Radiation from flames in gas turbines and rocket engines

RADIATION F R O M FLAMES IN GAS T U R B I N E S AND ROCKET E N G I N E S ARTHUR H. LEFEBVRE

Departme~t of Aircraft Propulsior~, The College of Aero~autics, CraT~field,England A brief description of the nature of flame radiation in gas-turbine combustors is followed by a discussion on the use and limitations of existing methods for estimating luminous emissivity. Since it is generally agreed that luminous radiation emanates from soot particles in the flame, consideration is given to the process of soot formation and to the influence of chemical composition on the soot-forming tendencies of fuels. Experimental data on the effects on flame radiation of variations in the operating conditions of pressure, temperature, velocity, and fuel-air ratio are presented and discussed. The important influence on radiation of the dist.ribution of air and fuel in the combustion zone is also considered. In rocket engines, problems of radiation are tess important because radiation normally represents only a small fraction of the total heat transferred to the walls. For this reason, interest in radiation in rocket engines has so far centered mainly on the exhaust gases, and flame radiation has received comparatively little attention. The small amount of avail.d)le experimental data is reviewed and attention is drawn to the need for further work to clarify one or two existing anonmlies. Introduction In aircraft gas turbines the continued trend toward higher flame temperatures and pressure ratios has led to a considerable increase in the amount of heat transferred to flame-tube walls by radiation. The problem that this presents is not new; in fact, the important influence of radiation on the temperature and durability of flame tubes has been recognized for many years. Indeed it might have been expected that by now the process of flame radiation in gas-turbine eombustors, if not fully understood, would at least have been thoroughly investigated. In the event, the number of reported studies is surprisingly small, and in many of these more emphasis has been placed on the various experimental techniques eml)loyed than on the actual results obtained. One reason for this apparent lack of interest is because, to be of value to the engine designer, flame-radiation studies must be carried out at the pressure levels encountered in modern turbojet eombustors--i.e., in the region of 30 atmospheres. However, the high cost of providing test facilities capable of supplying even modest amounts of air at such elevated pressures is daunting enough for many engine companies and prohibitive for almost all universities. Another dissuading factor has been the widespread belief that, because flame-tube lives are now measured

in thousands of hours, the problem of flame radiation in gas-turbine combustors has been effectively overcome. W h a t is often overlooked is t h a t very large quantities of air have to be employed in film cooling in order to achieve this very satisfactory flame-tube life. On some engines, this can amount to more than one-third of the total chamber air flow. The injection of such large quantities of relatively cold air along the inside walls of the flame tube has an adverse effect on combustion efficiency at high altitudes and also produces maldistribution of temperature in the exhaust gases. I t is imperative, therefore, that the amount of air employed in film cooling should be kept to an absolute minimum. The determination of this mininmm quantity is, of course, dependent upon a sound knowledge of the processes of flame radiation and film cooling. In rocket engines, the problem of flame radiation is much less severe, since radiation normally represents only a small fraction of the total heat transferred to the chamber walls. For this reason, the following discussion is mainly concerned with flame radiation in gas-turbine combustors, although much of the evidence on the nature of flame radiation and the iIffluence of certain operating variables is relevant to both types of combustion system. Experimental d a t a that relate exclusively to rocket engines are discussed separately in a short section at the end of the paper.

1247

1248

IIAI)IATION FIIOM PIIACTICAL FLAMES

affected by pressure, with the ample experimental evidence showing that soot formation increases In a gas turbine, the products of combustion appreciably with pressure. On the other hand, consist mainly of H~O, C02, and N2, with smaller these findings are fully consistent with vaporamounts of CO, 02, H~, and other minor species. phase cracking, which is known to be very deIn nonluminous flames, the banded spectra from pendent upon pressure. It would seem, therefore, H20 and CO2 are the most prominent featme at that soot is formed mainly by pyrolosis in the temperatures up to about 3000~ ,e At higher vapor phase, although the possibility that, at temperatures, H20 and CO2 are depleted by dis- high pressures, significant quantities of soot may sociation, but radiation from diatomie molecules, emerge as the product of flame reactions cannot notably CO, is increased. The contribution from be excluded. other heteropolar gases is quite small. Moreover, A comprehensive survey of current theories of gases with symmetrical molecules such as He, 0.~, soot formation is beyond the scope of this paper and N2 give no appreciable radiation even at the and, in any ease, several useful reviews already highest temperatures. exist. ',a,6 From a practical viewpoint, the imAt atmospheric pressure, the soot particles portant conclusion to be drawn from the literformed in combustion contribute a continuum in ature is that soot formation occurs most readily the visible spectral range, thereby producing a in rich mixtures at high temperature. In a gasluminous flame, but usually they are too small turbine combustor, these conditions are found in in size to radiate appreciable energy in the im- the center of the flame tube adjacent to the portant infrared region. With increase in pressure, atomizer. This region contains high temperature, the continuous radiation increases in intensity oxygen deficient, combustion products in which and the molecular bands become less pronounced. a high fuel concentration is maintained by the At the high levels of pressure encountered in evaporation of fuel drops that detach themselves modern gas turbines, the soot particles can attain from the inside of the spray. Thus, any modifisaffieient size to radiate as blackbodies in the cation to the flame tube or fuel injector that infrared region, and the flame is then character- reduces either the temperature or fuel concenized by a predominance of continuous radiation. tration in this zone will effectively reduce the rate It is under these conditions that severe radiant of soot formation and, hence, the flame radiation. heating is encountered with its attendant problems of flame-tube durability. Clearly the solution Calculation of Flame Radiation to this problem lies in controlling the size or rate of growth of soot particles. To achieve this control, the mechanism of soot formation must be The rate at which heat is transferred from a clearly understood, the optical properties and flame to its enclosure can be calculated if the size distribution of soot particles must be known, following are knownT: and their emissivity must be calculated or meas1. The size and shape of the flame; ured.3 2. Its mean or "bulk" conditions of pressure, temperature, and composition; 3. The extent to which radiation is influenced M e c h a n i s m of Soot Formation by flame lmninosity. Nature of Flame Radiation

Soot particles formed in combustion contain about 96% carbon and 1% hydrogen, with the remainder believed to be oxygen) They vary in diameter between 1 and 6 microns) The so-called "cracking" reaction, that governs the thermal decomposition of hydrocarbons, may be expressed in its simplest form as CI2H24"---~12C-']-"12H2. Cracking may occur in either the liquid or vapor phase. However, for distillate fuels at least, the temperature of the evaporating droplet is probably too low for liquid pyrolosis to occur. Moreover, it is difficult to reconcile the process of liquid-phase cracking, which is largely un-

Nonluminous Flames Reference 8 gives the general relation R = 0.5S(l+ew) (esT/-- aA'~4).

(1)

Investigation over a wide range of values of Pl has shown that, to a sufficiently close approximation, 7

aff ej= ( Tff Tw)' '~.

(2)

Substitution of as from Eq. (2) into Eq. (1) gives

R=O.5Z(l+ew)esT]'.5( Tf2.5-- T~,2.~).

(3)

RADIATION FROM FLAMES IN GAS TURBINES AND ROCKET ENGINES The variables in the above expression are essentially mean or effective quantities, representing the far-from-homogeneous conditions prevailing in actual chambers. Improvements in accuracy could be achieved by the method of zoning, as described by Hottel, 9 but this would demand a more exact knowledge of the distribution of fuel and temperature in the combustion zone than is available for most current chamber designs. The flame emissivity e/ can be obtained direedy from the standard charts, making suitable corrections for "total" pressure and mutual absorption, or, more conveniently, from the following general equation for nonluminous emissivity provided by Reeve#~

e/= 1--exp[--2.86XlO2P(rl)~

(4)

The "bulk" or mean flame temperature iv/ is obtained as the sum of the chamber en try temperature and the temperature rise due to combustion.

Luminous Flames Gas-turbine flames are lmninous to an extent that depends largely upon the type of fuel employed, the method of fuel injection, and the chamber pressure. Where the lmninosity is relatively low, as in kerosene flames at low pressure, its effeet may be accounted for by introducing a "luminosity factor" L, r into Eq. (4) to give e/: i-- e x p [ - - 2.86>( 102LP (rl)0.5Ti-~.5].

(5)

Thring and HollidayH carried out a munber of experiments in whieh various fuels were burned as a diffusion flame in a furnace-type chamber. Analysis of their data and those of Reeves m led to values which conform quite well to the expression

1249

I00~

80,

i

600

Z 0

400

< 2OC

1"5 ATMOS

o

->

;

i

,,

13

CARBON/HYDROGEN RATIO Fig. 1. Influence of pressure and fuel composition on flame radiation. Results obtained by Schirmer and Quigg, Ref. 16.

radiation are at least as important as variation in fuel type. In recent years, an alternative method of estimating luminous emissivity has been evolved which is based on a knowledge of the size, mass concentration, and optical properties of soot particles in the flame, a,mla Application of the Mie u theory permits curves to be drawn representing the variation of the monochromatic emissivity of a cloud of particles with the wavelength of radiation. This approach still needs considerable development, and in the meantime the designer has to rely mainly on the small amount of available experimental data t h a t is relevant to gas-turbine eombustors. The main conclusions to be drawn from these data are summarized in the following sections.

Influence of Fuel Composition L = 7.53 ( C / H - - 5.5) ~ where C / K is the carbon/hydrogen ratio of the fuel. For kerosene, L is sensibly independent of P and r, at least for pressure below 5 atm, but for gas, oil, and other heavier fuels, L increases with pressure in a manner which implies that total emissivity is increasing at a rate faster than the nonluminous emissivity# ,m Thus, although the concept of luminosity faetor is useful in rough esdraations of flame radiation in furnaces and other low-pressure systems, it has no practical significance in chambers operating at high pressure, where tJhe effects of atomization and fuel distribution on soot formation and, hence, flame

The important influence of fuel composition on flame emissivity and radiation intensity has been clearly brought out in a number of independent experimental investigations carried out in combustors ranging from 5-75 em in. in diameter. In nearly all these studies, attempts were made to relate radiation intensity either to fuel properties, such as chemical composition or molecular strueture, or to various fuel ratings, such as smoke point or luminometer nmnber. The most comprehensive study of the influence of fuel composition on flame radiation is that due to 8ehirmer and his colleagues15a6 who investigated a wide range of fuels at pressures up to 15 arm. The results of tests carried out by

1250

RADIATION FROM PRACTICAL FLAMES 50C 400

J 7

300

200

,< n,"

IOO o

/

x

/

8

16

PRESSURE ~

24

32

ATMOS

Fig. 2. Influence of pressure on flame radiation in a gas-turbine combustor. Results obtained by Jackson and Winter, Ref. 17.

Schirmer and Quigg 16 on the Phillips 5 cm combustor are shown in Fig. 1, in which the experimental points are omitted for the sake of clarity. This figure demonstrates a very satisfactory correlation between flame radiation and the hydrogen content of the fuel. Another interesting feature is that, at atmospheric pressure, where the radiation is mainly nonluminous, it is virtually independent of fltel composition. This study also showed that, for any given value of smoke point, there is no difference in radiation between monocyclic and multicyclic aromatics at the important pressures between 10 and 20 arm. Of equal practical importance is the relatively poor correlation obtained between radiation intensity and luminometer number. This result throws serious doubts on the value of luminometer number as a means of predicting the soot-forming tendencies of fuels.

Influence of Operating Conditions In a gas-turbine combustor, the operating variables are those of pressure, temperature, velocity, and air/fuel ratio.

Pressure Pressure may influence the emissivity of a luminous flame through its effects on the chemistry of soot formation, the quality of atomization, the distribution of fuel in the combustion zone, and fuel-air mixing. The nature and relative importance of these effects is largely unknown. However, there is no doubt that flame radiation increases markedly with pressure. This is demonstrated in Figs. 1 and 2. Figure 2 is based on

experimental data obtained by Jackson and Winter 17from a 9.5 cm tubular chamber at pressures up to 30 atm. In combustion chambers fitted with atomizers, the main factor contributing to high flame radiation at high chamber pressures is the influence of pressure on the characteristics of the fuel spray. An increase in gas pressure produces an increased resistance to the movement of individual fuel droplets and, at the same time, a reduction in mean droplet diameter. The net effect is to increase the surface-to-volume ratio of the spray so that both the rate of evaporation and the drag of the spray are increased. These two effects are cumulative in reducing the penetration of the spray. Thus, instead of the fuel distributing itself evenly across the primary zone, at high pressures it tends to concentrate at the center of the flame tube in the vicinity of the spray nozzle. This local fuel-rich zone, surrounded by flame, constitutes ideal conditions for the production of soot. This situation does not arise, of course, with the vaporizer system in which the distribution of fuel does not depend on its kinetic energy, but is governed instead by the airflow pattern. Another significant effect of an increase in pressure that applies to both types of combustion system, is to extend the rich limits of inflammability and thereby permit combustion to take place at the very rich mixture strengths that are so conducive to soot formation, is In an actual combustion chamber, it is probable that all these effects contribute in varying degrees, depending upon the combustor design, to the observed increase in flame radiation with pressure, as illustrated in Figs. 1 and 2.

Temperature All the available evidence suggests that flame radiation increases markedly with rise in air-inlet temperature, although the relative magnitude of the contributions made by emissivity and flame temperature to this increase is not yet clear. Data obtained in two separate investigations on turbojet combustors at high pressure provide a rough but useful guide to the influence of temperature. 16,19 The results suggests that, over the range from 500 to 1000~ an increase in inlet temperature of 100~ produces a 10% increase in radiation intensity.

Velocity Since, for reasons of size and pressure loss, most combustors are designed within a fairly small range of reference velocity, the influence of velocity on radi~.tion h~s no greaf practiet~l significance.

RADIATION FROM FLAMES IN GAS TURBINES AND ROCKET ENGINES

1251

The effect of a change in velocity on radiation depends upon how the change is brought about. For any given eombustor, operating at constant over-all air/fuel ratio, an increase in velocity is accompanied by a corresponding increase in fuel flow and a rise in fuel pressure. The increase in fuel flow tends to raise the soot concentration, while the rise in fuel pressure, by improving the atomization quality, tends to reduce it. The net result is that the influence of velocity is usually quite small. In general, an increase in velocity leads to a slight reduction in radiation intensity? 6 However, if the change in velocity is accomplished by a modification to the combustor flow area, with the fuel and air flows kept constant, then the influence of velocity is more pronounced. The effect of an increase in velocity is to raise the turbulence level in the combustion zone and probably also the proportion of the total airflow that is entrained in reeirculation. Both effects lead to improved aeration of the soot-forming regions in the flame and, hence, to reductions in emissivity and radiation.

maximum temperature and radiation. This point is clearly brought out in Fig. 3, which also serves to illustrate the dechne in radiation intensity at the downstream end of the chamberP This occurs because the soot particles are gradually consumed during their passage through the combustion zone. Macfarlane, Holderness, and Whitcher TM have shown that pre~ure and velocity have a negligible effect on the threshold mixture strength for soot formation in premixed flames, which for a variety of fuels lies in the region of 4)--1.5. If the burning zone is operated with richer mixtures, there is a rapid increase in the amount of soot formed, especially at pressures above 10 atm. For current chamber designs, these results are of academic interest only, since some imperfections in fuel-air mixing are tolerated--and sometimes deliberately contrived--in order to maintain combustion ox,e r a wide range of over-all air/fuel ratios.

A ir/Ft~el Ratio

In combustion chambers fitted with spray atomizers, the main soot-forming region lies at the center of the primary zone inside the fuel spray. Thus, any modification to either the fuel injector or flame tube that reduces the concentration of fuel in this region will effectively reduce soot formation and, hence, also flame emissivityY~ In the design of the flame tube, care should be taken to ensure that the maximum possible amount of air participates in primary-zone recirculation, consistent with adequate relighting capability at altitude. Ideally, the primary zone should be no richer than stoichiometric and preferably much weaker. It is, of course, important that the primary-zone air should have sufficient penetration t IOreach the soot-forming zone at the ('enter of the flame tube. This penetration is determined mainly by the size of the secondary air holes and the pressure drop across them. A high-pressure drop is particularly useful because it assists both in the penetration of the air jets and also, by raising the turbulence level, in their subsequent mixing with fuel and combustion products.

in a gas-turbine combustor, flame radiation is normally highest in the region of maximum flame temperature, which usually lies in the primary combustion zone. However, at high fuel flows, the supply of air to the primary zone may be inadequate for complete combustion. Under these conditions, burning continues into the intermediate zone, which then becomes the region of

%

6C A, ER.

55/l

I 4c Z C)

I00/I

F, 2O rr

o0

(o) I, 5

h

Ib

DISTANCE DOWN

IS

2b

I

2S

Combustor Design

Rocket Engines

FLAME TUP,E - - GITI.

Figure 3. Influence of over-all air-fuel ratio on flame radiation in a gas-turbine combustor. Stations (a), (b), and (c) denote the planes of the secondary, intermediate, and dilution holes, respectively. Resuits obtained by Talbot and Leathley, llef. 19.

The number of reported investigations of radiative heat transfer in rocket combustion chambers is surprisingly small. In contrast, thermal radiation in solid-propellant and liquid engine exhausts has been extensively studied, mainly as a result of the serious structural damage that

1252

I~ADIATION FROM PRACTICAL FLAMES

has occurred during launching and separation phases due to excessive heating by exhaust gases. A recent comprehensive review by Rochelle a lists over 600 references to work involving the experimental determination of emissivities, spectral radiation, and temperatures in luminous rocket exhausts. A certain amount of work has also been carried out on rocket-propellant flames at atmospheric pressure? ~ With solid-propellant motors, the metallic partitles in the propellant may raise the level of thermal radiation to values that are several times higher than the corresponding gas radiation. With aluminized propellants, the radiation is caused mainly by emission from A120~, with some contribution from carbon in the 1- to 3-micron wavelength region? Emission spectra obtained by Burrows and Povinelli~ from a gaseous hydrogen-gaseous oxygen chamber revealed the presence of a continium in addition to radiation from OH and 02. Radiation intensity varied with pressure as p~, where n varied from 2 to 3 over the pressure range 10 to 30 atm. The effect of variation in oxidantfuel ratio was smaller than expected from thermochemical equilibrium calculations. This may have been due to uneven distribution of propellants. The same propellant combination was also studied by Ziebland 2a in a 4-cm-diam chamber at pressures up to 60 arm. At 10-arm pressure, the measured values of radiation were up to five times greater than calculated values based on an equilibrium concentration of H20. According to Ziebland, this strong emission was due to a continuum emanating from the reaction zone. In a further series of tests, it was found that local values of radiative heat transfer at upstream axial stations were about 289 times greater than those observed further downstream. A sharp transition occurred between the two radiation regimes that moved downstream with increase in propellant flow rate. A similar effect with the same propellants was also observed in an independent investigation carried out by Burrows. 24 In general, the small amount of published data on radiative transfer in rocket engines show broad agreement with conventional calculations, the only unusual feature being the very high rates of radiant heat transfer measured at the upstream end of the chamber with H2/O2 gaseous propellants. This result requires further investigation. Discussion

All the experimental observations on the influence of design factors and operating variables on flame radiation in gas-turbine comt)ustors are

consistent with the view that luminous radiation emanates from soot particles produced in the high-temperature, fuel-rich regions of the combustion zone. Any change that increases either the temperature or fuel/air ratio in these regions will accelerate the rate of soot formation and thereby increase flame radiation. The most satisfactory way of minimizing radiation lies in the design of the flame tube and fuel injector. In particular, the primary combustion zone should be made "fuel weak," with the air supplied in the form of highly turbulent jets of sufficient penetration to reach the sootforming zones. Not the least advantage of a fuel-weak primary zone is that, by reducing the contribution made by radiation to the total heat transferred to the flame-tube walls, wall temperatures become less sensitive to variations in fuel composition. Minimum flame radiation is obtained with fuel injectors that provide some measure of fuel-air mixing prior to combustion but, if conventional pressure atomizers are employed, the cone angle should be as wide as possible, consistent with satisfactory altitude relighting performance. Fuel composition is unimportant at low pressures where nonluminous radiation predominates, and also at high pressures where the flame emissivity is close to unity for all fuels regardless of composition. However, within the pressure range of interest for gas turbines--i.e., between about 5 and 25 arm--flame radiation is very dependent upon fuel type and composition. Presumably, the recent developments in fuel additives for reducing exhaust smoke ~'25 should also effectively reduce flame radiation, but as yet no experimental data has been reported to confirm this. At the present time, the situation in regard to flame radiation in gas turbines is satisfactory only to the extent that the influence of the operating variables of pressure, temperature, velocity, and air/fuel ratio is qualitatively known, and that design rules for achieving minimum radiation have been established. If, in any given eombustor, these rules are not always strictly observed, it is usually out of consideration for other, conflicting performance requirements. Undoubtedly the major cause for concern is the lack of a reliable method of predicting the emissivity of luminous flames at high pressure. This is a serious weakness, since it is at high pressures, where a small increase in radiant energy can drastically reduce flame-tube life, that accurate estimates of radiation intensity are most needed. With the advances now being made toward a better understanding of the film-cooling process, the present inadequate knowledge of flame radiation is rapidly becoming the major

RADIATION FROM FLAMES IN GAS TURBINES AND ROCKET ENGINES obstacle to the prediction of flame-tube wall temperatures. Theoretical treatments of radiative heat transfer based on soot concentration and temperature may in due course find useful application. Unfortunately, nearly all existing aircraft eombustors employ pressure atomization, which creates serious complexities due to the wide spectrum of drop size, the steep axial and radial gradients of temperatare and fuel concentration, and a virtually unpredictable flow pattern. This situation should improve with developments in airblast atomizers and vaporizing systems, in which the fuel is mixed with air prior to combustion. In rocket engines, further research is needed to explain the very high local rates of thermal radiation that are measured close to the injector face.

Nomenclature

R S eI e,~ a~ T~ T~ P r r L l

radiation heat-transfer coefficient, kW/m 2 Stefan-Boltzmann constant= 57.24X 10-12 k W / m 2 (~ fame emissivity wall emissivity flame absorptivity flame temperature, ~ wall temperature, ~ gas pressure, k N / m 2 fuel/air ratio equivalence ratio r luminosity factor radiation beam length, m

REFERENCES 1. GAYDON,A. G.: The Spectroscopy of Flames, Chapman and ttall, 1957. 2. TOERIN, R. H.: Spectroscopic Gas Temperature Measurement, (J. M. Beer, Ed.), Elsevier, t966. 3. ROCHELLE, W. C.: Review of Thermal Radiation from Liquid and Solid Propellant Exhausts, NASA TM X-53579, 1967. 4. SCALLA,R. L., CLARK, T. P., AND 5IcDoNALD, G. E.: Formation and Combustion of Smoke in Laminar Flames, NACA Rept. 1186, 1954. 5. TOONE, B.: Unpublished Rolls Royce Report, 1964. 6. WItELAN,P. F.: The Mechanism of the Carboi~ Formation in Combustion, paper presented at an ordinary meeting of the Institution of Mechanical Engineers, London, March 1961.

1253

7. LEFEBVRE, A. H. AND HERBERT, M. V.: Proc. Inst. Mech. Engrs. 174, 463 (1960). S. FISttENDEN, M. AND SAUNDERS, O. A.: An Introduction to Heat Transfer, Oxford University Press, 1950. 9. HOTTEL, H. C.: Some Problems in Radiative Transport. International Developments in Heat Transfer, Am. So(.. Mech. Engrs., 1961. 10. HEEVES, D.: Unpublished Ministry of Supply lleport, 1956. 11. ]toLLII)A'C, I). K. AND THm-~G, M. W.: Tile Radiation From Flames ill a Small-Scale, OilFired Furnace, A.R.C. Report 18237, Aeronautical Research Council, England, 1956. 12. SIDDALL, l'~. (J. AND McGRATH, I. A.: A:it~th Syn~posium (Diternatiot~al) on Comb~tstion, p. 102, Academic Press, 1963. 13. THRIXG, M. W., FOSTER, P. J., MCGRATH, I. A., AND ASHTON,J. S.: Prediction of the Emissivity of Itydrocarbon Flames, Intenmtional l)evelopments in Heat Transfer, Part IV, p. 796, Am. Soc. Mech. Engrs., 1961. 14. 5h~, G.: Ann. Phys. 25, 377 (1908). 15. ScreaMER, R. M., MeREYNOl.nS, L A., AND I)ALEY, J. A.: SAE Trans. 68, 554 (1960). 16. ScreaMER, R. M. AND QUIGG, H. T.: ttigh Pressure Combustor Studies of Flame Radiation as Related to Hydrocarbon Structure, Phillips Petroleum Company, Research Division Report 3952--65R, 1965. 17. JACKSON, E. AND WINTER, J.: Unpublished Lucas Report, 1965. 18. MACFARLANE,J. J., HOLDERXESS, F. H., AND Wmci~Ea, F. S. E.: Combust. Flame 8, 215 (1964). 19. TALBOT, D., AND LEATHLEY, B. W.: Unpublished Lucas Report, 1962. 20. LEFEBVRE, A. H. AND DURRANT, T.: Esso Air World 13, No. 3 (1960). 21. PENZIAS, G. J., GILLMAN, S., LIANG, E. T., AND TOURIN, R. g.: An Atlas of Infrared Spectra of Flames, Reports part 2, 1961, aud part 3, 1963, Advanced Research Projects Agency, Department of Defense, Washington, D.C. 22. BVRaOWS, M. C., AND POVINELLI, L. A.: Emission Spectra From High-Pressure HydrogenOxygen Combustion, NASA TN I)1305, 1962. 23. ZIEBLAND, H.: Unpublished E.R.I).E. Repoi% 25/R/64, Waltham Abbey, England, 1964. 24. BoRRows, M. C.: Radiation Pro~'esses Related to Oxygen-Hydrogen Combustion at tiigh pressteres, NASA TN 1)-2541, 1964. 25. Aviation Week, p. 101, January 29, 1968.