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COMB USTION AND FLA ME 4 6 : 2 8 3 - 3 0 0 ( 1 9 8 2 ) 2 8 3
A utoigni t ion C haracter i s t ics of Aircraft -Typ e Fuels*L O U I S J . S P A D A C C I N I a n d J O H N A . T E V E L D E
U m t e d T e c h n o l o g ie s R e s e a r c h C e n t e r , E a s t H a rt J b rd , C T 0 6 1 0 8
An app lied research program was undertaken to evaluate the autoignition characteristics of five liquid hydrocarbonfuels in air ov er ranges o f air temperature, p ressure, and equivalence ratio appropriate to advanced aircraft gas turbineengines. Ignition delay times were m easured using a continuous flow test apparatus which permitted independentvariation and evaluation of the effects o f temperature, pressure, flow rate, and fuel/air ratio on ignition delay time. Amultiple conical tube fuel injector, consisting of 19 parallel venturi elements with independent fuel control to eachelem ent, was used for the testing. Measuremen ts of the spray distribution produced were made by isokineticallysampling the flow at reduced temperatures w ith water injection, and indicated that a nearly uniform fuel/air mixturedistribution would be obtained.Param etric tests to map the ignition delay characteristics of Jet-A, JP-4 , No . 2 diesel, cetane, and an experimen talreferee broad specification {E RBS) fuel were conducted at pressures of 10, 15, 20.2 5 and 30 atm, inlet air temperaturesup to 1000K, and fuel/air equivalence ratios of 0.3, 0 .5, 0.7, and 1.0. Ignition delay times in the range of 1-50 msec atfreestream flow v elocities ranging from 20 to 100 m/see w ere obtained. In accord with classical chemical kinetics, theignition delay times for all fuels tested appeared to correlate with the inverse of pressure and the inverse exponent o ftemperature, viz..
A E1 = - - e x p ~ pn R T
In general, the data w ere very repeatable. W ith the exception of pure cetane, which has the shortest ignition delaytimes, the difference b etween the fuels tested did not appea r to be significant. The appa rent global activation energies fo rthe typical gas turbine fuels ranged from 38 to 43 kcal/mole, while the activation energy determined for cetane was 50kcal/m ole. In addition, the data indicate that. for lean mixtures, ignition delay times decrease w ith increasingequivalence ratio. I t was also noted that physical {apparatus dependent) phenom ena, such as m ixing (i.e., length)andairstream cooling (due to fuel heating, vaporization, and convective heat loss) can have an important effect on theignition delay.
I N T R O D U C T I O NS t u d i e s o f l o w - e m i s s i o n c o m b u s t o r c o n c e p t s f o ra d v a n c e d g a s t u r b i n e e n g i n e s h a v e i n d i c a t e d t h a tl e a n c o m b u s t i o n o f p r e v a p o r i z e d / p r e m i x e d f u e l s isa m o s t p r o m i s i n g a p p r o a c h f o r r e d u c i n g N O x e m i s-s io n s . H o w e v e r , a n i n t r i n si c p r o b l e m t o b e t r e a t e di n t h e d e s i g n o f p r e v a p o r i z i n g / p r e m i x i n g c o m b u s -
* R e s e a rc h pe r f o r m e d f o r N A S A , L e w i s R e s e a rc h C e n t e r ,unde r C on t r a c t N A S 3- 20066 , C . J . M e r c k , P r o j e c tManager.
C opy r i gh t 1982 by T he C ombu s t i on I n s t i t u t ePubl i shed b y El sevie r Sc ienc e Publ ish ing Co. , I nc .52 V a nde r b i l t A ve nue , N e w Y or k , N Y 10017
t o r s i s t h e p o t e n t i a l f o r in a d v e r t e n t a u t o i g n i t i o no f t h e f u e l - a i r m i x t u r e p r i o r t o i n j e c t i o n i n t o t h ep r i m a r y c o m b u s t i o n z o n e . I n th i s c o n t e x t , t h eh i g h c o m b u s t o r i n le t t e m p e r a t u r e s a n d p r e ss u re sa s s o c i a t e d w i t h a d v a n c e d g a s t u r b i n e e n g i n e s c a ne a s il y p r o m o t e i g n i t i o n a n d f l a m e s ta b i l i z a t i o n inp r e m i x i n g p a s s a g e s , i f t h e r e s i d e n c e t i m e is s u f -f i c ie n t l y l o n g . C o n s e q u e n t l y , m i x i n g a n d v a p o r i-z a t i o n m u s t b e c o m p l e t e d r a p i d l y . I n a d d i t i o n , a l-t h o u g h t h e s p o n t a n e o u s i g n i ti o n c h ar a c te r is t ic s o fh y d r o c a r b o n f u e ls i n a ir h a v e b e e n a s u b j e c t o f i n-v e s t ig a t io n f o r m a n y y e a rs , n o n e o f t h e p re v i o u s
0 0 1 0 - 2 1 8 0 / 8 2 / 0 6 2 8 3 + 1 8 5 0 2 . 7 5
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2 8 4 L O U I S J . S P A D A C C I N I a n d J O H N A . T E V E L D Einves t iga to rs has been success fu l in i so la t ing ande v a lu a ti n g e a c h o f t h e e x p e r im e n t a l v a ri a b le s i n ac o n t r o l l e d m a n n e r o v e r r a n g e s o f c o n d i t i o n s r e p -r e s e n t a t i v e o f t h o s e e n c o u n t e r e d i n a d v a n c e d g a stu r b in e e n g in e s . T h u s , t h e e x i s t in g b o d y o f a u toig -n i t i o n d a t a d o e s n o t p e r m i t a s a t i s fa c to r y q u a n t i ta -t i v e e v a lu a t io n o f t h e p r e s u m e d e f f e c t s o f a ll t h ec o n t r o l l i n g p a r a m e te r s .
W entze l l [1 ] was o ne o f the f i r s t inves t iga to rst o c o n c lu d e t h a t t h e i g n i ti o n d e la y tim e c o m p r i s e sa se ries o f over lap p ing phys ica l and chem ica l p roc-esses . The phys ica l de lay i s the t ime requ i red fo rd r o p l e t f o r m a t io n , h e a t i n g , v a p o r i z a t i o n , d i f f u -s i o n , a n d m ix in g w i th t h e a i r . T h e c h e m ic a l d e l a yi s t h e t im e e l a p s e d f r o m th e i n s t a n t a c o m b u s t i b l em i x t u r e h a s b e e n f o r m e d u n t il t h e a p p e a r a n ce o f ah o t f l a m e ; it i n v o lv e s t h e k in e t i c s o f p r e f l a m e r e a c -t i o n s wh ic h r e s u l t in t h e d e c o m p o s i t i o n o f h ig h -m o le c u l a r - we ig h t h y d r o c a r b o n s p e ci e s a n d t h e f o r -m a t i o n o f c r i t i c a l c o n c e n t r a t i o n s o f i n t e r m e d i a t ef ree - rad ica l spec ies , so -cal led ign i t ion p recurso rs . I ti s be l ieved th a t the chem ica l p rocesses s ta r t im-m e d i a t e l y u p o n t h e i n t r o d u c t i o n o f fu e l a n d a ir i na c o m b u s t i o n c h a m b e r ; h o we v e r , i n i t i a l l y t h e yp r o c e e d a t a v e r y s l o w r a t e , a n d c o n s e q u e n t l y t h em a s s o f f u e l v a p o r wh ic h u n d e r g o e s c h e m ic a l r e -ac t ion i s ve ry smal l compared to the mass neces -s a r y t o c a u s e a d e t e c t a b l e t e m p e r a tu r e o r p r e s s u rer is e d u e t o c o m b u s t i o n . T h e r e f o r e , t h e v e r y e a r l ys tages o f the p re ign i t ion p rocesses a re p robab lyd o m in a t e d b y t h e p h y s i c a l p r o c e ss e s , a n d t h e l a t e rs tages b y the chem ica l p rocesses . Th e re la tive e f -f e c t s o f t h e p h y s i c a l a n d c h e m ic a l p ro c e s s e s o n t h em a g n i tu d e o f t h e i g n i t i o n d e l a y h a v e b e e n s t u d i e db y m a n y i n v e s ti g a to rs ( e .g . , [ 2 - 4 ] ) , a n d i t h a sb e e n c o n c l u d e d t h a t i n c o n v e n t i o n a l c o m b u s t i o nsys tems (e .g . , gas tu rb ine and d iese l eng ines ) thec h e m i c a l d e la y is t y p i ca l l y th e m o r e i m p o r t a n t o ft h e tw o p e r i o d s .Au to ig n i t i o n i s g e n e r a l ly d e t e c t e d b y m e a s u r in ga sudden inc rease in tempera tu re , p ressu re , l igh temiss ion , o r con cen t ra t ion o f f ree rad ica l spec ies.Co n s e q u e n t l y , m a n y o f t h e p r e v io u s i n v e s t i g a to r sd i f f e r i n t h e i r d e f i n i t i o n o f t h e d e l a y p e r i o d , m a in lyb e c a u s e d i f f e r e n t p h e n o m e n a w e r e u s e d t o i n d i c a t eth e e n d o f t h i s p e r i o d . In a d d i t i o n , a g r ea t v a r i e t yo f e q u i p m e n t a n d p r o c e d u r e s h a s b e e n u s e d t om e a s u r e t h e i g n i t io n d e l a y o f l i q u id h y d r o c a r b o n
f u e ls , i n c lu d in g c o n s t a n t v o lu m e b o m b s , r e c ip r o -ca t ing eng ines, and s teady f low tes t appara tus .H o w e v e r , t h e s p o n t a n e o u s i g n i ti o n t e m p e r a t u r e o fa c o m b u s t i b l e s u b s t a n c e is n o t a n a b s o lu t e p r o p e r tyo f t h e s u b s t a n c e a n d , c o n s e q u e n t l y , al l s p o n t a n e o u sig n i ti o n d a t a n e e d t o b e i n t e r p r e t e d c a r e f u l l y i nth e l i g h t o f t h e t e s t a p p a r a tu s a n d m e th o d s u s e df o r t h e i r d e t e r m in a t i o n . E x i s ti n g e x p e r im e n t a l d a taa r e g e n e r a ll y d e p e n d e n t o n t h e p a r t i c u l a r e x p e ri -m e n t a l c o n f ig u r a t i o n e m p lo y e d a n d a re , t h e r e f o r e ,too incons is ten t fo r un ive rsa l des ign use . For ex -a m p le , t h e a u to m o t iv e l i t e r a tu r e c o n t a in s n u m e r o u sa c c o u n t s o f i n v e s ti g a ti o n s o f a u to ig n i t i o n i n i n t e r -m i t t e n t c o m b u s t i o n s y s t e m s ; h o w e v e r , th e e f f e c tso f c o n t i n u o u s ly v a r y in g p r e s s u r e , t e m p e r a tu r e ,v e lo c i t y , a n d tu r b u l e n c e a n d o f i n j e c to r s p r a ycharac te r i s t ic s (d rop le t s ize and d is t r ibu t ion) p re -v e n t a n u n a m b ig u o u s d e t e r m in a t i o n o f t h e i n-f l u e n c e o f a n y o n e o f t h e s e v a ri a b le s b e ca u s e a u to -i g n i t i o n i s a p a th - d e p e n d e n t p h e n o m e n o n . Ra p idc o m p r e s s io n m a c h in e s l e s s en , b u t d o n o t e l im in a t eth e e f f e c t s o f t r a n s ie n t s . He a t e d b o m b t e c h n iq u e s ,o n t h e o th e r h a n d , u s u a l l y r e q u ir e r e la t i v ely l o n gf u e l - a i r m ix in g t im e s a n d y i e ld r e s u l t s wh ic h a r ec o n f ig u r a t i o n ( s h a p e , s u r f a c e, a n d v o lu m e ) a n ds u r f a ce m a te r i al d e p e n d e n t . S h o c k t u b e s t u d ie s a r el im i t e d b y s h o r t t e s t t im e s a n d l o c a l n o n u n i f o r m -i ti e s , a n d u s u a l l y a r e r e s t r i c t e d t o h o m o g e n e o u sg a s eo u s m ix tu r e s . I n c o n t r a s t , c o n t i n u o u s f l o w d e-v ices pe rmi t ample t ime fo r measur ing and regu-l a ti n g m a n y o f t h e p h y s i c a l v a r ia b le s o f i n t e r e stp r i o r t o s p o n t a n e o u s i g n i t i o n wh i l e p r o v id in g a no p p o r tu n i t y t o m in im iz e t h o s e e f f e c t s m o r e s ub -jec t to des ign va r ia t ion (e .g ., in jec to r sp ray char -ac te r i s t ic s and degree o f m ix ing) .
Co n t i n u o u s f l o w in v e st i g at i on s o f t h e s p o n t a -neous ign i t ion charac te r i s t ic s o f severa l pure andd i s t i l l a t e h y d r o c a r b o n f u e l s p r a y s i n j e c t e d i n toh igh- tempera tu re , h igh-ve loc i ty a i r s t reams werecon du c ted b y Mul lins [5 ] in v i t ia ted a i r a t p res -su res equa l to o r be low 1 a rm; S t r inger e t a l . [6 ]in an oxyg en- rep len ished v i t ia ted a i r s t ream a t p res -s u r e s r a n g in g f r o m 3 0 t o 6 0 a tm ; T a b a c k [7 ] i nv i t i a t e d a i r a t a m b ie n t p r e s s u re s o f 1 7 - 2 8 a r m ; a n dSpadacc in i [8 ] in d ry unv i t ia ted a i r a t p ressu res inth e r a n g e 6 . 8 - 1 6 . 3 a r m . O f t h e v a ri o u s p h y s i c a lfac to rs inves t iga ted , a i r t empera tu re and p ressurewe r e f o u n d t o e x e r t t h e m a jo r in f l u e n c e o n t h e ig -
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A U T O I G N I T I O N O F A I R C R A F T - T Y P E F U E L S 2 8 5n i t i o n d e l a y , wh i l e v e lo c i t y , f u e l / a i r r a t i o , a n dtu rb u len ce in ten s i ty had a negl ig ib le e f fec t . Vi t ia -t i o n w i t h o u t o x y g e n r e p l e n is h m e n t w as s h o w n t ohave a s ign i f ican t e f fec t on ign i t ion de lay , a s d idth e f u e l t e m p e r a tu r e a n d i n i t i a l s p r a y c h a r a c t e r i s -t ic s (e .g . , d rop le t s ize and s ize d i s t rub i t io n) . Th eign i t ion de lay da ta were co r re la ted us ing Ar-rhen ius - type express ions .
M o r e r e c e n t l y , s t u d i es o f a u to ig n i t i o n o f a ir -c r a f t - t y p e f u e ls i n p r e m ix in g - p r e v a p o r i z in g c o m -b u s t i o n s y s t e m s h a v e b e e n c o n d u c t e d b y M e s t r eand Ducourneau [9 , 10] a t p ressu res in the range5 .4 - 12 a tm ; Marek e t al . [11] a t p ressu res in ther a n g e 5 . 4 - 2 5 a tm ; a n d T a c in a [1 2 ] a t p r e ss u r es o f1 . 8 - 6 .6 a tm . A l th o u g h t h e r e i s c o n s id e r a b l e d a t asca t te r and d i f fe rences in the i r re su l t s , the fue l /a i rm ix tu r e r a t i o a p p e a rs t o b e a n im p o r t a n t f a c to r a f-fec t ing au to ign i t ion , pa r t icu la r ly a t nea r -s to ich i -o m e t r i c a n d f u e l -r i c h c o n d i t i o n s .
O f c o u r se , t h e a c h i e v em e n t o f a u n i f o r m m i x -tu r e i s a p r e r e q u i s i t e f o r a n e v a lu a ti o n o f t h e im -p o r t a n c e o f f u e l / a i r r a t i o . I n a d d i t i o n , i n t e s t a r -r a n g e m e n t s wh ic h s t ri v e t o p r o d u c e m ix tu r eh o m o g e n e i t y , t h e m e a s u r e m e n t o f d e l a y t im e sm a y b e a f f e c t e d b y c h e m i c a l r e a c ti o n s w h i c h c a no c c u r i n t h e b o u n d a r y l a y e r a l o n g t h e wal ls . No n eo f t h e p r e v io u s i n v e s ti g a to rs m a k e m e n t io n o f th eo c c u r r e n c e o f i g n i t i o n a n d c o m b u s t i o n i n t h eb o u n d a r y l a y e r ; h o we v e r , i t a p p e a r s t h a t a u to ig -n i t i o n a n d i t s p r e c u r s o r s m a y o c c u r i n t h e s l o we rm o v in g ( i. e . , l o n g e r r e s id e n c e t im e ) m ix tu r e i n t h eb o u n d a r y l a y e r i n a s i t u a t i o n i n wh ic h t h e wa llt e m p e r a tu r e i s a t o r n e a r t h e i n l e t a i r t e m p e r a tu r e .A l s o , f l o w d i s t u r b a n c e s, s u c h a s t h o s e p r o d u c e d b ylarge-size fuel in jectors or h igh-blockage f lame-h o ld e r s , s h o u ld b e a v o id e d s i n ce t h e y m a y c r e a t elo c a l r e g io n s o f f l o w r e c i r c u l a t i o n a n d , t h e r e f o r e ,h igh res idence t imes .
EXPERIMENTAL APPROACHDescription o f A pparatusA s c h e m a t i c r e p r e s e n t a t i o n o f t h e a u to ig n i t i o n t e s ta p p a r a tu s u s e d i n t h e p r e s e n t s t u d y i s s h o wn inFig . 1 . I t cons is t s o f ( l ) an e lec t r ica l re s i s tance- typea i r h e a t e r , ( 2 ) a n i n l e t p l e n u m a n d f l o w s t r aig h t e n e r,( 3 ) a p r e m ix in g - ty p e fu e l i n j e c to r , ( 4 ) a c y l in d r i c a l
mixer /vapor ize r sec t ion compr is ing severa l f langeds p o o l p i e c e s t o p e r m i t l e n g th v a r i a ti o n o v e r t h er a n ge 2 . 5 - 1 5 0 c m in i n c r e m e n t s o f 2 . 5 c m , ( 5 ) a ne x p a n d e r s e c t i o n wh ic h p r o v id e s a s u d d e n e x p a n -s io n a n d w a t e r q u e n c h a t t h e a u to ig n i t i o n s t a t io n ,(6) a variable area orif ice to isola te a fue l scavenginga f t e r b u r n e r f r o m th e e x p e r im e n t , ( 7 ) a s c a v e n g e ra f t e r b u r n e r , a n d ( 8 ) a r e m o t e l y o p e r a t e d t h r o t t l eva lve loca ted in the exhaus t duc t ing .
De t a il s o f t h e m ix e r /v a p o r i z e r a n d e x p a n d e rsec t ions a re shown in F ig . 2 . The inner su r face o ft h e m ix e r /v a p o r i z e r s e c t i o n s a r e s m o o th a n d f r e eo f b o u n d a r y d i s c o n t i n u it i e s c a p a b le o f p r o d u c in gwakes in the f low. Th is is accom pl i shed by in te r -n a l m a c h in in g a n d t h e u s e o f al i g n m e n t d o we l s f o re a c h s e c t i o n o f t h e m ix e r /v a p o r i z e r . T h e wa l ls o ft h e m ix e r /v a p o r i z e r s e c t i o n s we r e wa t e r c o o l e ddur ing a l l t e s t s in o rder to p rec lude the poss ib i l i tyo f i g n i ti o n a n d f l a s h b a c k v ia t h e b o u n d a r y l a y e re v e n t h o u g h t h e o r e t i c a l a n a ly s e s we r e n o t a b l eto d e m o n s t r a t e c o n c lu s iv e ly t h a t c o o l i n g wa s r e-q u i r e d . S in c e t h e f a c i li t y a f t e r b u r n e r , l o c a t e d inth e- e x h a u s t d u c t i n g , r e p r e s e n t e d a c o n t i n u o u s ig -n i t i o n s o u r c e a n d b e c a u s e a u to ig n i t i o n m a y b ein i t i a te d a t a n a x i al l o c a t i o n w i th in t h e s u d d e n -e x p a n s io n s e c t i o n , a d d i t i o n a l p r e c a u t i o n s we r et a k e n t o e l i m i n a t e a n y p a t h b y w h i c h t h e f l a m em ig h t p r o p a g a t e u p s t r e a m in t h e m ix e r /v a p o r i z e rs e c t i o n ( e . g . , t h r o u g h t h e wa l l b o u n d a r y l a y e ra n d /o r b y m e a n s o f r e c i r c u l a t i o n z o n e s ) . T h e s ep r e c a u t i o n a r y m e a s u r e s in c lu d e d ( 1 ) d i r e c t wa t e ri n j e c t i o n a t t h e s t e p r e g io n , t o p r e v e n t f l a m es t a b i li z a t io n a t t h e e x i t o f t h e m ix e r /v a p o r i z e r ,a n d ( 2 ) i n s t al l a ti o n o f a two - d im e n s io n a l , m u l t i p l es t r u t - t y p e f l o w n o z z l e j u s t u p s t r e a m o f t h e s ud -d e n e x p a n s io n , t o a c c e l e r a t e t h e f l o w lo c a l ly an dp r o v id e a d d i t i o n a l wa t e r i n j e c t i o n t o q u e n c h t h ec h e m ic a l r e a c t i o n s r a p id ly . T h e r m o c o u p l e s a n dp h o to d e t e c to r s we r e u s e d t o m o n i to r t h e s t e p r e -g io n a n d i d e n t i f y c o n d i t i o n s wh ic h wo u ld r e su l tin f lame s tab i l iza t ion . Dur ing p re l iminary te s t ingwi th J e t - A f u e l , i t wa s d e t e r m in e d t h a t u n s t e a d yc o m b u s t i o n i n t h e a f t e r b u r n e r g e n e r a t e d p r e s su r ef l u c tu a t i o n s wh ic h we r e t r a n s m i t t e d u p s t r e a m toth e m ix e r /v a p o r i z e r s e c t i o n a n d c a u s e d p r e m a tu r e( i . e ., low - tem pera tu re ) ign i t ion . The re fo re , a ll sub-s e q u e n t t e s t s we r e p e r f o r m e d w i th t h e a f t e r b u r n e rc o m b u s t i o n t e r m i n a t e d p r i o r t o a u t o ig n i t io n . H o w -
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e v e r , wa t e r i n j e c t i o n i n to t h e s t e p ( r e c i r c u l a t io n )r e g io n wa s c o n t i n u e d a n d wa s f o u n d t o h a v e n oe f f e c t o n t h e m e a s u r e m e n t s .
T e s t ProcedureT h e n o r m a l o p e r a t i n g p r o c e d u r e c o n s i s t e d o f es -tab l i sh ing a p resc r ibed cond i t ion (e .g . , p ressu rea n d f l o w r a t e ) w i th in t h e t e s t d u c t a n d g r a d u a l l yinc reas ing the in le t a i r t empera tu re un t i l au to ig -n i t i o n o c c u r r e d a t t h e e x i t o f t h e m ix e r /v a p o r i z e rs e c t i o n . T h e o c c u r r e n c e o f a u to ig n i t i o n wa s d e -t e r m i n e d i n d i re c t ly b y ( 1 ) a t h e r m o c o u p l e p ro b elo c a t e d i n t h e e x p a n d e r s e c t i o n , ( 2 ) p h o to d e t e c -to rs ( loca ted a t severa l pos i t ions in the te s t r ig ) ,( 3 ) a d i f f e r e n t i a l p r e s s u r e t r a n s d u c e r m o n i to r i n gth e p r e s su r e d r o p a c r os s t h e m ix e r /v a p o r i z e r s ec -t i o n , a n d ( 4 ) a b s o lu t e p r e s s u r e t r a n s d u c e r s i n t h em ixer /v apo r ize r ( see F ig . 2 ) . Upo n ign i t ion , thet e s t w a s a b r u p t l y t e r m i n a t e d b y s h u t ti n g o f f t h efue l f low, reduc ing the r ig p ressu re and tempera -t u r e , a n d p u r g in g th e f u e l in j e c to r w i th w a t e r . S u b -s e q u e n t t e s t s w e r e n o t p e r f o r m e d u n t i l th e s y s te mh a d b e e n p u r g e d o f r es i d u al f u e l b y t h e a i r f l o w ,wh ich was m ain ta ine d a t a l l t imes . T he te s t a r range-m e n t p e r m i t t e d i n d e p e n d e n t v a r i a ti o n o f ea c h o ft h e im p o r t a n t v a r ia b l es ( i. e ., p r e s s u re , t e m p e r a tu r e ,v e lo c i t y , r e si d e n c e t im e , a n d f u e l / a i r r a t io ) w i th ina FLxed range o f te s t c ond i t ion s . Dur ing da ta reduc-t ion , the ign i t ion t ime was equa ted to the res i -d e n c e t i m e o f t h e f u e l -a i r m i x t u r e b e t w e e n t h ep o in t o f f u e l i n j e c t i o n a n d t h e l o c a t i o n o f t h ewa te r q u e n c h j u s t u p s t r e a m o f t h e e x p a n d e r s ec -t i o n . I t wa s c o m p u te d b a se d u p o n t h e a v e ra gef lo w v e lo c i t y a s c a l c u l a te d f r o m th e i n l e t t e m p e r a -tu re , p ressu re , and a i r f low ra te .Injector Developm entS in c e t h e g e n e r a t i o n o f a u n i f o r m f u e l - a i r m ix tu r ein the shor tes t d i s tance ( t ime) poss ib le i s c ruc ia lf o r d e t e r m in in g t h e e f f e c t o f f u e l / a ir r a t i o o n a u to -ign i t ion , e spec ia l ly a t the m ore severe te s t cond i -t ions , severa l cand ida te fue l in jec to rs were fab r i -c a t e d a n d e v a lu a t e d e x p e r im e n t a l l y . E a c h i n j e c to rwa s d e s ig n e d t o p r o v id e r a p id v a p o r i z a t i o n a n dm ix in g wh i l e m in im iz in g t h e f l o w d i s t u r b a n ce s . Ato t a l o f f o u r d i f f e r e n t d i s t r i b u t e d - s o u r c e i n j e c to r
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