INDUSTRIAL AND ENGINEERING CHEMISTRY
2822
Flame Temperatures of Propellant Compositions with Different Enthalpies
Table II.
Pressure, Atmosvheres
Specific Enthalpy, Calories per Gram -368 - 644 -876 -368 - 644
x.
CONCLUSION
2817 2689 2461 3014 2847 2544 3056 2906 2661 3099 2974 2578
-368 - 644 -672 - 368 - 644 -875
The adiabatic flame temperature of a propellant is an important parameter in the calculation of theoretical performance characteristics. The computation of the flame temperature requires a knowledge of the composition of the product gases which may be determined by one of several systematic mathematical procedures, The flame temperature is but little dependent on pressure for propellants with low enthalpies but it is dependent markedly for propellant of high enthalpies. The effect of ernpirical formulas of the same enthalpy on flame temperature is not, predictable without a knovledge of the composition of the product gases.
Table 111. Flame Temperatures as Function of Pressure for Different Propellant Compositiona of Assumed Equal Enthalpies Pressure, Atmospheres Propellant Flame Temperature, O K. 2915 2881 2838 2500 2137 3000 2905 2876 2502 2139
A
B C
D
E A B C
20
D E
Propellant
C
H
A B C
0.1170
0,4006 0.2787 0.4712 0.3004 0.3247
D E
0,2086 0,0341 0,2451 0,2497
0 0.3573 0.3967 0.3039 0.3452 0.3349
ACKNOWLEDGMENT
The large number of calculations involved in obtaining the data given in the tables mere performed by Jane McCuistion. LITERATURE CITED
(1: BrinMey, S. R., and Kandiner, H. J., IXD.ENG.CHEM.,42, 860 (1950).
(2) Edse, R., Air Tech. Service Command, PiogressRegt., No. IRE-47 (1946).
Atom % a
compositions for which equal enthalpies were assumed. The importance of a knowledge of the composition of product gase-, are emphasized by these results.
Flame Temperature,
- 875
10
Vol. 43, No. 12
S
0.1260 0.1159 0,1913 0.1093 0.0935
(3) Hottel, H. C., Williams, 6. C., and Satterfield, C. N., “Thermodynamic C h a r t s for Combustion Processes, P a r t I,”New York. John Wiley & Sons, 1949. (4)Kreiger, F. J., and K h i t e , W.B., Project Rand Rept., RA15 055 (October 1942).
(5) McEwan, W. S.,NavOrd Rept. 1239, U. 3. Yaval Ordnance Test Sta. (July 1960). (6) McEwan, W. S.,and Skolnik, 801, Ren. Sei. Instrument, 22, No. 3, 125-32 (1951).
( 7 ) Sachsel, G. E”., Bell, J. C., and Xantis, M. E., “Third Symposium
If compositions having the same enthalpy hut differing in empirical formulas are considered, then no simple correlation between flame temperature and composition appears possible. Table I11 lists the calculated flame temperature of five different
on Combustion, Flame and Explosive Phenomena,” p. 620, Baltimore. Williams R- milkins Co., 1949. (8) Wimpress, A. K.,“Internal Ballistics of Solid-Fuel Rockets,” p. 4,New York, McGraw-Hill Book Co., 1950. RECEIVED July 3, 1951.
E. S. STARKMAN’, A. G. CATTANEO, AND S. H. MCALLISTER Shell Development Co., Emeryville, Calif.
To
investigate gas turbine combustion problems separate from the complexities of compressor and turbine, a small combustion chamber was designed to incorporate the principles of contemporary full scale burner tubes. More specifically, i t was used to study the influence of fuel and operating factors on the carbon deposits found in combustion chamber liners. The carbon deposition tendencies of paraffinic, naphthenic, and aromatic fuels increased in that order. For a gven ratio of carbon to hydrogen, deposits decrease as volatility increases, more generally increasing with specific gravity of the fuel. Deposits are influenced by operating conditions. Increased air temperature, increased air-fuel ratio, and decreased pressure minimize them. For a given combustion chamber, normally encountered operational ranges of air temperature and pressure influence the amount of carbon as much as changes in fuel composition. 1 Present address, Department of Mechanical Engineering, University of California. Berkeley, Calif.
Besides providing a laboratory test for carbon deposits, this work resulted in the derivation of a formula by which the carbon deposition tendency of a fuel can be predicted from its characteristics.
URING the development stage of aircraft gas turbine engines, kerosene has generally been used a s the fuel, Choice of kerosene was prompted by a number of reasons, of which low vapor pressure was the primary one (1). It had not been ascertained from an operational standpoint whether fuels of the kerosene type were really the most desirable, and a survey in laboratory apparatus of various potential gas turbine fuel components appeared necessary. Same of the desired qualities wer? apparent from piston engine experience (%). One aspect of performance which could not suitably be evaluated from piston engine data, because of the difference in mode of combustion, was the tendency for coking or carbon formation in the gas turbine combustor. Carbon formation is objectionable for many reasons, among which are low combustion efficiency, local hot spots on the
December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 1. Combustion Apparatus flame tube and possibly subsequent buckling, and the breaking loose of chunks which may erode the turbine blades or reduce the turbine nozzle area by blocking the nozzle ring. For studying fuels in regard to deposit formation, a full scale combustion chamber of the jet propulsion type can be used. However, such an installation is not practical for laboratory investigations-for example, its fuel consumption is too large to permit the study of individual, often very expensive, fuel components. A small scale combustion apparatus was built which incorporates the fundamental features of the full scale engine burner and consumes fuel a t a rate of only 1 gallon per hour. The unit lends itself to an investigation of the effect of fuel properties incorporating relatively pure compounds if need be. It was intended t o determine how fuel properties influenced carbon deposition and to compare the influence of fuel properties to the influence of operational characteristics. No attempt was made, however, to determine the effects of combustor configuration or of modifications in the fuel-air mixing process, which could undoubtedly overshadow the effect of fuel composition. The data obtained have, therefore, only relative value.
2823
Initial ignition of the iuel IGXITIOK. is accomplished by means or 3 high tension spark across a gap of platinum electrodes. Tungsten electrodes eroded elcessively during t c m and commercial spark plug electrode inatciial melted. Platinum mas found satisIactory in both respects. 11 hile the spark is necessary only ior the initial ignition, it was continued throughout each test to ensure immediate reignition in case of an wcidental blonout of the flame. 0PER.ITIOS. Thermocouples ale illstalled to determine the ail temperature before the air rotameter and the flame tube. The exhaust ea3 tenmerature is measured just after -the exhaust nozzle by a radiation shielded thermocouple. A surface thermocouple indicates the temperature of the flame tube a t one point. The fuel flow rate can be varied between the limits of 0.5 and 13 pounds per hour, A good spray pattern can be obtained over this whole range with the present Monarch-type F-80 nozzles. Air, supplied in quantities up to 500 pounds per hour, can be heated to 250' F. Higher temperatures are possible a t intermediate flow rates. Pressurizing of the burner section is accomplished by regulation of the air flow rate and size of the exhaust nozzle. Pressures UD to 60 pounds per square inch can be maintained in the burner cube with the present arrangement. DISCUSSION
EFFECTOF AIR-FUELRATIO. Before carbon deposition could be evaluated, some knowledge of the effect of air-fuel ratio on carbon deposition was necessary. Accordingly, a survey was made to determine the amount of carbon deposits obtained from kerosene as a function of air and fuel flow rates.
APPARATUS
Figure 1 is a photograph of the small scale combustion chamber. BURNERSECTION. The Monel liner or flame tube is 14 inches in length and 2 inches in outside diameter with a wall thickness of '/I: inch. Two hundred and forty air holes of '/*-inch diameter are drilled a t an angle of 60" to the center line of the tube. The hemispherical cap is removable for inspection, weighing of deposits, and cleaning as shown in Fi Borosilicate glass tubing, witpiE2dutside diameter of 75 mm. is used for the enclosure in order to make the combustion process visible to the operator. The glass tubing section is held in place between the air and fuel headers by spring-loaded tie rods, using thick asbestos gaskets as cushions to allow thermal differential expansion of the burner tube and glass enclosure. The fuel nozzle is attached to the fuel header and inserted into the burner tube through a loosely fitting hole in the burner cap. Thus the nozzle can move with the thermal differential expansion of the lass enclosure. At tke air header, an exhaust nozzle is installed through which the combustion gases pass into the exhaust stack. Present design of this nozzle limits the issuance of exhaust gases to subsonic velocities. AIR SUPPLY. Air, a t a pressure of 100 pounds per square inch, from the laboratory service supply is used in the ap aratus. After entrained moisture is removed, the air passes &rough a section containing electrical strip heaters for elevating the temperature. It is then distributed by the manifolding system into the burner unit, whose counterflow system reflects the design found in some full scale engines. Direction of air flow and distribution in the combustion chamber are illustrated by Figure 3. FUELSUPPLY. Monarch-type F-80 commercial oil burner nozzles in various sizes (ranging from 0.4 to 2.0 gallons per hour a t 100 pounds per square inch supply pressure) inject the fuel into the burner tube. In the 1 allon per hour nozzle the fuel issues from an orifice 0.008 inch in fiameter. The fuel is supplied from a reservoir which is under nitrogen pressure in order to provide steady flow a t the pressures necessary to operate the nozzle over a wide range of flow rates.
Figure
2. Carbon Deposits, Kerosene
Figure 4 shows that these deposits increase with increasing richness-i.e., with decreasing air-fuel ratios. At rich mixtures (60 to 1 and 68 to 1 air-fuel ratios) the amount of carbon deposited was decreased with increasing air flow rate. At leaner mixtures (75 to 1, 80 to 1,and 120 to 1air-fuel ratios) there was a particular air flow rate for which carbon deposition appeared to be a t a maximum. It is probable that the shapes of the curves plotted in Figure 4 are largely dependent upon the air flow pattern within the burner tube. Thus the results outlined will be applicable only to this particular burner tube. The trends, however, may be expected to be similar in other burner tubes of the same general type, as well as in full scale burners of comparable design and chamber pressures. The survey outlined above could not be repeated on all fuels under test because of the volume of sample necessary and the amount of time involved. Therefore, the data collected for kerosene were used as a basis for outlining a test procedure to determine carbon for any particular fuel. The chosen duration of the test was short-15 minutes-to allow testing of small samples.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2824
High rates of carbon deposition were desirable to obtain significant measurements in this short period. On the basis of the data in Figure 4, the following test conditions were chosen for the balance of the investigation: Test duration Air flow rate Fuel flow rate
15 minutes 280 pounds Per hour Air-fuel ratio 60 4.67 pounds per hour}
The following conditions were fixed by the above conditions, varying slightly however with different fuels : Fuel nozzle supply pressure 150 pounds per square inch Air supply pressure 40 pounds per square inch Combustion chamber pressure 35 pounds per square inch absolute Exhaust temperature 1100O F. 8ample required for one determination 2000 ml.
,-PYREX
EXHAUST
TUBE
24 HOLES
r
fuel composition. Three somewhat arbitrary lines have been drawn in Figure 6 to represent parameters of fuels of completely paraffinic, completely naphthenic, and completely aromatic nature (carbon-hydrogen ratios of 5 to 5.6, 6, and 12, respectively). These three lines define a framework within which fuels of any particular composition or type can be located. All fuels thus far tested correlate well with this composition framework. Toluene, cumene, and amyl benzene, which are aromatics with side chains of paraffinic structure, have lesser tendencies toward carbon deposition than the aromatics without side chains. These fuels fall within the framework in a position which qualitatively reflects the ratio of carbon to hydrogen in their structure.
r
2 " O.D.M O N E L LINER
rAIR HEADER
&" EXHAUST
Vol. 43, No. 12
IGNITER
T t i E R hl CC 0 UP L E S
NOZZLE Figure 3.
Combustion Chamber Unit Burner section
Reproducibility of this test, judged by the weight of the carbon deposit in duplicate runs, was within &IO%. A plot of the amount of carbon EFFECT OF FUEL COMPOSITION. deposited in the combustion chamber as a function of fuel volatility is shown in Figure 5 for a number of hydrocarbon fuels tested in the apparatus; data are in Table I. The amount of carbon deposited is a function not only of volatility, but also of
Table
I.
Combustion Chamber Tests on Various Hydrocarbons Air-fuel ratio 60, 15 min. duration, 280 lb. air per hour Boiling Point, F.
Carbon Deposit, Carbon-Hydrogen Fuel Grams Ratio Alkylate bottoms 1.56 Amylbenzene 1.77 0.19 Aviation gasoline Kerosene 0.83 Benzene 1.64 9.0 309 Cumene 1.71 177 6.0 Cyclohexane 0.45 121 6.0 Cyclopentane 0.28 Decalin 1.65 6.6 381 6.5 1.79 48Ia Diesel fuel UO-9 6.4 1.79 460a Diesel fuel-UO-10 5.3 209 Heptane negl. Hexene negl. 6.0 149 5.5 Isododecane 0.40 394 212 5.3 Iso-octane negl. 13.2 469 Methylnaphthalene 2.79b 10.0 2.36b Tetralin 405 10.5 Toluene 1.15C 230 10% ASTM distillation temperature. b Test not run full 15 minutes because of flame emission from back end of tube caused by excessive deposits. Large quantities of carbon formation blowing out exhaust d.uring test.
Mixtures of hydrocarbons, such as kerosene or Diesel fuel, fall into the framework in accordance with their composition, which is mainly paraffinic and naphthenic with small proportions of aromatics. Comparison of the relative significance of volatility and oarbonhydrogen ratio is available as a result of the data obtained from this survey of fuels and fuel components. The simplest approach involved the determination of an empirical equation expressing carbon formation in terms of volatility and carbon-hydrogen ratio which best agreed with the experimental results, The following equation resulted: ln(0.83 R - 1.5) W = - 3.0 (1) 0.54 225 where W = carbon deposit grams ~n = natural logarithm R = carbon to hydrogen weight ratio 5" = boiling point for a pure compound or 10% ASTM distillation temperature for a normal mixture of hydrocarbons, +
O
F.
The relation between this expression and the data from which it was obtained is illustrated by Figure 6. Equation 1, of course, expresses the carbon-€arming tendencies of fuels for only one specific combustion chamber, operating a t a given set of test conditions. The coking tendencies for other chambers and other conditions could be expressed, however, by a more general expression, such as In(KIR - Kz) T W = KB - K5 K4
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
Deaember 1951
2825
3
lOLUENE 0
75
m
( 1 : -
3
n
360
320 L E . PER H R .
AIR FLOW,
Figure 4.
m
90
200 BOILING
Effect of Air and Fuel Flow Rates on Carbon Deposit for Kerosene
Figure 5.
300 400 TEMPERATURE, OF.
500
Carbon Deposit for Various Hydrocarbons
0
I
0
I W E
In (0.83R-1.5) 0.54
Figure
$
' +
I
,
I
2
.65
.70
J--3,0 225
Figure 7.
6.
Correlation between Carbon Deposit and Deposit Prediction Equation
.75
I
.90
.80 .E5 SPECIFIC GRAVITY OF FUEL
Relation between Specific Gravity and Carbon Deposition
$1 a
9
0'
,
I
100
0
I
260
1
300
60
Figure 8.
Effect of Air Temperature on Carbon Deposit for Kerosene
where the constants K I ,Kz,etc., are determined by apparatus design and operation, and R and T a r e as in Equation l. * By noting that carbon-hydrogen ratio and fuel volatility influence the specific gravity of the hydrocarbon, simplification may be made of the relationship between carbon deposition tendencies and fuel properties. Thus, plotting the carbon deposited by the fuels of Figure 6 against their specific gravity also gives a reasonable correlation as shown in Figure 7. Because of some unreliability in the variation of specific gravity with respect to other fuel properties, using this relationship in preference to the carbonhydrogen ratio and fuel volatility for predicting carbon does not, however, seem advisable. To illustrate the relative effects of aromaticity (or carbonhydrogen ratio) and volatility, Equation 1 can be applied to com-
70
80
90
AIR-FUEL RATIO
AIR TEMPERATURE, O F
Figure 9.
Effect of Pressure on Carbon Deposit for Kerosene
pare the contribution to total carbon of a change in the carbonhydrogen ratio to a change in volatility. Pentane (CsHI2)and benzene ( CBH~), are two extremes in carbon-hydrogen ratio which might occur in potential gas turbine fuel. Aviation gasoline and Diesel fuel are the extremes which might be expected in volatility. Pentane, by the first right hand member of Equation I, yields a value of 1.8 grams carbon, while benzene gives 3.9, a difference of roughly 2 grams. Aviation gasoline, by the second right hand member of Equation 1, gives 0.6 gram carbon and Diesel fuel gives 2.1 grams, a difference of about 1.5 grams. Thus, the maximum changes to be expected in volatility or aromaticity will contribute roughly the same quantities of carbon. The advent of volume-limited high-speed aircraft has suggested the advantages of high volumetric heat of combustion
2826
INDUSTRIAL AND ENGINEERING CHEMISTRY
fuels. However, fuels of this type such as tetralin (ClOHl2) and naphthalene ( CloHs) have high carbon-hydrogen ratios and, according to Table I and Equation 1, high carbon deposit tendencies. They would thus tend to enhance combustion chamber deposition problems. EFFECT OF OPERATIONAL CONDITIONS.Since carbon deposition is profoundly affected by the volatility of the fuel as shown previously, it could be assumed that evaporation might be a principal factor. Thus the temperature of the air into which the fuel is injected should be important. To investigate the effect of air temperature on coking when burning a mixture of hydrocarbons, kerosene was tested with results as shown in Figure 8. Deposits decreased with increasing temperature. Examination of Equation 1and the data of Figure 8 shows that there is probably some relation between vapor pressure of the fuel and carbon deposits. Thus the second member of Equation 1 could be replaced by an expression involving the vapor pressure rather than the boiling temperature. The derivation of an all-inclusive expression for carbon in terms of vapor pressure is made difficult however by the same factors which govern the laws of evaporation of fuel droplets. Therefore, no attempt has been made to incorporate such a relationship between vapor pressure and carbon formation.
TRAETHY LLE
Vol. 43, No. 12
The effect of combustion chamber pressure on coking is as pronounced as the effect of temperature, as shown in Figure 9. This could be expected, if fuel evaporation rate is important, since the evaporation rates of fuel droplets are functions of pressure as well as temperature. A comparison between fuel factors and operational characteristics shows that changes in operation can have as much influence on carbon deposits as maximum variations in fuel composition and volatility. Thus wide variations in volatility and composition of the fuel can cause up to 2 grams difference in carbon deposits out of 3 grams total, whereas raising the temperature of the inlet air from ambient to 300 F. results in a decrease in deposits of over 0.5 gram. Raising the combustion chamber pressure from 33 5 to 52.0 pounds absolute increased the deposits by as much as 2 grams out of 3 grams. O
LITERATURE CITED
(1) Bollo, F. G., Stanly, A. L., a n d Cattaneo, A. G., S.A.E. Journal, 54, 56-63 (1946). ( 2 ) Johnson, C. R , paper presented a t t h e i i a t i o n a l Aviation Mecting, American Society of Mechanical Engineers, Los Angeles, 1947. R E C E I V EMay D 2 , 1651
I CARBONS
Antiknock Effectiveness WANDA 1. ZANG AND WHEELER G. LOVELL Ethyl Corp., Detroit, Mich. T h e effectiveness of tetraethyllead in suppressing lcnoclr varies greatly with different gasolines and engines. Using the data on pure hydrocarbons from American Petroleum Institute Research Project 45, it has been found that the effectivenessof the addition of tetraethyllead may be evaluated simply in terms of the potential increase in engine power. The addition of a given amount of tetraethyllead to paraffins and naphthenes results, gerierally, in a constant percentage gain in relative potential power or performance number, regardless of the clear antiknock 'level or engine operating conditions. The effectiveness of the addition of tetraethyllead to aromatics and olefins is variable, but may be related in fairly simple ways to the molecular structure and conditions of test. Such data on pure compounds may serve as a basis for estimating the potentialities for improvementsin the utility of tetraethyllead in commercial gasolines.
H E chemical mechanisms by which the addition of tetraethyllead to motor fuels so effectively suppresses knock in engines are not well known. However, an empirical evaluation of the effect of tetraethyllead in various pure hydrocarbons, under a variety of engine operating conditions, yields results of practical interest and of speculative significance. The work of the American Petroleum Institute Research Project 45, which deals with the synthesis, purification, and properties of hydrocarbons of low molecular weight ( I ) , involves the preparation and knock-testing of hydrocarbons of high purity. The knock tests are conducted in the General Motors and Ethyl
T
Corp. Research Laboratories to evaluate the potential utility of these hydrocarbons in internal-combustion engines ( 4 ) . Almost 300 pure hydrocarbons and other compounds have been engine tested in this cooperative program under as many as 29 test conditions. The data used for the study of these pure hydrocarbons have been taken from tabulated knock-test engine tables which were assembled by the American Petroleum Institute Research Project 45; these tables, which represent about 4000 separate determinations, have been published in the Eleventh Annual Report of API Research Project 45 ( 1 ) Correlations existing betreen hydrocarbon structure and knock behavior with different methods of engine testing, between the knock behavior of pure hydrocarbons and their blends, and between the chemical natures of the hydrocarbons and the effects of the addition of tetraethyllead have already been published (7). The engine characteristics of pure hydrocarbons have been correbted with molecular structure by a study of such concepts as the free radical chain mechanism, relative reaction rates of primary, secondary, and tertiary hydrogen atoms, and the activating effect of methyl groups ( 3 ) . This paper presents further correlations of the relationships which exist among hydrocarbons with regard to gains in antiknock quality attributable to the addition of tetraethyllead. M E T H O D OF M E A S U R E M E N T
In order to deal with data obtained with a variety of engines and operating conditions on a basis which results in simple relationships, use has been made throughout this paper of the performance number scale. This scale, used to express the antiknock
