Review of jet engine emissions

N A S A T E C H N D C A L

NASA TM X- 68064

M E M O R A N D U M

s

I - N 7 2-2483C: REVIEW OF JZT E N G I N E v) {NASA-T3-X-68064)

ICc EHIISSIONS J. Grobman ( N A S A ) 7972 17 p z CSCL 21E

Unclas 6 3 / 2 8 29831

REVIEW OF JET ENGINE EMISSIONS

by Jack Grobman Lewis Research Center Cleveland, Ohio

TECHNICAL PAPER presented at the Department af

8 T r a n s p o r d i o n Survey Conference: Climatic Impact Assessment P r o g r a m t Cambridge, Massachusetts, February 15- 16 ~ 1972 k BS TRACT The purpose of this presentation i s t o provide a b r i e f t u t o r i a l review on t h e s u b j e c t of the emission c h s r a c t e r i s t i c s of j e t engines. The sources and concentrations of the various c o n s t i t u e n t s i n t h e engine exhaust and t h e i n f l u - ence of engine operating conditions on emissions are discussed. Crutse emissions to be expected from sapersonic engines a r e compared with emissions from subsonic engines. The basic operating p r i n c i p l e s of t h e gas t u r b i n e combustor are re- viewed together w i t h the e f f e c t s of combustor operating conditions on emissions.

a t h a t determine t h e design of gas t u r b i n e combustors a r e W The performance c r i t e r i a discussed. Combustor design techniques are considered t h a t may be used t o re- & duce emissions.

KEVIEW OF YET ENGINE BMISSIONS by Jack Grobmun Lewis Reeearch Center SUMMARY A considerable crmount of d a t a has been reported i n t h e literature d e s c r i b i n g t h e emission c h a r a c t e r i s t i c s of commercial subsonic j e t a i r c r a f t f o r o p e r a t i n g modes below an a l t i t u d e of 900 meters (3000 f t ) , which include i d l e , t a k e o f f , climbout, descent, m d landing, These d a t a were obtained by ground-based sam- p l i n g of t h e exhaust from jet engines t e s t e d over a range of engine speeds t o simulate t h e various o p e r a t i n g modes. V i r t u a l l y no d a t a o r e p r e s e n t l y a v a i l a b l e on t h e emissions oE either subsonic or supersonic a i r c r a f t a t c r u i s e conditions.

The c o n s t i t u e n t s i n t h e exhaust from t h e engine are mainly a f u n c t i o n of t h e combustor operating conditions including combustor inlet t o t a l temperaturz and pressure, combustor r e f e r e n c e v e l o c i t y o r r e a c t i o n d x e l l t i m e , and f u e l - a i r r a t i o .

Cruise emissions from either subsonic o r supersonic j e t a i r c r a f t may be predicted

%

from a knowledge of combustor o p e r a t i n g c o n d i t i o n s during cruise using t y p i c a l % combustor emission drtc. c o r r e l a t i o n s obtained over a range of combustor o p e r a t i n g

&

conditions.

The c o n s t i t u e n t s i n the exhaust rn~y b e divided i n t o f i v e c a t e g o r i e s i n c l u d i n g t h e unreacted c o n s t i t u e n t s in t h e ;ir, t h e p o d u c t s of complete combustion, t h e products of incomplete c m b u s t i o n , oxides of nitrogen, and c o n s t i t u e n t s due t o t r a c e elements i n the f u e l . b r i n g c r u i s e , t h e products of incomplete combustion, carbon monoxide and hydrowrbons, ;re re1.7 t i v e l y iott in concentration because gas t u r b i n e combustors operate ..t a combustion e f f i c i e n c y near 100 percent a t c r u i s e i n t h e exhaust conditions. The concentrations of csrbon monoxide and hydrocarbons be s l i g h t l y higher than of a f t e r b u r n i n g t u r b i n e engine would be expected t o those of an unaugmented engine becurise ,tfterburners tend t o burn less ef fic I e n t i y than main combustors.

Nitric oxide i s considered t o b e t h e most signf i i c :nt emission product fomc..d during a l t i t u d e c r u i s e . The 1,uantity ok n i t r i c oxide formed i n the combustor i s R s t r o n g function of flame temperntiire which increiises w i t h i n c r e a s i n g combustor i n l e t t o t a l temperature. Combustor i n l e t total ternparotore i n c r e a s e s with i n - c r e a s e s in e i t h e r campressor pressure ratict o r klighr X- c h number. A l t e r i n g gas t u r b i n e combustor designs i n osder t o make substanti.ii reductions i n oxides of nitrogen w i l l b e art extremely d i f i i c u l t Eiisk, Reser-rch is i n progress to develop aad e v a l u a t e experimental combustor8 t h a t reduce oxides of n i t r o g e n without com- promising on engine performance.

The purpose of this prc.sent:.ciun is to prrlVi& * . !.rief t u t o r i a l review on the s u b j e c t of the emission ch; ficreristics oi jet cznkiiies, p a r t i c u l a r l y during high a l t i t u d e c r u i s e . Many r e c e n t r e p o r t s ( r e f s . 1 t o 3) have. discussed t h e relative c o n t r i b u t i o n of j e t a i r c r a f t emissioos from ground i:nd low a l t i t u d e t o urban p o l l u t i o n . Even though t h e overall c o n t r i b u t i o n of ciircrrrft maneuvers t o urban p o l l u t i o n is small, t h e a i r q u a l i t y i n t h e v i z i n i t y of commercial G i r - p o r t s i s considered t o be s i g n i f i c a n t l y a f f e c t e d bj, jet a i r c r r f t emissions.

During i d l i n g and t a x i i n g , t h e p r i n c i p a l p o l l u t r n t s ,.re unburned hydrocarbons and carbon monoxide wnile d u r i n b landing and t a k e o f f , t h e mr!in p o l l u t a n t s &re oxiues ol: n i t r o g e n and smoke. Emission d a t a f o r a number of commercisl j e t engines f o r subsonic i i i r c r a i t have been documented i n r e f e r e n c e 4 . These d a t a apply mainly t o t h e emissions from j e t a i r c r a f t durin; i d l e , t a k e o f € , 2nd l;.ndint, operations. Similar data on c r u i s e emissions from subsonic and supersonic a i r - craf't a r e p r e s e n t l y n o t documented. W e a r e j u s t beginning t o o b t a i n t h e neces- s a r y c r u i s e emission d a t a t o h e l p determine t h e n a t u r e of t h e global problem.

This paper d i s c u s s e s t h e sources and concentri.tims of tile vasious con- s t i t u e n t s i n t h e engine exhaust and t h e i n f l u e n c e of engine o p e r a t i n b c o n d i t i o n s on emissions. P r e d i c t i o n s are made of c r u i s e emissions t o be expected from supersonic and subsonic engines oased on e x i s t i n g d a t a r e l z t i n g emissions w t t h combustor o p e r a t i n g c o n d i t i o n s . A b r i e f review is made of t h e b a s i c o p e r a t i n & p r i n c i p l e s and performance criteria t h a t determine t h e design of gas t u r b i n e con- b u s t o r s . A b r i e f survey is made of recent r e p o r t s ( r e f s . 5 t o 9) t h a t consider combustor design t e c m i q u e s t h a t may be used t o reduce emissions.

CONSTITUENTS IN JET ENGINE EXHAUST j e t engine exhaust d u r i n g t y p i c a l takeoff or The v a r i o u s c o n s t i t u e n t s i n c r u i s e c o n d i t i o n s a r e t a b u l a t e d i n f i g u r e 1. Each c o n s t i t u e n t has been grouped i n t o f i v e diiEerenc categories d e t e m i n e d by its source. These include (1) i n e r t s and unreacted oxygen from a i r , (2) producrs of complete combustion of i u e l , (3) products of incomplete combustion, ( 4 ) oxides of n i t r o g e n formed d u r i n g tile heat- i n g of a i r , and ( 5 ) elements o r compounds derived from s u l f u r and t r a c e metals present i n kerosene f u e l . T h e concentration of t h e components i n t h e a i r znd products of complete combustion were determined €or an o v e r a l l f u e l - a i r r a t i o of 0,014 using commercial J e t A - l kerosene f u e l . The o v e r a l l f u e l - a i r r a t i o d u r i n g cruise g e n e r a l l y h a s a range of 0.01 t o 0,03. The products of incomplete com- bustion include carbon monoxide, unburned f u e l , p r t i a l l y oxidized hydrocarbons Sucil a s aldeiiydes , hydrogen, and p a r t i c u l a t e s (soot) c o n s i s t i n g mainly 02 carbon.

Combustors f o r gas t u r b i n e engines a r e designed t o o p e r c t e with maximum perform- aizce during c r u i s e , and t h e estimated concentrations of t h e products of incomplete cornbustion shown i n t h e t a b l e are equivalent Lo :in u v e r i l l combustion i n e z f i c i e n c p of about 1 percent o r less. Afterburners f o r gas t u r b i n e engines tenu t~ be l e s s e f f i c i e n t than t h e main comubstor; and t h e r e f o r e , the concentrations of t h e pro- d u c t s of incomplete combustion a r e generally s e v e r h l times g r e ; t e r f o r auhniented engines.

Oxides of n i t r o g e n are formed from t h e r e a c t i o n s o t oxygen and n i t r o g e n a t elevated temperatures generated i n the r e a c t i o n zone of h e combustor Thcsc oxides of n i t r o g e n (NOx) g e n e r a l l y c o n s i s t of about 90 t o 95 percent n i t r i c u ~ i d c (NO) with t h e remainder being n i t r o g e n dioxide (NO21 The q u a n t i t y of n i t r i c oxide formed is affected by a number of factors including engine compressor pres- sure ratio, flight Mach number, fuel-air ratio, and combustor design.

Commercial specifications for Jet A-1 kerosene require that the sulfur con- centration in the fuel not exceed a value of 0.3 percent by weight. In practice, the sulfur concentration is generally less than 25 percent of this value.

The sulf.Jr in the fuel is converted into mostly sulfur dioxide (Sop) and lesser amounts of sulfur trioxide (SOg). A variety of trace metals are present in the fuel such as aluminum, iron, manganese, nickel, sodium, potassium, and vanadium.

The total concentration of t i l e trace metals in jet engine exhaust is estimated to be about 5 to 20 ppb (parts per billion).

The remaining discussion on the constituents in jet engine exhaust will be limited to the products of incomplete combustion and nitric oxide.

The factors affecting the formation and control of these exhaust components will be covered.

TYPICAL JET ENGINE EMISSION CHARACTERISTICS Figure 2 illustrates the carbon monoxide (GO), total hydrocarbon, and nitro- gen oxide (NOx) emissions over a range of engine speeds for a typical gas turbine engine for a subsonic aircraft with a compressor pressure ratio of 13.4 using 3P-5 fuel (similar to commercial Jet A-1 f u e l ) . The term emission index, grams of pollutant per kilogram o i fuel burned, is used instead of the more familiar volumetric concentration term, parts per million, in order to normalize emissions on the basis of fuel flow, At a fuel-air ratio of 0.015, an emission index of unity is equivalent to either iS ppm CO, or 30 ppmC total hydrocarbons, or 10 The NOX emission index is calculated by assuming the conversion of all ppm NOx.

nitric oxide (NO) in the exhaust to nitrogen dioxide (N02). At low engine speeds corresponding to idle operation, the CO and hydrocarbon emissions are at their highest whereas nitrogen oxide emissions are at their lowest. At an engine speed 100 percent corresponding t.o takeoff conditions, the nitrogen oxide emissions of are highest, whereas CO and hydrocarbon emissions approach their minimum. For a subsonic aircraft the CO and hydrocarbon emission levels at cruise would be expected to be similar to the levels during takeoff, but NOX emissions would be lower than at takeoff.

Figure 3 shows a tabulation of the main emission products, together with their major causeso A range of emissions at either idle or takeoff is shown for typical subsonic commercial engines. High hydrocarbon and CO emissions that Inefficient combustion occur only during idle are due to inefficient combustion.

is caused by a combination of poor fuel atomization at low fuel flow rates, lean reaction zone fuel-air ratios, and low combustor-inlet pressure and temperatures, Higher NOX emissions during takeoff are caused by the increased reaction rates of oxygen and nitrogen at higher flame temperatures as a result of higher combustor inlet temperatures, Smoke density is higher at takeoff because of higher com- bustor pressures and richer reaction zone fuel-air ratios. Cnly nitric oxide is formed in any significant quantities during subsonic cruise, but the NOX emission index during cruise is less than that during takeoff because the combustor-inlet temperature and pressure are lower.

COMPARISON OF SUBSONIC AND SUPERSONIC ENGINES Figure 4 shows a drawing of a t y p i c a l turbofan engine used on subsonic a i r c r a f t . The main engine components include a fan, compressor, combustor, and turbine. A portion of the a i r passing through t h e fan e n t e r s the compres- s o r , while t h e remaining a i r i s by-passed around t h e core engine t o provide a d d i t i o n a l t h r u s t . The flowrate and t o t a l temperature and pressure of t h e a i r e n t e r i n g t h e combustor from t h e compressor discharge are e s t a b l i s h e d by the o v e r a l l fan and compressor pressure r a t i o , f l i g h t a l t i t u d e , and f l i g h t speed.

The f u e l - a i r r a t i o i s determined by t h e combustor temperature r i s e required t o obtain t h e designed t u r b i n e i n l e t temperature.

The f r o n t a l area of t h e combustor generally does not exceed t h e required f r o n t a l a r e a of t h e compres- s o r o r t u r b i n e i n order t o avoid unnecessary engine drag.

The o v e r a l l length of t h e combustor is kept as s h o r t a s p r a c t i c a l t o minimize engine s h a f t length and bearing requirements. The combustor shown i n t h e drawing is of t h e can- annular type i n which a number of tubular combustion l i n e r s a r e arranged within a common annular housing. Most r e c e n t engines use an annular type combustor i n which a continuous annular combustion l i n e r is i n s t a l l e d within an annular housing e A schematic drawing of a conventional annular combustor is shown i n f i g u r e 5. The combustor c o n s i s t s of t h r e e main parts: A d i f f u s e r , t h e primary (reac- t i o n ) zone, and t h e secondary ( d i l u t i o n ) zonep The d i f f u s e r is used t o d i f f u s e t h e r e l a t i v e l y high v e l o c i t y a i r f l o w discharging from t h e compressor t o a high s t a t i c pressure, are necessary t o o b t a i n s t a b l e Lower combustor v e l o c i t i e s combustion and t o avoid excessive,combustor pressure l o s s . Fuel is generally i n j e c t e d by pressure atomizing nozzles and combustion is i n i t i a t e d and s t a b i l i z e d i n t h e primary zone. Enough air i s introduced i n t o t h e primary zone through swirlers around t h e f u e l nozzles o r through openings i n t h e l i n e r w a l l t o main- t a i n a near stoichiometric mixture of f u e l and air. A i r by-passing the primary zone is i n j e c t e d i n t o the secondary zone through a d d i t i o n a l openings i n t h e l i n e r and mixes with the hot gases from the primary zone t o achieve a d e s i r e d turbine i n l e t temperature d i s t r i b u t i o n , The remai9ing a i r f l o w is used t o f i l m Most of t h e chemical r e a c t i o n s are completed cool t h e walls of t h e chamber.

Consequently the emission products a r e p r i o r t o d i l u t i o n i n the secondary zoneo e s s e n t i a l l y frozen near t h e e x i t of the combustor, A drawing of one type of engine s u i t a b l e f o r powering a supersonic a i r c r a f t i s show1 i n f i g u r e 6. The main d i f f e r e n c e s between t h e a f t e r b u r n i n g t u r b o j e t shown i n t h i s f i g u r e and t h e turbofan engine shown i n a previous f i g u r e are: (1) incorporation of an i n l e t d i f f u s e r (not shown) t o slow t h e a i r t o subsonic speeds before it e n t e r s t h e engine, (2) omission of fan and by-puss duct, (3) a d d i t i o n of a f t e r b u r n e r t o provide t h r u s t augmentation, and (4) i n s t a l l a t i o n of Afterburning t u r b o j e t engines s i m i l a r t o t h a t a supersonic exhaust nozzle, shown i n f i g u r e 6 are used on t h e Concorde SST and were t o be used on t h e Boeing Recent SST Other engine t y p e s could be considered f o r f u t u r e SST designs.

s t u d i e s ( r e f , 10) have shown t h a t an afterburning turbofan engine may o f f e r advantages i n reducing j e t noise.

.The a f t e r b u r n e r c o n s i s t s of an i n l e t d i f f u s e r t o slow down t h e t u r b i n e discharge gases, an a r r a y of fu-el-spray b a r s followed by an a r r a y of flameholders, and a combustion chamber t h a t i s a i r cooled and a c c o u s t i c a l l y damped t o prevent screech. From a combustion point of view, t h e operating conditions i n an a f t e r - burner a r e generally more severe than i n the main combustor, and therefore, t h e afterburner combustion e f f i c i e n c y tends t o be lower.

A comparison of t h e c r u i s e operating conditions of r e p r e s e n t a t i v e subsonic turbofan and supersonic afterburning (AB) t u r b o j e t engines is shown i n f i g u r e 7.

The f l i g h t conditions chosen f o r t h e supersonic engine were those f o r t h e GE-4/Boeing SST. A combustion e f f i c i e n c y of nearly 100 percent i s achieved during c r u i s e f o r e i t h e r of these engines; and, thus a m i n i m u m q u a n t i t y of hydrocarbons and CO i s formed. The a f t e r b u r n e r combustion e f f i c i e n c y would be expected t o be several percent less than t h a t of t h e main combustor; however, the q u a n t i t y of f u e l burned i n the a f t e r b u r n e r du::ing c r u i s e amounts t o only about 22 percent of the t o t a l f u e l burned i n the engine. The main f a c t o r a f f e c t i n g the formation of n i t r i c oxide i n e i t h e r engine i s t h e combustor i n l e t t o t a l temperature. Increas- ing e i t h e r the compressor pressure r a t i o o r the f l i g h t speed r e s u l t s i n an i n - crease i n combustor i n l e t t o t a l temperature. P r e l i m i n a r y data ( r e f , 11) i n d i - cate t h a t afterburning does n o t s i g n i f i c a n t l y add t o n i t r i c oxide emissions from supersonic a i r c r a f t EFFECT OF OPERATING VARIABLES ON EMISSIONS Hydrocarbons and Carbon Monoxide Combustion e f f i c i e n c y can be defined as t h e r a t i o of t h e a c t u a l enthalpy rise t o the t h e o r e t i c a l enthalpy rise a t t a i n a b l e f o r t h e amount of f u e l used.

The t h e o r e t i c a l enthalpy rise assumes t h a t t h e combustion r e a c t i o n proceeds t o completion t o form gaseous products of combustion i n e q u i l i b r i u m a t the combustor e x i t temperature. Previous s t u d i e s ( r e f . 12) have c o r r e l a t e d combustion e f f i - ciency a g a i n s t a combustion parameter composed of i n l e t t o t a l pressure, (Pinlet or P3), multiplied by i n l e t t o t a l temperature, (Tinlet or Tg), and divided by reference v e l o c i t y (VR) where reference v e l o c i t y is equal t o t h e combustor a i r - f!ow rate divided by the product of t h e a i r density a t the combustor i n l e t and ' .e maximum cross-sectional flow area of t h e combustor. A t y p i c a l combustion e f f i c i e n c y c o r r e l a t i o n is shown p l o t t e d i n f i g u r e 8. The f i g u r e shows t h a t combustion e f f i c i e n c y increases a s i n l e t pressure and i n l e t temperature i n c r e a s e and decreases a s combustor v e l o c i t y increases, Typical values of t h e c o r r e l a t i n g parameter f o r takeoff and cruise f a l l f a r t o t h e r i g h t of the bend i n t h e curve.

Therefore, t h e r e a r e n o t any s i g n i f i c a n t problems i n obtaining good combustion e f f i c i e n c y f o r takeoff and c r u i s e . A t engine i d l e conditions, t h e value f o r t h e c o r r e l a t i n g parameter i s low, and i n a d d i t i o n f u e l flow is low; t h e r e f o r e , obtain- A s expected, t h e emission ing good combustion e f f i c i e n c y is a d i f f i c u l t : problem, indices f o r CO and t o t a l hydrocarbons decrease with increasing values f o r t h e A p l o t of t h e CO emission index a g a i n s t t h e c o r r e l a t i n g Correlating parameter.

a t y p i c a l combustor obtained from reference 13 is shown i n f i g u r e 9.

parameter f o r The c o r r e l a t i o n p l o t f o r the hydrocarbon emission index follows a similar trend, Oxides of Nitrogen Figure 10 obtained from reEerence 2 shows the e f f e c t of engine pressure r a t i o on NOX e m i s s i o n s , a t t a k e o f f , The i n c r e a s e i n the NOX emission index with engine pressure r a t i o i s mainly due t o increasing combustor i n l e t temperature, For a given subsonic engine, the combustor i n l e t temperature during c r u i s e i s less than a t takeoff because the ambient temperature i s lower a t the c r u i s e a l t i t u d e and because corrected engine speed i s lower; and therefore, NOX emissions a t c r u i s e a r e less than a t takeoff. For a supersonic a i r c r a f t , the combustor i n l e t t o t a l temperature increases sj-gnificantly w i t h increases i n f l i g h t Mach number, and thus, can result i n increases i n NOX emissions. Combustor i n l e t t o t a l pressure is lower during c r u i s e than a t takeoff f o r both subsonic and supersonic engines.

Preliminary data i n d i c a t e t h a t n i t r i c oxide emissions tend t o be lower as com- bustor pressure is reduced, The e f f e c t of combustor operating v a r i a b l e s on NOX formation is summarized

The strong iniluence of combustor i n l e t temperature on N a forma-

i n f i g u r e 11, t i o n i s the result of the f a c t t h a t increasing combustor i n l e t temperature increases the flame temperature and hence the r e a c t i o n r a t e . Increasing combustor i n l e t pressure a l s o tends t o increase the formation of NOX, but the e f f e c t i s not as s i g n i f i c a n t a s t h e e f f e c t of i n l e t temperature. The e f f e c t of pressure might be z t t r i b u t e d t o e i t h e r chemical k i n e t i c s o r f u e l - a i r mixing, Increasing com- bustor reference v e l o c i t y tends t o reduce r e a c t i o n zone dwell t i m e thus reducing the q u s n t i t y of NOX formed s i n c e t h e formation of NOX i s r e a c t i o n r a t e l i m i t e d .

In general, increasing t h e o v e r a l l f u e l - a i r r a t i o has been observed t o increase NOx formation. T h e o r e t i c a l l y , t h e l o c a l f u e l - a i r r a t i o i n t h e primary (reaction) zone should have a very s i g n i f i c a n t e f f e c t on NOX formation by i t s e i f e c t on flame temperature. Flame temperature should be near a maximum a t stoichiometric conditions and should be lower f o r f u e l - a i r mixtures t h a t a r e e i t h e r l e a n e r or r i c h e r than stoichiometric, Gas turbine combustors a r e gener- a l l y designed so t h a t the primary zone f u e l - a i r r a t i o i s near stoichiometric i n order t o optimize combustion performance. Operating with a primary zone t h a t is lean i n f u e l leads t o unstable combustion while operating on the f u e l r i c h s i d e causes excessive carbon formation, Even i f t h e r e a c t i o n zone were operated e i t h e r l e a n e r o r r i c h e r than stoichiometric, f a c t o r s t h a t a f f e c t t h e homogeneity of the r e a c t a n t s such a s t h e f u e l - a i r mixing i n t e n s i t y , f u e l d r o p l e t s i z e d i s t r i - bution, and f u e l evaporation r a t e tend t o be as important an e f f e c t on NOx forma- t i o n as the average l o c a l f u e l - a i r r a t i o i n t h e r e a c t i o n zone, COPBUSTOR DESIGN TECHNIQUES TO REDUCE EMISSIONS such as combustor i n l e t t o t a l pressure and Combustor operating v a r i a b l e s fuel-air r a t i o are fixed by the engine design. Factors temperature, and o v e r a l l Combustor reference a f f e c t i n g engine design a r e discussed i n reference 10.

v e l o c i t y m u s t be limited t o prevent excessive pressure l o s s e s . Thus t h e only means r e a d i l y a v a i l a b l e t o the combustor designer t o c o n t r o l emissions are t h e method of f u e l atomization, t h e mixing of f u e l and a i r , and t h e general geometry i n of the combustor. Preliminary research on the s u b j e c t of emission reductions turbine combustors i s discussed i n references 5 t o 3 , Hydrocarbon end CO gas emissions were shown t o be mainly a problem during i d l e while smoke i s mainly a takeoff problem, end s i n c e t h i s paper i,s mainly concerned with emissions i n t h e u p p e r atmosphere, t h e discussion in t h i s s e c t i o n w i l l be l i m i t e d t o methods t o reduce n i t r i c oxide emissions The two p r i n c i p a l methods f o r reducing n i t r i c oxide emi.ssions a r e t o reduce t h e r e a c t i o n zone temperature (flame temperature) and t o reduce t h e r e a c t i o n zone dwell time, The r e a c t i o n zone temperature may be reduced e i t h e r (1) by o p e r a t i n g with e i t h e r a f u e l - r i c h o r f u e l - l e a n reaccion zone, (2) by o p e r a t i n g with a niore homogeneous f u e l - a i r mixture, o r (3) by introducing inerts i n t o t h e r e a c t i o n zone.

be s h i f t e d by a l t e r i n g t h e combustor a i r f l o w The r e a c t i o n zone f u e l - a i r r a t i o may d i s t r i b u t i o n , However, r i c h r e a c t i o n zones tend t o form excessive amounts of carbon monoxide, hydrocarbons, and smoke while l e a n r e a c t i o n zones present s e v e r e combustion s t a b i l i t y problems, I f t h i s approach is t o be u s e d , v a r i a b l e geometry might be required t o continuously c o n t r o l combustor a i r f l o w d i s t r i b u t i o n t o avoid poor combustion e f f i c i e n c y and t h e emission products due t o incomplete combustion.

The f u e l - a i r mixture could be made more homogeneous e i t h e r by i n c r e a s i n g mixing i n t e n s i t y , by premixing t h e f u e l and a i r before they e n t e r t h e r e a c t i o n zone, by p r e v a p r i z i n g t h e f u e l b e f o r e i t e n t e r s t h e r e a c t i o n zone, o r by i n c r e a s - i n g t h e number of f u e l i n j e c t i o n p o i n t s . I n e r t s t h a t might oe used t o reduce flame temperature include water o r r e c i r c u l a t e d combustion products, W t e r i n j e c t i o n would be impracticai d u r i n g c r u i s e because of payload p e n a l t i e s . A s i g n i f i c a n t i n c r e a s e i n t h e amount of combustion products t h a t a r e already r e c i r - c u l a t e d i n t h e r e a c t i o n zone might r e q u i r e excessive p r e s s u r e losses, The r e a c t i o n zone dwell t i m e could be reduced e i t h e r by shortening t h e l e n g t h o f t h e r e a c t i o n zone o r by i n c r e a s i n g t h e number of l o c a l burning zones i n o r d e r t o reduce t h e r e c i r c u l a t i o n path length. One experimental combustor being s t u d i e d a t NASA-Lewis t h a t incorporates many of t h e above approaches is shown i n t h e photograph of f i g u r e 1 2 . This t.nnul2.r combustor which i s about 30 percent s h o r t e r t h m conventional combustors incorporates 120 small i n d i v i d u a l burners, r e f e r r e d t u a s swirl-cans, i.nto a modular a r r a y arranged i n t h r e e concentric annular rows a s shown i n t h e c r o s s - s e c t i o n a l sketch i n f i g u r e 13. The modular combustor has no well-defined primary o r secondary zone a s i n t h e conventional N e a r l y a l l the a i r f l o w , except f o r t h a t required t o cool t h e l i n e r , combustor.

passes d i r e c t l y through o r around t h e modules, The d e t a i l s of each swirl-can element are i l l u s t r a t e d i n f i g u r e 14. Each swirl-can, which is ribout 5 c e n t i - meters ( 2 i n . ) i n diameter, c o n s i s t s of a c a r b u r e t o r , swirler, and fliime s t a b i - l i z e r , The carburecor premixes t h e f u e l with a p o r t i o n of a i r e n t e r i n g an o r i - The swirler a c t s t o f u r t h e r mix t h e f i c e Fn t h e upstream end oJ; the chamber.

f u e l and a i r and t o impart a s w i r l t u t h e mixture, Combustion i s i . n i t i a t e d and maintained a t the flame s t l b i l i z e r . The a i r flowing around t h e o u t s i d e of t h e module mixes with the: h o t combustion gases i n t h e wake of each module. There t h e combustion r e a c t i o n i s completed and mixing of t h e gases t o t h e d e s i r e d t u r b i n e l n l e t temperature d i s t r i . b u t i o n i s achieved. Further d e t a i l s on t h e oper- s t i o n and performance of t h e swirl-can combustor a r e described i n references 14 and 15.

The main advantages of t h i s type of combustor a r e t h a t f u e l and a i r a r e p a r t i a l l y premixed p r i o r t o burning and t h a t burning and mixing downstream of Nitri.c oxide emission d a t a f o r t h e experimental swirl- each module is very rapid.

can combustor a r e compared t o those from a inore conventional combustor i n f i g u r e 25. The ZVOx t ? m i S s i G i l index is p l o r t e d a g a i n s t combustor i n l e t t o t a l temperature, Even though these r e s u l t s are q u i t e preliminary and even though t h e r e a r e a I i number of problems remaining t o be solved befare such B combustor may be incorpo- rated i n t o ' a n a c t u a l engine, t h e swirl-can concept appears t o b e A promising approach f o r obtaining reduced n i t r i c oxide emissions, CONCLUDING REMARKS N i t r i c oxide appears t o be the only p o l l u t a n t emission product t h a t is formed during h i g h a l t i t u d e c r u i s e t h a t has a s i g n i f i c a n t ccncentration, A g r e a t deal of research is c u r r e n t l y i n progress t o develop methods of reducing n i t r i c oxide emi.ssions from gas turbine combustors. Ocher research being perFormed t o determine the c o n s t i t u e n t s and chemical r e a c t i o r s i n t h e upper atmosphere may provide a c l e a r e r answer t o whethey- or not reductions i n c r u i s e emissions 3re required, and i f so, how l a r g e a reduction i s required. X t i s EairLy c l e a r , the approaches described herein o r some s i m i l a r epproach however, that w h e t h e r i s required t o reduce n i t r i c oxide emissions, a signij-icant modification w i l l be requi.red t o conventional gas turbine combustors. Furthermore, H considerable e f f o r t w i l l b e requi.red t o accomplish these emission reductions without compro- mising on o t h e r combustor performznce requirements.

REFERENCES 1. F l a t t , M.; Baker, R. C . ; Bastress, E. K.; Chng, K. M.; and Siegel, R. D o : The P o t e n t i a l &pact of A i r c r a f t Emissions Upon A i r Quality.

Rep 1167-1, Ncrthern Research and Eng. Corp , Dec. 29, 1971.

2. Anon.: A Study of A i r c r a f t Gas Turbine Engine Exhaust Emissions. Aerospace Industries AssociatLon of America, Aug. 1971.

3 . Sawyer, R. F.: Atmospheric Pollution by A i r c r a f t Engines and Fuels. AGARD- AR-40, M a r . 1972, 4. Bogdm9 Leonard; and McAdams, H. T.: Analysis of A i r c r a f t Exhaust Emission Measurements CAL-NA-5007-K-1, Cornell Aeronautical. Lab. , @ Octr 1971.

5. Butze, Hehut, F . : Methods f o r Reducing Pollutant Emissions from Jet A i r c r a f t , N A S A TM-X-CiSOOO, 2971.

60 Bastress, E. K.; e t al, : Assessment of Aircraft Emission Control Technology.

Rep, ll68-l, Northern Research and f i g . Corp., J u l y 1-971.

7. Norster, E. R.; and Lelebvre, A. H. :: E f f e c t s of Fuel. I n j e c t i o n Methods on Gas Turbine Combustor Emissions. Presented at General. Motors Research Lab Symposium, Wamen, Mich., Sept. 27-28, 1971.

8. Bahr, Do W.: Control and Reductiun of Aircraf'c Exhause h i s s i o n s from A i r c r a f t Turbine Engines, Presented a% General Motors Research Lab Symposium, Vamen) Mich, Sept. 27-28, 1971.

9. Grobman, Jack: Effect of Operating Variables on Pollutant h i s s i o n s from A i r c r a f t Turbine Engine Combustors.

Presented at General. Motors Research Lab Symposium, Warren, Mich., Sept. 27-28, 1971.

10. Beheim, Milton A.; Cunmings, Robert L.; Dugan, James F., Jr.; F e i l e r , Charles E.; Grobman, Jack S. ; and Stewart, Wasner L . : Subsonic a d Super- .*. sonic Propulsion. Vehicle Technology f o r C i v i l Aviation: The Seventies and Beyond. NASA SP-292, pp. 107-156.

11. Di&l., Larry A . : Preliminary Investigation of Gaseous Emissions from Jet Engine Afterbmners. N A S A T M X-2323, 1971.

1 2 . Childs, J. Howard; Reynolds, Thaine W.; and Graves, Charles C. : Relatior, of TurboJet and Ramjet Combustion Efficiency t o Second-Order Reaction NACA Rep, 1334, 1957, Kinetics and Fundamental Flame Speed.

13. B r i e h l , Daniel; Papathakos, Leonidas; and Strancar, Richard J. : E f f e c t of 0perat;ing Conditions on the Exhaust Emissions from a Gas Turbine Combustor, N A S A TN D-6661, 1972.

14. Niedzwiecki, Riciiwd W.; Juhasz, Albert J. j and Anderson, David N. : Performance ot a Swirl-Can Primary Combustor t o O u t l e t Temperatures of 3600' F (2256 K ) . N A S A TM X-52902, 1970- 15. Grobman, Jack; Jones, Robert E.; Marek, Cecil 3.; and Niedzwiecki, Richard W, : Combustion. A i r c r a f t Psopulsion. NASA SP-259, 1971, pp. 97-134.

----____-I--- CONSTITUENTS SOURCE I ESTIMAIED

I CONCENTRATION

- - ~ - AIR M IVOLl % !

AIR 16, LS ivau AIR 0.9% Nail A _I___ C F F COMBUSTION 2 . 7 % IVOLI EFF GOMRUSTION 2. S I Miil

-

co I N E F F COAISUSTION. * io-% P P ~ INEFF COMBUSTION UNBURNED HC

1 5-25 pPMC

PARTIALLY OXIDIZED HG INEFF CDMBUSTlOFi H2 INEFF COMCUSTION 5-lio PPM SMOKE WARTICUIATESI INEFF COMBUSTION 0.4-50 PPhl (MASS -- - NO. NO2 HEATING OF AIR 5a-400 PPAl -- so SO FUEL 1-10 PFhl

I ~ R B E J~ALS

FUEL 5-20 PPR __ T y P l C 4 C ENGINE EXUWST EMISSION CHARACTERISTICS EMISSION INDEX.

n r-F.Li239 i o 80 90 IIDLEI ENGINE SPEED. I (TAKEOFF) Firrure 2.