et of LOUIS C. GIBBOXS, HESRY C. BARNETT, ISD MELVIS GERSTEPN .VVational Advisory C o m m i t t e e j o r Aeronautics, Lewis Flight Propulsion Laboratory, Cleveland, Ohio
-4survey of investigations of the effect of fuel molecular structure on combustion parameters is presented in this paper. These parameters presumably play a significant role in the continuous combustion process that occurs in a gas turbine combustor. Data are presented to show the effect of the molecular structure of hydrocarbons and oxygen-containing organic compounds on minim u m ignition energy, spontaneous ignition temperature, limits of flammability, flame velocity, and smoke formation. The relationship of these fundamental combustion parameters to gas turbine combustor performance is showm. CONSIDERABLE amount of research effort is now being exp-nded in this count'ry and abroad in an effort to learn something about the mechanisms that may control t,hc combustion process in continuous-flow engines such as the gas turbine. One attack on this problem is to relate t'he so-called fundament,al combustion parameters of a seriee of fuels t o the behavior of these fuels in a gas turbine combustor. It seems necessary to evaluate a reasonably large number of compound?, because the establishment of any relationship between a fundamental combustion property and performance in a combustor would have little significance if based on only one or two materials. In addition: if several combustion properties are known for a, series of fuels, it may be possible to establish relationships betn.een these parameters and, ultimately, contribute to an understanding of combustion processes applicable to any engine or combustion application. 9 third reason for studying the basic combustion properties of fuels is t o attempt, to find laboratory tests that Fill predict the performance of fuels in gas turbines. There is nothing new in this approach. For many gears fuels have been evaluated in laboratory equipment for essentially the same reasons. I n the past the applications have been to Otto cycle and Diesel engines, and an addirional effort is now being made to relate laboratory combustion t,ests to the gas turbine engine. This paper reviews briefly available information on the effect of fuel molecular st,ructure on some combustion properties, including critical ignition cnergy, spontaneous ignition temperature, ignition delay, flammability limit's, quenching distance, flame velocity, and smoke formation. In addition, the qualitative relationships between fundamental coinbustion properties and performance in a gas turbine combustor are shown for five hydrocarbons.
trical energy requiremenw and the chemistry of the combustion process is less evident but the minimum electrical energy for ignition can be correlated with ot,her conibustion propert,ies. As t,he amount of electrical energy added to a combustible mixture by means of a spark ii increased, a critical energy is reached at which a flame appear:: and propagates a w y from the electrodes. This is the critical ignition energy under the environment and conditious of t,he test. This critical energy is influenced principally by parameters n-hich may be classed as electrode variables, initial mixture variables, and fuel variables. In a study of fuels for turbojet applications, the effect of electrode variables on the critical ignition energj- is of minor inkrest unless the effect of such a variable obscures or alters the relationship among fuels. Of the electrode variables, some ol %-hichare discussed by Swett (341, electrode spacing is one of the most
IGNITION
Critical Ignition Energy. I n order to obtain a self-sustaining flame, it is necessary t o supply to a combustible gas mixture sufficient energy to produce a rapid chemical reaction so that heat' generat,ion within the mixture exceeds heat transfer away from t'he mixture. The addition of t'he necessary energy may be made thcrmallp or electrically. An important application of electrical ignition is t,o start an engine by uaing a spark to initiate combustion. The relationship between minimum elec-
2150
Atmospheric pressure I
I
I
.I
.2
.4
l
1
,
.6 .E 1
2
A
I
l
P R E S S U Y otm. 15' F.
Figure 1. EfTeFect of Temperalure and Pressure on Critical Ignition Energy of Propane-Air Mixture ( 4 )
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 46.No. 10
-Turbine
5
g
E
2 -
(1
5.6
w
such as pentane and 2-pentene have approximately the same critical ignition energies. Branching usually increases the critical ignition energy, but the effect is often small. Values for IHEPTIVUE hexane and 2,2-dimethylbutane are compared in Figure 3 (22,23). / /" Spontaneous Ignition Temperature. I n considering param/ eters that might have an effect on the continuous combuPtion process, it might readily be imagined that the rate of burning in BUTANE
4-
8
5
-
4-
i
3.2
-
P
flowing systems, the effect of velocity on critical ignition energy is also important. The critical ignition energy for propane-air mixtures a t reduced pressure varies almost linearly with gas velocity ($5). The critical ignition energy also de-. pends on such fuel variables as concentration and type. The variation for a
!I,,,&
ETHANE
mm. Hp
+ ,I & P .Q ? l\ /ETHYLENE
z
-
_/ACETYLENE
a c
20PRESSURE.
0 4
value of the flame velocity is onlyslightly affected by molecular weight. The marked shift of the curves for critical ignition energy makes a comparison of different fuels difficult. If the fuels are compared a t stoichiometric percentage of combustibles in air, the critical ignition energy varies from about 0.3 millijoule for methane and ethane, to about 1.1 millijoule for heptane. If compared at the minimum, however, most of the fuels have almost the same ignition energy, about 0.25 millijoule, while the value for methane is slightly higher, about 0.29 millijoule. The choice of the composition at which comparisons should be made may depend, to a large extent, on the system under consideration. If the fuel and air are premixed, the energies at the specific composition under consideration certainly should
October 1954
Fuels-
I
,
;L
IO
1
1
1
2
ignition was slightly higher than for the corresponding compound containing only single bonds. In the same figure is shown a comparison between mono- and bicyclic aromatics and the alkanes and alkenes. Benzene has a spontaneous ignition temperature almost 600' F. above that of hexane. The spontaneous ignition temperature of alkylbenzenes drops as the length of the side chain is increased. The same trend is shown for alkyl derivatives of biphenyl. The effect of branching the alkanes on spontaneous ignition temperature is shown in Figure 4,B. The general trends look like those that have been plotted for octane number in that as branching of the molecule is increased the spontaneous ignition
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2151
I
2
Figure 4. Effect of AIoleciilar Structure on Spontaneous Ignition Temperature ( I 6 ) 1
I
I
5
6
8
7
9
1 10
6
D NO. O F C A R B O N ATOMS IN MOLECLLE
temperature is increased. The branching of alkenes s1ion.s the same trends (Figure 4,D). A further comparison of alkanes and alkenes is shown iii Figure 4,C. For straight-chain compounds, apparently a slightly higher temperature is required for spontaneous ignition when a double bond is present in the molecule than when only single bonds are present, However, Jyith highly branched compounds the presence of a double bond in the molecule allows spontaneous igiiition at, a temperature about 100" F. lower than the corresponding alkane, The trend observed with ortho and para isomets of alkylbenaenes is indicat,ed in Figure 4,E. The para isomers require considerably higher temperatures for self-ignition than the ortho derivatives.
I
I I I 1 I I l l 4 6 IO 20 4 0 60 I O 0 PROPANE, % BY VOL. 1 1 I 1 1 .50 1.0 25 5 10 2 5 100 EOUIVALENCE RATIO
I
I 1 25
2
Figure 5 . Variation of Ignition Lag with Propane Concentration (17)
compression ( $ I ) , injection of fuel into a hot gas stream (65), preheating of fuel and air and rapid mixing in a flow system ( 1 7 ) , and injection of fuel into a heated bomb (15). Jackson and Broliuw ( 1 7 ) reported on the effect of fuel conccntlat,ion on ignit~iondelay. Fuel and air vere preheated separatcly and mixed very rapidly in a flow system. Ignition n-as detected ure recorder. The trends obby means of a photocell or a p served in the study are shon-n Figure 5 for propane-air mixtures. -4s fuel concentration increases the ignition lag de Ignition w-as obtained at several temperatures at concentrations ranging to 40y0 propane. This concentration is more than 10 times the st,oichiomctric fuel-air ratio. Tmo curves are dot,t#ed to 60 and 80% propane. Pressure pukes were noted a t t,hesi.fuel concentration., hut it, is uncertain that these data represent t,rue ignitions. The study of Hum wid Smith (15)included a largo number of hydrocarbons of various mo1ec:ular structures. The initial temiiemture and pressure of the bomh could be controllod indcperideiitll-. The fuel \\-as injectotl uiiticr high pressure irit,o tho h m h ,
O I a It
0'5-
*( (
I-0CTENE
I n general, in the liquid fuel range, straight-chain hydl'oc:iilJoiis have considerably lower spontaneous ignition temperatures than highly branched paraffins, olefins, or aromatics. IGUITIOY D E L 4 Y
It has been known for many vears that the time requued to cause ignition of a fuel-air mixture a t a constant tempeiature is influenced by the fuel-air mixture temperatuie, piessure, and concentration, and the molecular stiuctuie of the fuel. Ignition lag studies have been conducted in several ways, including rapid 2152
--
nI - CCTANS OCTACECEhE
.1
,
1
n -0CTbDECANE 1
t
B O M B PRESSURE 300 LB. SO. INCH
aA'?AFFlU.NAPHTHENE PORTIONS OF CL1
T E X A S CRUDE O I L BOMB PRESSURE 525 LBJSQ. INCH
Figure 6. Effect of 3Iolecular Structure on Ignition Lag (15)
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Voi. 46, No. 10
-Turbine a n d the ignition lag mas measured by means of instrumentation which indicated a change of pressure. The effect of bomb temperature on ignition lag for this equipment is shown in Figure 6. 1-Octene and 1-octadecene have longer ignition lags than the corresponding paraffins. This trend is shown more completely in Figure 7 , a plot of ignition lag versus number of carbon atoms in the molecule. Data are presented for paraffins, or-olefins, methyl-, ethyl-, and butylcyclohexane, and two dicyclic compounds. .020,-
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0DECALIN 03
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8 IO 12 CARBON ATOMS IN
1
14
1
-
16
18
STRUCTURE
Figure 7. Effect of Hydrocarbon Type o n Ignition Lag (15) Bomb pressure 300 lb./sq. inch, temperature 900O F.
The trends shown in Figure 7 are similar to the effect of moleculsr st’ructure on spontaneous ignition temperature. I n both cases the values for the n-paraffins are the lowest and for the a-olefins somewhat higher; the bicyclic compounds have relatively high ignition temperatures. The ignition lags for cyclohexanes and the corresponding a-olefins are approximately the same. I n addition to pure hydrocarbons, Hurn and Smith ( 1 5 ) determined the ignition lags of a Diesel fuel and two fractions of the fuel, also shown in Figure 6. The aromatic fraction exhibited considerably longer ignition lags than did the total Diesel cut, and the cut containing the paraffinic and naphthenic hydrocarbons showed the shortest ignition lag. Thus, for both pure hydrocarbons and mixed fuels t’hest’raight-chainparaffins exhibit t,he shortest, the a-olefins slightly longer, and aromatics the longest, ignition delays.
those that will not. These limits may be either lean or rich depending on the fuel-air ratio; mixtures in the range betlveen the lean and rich limits are flammable. Molecular Structure. The flammability limits of nunieroua hydrocarbons have been reported by the U. S. Bureau of Mines (6) and the National -4dvisory Committee for iieronautics (?’, 33). A portion of the XACA data is presented in Figure 8 1,o illustrate the relationship between molecular weight and flaniinability limits for paraffins. The rich side of the curve of Figure 8 contains an additional lobe; this was found for all hydrocarbon fuels except methane. It has been suggested t h a t the lean lobe corresponds to the limits of propagation of normal flames, while the rich lobe is due to “cool” flames that are capable of propagating in rich hydrocarbon-air mixtures by means of a different mechanism. The data indicate that the influence of hydrocarbon type on lean flammability limits is relatively small, although appreciable variations are found for rich limits. The rich limits for the normal paraffins increase as molecular m i g h t increases. Since the lean limits are approximately the same for all fuels, the range of flammability increases with molecular weight. However, other data shown in Figure 9 ( 3 3 ) shon- t h a t this trend holds only through n-heptane a n d for heavier fuels such aa n-oct,ane, n-nonane, and n-decane the trend reverses-the range of flammability decreases as molecular weight increases. The effect of branching on flammability limits was also investigated and the data indicate that the branched compounds have flammability ranges somewhat smaller than those of the corresponding normal paraffins. 2,2-Dimethylpropane and 2-methylpentane have smaller ranges than n-pentane and n-hexane, respectively (Figure 8). Figure 9 indicates that for st,raight-chain hydrocarbons the influence of unsaturation on flammability limits is small. There is, however, one outst,anding exception-ethene. The rich limit of ethene is greater than 610% of Ft.oichiometric compared with 272% for ethane.
36 0 400:
@
240
FLA1ZRZABILITY LIMITS
80t
I n consideration of the combustion characteristics of fuels it is found that physical and chemical influences may act to set limits on properties such as ignition and flame propagation. These limits are important in jet propulsion engines because conditions ma): arise where a fuel-air mixture cannot be ignited or, if ignited, is incapable of sustained burning. For these reasons considerable effort has been devoted to research on the flammability limits of fuels in order to determine the reasons for such limits and xvaj s tq avoid them in practical combustion devices A furl-air mixture capable of propagating a flame indefinitely aivay from, and in the absence of, a n ignition source is called a flammable mixture. Ignition must be provided initially, but a flammable mixture continues to propagate the flame even after the ignition source has been extinguished. If a flammable mixture of fuel and air is progressively diluted with either constituent a nonflammable mixture is eventually obtained. The so-called flammability limit is the borderline composition that distinguishes between mixtures of fuel and air that will propagate flame and
40‘
October 1954
Fuels-
I
, 0
40
80
---------L_L-.I-d
120
/ -
160 200 2 4 0 280 320 360 400 4 4 0 480 % STOICH FUEL
Figure 8. Effect of Molecular Weight o n Flanr~nability Limits (7) Additives. The subject of the effects of additives anti diluents on flammability limits has been of interest for many gears in the field of fire extinguishing research. The efforts in research on this problem have been devoted to the search for mateiials that narrow the flammable limits of a fuel and thus bring about its extinction. I n recent years the emphasis has been reversed in that the search is now directed toward additives that widen the flammable limits and bring about greater combustion stability in jet propulsion engines. I n one study ( 1 ) the effects of hydrogen, ethyl nitiate, chloro-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2153
picrin, hydrogen sulfide, carbon disulfide, and methyl bromide on the flammable limits of propane are described. Although the effects found in these tests were small it was concluded that ethyl nitrate and chloropicrin definitelv promote flame propagation at rich mixtures. Both chloropicrin and methyl bromide inhibit propagation a t lean mixtures. Carbon disulfide has a large inhibitory effect on flame propagation in lean mixtures. Although these compounds affect only lean and/or rich limits, methyl bromide effects the minimum pressure for flame propagation. I n this respect it was found that the addition of methyl bromide increases the minimum pressure for flame propagation.
1
700
~
-
$
0
6001
500
z
FALKANES
ETHENE 1.610)
k" ." 400- /\RICH
i
300
9
200 2 001
i:
j
r'
5U
LIWT
( 0
FLAMMASLE QECIIOL
o
0
0
LEAN LIMIT L
2 4 6 8 1 0 1 CARBON ATOMS I N UOLECULE
2
Figure 9. Effect of Number of Carbon Atoms on Flammability Limits (33)
I n studies of additives there is alwaj the question of whether the effect is chemical or diluting. Figule 10 (20, 24) illustrates the effect of several materials on flammability of gasoline. Methyl bromide and bromochloromethane are much more influential than nitrogen, carbon dioxide. and exhaust gas. For this reason it is believed that the former materials evert a chemical effect while the latter materials act merely as diluents.
b e> .o.
Figure 11. Variation of Quenching Distance with Equivalence Ratio for Prop a n e - . & - 3Iixtures at 1 Atmosphere ( 1 1 )
Figure 10. Effect of Diluents on Flammability Limits of Gasoline (20, 24)
Estimation of Flammability Limits. I n any systematic research investigation there is always the fond hope that sound relationships may be established to permit the prediction of additional data. For this reason efforts have been made to correlate flammable limit data for pure hydrocarbon fuels. The results of these efforts have been reasonably satisfactory. The following empirical equations nere developed from KACA pure hodrocarbon data iSJ'i:
L =
2154
1.87
x
AH X
106
M
where L = lean limit concentration, % by volume, AH net heat of cornbustion, B.t.u. per pound, aild JI = molecular weight.
R=L+-
143
IIP.~
where R = rich limit, % by volume. These equations may also be applied t o petroleum stocks a hen sufficient data are available t u estimate molecular w i g h t s . QUEYCHING DISTANCE
The normal propagation of flame may be diminished or even stopped by the action of cold surfaces which serve to remove heat and active radicals from the reaction zone I n a flame propagating in a tube or near a cold surface, there exists a region near the surface, called the dead space, in which no flame is visible. As the surface to volume ratio is increased-for example, by reducing the radius of a tube-there occurs a ciitical radius a t which flame can no longer propagate in the tube. This radius is called the quenching distance for the given system. The quenching distance is relatively independent of the material nature of the surface ( 1 0 )but does depend on the geometry of the system, the initial temperatwe and pressure of the surface of the mixture, and on the fuel type and concentration. It has been shovn ( 8 ) that flame velocitj can be related to the concentration of eel tain active radicals, principally H, 0, and OH. It would appear reasonable, then, that the quenching of a flame may be relatcd to the destruction of these radicals by the quenching surface. A theory based on this concept has been developed (31),and the quenching distance is related to the properties of the system by the equation.
L
!There d
"
' 2
= quenching distance
G = geometricfactor 4 = fraction of molecules present in gas phase JThich must react for flame to continue to propagate totalpressure number of fuel molecules per cc. partial pressure of one kind of active particle specific rate constant for reaction of active particles of one kind and fuel molecule Di = diffusion coefficient of act'ive particles of one kind into gas a t r e a d o n zone temperature and pressure
P
= Si = pi. = hi =
T h e geometric factor, G, depends on the shape of the slit or tube and may become fairly complicated for shapes other than cylinders or parallel plate.. Berlad and Potter ( 2 ) discuss the effect of geometry on the critical quenching distance. The rffect of pressure on quenching distance is pronounced. As a first approximat,ion, quenching distance varies inversely with pressure, but a closer examination of both the theory and experimental data shows that d m P - " where is a constant near unity which depends on fuel type and fuel-air ratio. I n all cases the quenching distance increases as pressure is decreased. This may be used to explain the change in flammability limit with pressure shonn in Figure 8. The flammability limits are essentially constant as pressure is reduced until a critical pressure is reached a t which the region of flammability decreases as prcssure decreases. It is in this region that the propagation is inhibited by quenching, and if the tube size is increased the fiammability limits remain constant to lower pressuree. The init,ial temperature of the mixture has much lcss effect on the quenching distance-a change in temperature from 80" to 545" F. changes the quenching distance of a stoichiometric propane-air flame from 0.82 to 0.57 inch (11). Quenching dist,ance is roughly proportional to the square root of the absolute t'emperature. For a given set of experimental conditions, the quenching distance varies with equivalence ratio and fuel type. A typical
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 10
-Turbine
Fuels-
FLAME VELOCITY
TABLE I. QUENCHISODISTANCES AT 200 M u . OF MERCURY PRESSURE (23) Fuel Methanol Propylene oxide Diethyl ether Isopentane Pentane 2-Pentene Heptane 2 ,2-Dimethylbutane Jlcthylcyclohexane
Equivalence Ratio 1.00 0.99 0.98 1.00 1.01
1.00
1.01
0.99 1.00
Quenching Distance, Cm. 0.53 0.56 0.66 0.71 0.74 0.96 0.97 0.99 1.11
curve of quenching distance as a function of equivalence ratio is shown in Figure 11 for a propane-air flame at 1 atmosphere pressure (11). There is a minimum quenching distance a t a fuelair mixture slightly richer than stoichiometric. Comparisons of different fuel types are often made at this minimum, although most of the data report quenching distance only at the stoichiometric fuel-air ratio. Unlike the extensive studies of the variation of flame velocity, flammability limits, and minimum ignition energy with fuel type, investigators of the effect of fuel type on quenching distance has been limited to only a few substances. Some of the available data are listed in Table I. The data were obtained by minimum electrode spacing measurements ( 2 5 ) . I n general, the variation of quenching distance with fuel t9pe is inverse to that of flame velocity. Such an inverse relationship would be predicted from theoretical considerations (0).
Considerable research effort has been devoted to studies of the effects of fuel composition on flame velocities. Unfortunately, however, variations in apparatus and photographic techniques among investigators have somewhat hindered development of knowledge concerning the effect of fuel type on rate of flame propagation. I n order to avoid these difficulties the KACA, in 1947, initiated a prograni t o study the flame velocities of many fuels i n a fixed apparatus and by uniform procedures. Some of this research is still in progress. Laminar Flame Velocity. Studies of laminar flame propagation were devoted to the measurement of flame velocities of pure hydrocarbons in quiescent mixtures of fuel and air ( I S , 14, 26). These measurements were made in a glass tube rhich has been described in detail (14). Flame velocities were measured with a Bunsen burner for those fuels with insufficient vapor pressure for satisfactory measurement in the tube (86). Measurements by this method gave values about 10% higher than values determined by the tube method. For each fuel the flame velocities were determined for a range of fuel-air ratios. At some specific fuel-air ratio a maximum flame velocity was found, and these maximum velocities, determined by both methods, are presented in Table 11. Molecular Structure. The effects of molecular structure on flame speed are illustrated in Figure 12,A for the normal aliphatic hydrocarbons. For the normal paraffins there is little change in the maximum flame velocity as the number of carbon atoms is increased from two to seven. However, the flame velocity of
TABLE 11. FLAME VELOCITIES OF HYDROCARBONS Fuel Methane Ethane Propane n-Butane n-Pentane n-Hexane n-Heptane n-Decane n-Hexadecane 2-Methylpropane 2,Z-Dimethylpropane Z-Methylbutane 2,2-Dimethylbutane 2,3-Dirnethylbutane 2,2,3-Trimethylbutane 2-Methylpentane 3-Methylpentane 2 ,2-Dimethylpentane 2,3-Dimethylpentane 2,4-Dim ethylpentane 3,3-D1methylpentane 2,2,4-Trimethylpentane Ethene Propene 1-Butene I-Pentene 1-Hexene I-Decene 2-Pentene 2-Metbyl-1-propene 2-Methyl-I-butene 3-Methyl-1 -butene 2-Ethvl-1-butene Z-Mel-l:3-pentadiene 2,3-Dimethyl-1,3-butadiene
October 1954
Max. Flame Velocity (Uj), Cm./Seo. 33.8 40.1 39.0 37.9 38.5 38.5 38.6 40. Za 40.7a 34.9 33.3 36.6 35.7 36.3 35.9 36.8 36.7 34.8 36.5 35.7 35.3 34.6 68.3 43.8 43.2 42.6 42.1
41,2a 43.1 37.6 39.0 41.5 39.3 39.6 40.6 39.2 37.2 73.8 54.5 68.0 46.5 45.6 46.6 50.7 51.8 44.2 46.0 39.0 41.6
Fuel a t iMax. Flame Velocity, VOl. % 9.96 6.28 4.54 3.62 2.92 2.51 2.26 1.40 0.92 3.48 2.85 2.89 2.43 2.45 2.15 2.46 2.48 2.13 2.22 2.17 2.13 1.90 7.40 5 04 3.87 3.07 2.67 1.55 3.38 3.83 3.12 3.11 2.65 2.80 2.62 . 3.83 3.36 6.04
4.34 4.27 3.47 3.37 3.33 3.43 3.45 2.83 3.41 2.78 2.86
Fuel Ethyne Propyne I-Butyne I-Pentyne I-Hexyne 2-Butyne 2-Pentyne 3-Hexyne
Max. Flame Velocit (Uj), Cm.,%ec.
Fuel at Max. Flame Velocity, 5701. yo
141 69.9 58.1 52.9 48.5 51.5 51.3 45.4
10.1 5.86 4.36 3.51 2.97 4.36 3.36 3.05
I-Butene-3-yne 3,3-Dimethyl-l-butyne 4-Methyl-1-pentyne
75.5 47.7 45.6
4.40 2.89 2.87
Cyclopropane Methyloyclopropane Ethylcyclopropane cis-l,2-Dimethylcyolopropane tran+l,2-Dimethylcyclopropane 2-Cyclopropylpropane 1,1,2-Trimethylcyclopropane 2-C yclopropylpropene 2-Cyclopropylbutane 2-Cyclopropyl-1-butene Cyolobutane Methylcyclobutane Ethylcyclobutane Isopropylcyclobutane Methyleneoyclobutane Spiropentane Cyclopentane Methylcyolopentane Cyclopentene Cyclohexane Methylcyclohexane Cyclohexene Benzene Toluene o-X y 1ene 1,2,4-Trimethylbensene n-Butylbenzene tert-Butylbenzene Diphenylmethane Tetralin trans-Decalin
4Q.5 49.2 47.5 46.5 46.2 42.7 43.5 44.9 39.8 42.5 56.6 44.6 44.7 39.1 51.5 59.9 37.3 36.0 40.4 38.7
4.97 3.93 3.40 3.16 3.19 2.66 2.62 2.85
2.37 3.88 3.18 2.62 2.66 3.55 3.46 3.16 2.75 3.48 2.65
40.3 44. ea 38.8a 34.4a 34.35 36.95 36,6a 33.P 36. Z a 33.95
2.94 2.39 2.12 1.87 1.66 1.61 1.39 I .til 1.58
2 51
...
Determined b y Bunaen burner method, which gives velocities 10% higher than tube method. Q
INDUSTRIAL AND ENGINEERING CHEMISTRY
2155
OCr:
:t c
c:c.c.c.c.: c c.:-c :c
fuels. Compountla included in hi? studics were acetone, acetaldehyde, benzaldehyde, diethj-1 ether, benm w , and carhoii disulfide. It was found in t,hese st,udics that: despite apparent t1ifi'erener:s in oxidation piopci,ticLs of the coinpoundp, all cornpounds change thc flame velocity in c?xact,ly the silnic
5
(1
0
c-c-c c
I:
c.2
2 ED
Figure 12.
methane is somewhat lower than that of the ot,her compounds in the series. The highest flame velociiies a-ere obtaincd n-ith the alkynes znd alkenes, particularly when the number of carbon atoms in the chain is small. For esamplc, the flame velocity of acetylene is approximately 3 1 / 2 times that, of ethane. A s t,he number of carbon atoms is increased in the chain all classes of compounds tend to approach the same maximum flrtnie velocitv. ,230,-
f f looki
\\
hi \ l V d F / S W 3 K - l G N I T l O N ENEqGY, mil 1 1 3 u I e s
Figure 13. \ ariation of Alaximum Flame Yelocity with XIinimum Spark Ignition Energy (23)
bXaximuin flame velocities for ki~ancficd cornpounds arc compared in Figure 12,B. 1 1 ased methyl substit,utioii decrca3es flame velocity in much the mmc manner as lengthening of thc primary chain. \Tithin any giver1 series of compounds containing the same number of carbon atoms in the molecule. increased uiisaturation increases the flame velocity. For csaniplc. a diene has a higher velocity than the corresponding alkcne but a lower velocity than the corresponding alkyne. I n the caee of the diencs, homver, the flame velocity is dcpendent upon the nature of unsaturation-that is, whether the doublebond3 are isolated, conjugated, or cumulated (Figurp 12,Cj . I n general. the cumulated double bond systcm produces the highest fl:ime veiocity whereas the ot,hcr systems, although loxyer in velocity. still produce fla.me velocit,ies higher than t,hoPe of the corresponcliiig alkenes. The flame velocities for cyclic compounds are givcn in Tsble 111. t,ogrther v i t h velocities for t,he corresponding aliphatic isomers, Thrse data indicatc that thrw- and four-membcred ring compounds hare velocit,ies slightly higher than those of the striiight-c!:ain zilkene isomers. The single case shown for a six-membered ring indimtep that the velocity is slightly lower than thnt of t'he c,otresponding dkene isomer. Spiropentanc, \\-hic,h has two three-membered rings joined together, has a fltinic velocity about 13% higher thxn t,he fastest isomeric pcntadieiie flame. From this example, it is indicntrd that thrcc-mcmberctl rings are perhaps more effective than double bonds. Additives. The effect of fuel additives on flamo velocity has not been investigated extensively: however, some information has beer! reported in the literature. Leason (19) studied the effects of sni:iil :tdditions (less t,han 3'%) of several materials t o
2156
quircnieiit. Thew was no pheriomcnun cauivalent to the effect of aldeliyde on reaction rate a t the low t,c?mpcr:rturr: oxidation oE hydrocarbons. On the basis of fuel antiknock studies it has iiwn expected that additives might, influence the flume vrlocities o l furls; howc.i csperinicntd st,udies ha,ve iiot confirmed this belief. Uric vcstigation (32) indicates that tet,r:ictliyllead has no effect on the flame velocities of iro-octane or n-butane. Above 300' C. the flame velocity of n-butme iq appreciably reduced by prcflame oxidation. xhieh in turn is suppressed by tehethyllead. Othcr investigators (18, 67) concludc that siiinll percentages of tctraethyllcad. ethyl nitrate, and iron pentacarbonyl do not, affect flame velocity. Turbulent Flame Velocity. The foregoing discussion has been concerned with laminar flames in which a smooth, discrete flame zone exists. Small flow disturbances map distort the flame surfacc and may influence the rate of flame propagation somewhat, but the discret'e reaction zone remains. If, holTever, the unburned gas flow is made t,urbulent,,a diffuse brushy flame results and the rate at, which the c,ombustible mixture is consumed increases greatly. Furthermore, this turbulent flame, unlike the laminar, is often accompanied by noise and rapid fluctuations of the Rame envelope. Flames in jet propulsion cngincs a r e t , u r b u l e n t i n 103nature. Consequently, flame vcllwity studies in the turhulcnt rcgime are more applivable to engine performance evaluations. However, very lit& research has been coiidurted t o determine the influence of molecular st,ruct u r e o n t u r b u l e n t flame velocity. The reason for this lwli of study may! of coursc, lie the difficulty of obtaining siitisi'iictory measurements. For the lamiiiar flame it ii .I .2 .4 .6.8 2 4 6 8 '0 Q J E N C H I N G DlSTA'.SE. pwsihk t,o dcfinc a Aamr velocity which, within reaFigure 14,. C-ariation of 8ollable limits, is a functiorl Critical Spark Ignition only of t8he fuel-air mixture Energ). with aria hi^^ Distance (23) and r e l a t , i v e l y independent of t,he cxperiment,al appamtus. This simplification has not been possible in turbulent flame propagation studies for it' has been found that numcrical values of turbulent flame ve1ac-itics depend not only on the experimental technique but a130 on the concept, of turbulent flames assumed by the investigator. I n spite of these rather imposing difficulties some progress 11:~s been i&%dc by investigators, and it may be anticipated that research now in progress will lead to an understanding of the cffccts of molerulnr struct.uro on turbulent flame velocit,y. One investigation conducted by Williams and Bollinger ( 3 7 ) confirms this belief. I n studies with propane, ethylene, and acetylene-air mixtures they found that t,he turbulent flamc: velocities can t i c
RIolecular Structure and RIaximum Flame Velocity
CT
INDUSTRIAL AND ENGINEERING CHEMISTRY
,
Vol. 46, No. 10
-Turbine
Fuels-
This relationship was applied to the data of Metzler ( 2 3 ) for
TABLE111. FLAXE VELOCITIES OF CYCLOALKANES AND CORRESPOSDING ALKENES Cycloalkanes Flame Velooity, Cni./Sec. Structuie C 47.3 kJC
Alkenes Structure
c-c=c
c-e-c
c
cc;:
‘ I c=c-c-c-c
42.7
Flame Velocity Cm./Seo: 43.8
b
A c-c
c=c-c-c-c
I
c
41
42.6
C
44 6
c=c-c-cI
41.5
C
‘ 6 5
I
c&-c-c c-c-c=c=c c=c-c-c=c
C
A
Y‘
39.0 51.8 48.6
C
51.5
b-c=c
C=
45.0
38.7
C=C-C-C-C-C
42.1
59.9
c-c=c=c-c
50.7
c
/ \
c c cI c/
y
c-c
‘d /\
C-C
correlated by an expression showing direct proportionality with laminar flame velocity. This relat,ionship is applicable over a range of Reynolds numbers between 3000 and 40,000. Interrelations of Combustion Properties. The flame represents a zone in which a rapid chemical reaction takes place. It appears reasonable that properties of the flame-ignition, quenching distance, and velocity--may be interrelated. I t has been shown (23), for example, that flame velocity and critical ignition energy can be related by an equation of the form
HGY)
Uf(msx.)
where lij(mbx.) = maximum flame velocity and H(min.) = minimum critical ignition energy. -4 plot of the data ($3)is shown in Figure 13 illustrating this correlation for a number of fuels. Similarly, it has been shown (20, 23) that critical ignition energy and quenching distance are related (Figure 14). Metzler (23) also develops the equation
€I = 6.36 X
SMOKE FORMATIOY
Most of the topics discussed in this paper are related t o maintaining a stable and efficient flame. There are other fuel properties that are important in the combustion process but that are not directly related to the efficiency of the process. One of these properties is the smoking tendency of the fuel. Long recognized as a problem in industrial combustion systems, smoke and carbon formation are also undesirable in propulsion systems since the deposition of smoke or carbon may affect the operat:on of certain essential parts of the combustion chamber. While the over-all fuel-air ratio in a turbojet combustor is much too lean to give smoke if the fuel and air were homogeneously mixed, the existence of smoke and carbon indicates that regions do exist in which there is little air. Since such regions may burn as diffusion flames, much of the research on smoke formation has been performed on simple diffusion flames. The conditions necessary to give smoke and, subsequently, the quantity of smoke formed are functions of the burner geometry, flow variables, initial conditions of temperature and pressure and fuel type. As with other properties, in fuels research the dependence oi smoke formation on the fuel type and on the initial conditions is the principal interest. Some of the other factors are discussed by Clark ( 5 ) . Smoking tendency may be determined from the maximum fuel flow which can burn without smoke iu a simple diffusion flame. This fuel flow is related to flame height, which has been used by other investigators (18) as a criterion of relative smoking tendency. The greater the fuel flow or flame height that can exist before smoke appears, the less is the smoking tendency of the particular fuels.
TABLE Iv.
CORRELlTION O F
rrd2N
(Evaluation of HU’ (Tb
COMBUSTION
DAT.4
= constant with data of Jletzler, 29
Fuel Ethane n-Pentane n-Hexane n-Heptane Isopentane 2 2-Dimethylbutane dsolohexane Benzene Ethylene 2-Pentene
Conetant 4.1 4.6 4.8 4.5 5.0 5.9 4.7 4.6 2.3 3.0
3.6 1.2 2.8 3.7 2.6 3.7 2.3
d1.7g
where d = quenching distance in cm. A theory has been proposed (20) that relates flame velocity, quenching distance, and critical ignition energy with other properties of the fuel-air mixture. The folloxing simple equation results:
where H = critical ignition energy U, = flame velocity Tb - Tu = temperature rise on combustion d = quenching distance 1 = viscoeity
October 1954
constant but is approximately equal to 4 rather than 1. Nevertheless, with the use of a single empirical constant it is possible to use this relationship to correlate combustion data for most hydrocarbons; the major exceptions are nonhydrocarbons, as well as ethylene and acetylene.
40.5
c-c=c
C
a number of stoichiometric fuel-air mixtures. It was found (Table IV) that the left-hand side of the equation is relatively
While temperature appears to have little or no effect on the smoking tendency of a diffusion flame (68), pressure has a very pronounced effect. Figure 15 (SO) illustrates for two fuels the sharp increase in smoking tendency as pressure increases particularly in the range of 0.5 to 4 atmospheres; the curves level off a t higher pressures. The curves show a reciprocal relationship, and the product of smoke-free fuel flow and pressure is reasonably constant in the region of pressure studied. In a diffusion flame, fuel-air ratio is, of course, meaningless, but since the zones from which smoke is evolved in the combustor may be partially aerated, the smoking tendency depends on the local fuel-air ratio. Clark (6) showed that a t a given set of conditions the fuel-air ratio at which smoke appears is r e r y criti-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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under the conditions of operation and the difference in combustion efficiency increases as the inlet air velocity increases. A similar trend is shown for n-hexane and 2,3-dimethylbutane. The temperature rise through an aircraft gas turbine combustor is plotted against energy input in B.t.u. per pound of air burned in Figure 18,A, The plot shows the temperature rise that obtained in the combustor with three different fuels if the combustor is operated a t a high inlet air velocity. Benzene allom a higher temperature rise than heptane and it, in turn, produces a higher temperature rise than iso-octane. However, when the combustor is operated a t a low inlet air velocity the three fuels TOG exhibit essentially the same temperature rise, as shown in Figure 18,B. Thus the very limited data indicate t8hat',under some severe operating conditions, benzene shows a higher heat release than a normal paraffin and in turn the normal paraffin shows a higher heat release than a branched paraffin. However, if the combustor is operated a t conditions that are favorable to combustion, such a8 high inlet air temperature and low inlet air velocity, the molecular , 6 9 , C L 4 structure of hydrocarbon fuels has no U96CU n-c:,5 apparent effect on combustor performFigure 15. Effect of Pressure Figure 16. C o m b u s t i o n Figure 17. Relative ance. Another combustor performance on Smoke-Free Fuel Flow Efficiency us. Reference S m o k i n g Tendency of parameter that seems to be influenced by (30) Velocity Hydrocarbons (29) fuel type is t'hc rich blowout point. Combustor inlet conditions, The curves of Figure 18 terminate a t low temp. and pressure t,he rich blowout point for t,he fuels. Dat'a that show temperature rise related to carbon to hydrogen ratio but a closer examination, attained through a combustor as a function of inlet air flow are particularly of such isomers as butane and isobutane, shows that shown in Figure 19. As the air velocity is increased the temcarbon t o hydrogen ratio alone cannot explain all of the variaperature rise before blowout decreases. However, benzene givea tions. On the basis of spectroscopic studies it has been suggested ( f 2 ) that in a diffosion flame the fuel is cracked prior to combustion. This consideration has been used (89) to suggest that the smoking tendency of a fuel mag be related to the stability of the carbon skeleton of the fuel molecule. If the hyI :@adrogen atoms are removed, leaving a BEEIZENE 830r stable carbon skeleton that can later hEC-AYE polymerize to form smoke, the fuel may 630have a high smoking tendency. If, on 8 50-3CT'UE the other hand, the carbon skeleton is F AOObroken into small fragments during the preflame cracking the fuel has less roar 20Ok -I-1 -L , L,-~L--L.--L-..-.~ .-_1. tendency to smoke. There appears to 60 4C Pi0 303 380 4% 540 IO@ 180 260 340 4 h C 500 583 be some correlation between smoking ENERGY lU?UT 3 f~ f i b o r EhERGY INPUT, B . ' u / l b o r B. LOW INLET AIR VELOCITY tendency and carbon-to-carbon bond A. HIGH INLET AIR VELOCITY strengths, but the data ( 2 9 ) are inFigure 18. T e m p e r a t u r e Rise of Fuels sufficient t o verify a definite conclusion.
cal. This fuel-air ratio appears to be relatively insensitive to external variables, excluding those which can add additional air to the flame. Fuel type, however, has a strong influence on smoking tendency. A comprehensive study of many fuels was reported by Schalla and McDonald ( 2 s ) . The results for a series of compounds are shown in Figure 17. In general, the smoking tendency for hydrocarbon types varies as follows: aromatics > alkynes > mono-olefins > isoparaffins > n-paraffins. As a first approximation it would appear that the smoking tendency is d l l
11
,
I
l i
i
i
LC
F
-
FUEL STRUCTURE AND COMBUSTOR PERFORM4NCE
Only very limited data are available a t the present time on the effect of fuel properties on gas turbine combustor performance. I n general, fuel volatility and viicositv are more likelv to influence the behavior of an aircraft gas turbine cornbustor than fuel molecular structure. However the limited information available indicates that under adverse operating conditions the molecular Rtructure of fuels may influence combustor performance. Adverse conditions include high inlet air velocity and low inlet air pressure. The trends are illustrated in Figure 16 in which combustion efficiency is plotted against relative inlet air velocity. Heptane has a higher combustion efficiency than 2,2,4-tiimethylpentane ~
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a higher temperature rise than heptane before blowout, and the heptanp gives a higher temperature rise than Iso-octane a t each relative air flo~v. Combustion properties are shown in Table V for the five fuels for which combustor performance data are presented in this paper. It appeals that a classification by burning velocities results in ranking the fuels in the same order as their performance in the combustor. This implicv that. under conditions in the burner that are conducive t o poor combustion. the fuel with the fastest fundamental burning velocity xi11 give the bcst performance. Ho\5-ever, the burning velocities of the fuels differ bl' such a small amount that it is difficult to understand how this property could influence combustor performance to any noticeable extent.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 10
-Turbine
Fuels-
(6) Coward, H. F., and Jones, G. W., U. S.
Bur. Mines, Bull. 503, 1952.
TABLE V. FUNDAMEKTAL COMBUSTION PROPERTIES OF FUELS Critical Ignition Energy
(7) DiPiazza, James T., Natl. Advisory
Comm. Aeronaut., Research Mem. E51C28, 1951. (8) Dugger, G. Ill., and Simon, D. RI., Cm./Sec: 9 = 1 (amin. (Rich - Lean) F. Fourth Symposium (International) Benzene 40.7 0 . 2 9 0.17 293 1097 on C o m b u s t i o n , p p . 336-45, 38.6 0.32 0.18 Heptane 397 477 Williams and Wilkins, Baltimore, 33.6 0.50 312 837 2,2,4-Trimethylpentane 1953. 38.5 0 : is 350 501 Hexane ., 296 790 (9) Friedman, R., Third SsmDosiuin on 36.3 O:k3 2.3-Dimethvlbutane Combustion and Flame and Explos i o n P h e n o m e n a . DO. 110-20, Williams and Wilkins; +Baltimore, 1949. (10) Friedman, Raymond, and Johnston, W. C., J . A p p l . Phys., 21, 791-6 (1960). (11) Friedman, Raymond, and Johnston, W. C., Westinghouse Research Rept. R-94451-4-B, 1949. (12) Gaydon, A. G., and Wolfhard, H. G., "Flames, Their Structure, Radiation and Temperature," Chapman and Hall, London, 1953. (13) Gerstein, Melvin, Levine, Oscar, and M'ong, Edgar L., IND. ENG.CHEU,,43. 2770 (1961). (14) Gerstein, Melvin, Levine, Oscar, and Wong, Edgar L., J . A m . Chem. SOC.,73, 418 (1951). Hum, R. W., and Smith, H. RI., TND. ENG.CHEM.,43, 2788 BENZENE (1951). :.HEPTANE Jackson, Joseph L., Natl. Advisory Comm. Aeronaut., Research Mem. E50J10,1950. Jackson, Joseph L., aiid Brokaw, Richard S..I b i d . , E54B19, 4001 \ 150-OCTANE 1954. L Jost, Wilhelm, "Explosion and Combustion Processes in 44 52 6C 68 76 84 92 100 RELATIVE INLET LNR '/EL@:ITY, Gases," 11IcGraa-Hill, New York, 1946. Leason, D. B., Div. Aeronaut., Australian. Council Sei. and Figure 19. Temperature Rise a t Rich Ind. Research (Melbourne), Rept. E. 6 2 , Xovember 1948. Blowout Lewis, Bernard, and Von Elbe. Guenther, "Combustion Flames, and Explosions of Gases," Academic Press, Sew York, 1951. Livengood, J. C., and Leary, W. A , , IND. EKG.CHEM.,43, 2797 (1961). It is also indicated in Table V that. in a qualitative manner, Metzler, Allen J., Satl. Advisory Comm. Aeronaut., Research both critical ignition energy and quenching distance 11 ill rank Mem. E52F27, 1952. the fuels in the inverse order as they ranked in combustor perIbid..E53H31.1953. formance. Benzene, with the smallest ignition energy and the RIoran, H. E., Jr., and Bertschy, A. W., Saval Research Lab., Rept. 4121, Feb. 25, 1953. smallest quenching distance, gave the best combustor perforinLIullins, B. P., National Gas Turbine Establishment, Studies ance. and heptane and 2,2,4-trimethylpentane follow in order. on the Spontaneous Ignition of Fuels Injected into a Hot At the present time it is not apparent whether flame speed, Air Stream, Pt. I-VIII, 1900. ignition energy, or quenching distance may be the controlling Reynolds, Thaine W,, and Ebersole, Earl R., Natl. Advisory Comm. rleronaut., Tech. Note 1609,1948. step in the combustor performance. It may be t h a t all three measSachsse. H., and Bartholome, E., 2. Electrochem., 53, 183-90 urements are controlled by one mechanism such 2s diffwion of (1949). active particles or heat transfer and, conscquently, actually Schalla, Rose I,., Katl. Advisory Comm. Aeronaut., Research only one process is being measured. These qualitative relationMem. E53J12, 1953. Schalla, Rose L., and McDonald, Glen E., Ibid., E52122, 1952. ships have been shown for only three fuels in one series of comI b i d . , E53E05,1953. parisons and for two fuels in another comparison. I l u c h reSimon, Dorothy bl.,and associates, Fourth Symposium (Intersearch must be done before generalizations can be dran-n. It national) on Combustion, pp. 126-38, Williams and Wilkins, seems reasonable to expect that future research will clarify the Baltimore, 1953. ' Smittenberg, J., and Kooijman, P.L., Rec. trav. chi?%, 59, effects and the reasons for the effects of fuel molecular structure 593 (1940). on gas turbine combustor performance. Spakowski, Adolph E., Satl. Advisory Comm. Aeronaut., Research Nem. E52H15,1952. LITERATURE CITED Swett, Clyde C., Jr., Ibid.,E51512, 1951. Swett, Clyde, C., Jr., Third Symposium on Combustion and Belles, Frank E., and Simon, Dorothy nl., Satl. Advisory Flame and Explosion Phenomena, pp. 353-61, Williams Comm. Aeronaut., Research Mem. E53129, 1953. and Wilkins, Baltimore, 1949. Berlad, A. L., and Potter, A. E., Jr., Ibid., E54C05, 1954. Wagner, Paul, and Dugger, Gordon L., J . A m . Chen. SOC.,t o be Blanc, SI. V., Guest, P. G., and associates, Third Symposium published. on Combustion and Flame and Explosion Phenomena, pp. Williams. David T.. and Bollinner. Lowell M.. Natl. Advisorv 363-7, Williams and Wilkins, Baltimore, 1949. Comni. Aeronaut., Rept. 932, 1949 (formerly Tech. Sotk (4) Calcote, H. F., Gregory, C. A,, Jr., and King, I. R., presented 1707, 1948). at 119th Meeting, AM. CHEM.SOC., Cleveland, Ohio. (5) Clark, Thomas P., Kat]. Advisory Comm. Aeronaut., Research RECEIVED for review June 9, 1954. ACCEPTBDJuly 23, 1954. Mem. E52G24,1952. Vzl%$
October 1954
Millijoulka (a = 1 (amin. 0.55 0.22 0.70 0.25 1 . 3 8 0.28 0.50 0.25 1.64 0.25
Qllenching Distance, Cm.
Flammability Range
% StoiohioAetric,
Spontaneous Ignition Temp., 0
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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