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COMBUSTION DRIVEN ATOMIC FLUORINE
GENERATORS FOR DF CHEMICAL LASERS
C. Verdier, B. Leporcq
To cite this version:
COMBUSTION DRIVEN ATOMIC FLUORINE GENERATORS FOR DF CHEMICAL LASERS
Abstract - Conventional high pressure corabustors delivering atomic fluorine for DF chemical lasers have been designed and tested. A method for routine estimation of the output F atom mass flow rate is presented. It also provides means to select fuels from the computation of the theoretical maximum atomic production for various input chemical compositions assuming frozen isentropic expansion and with or without heat losses in the combustor. The values are presented for one mole of combustion products including the diluent and for a .75 ty-pical value of the molar fraction of diluent. The best equivalence ratio for various fuel/ oxidizer/diluent system is calculated for adiabatic conditions and for the effective con-ditions of heat losses.
Good correlations have been obtained between the maximum laser power and the predicted best conditions of atomic fluorine production at good overall laser power.
1. INTRODUCTION
Available references describing combustors for also been demonstrated at temperature as low as
F atom production concern either cylindrical[ls 2] 700 K [7] .
or rectangular devices [3} which operate with This paper is concerned with fuel selection for gaseous or liquid reactants. Solid propellant com- high production of F atom by thermal equilibrium
positions such as NF^BFij ,(CF2)n have also been dissociation of F2 at high pressure. The advantage
proposed |/t, 5 ] . of using highly energetic fuels is confir-In most cases the pressure level is high (sev- med when a comparison is made with conventional
eral atm) and the atomic fluorine production can H2/F2 combustion [8] . The determination of the
be estimated from thermodynamic equilibrium calcu- best equivalence ratio is made for various input lation for the experimental pressure and input com- chemical compositions (fuel/oxidizer/diluent) and position. The F atom recombination in the laser for experimental conditions with heat losses.
nozzle has been demonstrated to be small [6]. In T h e validity of fuel selection is determined
the case of low pressure combustion, typically .05 from comparison between the equivalence ratio at
to .1 atm nonequilibrium F atom production has which the maximum F atom production is predicted
JOURNAL DE PHYSIQUE Colloque C9, supplément au n°ll, Tome 41, novembre 1980, page C9-31
C. Verdier and B. Leporcq
Office National d'Etudes et de Recherches Aérospatiales, 9-1120 Palaiseau, France.
Résumé - Des foyers de combustion conventionnels à haute pression délivrant du fluor atomi-que pour des lasers' chimiatomi-ques à DF ont été réalisés et essayés. Une méthode pour déterminer la production de fluor atomique est présentée ; elle permet aussi la sélection des com-bustibles dans le cas d'une détente figée isentropique et avec ou sans pertes de chaleur dans le foyer. Des valeurs sont présentées pour une mole de produits finals.et une fraction molaire de diluent de 0,75. Les meilleures conditions de richesse pour différents systèmes combustible/comburant/diluant sont déterminées dans le cas adiabatique et dans le cas réel.
De bonnes corrélations ont été obtenues entre le maximum de puissance laser et le maximum de production de fluor atomique à un bon niveau de puissance spécifique.
C9-32 JOURNAL DE PHYSIQUE
and the equivalence ratio at which the maximum overall laser power is obtained. The lasing effect power is described and discussed in an other paper at the same congress L9]
.
Comparison between the laser power obtained with various input composi-tions and the predicted F atom mass flow rate is
also a method of control.
It is assumed in the comparison between the fuel F atom production efficiency and the laser power that the aerothermochemical phenomena oc- curing in the laser nozzle and in the laser cavity are not strongly influenced by the optimisation of the combustor, and in particular by variation of F atom production.
2. F ATOM
PRODUCTION
The knowledge of the combustor inlet composi- tion enables to compute as a reference the adiaba- tic equilibrium. From this result and for a frozen flow through the nozzle the maximum F atom mass flow rate, which can be achieved if no heat losses occur in the combustor, is deduced for each inlet compo- sition.
In the actual combustion conditions important heat losses occur; they are reinforced by atom recombi- natibn on the walls as will be seen later, they can be estimated from the knowledge of the combustor pressure and from direct measurements.
The effective mean experimental temperature is
.
calculated with the equation of mass flow m through a sonic nozzle
where A, eff is the effective aerodynamic area of the throat
T is the mean temperature of the combustion
*is the mean molar mass of the combustion
gases.
Aerodynamic considerations [lo] show that for
a properly designed nozzle geometry Ac.eff is not very different from the geometric area A even for very small nozzles a few tenths of a millimeter in diameter. It has been checked that the geometric area remains nearly constant when the temperature of the gases varies.
From the mean temperature deduced from ( I ) , the
composition and the F atom experimental production are calculated. Unfortunately a convenient analy- tical technique is not available for F atom pro- duction estimation C111 ; thus a direct verifica- tion is not possible and only correlation with laser power can be made easily.
An input chemical composition (fuel/oxidizer/ diluent) is characterized by
- its equivalence ratio
;
fuel/; oxidizery =
(mfuel/&oxidizer).
stoichiometric.
where m. is the mass flow rate of species i
- and by the mass flow rate of diluent which
goes unaffected through the combustion. Its molar fraction also characterizes the final composition.
For calculation and comparison purposes the F atom productions (theoretical and adiabatic or experimental with heat losses) are calculated when
lf is varied and when the molar fraction of diluent
= fidil
dil
-
'total
is kept constant in the combustion products (.75 is a typical value in DF laser application which has been chosen in the examples). The calculated F atom production is expressed in term of molar fraction
Computations have been performed with various input conditions and the final combustion parame- ters such as temperature, composition have been determined under adiabatic conditions.
As an example the results for NF3/C2H4/He are presented on fig. 1 and 2.
'P max
r
nF/n tot
""t
Figure 1 : Computation examples : T, /;I tot. Pressure influence, xHe influence. N F ~ / E ~ H ~ / H ~ .
On figure 1 the effect of pressure (in the high domain of a few atm where equilibrium is achieved) is presented. It can be noticed that the molar fraction of diluent is an important parameter as is evident at first.
The F atom production is very sensitive to the equivalence ratio, as a consequence the inlet chamber mass flow rate must be very accurate. The maximum F atom production arises at temperature close to 1500K.
On figure 2 the results are presented in the case when the molar fraction of diluent is varied. The diagram gives the equivalence ratio at which the maximum adiabatic production of F atoms arises, the corresponding temperature and the amount of F atom expressed in molar fraction.
Molar fract~m of drluant ( x ~ e )
2 : xF
.maxJ
TthIF.
maxy
F. maxversus xHe for NF3, C2H4, He.Fuel selection can be achieved through calcula- tion of the maximum F atom production (adiabatic or with heat losses) for typical values of pressure
and molar fraction of diluent. The results are sum-
marized in figure 3, in this case the production of
F atom is assumed to be adiabatic. When calculations
are made with heat losses the production is lowered but the relative order of interest remains the same. It can be noticed that :
-
important variations for F atom productionarise when the fuel is changed, for instance
n ~ ( ~ 2 ~ 2 - ~ 2 )
#
3 n ~ ( ~ 2 - ~ ~ 3 ) this result si due to the high value of the standard heat of formation of combustion products such as -220,5 Kcal/mole for CFL, and -228,5 for SF6 ; in the case of HF the value is only -64,5 Kcal/ mole.-
the relative interest of the fuel do not de-JOURNAL DE PHYSIQUE
F i g u r e 3 : Fuel s e l e c t i o n (computation s t e p f o r \Q = 0.01) p
=
3 a t m , xHe = 0.75, no h e a t l o s s e sThe i n t e r e s t of h i g h power combustion can a l s o be deduced from f i g u r e 4 and 5. From such f i g u r e s it i s c l e a r t h a t t h e e f f e c t i v e experimen- t a l v a l u e f o r F atom production i s lower t h a n t h e
a d i a b a t i c production. An o v e r a l l p r o d u c t i o n y i e l d qF can be d e f i n e d a s t h e r a t i o o f t h e expe- r i m e n t a l maximum production t o t h e t h e o r e t i c a l one. ;1 exp. max
b =
FAF
theo. maxThe experimental maximum v a l u e i s o b t a i n e d a t a g r e a t e r equivalence r a t i o v a l u e t h a n t h e a d i a - b a t i c one. When t h e h e a t of t h e r e a c t i o n of com- b u s t i o n i s v a r i e d important v a r i a t i o n s of q occur,
it can be seen from f i g u r e 4 and 5 t h a t
The i n t e r e s t of u t i l i z a t i o n o f e n e r g e t i c f u e l f o r F
atom p r o d u c t i o n i s confirmed, moreover t h e produc- t i o n of non-deactivating s p e c i e s such a s CFq, SF6 i n s t e a d o f t h e h i g h l y d e a c t i v a t i n g u s u a l HF is a n
F i g u r e 4 : Combustion C2H4-F2. xHe
#
0.75.I n t h e c a s e of C2H4, f o r which a h i g h v a l u e of - 7 5 f o r q i s o b t a i n e d , experiments have been performed i n a l a r g e domain of v a r i a t i o n of t h e equivalence r a t i o . An experimental peak v a l u e f o r
.
t h e r m a l l o s s e s ( c u r v e AW/ntot) is p r e s e n t a t t h e same v a l u e of t h e e q u i v a l e n c e r a t i o t h a n t h e one a t which a d i a b a t i c maximum p r o d u c t i o n o c c u r s . Moreover it i s obvious t h a t t h e t e m p e r a t u r e of t h e combustion g a s e s i n c r e a s e s with t h e e q u i v a l e n c e r a t i o and t h a t t h e h e a t l o s s e s should a l s o i n c r e a s e s i n c e no m o d i f i c a t i o n was i n t r o d u c e d i n t h e i n j e c - t i o n d e v i c e and i n t h e i n j e c t i o n c o n d i t i o n s . Thus t h e peak v a l u e f o r t h e r m a l l o s s e s i s an i n d i c a t i o n t h a t a high p r o d u c t i o n o f F atom occurs e f f e c t i v e l y i n t h e c o r e of t h e combustor i n t h e mixing r e g i o n . Furthermore, a t t h e c o l d w a l l s of t h e combustor recombination 2F+F2 o c c u r s with h i g h h e a t r e l e a s e t h e r e f o r e i t can be s e e n from f i g u r e 4 t h a t a ttal equivalence ratio leading to the maximum of of a few milliseconds must be kept as low as pos-
heat losses is also an indication of accurate sible. As a result the geometric length of the
input mass flow rate measurements. combustor is only a few centimeters ) moreover to
avoid an impact of the individual triplet flows on the laser nozzle which could generate thermal
I
I
and chemical heterogeneity it has been necessaryFig. 5 : Combustion Hz-F2 .x~e#0.75.
to choose low injection velocities. As a result a low pressure drop through the injectors is achieved and no transversal pressure profile in the tube ahead of the injectors must exist.
Although it is well-known that the relative heat losses of conventional combustors decrease when the size of the combustor encrease it can be noti- ced that this reduction is reinforced in the pre- sent case by the high value of thermal losses due to atom recombination on the walls.
3. TECHNOLOGICAL CONSIDERATIONS
A typical combustor is shown on figure 6. When such bidimensional combustors are used the experi-
mental peak effect for F atom production versus
equivalence ratio which has just been described requires that a uniform equivalence ratio be
1
achieved along both the injector and nozzle banks.
Moreover because of the important F atom recom-
bination on the cold walls the characteristic
length
L*
( =Volume/Throat effective area) whichis usually of the order of a few meters and the associated residence time which is of the order
Figure 6 : Combustor for F atom production. It was convenient to use gaseous tracers, sampling and subsequent analysis by classical methods such as gas chromotography or mass spectrometry. With the injection device used the typical results of fig. 7 have been obtained which show the importance of the injection conditions on F atom production uniformity which depends sharply on the equivalence
c9-36 JOURNAL DE PHYSIQUE
low and n e g l i g i b l e with r e s p e c t t o t h e a m p l i f i c a - t i o n c o e f f i c i e n t of t h e l a s e r a t t h e considered wavelength.
-SAMPLING POINT ALONG NOZZLE BANK
F i g u r e 7 : Example of i n j e c t i o n condi- t i o n s t u d y . ( d e t e r m i n a t i o n of l o c a l equi- v a l e n c e r a t i o p a t t e r n ) . LASER PDWER
f
Arbctrary unitsFigure 8 : Atomic f l u o r i n e production and l a s e r power e f f i c i e n c y
4. CONCLUSION
Comparison of t h e l a s e r e f f e c t with t h e p r e - d i c t e d experimental F atom production was made. F i g u r e 8 i s an example. The p r e d i c t e d equivalence r a t i o f o r e f f e c t i v e maximum F atom production i s
almost t h e same a s t h e quivalence r a t i o a t which maximum l a s e r power occurs. A s h a r p peak
i s observed when equivalence r a t i o i s v a r i e d . The l i g h t discrepancy between t h e maxima was a t t r i b u t e d t o t h e f a c t t h a t two d i f f e r e n t n o z z l e banks were used, one i n t h e combustion t e s t s and a second i n t h e l a s e r t e s t s , t h e h e a t l o s s e s being s l i g h t l y g r e a t e r i n t h e l a t t e r c a s e .
The o v e r a l l l a s e r e f f e c t s obtained were of t h e o r d e r of 2 KW/mole with C2Hk/F2/He and 1 . 5 KW/mole with H2/F2/He.
The o v e r a l l l a s e r power was over 20 KW/mole F.
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Lasings T e s t s with Advanced R e a c t a n t s Doc. AD A 0305 42 (1977)
2 H.A. A R B I T , D.T. CAMPBELL
-
Combustor f o r Gas Dynamic Laser 1 1 t h JANNAF ( S e p t . 1974)./3/ R.A. ACKERMAN
-
Advanced R e a c t a n t s Laser Performance Evaluation Doc. AD A018627.4 R.E. CHANNEL, J.E. O'PRAY, F.Q. ROBERTO F l u o r i n e Generating Formulation f o r use i n chemical l a s e r s U.S. P a t e n t 4001 136 Jan. 4 (1977).
/ 5 / D.A. PILOPOVITCH
-
Oxidizer Compatible S o l i d P r o p e l l a n t F l u o r i n e Atom Generator U.S. P a t e n t 3963542 June 15(1976).
/ 6 / J.E. FERRELL, R.M. KENDALL, H. TONG
-
Recombination E f f e c t s i n chemical Laser Nozzles A I A A paper 76-643(1973).
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Handbook of Chemical Lasers John Wiley (1976). /9/ F. VOIGNIER,
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