n e and t h e s o l i d l i n e p r o v i d e s t h e optimum v a l u e o f t h e d i l u e n t c o n c e n t r a t i o n (He

+

CF4

=

7 9 , 5 and t h e c o r r e s - p o n d i n g F/HF r a t i o v a l u e ( 0 . 5 5 ) . By t h e r m o c h e m i c a l c a l c u l a t i o n we d e t e r m i n e d t h e n t h e mass f l o w r a t e s o f F2, CH4 and He p r o v i d i n g a t h e o r e t i c a l tempera- t u r e v a l u e of 2500 K and a He + CF4 c o n c e n k r a t i o n v a l u e o f 79.5

X.

E x p e r i m e n t s showed t h a t t h e mass f l o w rates o f F2, CHq and He d e t e r m i n e d by t h i s way p r o v i d e d t h e maximum l a s e r power. F i g u r e 1 0 shows t h a t t h e maximum l a s e r power was a c h i e v e d w h e n - t h e he- l i u m c o n c e n t r a t i o n was 77 % ( c o n c e n t r a t i o n o f CF4

=

3

X).

C9-48 JOURNAL DE PHYSIQUE

p- cavlty pressure (Torr) P- chemical efllclency (%)

W- specifoc power(arb.un~t/cm~ of nozzle exlt area) .P..F*

LASCO 1C (Fr&S -He)

P : cavlty pressure (Torr)

P : chemical efficiency ( % )

W : apecHlc power ( arb.unitlcm2 of nozzb exn wen ) O

LASCO 1C (NF, -4s -He )

Molar fraction of Me (96)

P cavlty pressure (Torr)

P . chemlcal elflciency(%)

W specific power(arb.un11lcdof nozzle ex11 area)

LASCO 1C 9 '

Fig.4

2 -

1 - W

P:cavity pressure (Tom) p:chem~cal efflc~ency (%)

W.speclfic power (arb.unit/cm20f nozzle exit area)

20 19

1

18 - 17 - 16 - 15 - LASCO 1C

:j

1

F2-H2-He) 12 - 11 - 10 - LASCO 1C (NF,-H,S-He) 6b 65 70 75 w

Molar fraction of He(%)

Speclflc powera

a (wlgls of F)

t\ h l g l a of total flow xio.')

Fig. 7 m- 1!mo- l a - i300- LASCO 1C lao. (F,-H,s-H~) ,,OO. Fig.6 %a- BDO- m- 1- , , , , , , 0 (15 m 7s 80 I

Molar fractlon of He(%)

//d

Fig.

2

t o

7

-

Cav@

p a w w l e , chemical eddiciency and npecidic

lacla

powun

dependence on diluent concentrration

m , Molar 75 fraction

ii:

m of He (%) a2 , , , , 00

sp.cmC powera a (wlgls of F)

q (w/g/s of total flow x 7 0 . l ~

F i g .

8

- Dependence ad l a n u powen on

diluent concentrration

LASER POWER FIHF (MOLE RATIO)

Fig.

9

-

Dependence

05

F / H F

&&o o n

fie

He

+

N2 + At +

CFq

+

SF6

concen-

&ation

4.

50- 0 (arb.unit) 1.5- 1. MOLAR FRACTION OF DILUENT (He,Ne orAr)

(He+N,+Ar+CF, +SF.) %

P

.

cavity pressure (Torr)

p : chern~cal efficiency (%)

W

.

speclfic Dower (arb.unitlcm2 of nozzle exit area)

LASCO (h

-

CH. ,sO

P

*,*

n/ d I " ' . ' ' ' ' ' 1 (U 70 ?a 80 Molar fraction of He (%)

Fig.

10

-

F 2

-

CHq

-

ffe

neactant ayatetn

I n t h i s combustor h e a t l o s s e s v a l u e s c o r r e s - ponding t o 0.60 o f t h e t o t a l energy a v a i l a b l e i n t h e combustion p r o c e s s e s were measured. I n o r d e r t o p r o - v i d e f u l l y d i s s o c i a t e d f l u o r i n e i n t h e plenum con- d i t i o n s t h e t h e o r e t i c a l combustion t e m p e r a t u r e must n o t b e l e s s t h a n 2900 K ( i n s t e a d o f 2500 K i n a l l p r e v i o u s t e s t s ) . A s shown i n f i g u r e 9, t h e t h e o r e t i c a l y p r e - d i c t e d v a l u e o f t h e optimum d i l u e n t c o n c e n t r a t i o n (He,

+

CF4

=

78 ?A, He

=

75 ?A) is i n good agreement w i t h t h e e x p e r i m e n t a l one ( s e e f i g . 1 0 ) .

C o n c ~ i o n

We have shown t h a t i n a DF combustion d r i v e n c h e m i c a l l a s e r t h e l a s i n g performances were s t r o n - g l y i n f l u e n c e d by t h e d i l u e n t c o n c e n t r a t i o n .

We found a very promising e m p i r i c a l method f o r t h e e s t i m a t i o n o f t h e b e s t d i l u e n t c o n c e n t r a - t i o n l e a d i n g t o t h e maximum l a s i n g power. Except f o r extreme c a s e s (Ar a s d i l u a n t o r SF6 a s combus- t e r * p r o d u c t ) t h i s method can b e used i n any r e - a c t a n t system and i n any combustor.

Acknowledgemevu2

W wish t o thank Dr C.Uerdier and h i s c o l l e a - e g u e s o f t h e ONERA ( F r a n c e ) f o r t h e d e s i g n , t h e

Redmenee

( 1 ) C. V e r d i e r and B. Leporcq

"Combustion d r i v e n atomic f l u o r i n e g e n e r a t o r s f o r OF chemical l a s e r s "

-

3 t h I n t e r n a t i o n a l Symposium on Gas Flow and Chemical L a s e r s , M a r s e i l l e , 1980.

c o n s t r u c t i o n and t h e i r c o n t r i b u t i o n t o t h e charac- t e r i z a t i o n o f t h e combustion chambers.