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FUEL SYSTEMS TECHNOLOGY OVERVIEW Rob e r t F riedma n National Aeronautics and Space Administration L e w i s Research Center F u e l s y s t e m r e s e a r c h and technology s t u d i e s a r e being conducted t o in- v e s t i g a t e t h e c o r r e l a t i o n s and i n t e r a c t i o n s of a i r c r a f t f u e l s y s t e m design and environment with a p p l i c a b l e c h a r a c t e r i s t i c s of t h e f u e l .
F u e l P r o p e r t i e s and t h e Fuel System Voluntary i n d u s t r y standards f o r a v i a t i o n t u r b i n e f u e l (ASTM D 1655, r e f . 1) include over 25 i t e m s of s p e c i f i c a t i o n , but only a f e w of t h e s e are of concern t o t h e f u e l s y s t e m design and operation. The proceedings of a 2) i d e n t i f i e d s e v e r a l f u e l prop- 1977 NASA-sponsored workshop on f u e l s ( r e f .
e r t i e s worthy of f u r t h e r research with respect t o t h e i r influence on t h e performance of present and f u t u r e a i r c r a f t f u e l systems. These p r o p e r t i e s include water s o l u b i l i t y , v i s c o s i t y , f l a s h p o i n t , aromatics c o n t e n t , and f r e e z i n g point. Water s o l u b i l i t y i s a minor c h a r a c t e r i s t i c , but it i s a property s e n s i t i v e t o f u e l composition; and changes i n t h e f u e l chemical c o n s t i t u e n t s may i n c r e a s e t h e s o l u b i l i t y and cause c l e a n l i n e s s problems.
Viscosity i s of concern with respect t o lowtemperature pumpability, but praposed research on v i s c o s i t y can be included with t h e freezing-point s t u - d i e s discussed l a t e r . Flashpoint was not included among t h e c i t e d p r o p e r ties i n t h e r e f e r e n c e 2 workshop proceedings. The workshop p a r t i c i p a n t s discussed f l a s h p o i n t but concluded t h a t s a f e t y and a l t i t u d e b o i l o f f l i m i t s made any changes o r research on f l a s h p o i n t unlikely. Subsequent t o t h i s workshop, an A S T M symposium reviewed t h e question of j e t f u e l f l a s h p o i n t , i t s measurement, and t h e advantages and disadvantages of changes i n t h e f l a s h p o i n t s p e c i f i c a t i o n . A compilation of t h e f lashpoint symposium papers has been r e c e n t l y published ( r e f . 3 ) .
The s i g n i f i c a n c e of increased content of aromatic compounds i n j e t f u e l s has already been discussed i n t h i s symposium, with respect t o combus- t i o n and emission e f f e c t s . Aromatic compounds c a n a l s o degrade t h e perform- ance of polymeric m a t e r i a l s used a s s e a l s i n t h e f u e l system. Limited laboratory t e s t s of m a t e r i a l c o m p a t i b i l i t y have been conducted by t h e Jet Propulsion Laboratory ( r e f . 4 ) and o t h e r organizations. I n some of t h e JPL t e s t s a t e s t specimen was supported i n a bath of f u e l w i t h i n a constant- temperature oven. The t e n s i o n required t o maintain a f i x e d gage length was measured by a load c e l l . . T e s t r e s u l t s on one elastomer are shown i n f i g - ure 1 as a p l o t of stress r e l a x a t i o n over a period of time. T e s t s were con- ducted i n a i r and with t h e elastomer exposed t o Jet A (20 percent aromatics) and Jet A blended with t e t r a l i n (40 and 60 percent aromatics). The rapid degradation of e l a s t i c q u a l i t i e s with increased aromatics content i s obvious i n t h e r e s u l t s of t h e s e a c c e l e r a t e d t e s t s . Figure 2 shows test r e s u l t s with t h e 60-percent a r o m a t i c s f u e l a t t h r e e temperatures. Comparison of f i g u r e s 1 and 2 shows t h a t a n i n c r e a s e from 20 t o 60 p e r c e n t a r o m a t i c s i s analogous t o a d e c r e a s e of 30° C i n o p e r a t i n g temperature.
F r e e z i n g P o i n t j e t f u e l p r o p e r t y which i s of g r e a t e s t concern t o t h e f u e l s y s t e m The performance i s , of c o u r s e , t h e f r e e z i n g p o i n t ( r e f s . 5 and 6 ) . The 1977 N A S A f u e l s workshop ( r e f . 2) made f i v e recommendations f o r c o n t i n u i n g and f u t u r e r e s e a r c h on f r e e z i n g p o i n t and i t s r e l a t i o n t o f u e l system perform- ance. These are (1) Analyses of i n - f l i g h t t e m p e r a t u r e s systems f o r u s e w i t h high-freezing-point ( 2 ) Design of a i r c r a f t f u e l f u e l s ( 3 ) Experimental s t u d y of l o w t e m p e r a t u r e pumpability ( 4 ) F u l l - s c a l e f u e l t a n k s i m u l a t o r s t u d i e s ( 5 ) Development of a r a p i d f reezing-point measurement technique A n a l y s i s of i n - f l i g h t f u e l t e m p e r a t u r e s . - I n t h e p a s t two decades, some f l i g h t f u e l temperature d a t a have been c o l l e c t e d by airlines and air- frame companies. Recently, t h e Boeing Company made a comprehensive a n a l y s i s of 8000 f u e l and ambient minimum temperature measurements f u r n i s h e d by f l i g h t crews of c o o p e r a t i n g I n t e r n a t i o n a l A i r T r a n s p o r t A s s o c i a t i o n a i r l i n e s ( r e f . 7). F i g u r e 3 i s one example from t h i s survey, a d i s t r i b u t i o n p l o t of p e r c e n t of f l i g h t s f o r s t a t e d minimurn f u e l t e m p e r a t u r e s o v e r a g i v e n route.
The d a t a are s e p a r a t e d according t o a i r p l a n e t y p e , b u t t h e d i f f e r e n t c u r v e s r e f l e c t t h e v a r i a t i o n s i n f l i g h t speed and f u e l management r a t h e r t h a n in- h e r e n t d i f f e r e n c e s i n f u e l system design. D e s p i t e t h e large number of d a t a p o i n t s , t h i s study s t i l l c o v e r s a s m a l l s t a t i s t i c a l range of l i m i t e d sea- s o n a l and geographic v a r i a t i o n s . The p r i n c i p a l purpose of t h e c o m p i l a t i o n was t h e c o r r o b o r a t i o n of a Boeing i n - f l i g h t f u e l temperature computing rou- t i n e ( r e f s . 8 and 9). The f u e l temperature c a l c u l a t i o n s were shown t o be a c c u r a t e by comparison w i t h t h e d a t a . The f u e l t e m p e r a t u r e computer program p e r m i t s t h e p r e d i c t i o n of f u e l temperatures f o r a l a r g e range of ambient c o n d i t i o n s , f l i g h t times, f u e l management, and t a n k c o n f i g u r a t i o n s , includ- m i l i t a r y and g e n e r a l - a v i a t i o n a i r p l a n e s as w e l l a s commercial models.
ing The c a l c u l a t i o n s and t h e measurement d a t a are s i n g l e - p o i n t , b u l k f u e l tem- p e r a t u r e s . ‘ A refinement of t h e computing program i s being d e v i s e d t o in- c l u d e r e a l i s t i c t e m p e r a t u r e g r a d i e n t s w i t h i n t h e f u e l tank. The paper by F r e d e r i c k T o l l e of Boeing d e s c r i b e s t h e computing program modification. A NASA program is now under way t o o b t a i n s e l e c t e d i n - f l i g h t measurements of t h e s e t e m p e r a t u r e g r a d i e n t s .
F u e l systems f o r high-freezing-point f u e l s . - Heat t r a n s f e r from t h e
f u e l d u r i n g f l i g h t d a n b e reduced by i n s u l a t i o n o r t h e f u e l temperature c a n be i n c r e a s e d by h e a t i n g i n o r d e r t o i n c r e a s e t h e minimum f u e l temperature and remove some r e s t r i c t i o n s on t h e freezing-point s p e c i f i c a t i o n . Boeing d e s i g n a n a l y s e s ( r e f s . 8 t o 10) have concluded t h a t , a t p r e s e n t , f u e l a s m a l l f r a c t i o n of h e a t i n g , u s i n g e n g i n e h e a t r e j e c t i o n o r c o n v e r s i o n of e n g i n e power, i s most f e a s i b l e .
Suggested techniques are d i s c u s s e d i n t h e paper by T o l l e .
Experimental s t u d y of l o w t e m p e r a t u r e pumpability. - The r e l a t i o n s h i p of f u e l flow o r pumpability at low temperature t o t h e f r e e z i n g p o i n t has been i n v e s t i g a t e d i n s e v e r a l programs. A NASA-supported study by t h e Lockheed-California Company ( r e f s . 11 and 12) used a scale-model a p p a r a t u s t h a t r e p r e s e n t e d a segment of a n a i r p l a n e f u e l tank. The upper and lower s u r f a c e s of t h e t a n k were c h i l l e d t o d u p l i c a t e t h e i n - f l i g h t f u e l t a n k envi- ronment. The paper by F r a n c i s Stockemer d i s c u s s e s t h e s e tests and t h e i r r e s u l t s i n terms of t h e unpumpable "holdup" of f r o z e n f u e l . Other r e s u l t s from l a b o r a t o r y and scale-model tests conducted by t h e Boeing Company are i n c l u d e d i n T o l l e ' s paper. Continuing s t u d i e s by Lockheed u s e an a p p a r a t u s which adds a heated f u e l r e c i r c u l a t i n g system t o t h e c h i l l e d t a n k m o d e l These tests c a n t h u s s i m u l a t e t h e behavior of f u e l i n a n advanced ( f i g . 4 ) .
f u e l system w i t h i n - f l i g h t f u e l heating.
F u l l - s c a l e f u e l t a n k simulation. -The 1977 f u e l s workshop recommended t h a t s t u d i e s be conducted i n a f u l l - s c a l e f u e l t a n k demonstrator t o r e l a t e t h e f u e l pumpability f i n d i n g s t o behavior a t r e a l i s t i c c o n d i t i o n s and con- f i g u r a t i o n s . No program of t h i s scope h a s been i n i t i a t e d , however.
Rapid f r e e z i n g - p o i n t measurement. - I f t h e f r e e z i n g p o i n t of a j e t f u e l were determined a t t h e t i m e of d e l i v e r y t o t h e a i r p l a n e , minimum i n - f l i g h t temperature l i m i t s c o u l d b e p e r m i t t e d n e a r t h e a c t u a l r a t h e r than t h e con- s e r v a t i v e s p e c i f i c a t i o n f r e e z i n g p o i n t . The p r e s e n t means of measurement, w h i l e a c c u r a t e and precise, i s cumbersome and u n s u i t e d f o r f i e l d measure- ments. A N A S A program, t o start i n 1980, w i l l i n v e s t i g a t e t e c h n i q u e s s u i t - a b l e f o r r a p i d f reezing-point measurement s.
REFERENCES 1. 1979 Annual Book of ASTM Standards. P a r t 23, Petroleum Products and L u b r i c a n t s (I). American S o c i e t y f o r T e s t i n g and Materials, Phila- d e l p h i a , 1979.
2. Longwell, John P., ed. : Jet A i r c r a f t Hydrocarbon F u e l s Technology.
NASA CP-2033, 1978.
3 . Dukek, W. G.; and S t r a u s s , K. H . , eds. : F a c t o r s i n Using Kerosine Jet F u e l of Reduced F l a s h P o i n t . ASTM S p e c i a l P u b l i c a t i o n 688, American S o c i e t y f o r T e s t i n g and M a t e r i a l s , P h i l a d e l p h i a , 1979.
4. Kalfayan, S. H.; Fedors, R. F.; and R e i l l y , W. W . : C o m p a t i b i l i t y of Elastomers i n A l t e r n a t e J e t Fuels. Jet P r o p u l s i o n Laboratory P u b l i c a t i o n 79-28, 1979.
5. Ford, P. T.; and Robertson, A. G . : J e t F u e l s - Redefining t h e Low Tem-
p e r a t u r e Requirements. S h e l l A v i a t i o n N e w s , No. 441, 1977, pp. 22-26.
6. Friedman, R. : High-Freezing-Point F u e l s Used f o r A v i a t i o n Turbine Engines. A S M E Paper 79-GT-141, March 1979.
7. Pasion, A. J.: I n - F l i g h t F u e l Tank Temperature Survey Data. Boeing Commercial A i r p l a n e Company, NASA CR- 159569, 1979.
8. Pasion, A. J. ; and Thomas, I. : P r e l i m i n a r y A n a l y s i s of A i r c r a f t F u e l Systems f o r Use w i t h Broadened S p e c i f i c a t i o n Jet Fuels. Boe m e r c i a l A i r p l a n e Company, NASA CR-135198, 1977.
9. Thomas, I v o r : Broadened Jet F u e l S p e c i f i c a t i o n s : T h e i r E f f e c t upon Commercial A i r p l a n e Design and Operation. S h e l l A v i a t i o n N e w s , No.
450, 1978, pp. 32-35.
10. Pasion, A. J. : Design and E v a l u a t i o n of A i r c r a f t Heat Source Systems f o r Use w i t h High-Freezing-Point Fuels. Boeing Commercial A i r p l a n e Company, N A S A CR- 159568, 19 79.
11. Stockemer, F r a n c i s J. : Experimental Study 0-f Low Temperature Behavior of A v i a t i o n Turbine F u e l s i n a Wing Tank Model. Lockheed-California Company, N A S A CR- 159615, 1979.
12. Friedman, R.; and Stockemer, F. J. : Temperature and Flow Measurements on N e a r F r e e z i n g A v i a t i o n F u e l s i n a Wing-Tank Model. A S M E Paper 80-GT-6 3, March 1980.
AIRCRAFT RESEARCH AND TECHNOLOGY FOR FUTURE FUELS
0 FUEL - FLEXIBLE
AIRCRAFT TECHNOLOGY 0 GUIDANCE TO FUELS USERS & SUPPLIERS SYSTEMS ANALYSIS 0 ASTM FUEL S PEClFlCATlON GUIDANCE ~ 0 EMISSION STANDARDS GUIDANCE
FUELS I
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STRESS RELAXATION OF ELASTOMER AT 150" C
BUTADIENE-ACRYLONITRILE RUBBER AIR
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0 1 2 3 4 5 6 7 CS-80-1486 TIME, h r
STRESS RELAXATION OF ELASTOMER IN 60% AROMATICS FUEL
BUTADIENE-ACRYLONITRILE RUBBER
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1 1 1 1 1 1 2 3 4 5 6 7 CS-80-1484 TIME, hr S T A T U S O F R E C O M M E N D A T I O N S O N F R E E Z I N G P O I N T R E S E A R C H RECOMMENDATION RESEARCH PROGRESS 4NALYSIS OF IN-FLIGHT FUEL COMPLETED BOEING SURVEY; TEMPERATURES COMPLETED BUSINESS JET CALCULATIONS NASA IN-FLIGHT TEMP PROFILE SURVEY IN PROGRESS DESIGN OF AIRCRAFT SYSTEMS COMPLETED BOEING STUDIES FOR USE WITH HIGH F . P . FUELS OF LOW COMPLETED LOCKHEED STUDY; EXPERIMENTAL STUDY COMPLETED BOEING-A. F . STUDY; TEMPERATURE PUMPABILITY LOCKHEED HEATED FUEL STUDY IN PROGRESS FULL-SCALE FUEL TANK SIMULATOR FUTURE STUDIES RAPID FREEZING POINT MEASUREMENT PROJECT TO START IN 1980 ~ ~~~ CS-80-1487
-
99.99 9 9 . 9 POLAR ROUTE % OF TOTAL 1 0
FLIGHTS i; .l- 1
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FUEL HEATNG SYSTEM STUDY-BOEJNG FUELlOlL HEAT EXCHANGER SYSTEM ADVANTAGES * USES EXISTING HEAT REJECTION HARDWARE LOW COST - LOW PENALTY SIMPLE SYSTEM, RELATIVELY REClR CU LATlON MINOR MODIFICATION DISADVANTAGES LlMRATlON TO QUANTITY , OF HEAT AVAILABLE (-34' C FREEZE PT MAX. 1 *VARIABLE HEAT AVAILABLE, DEPENDENT ON ENGINE OPERATING P O l M CD-12314-07 CS-78-2880 FUEL HEATING SYSTEM STUDY-BOENG ELECTRICAL HEATING SYSTEM ADVANTAGES 0 UNLIMITED HEAT AVAILABLE ( U P TO -Hoc FREEZE PT) 0 INDEPENDENT CONTROL 0 ADAPTABLE TO GROUND HEATING FOR LAYOVER DISADVANTAGES 0 MAJOR MODIFICATION-ADDITICn~AL SYSTEM .WEIGHT PENALTY CYCLE PENALTY WHEN USED CD-12315-07 CS-78-2881 FUEL PUMPABILITY APPARATUS WITH ADDED HEATING LOOP TEST FUEL TANK 7