2490
Vol. 44, No. 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
was essentially no flavor loss to the kettle as determined by an organoleptic appraisal of the kettle residues.
solution shows the still to be well suited to the requirements of essence concentration. l i t e r a t u r e Cited
Conclusion
Although it has been reported that the Oldershaw column in its original form ia unsuitable for distillation of aqueous solutions because of the high surface tension of water (W), the modifications used in the present experiments proved satisfactory for such distillations. The evaluation of the operating characteristics of the 40-plate continuous column through the use of the phenol test solution and the recovery measurements of a strawberry flavor
(1) Christensen, B. E., Wong, R., and Facer, J. F., ISD.ENG.CHEM., A N A L . ED.,12, 364 (1940). (2) Collins, I”. C . , and Lantz, V., Ibid., 18, 673 (1946). (3) Dimick, K., and Makower, B., Food Technol., 5, 517 (1951). (4) Helm, E., and Trolle, B., Wallsrstein Lab. Communs., 9, 181
(1946).
(5) Horsley, L. H., Anal. Chem., 19, 508 (1947). (6) Oldershaw, C. F., IND.ENG.CHEX, ANAL. ED.,13, 265 (1941). R ~ C E I V for E Dreview February 11, 1952.
ACCBPTED May 23, 1952.
Flame Velocities of liquid Hydrocarbons
development
R. E. ALBRIGHT, D. P. HEATH,
AND
R. H. THENA
Socony-Vacuum Laborafories, Paulsboro, N. J.
HE evaluation of fuels for gas turbine engines requires an experimental or laboratory test method Yhich can be made to correlate with performance in full scale equipment. Over a period of several years many methods have been devised for the evaluation of conventional aviation, motor, and Diesel fuels. Presumably, a similar evolution of methods \vi11 grow out of gas turbine fuel studies. Present knowledge indicates that the velocity of flame propagation of a fuel is an important rharacteristic of gas turbine fuels. Correlations between flame stability (blowout limits) and relative flame velocities have been obtained. In addition to the basic fuel flame propagation rate, the minimum ignition energy
T
requirements, flame temperature characteristics, and the quenching effect of relatively cold solid surfaces upon laminar gas flames have been investigated. On the basis of this work it appears that some combination of these fuel characteristics may provide a more complete expression of fuel combustion quality. The present discussion is limited to the study of the flame velocities of normally liquid hydrocarbons. The data presented are the results of an investigation initiated at the SoconyVacuum Laboratories several years ago as part of the general studies on the combustion characteristics of aircraft gas turbine fuels, Only a limited amount of information was available in the literature on the flame velocities of normally liquid hydro-
MIXER a VAPOR I 2 0R
AIR
I
I
TEAM I N L E T
-
. I C E TOWER CRITICAL-FLOW ORIFICES
a 3
SUPPLY
,-+clh
L
AIR
FUE
FLASH BACK
I
RESERVOIR HEATER AIR SURGE TANK
STEAM
OUTLET
____ Figure 1.
Diagram of Flame Speed Apparaiur
October 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
2491
feet8 on flame shape a t the Port ( l a ) . A spark Coil, connected to electrodes above the burner, was used for ignition. The electrodes were placed a t a distance from the burner port so as to avoid flow and temperature disturbances. An exhaust chamber containing baffles was used to minimize outside air currents. For Operation a t below atmos heric pressure the exhaust gases were evacuated through the p g n t vacuum system. The burner exhaust chamber was sealed sufficiently to permit operation a t a pressure of 0.33 atmosphere. The flame a t the burner port was photographed through an optically flat window in the side of the exhaust chamber. I n addition to the direct Dhotofraph of the luminous flame, means were pr&ded STANDARD MONARCH or photographing the shadow formed by the heated FUEL NOZZLE 3/32" HOLE, gases in the inner cone refracting light from the point light source provided by a 2-watt concentrated arc lamp. A Speed Graphic focal plane I" HEX. COLD ROLLED shutter camera, with the lens removed, and the STEEL BAR point light source were placed 12 and 24 inches, respectively, from the axis of the burner. The diAIR I N L E T I rect photograph magnified the flames 1.40 times while the magnification obtained from the shadow I /4" PYREX CAP1 LLARY photographs was 1.62. TUBE An angle measurement was required to determine the flame velocity, and a goniometer was provided for this purpose. Flame negatives were mounted on the goniometer face which was adjusted until the cross hair in a telescope &xed the desired tangent. The differences between two such I /4" O.D. TUBE BRASS I /4" B R A S S settings on opposite slopes of the flame constituted COMPRESS ION F I T T I NG a determination of the desired angle. This method of measuring cone angles was similar to that used by Smith and Pickering (18). An average of eight negatives was obtained for each run, covering the fuel-air ratio range from lean blowoff to the richest mixture a t which a stable flame Figure 2. Detail of Mixing and Atomizing Nozzle could be maintained on the burner port. The flame velocity value for each negative-calculated by the procedure given behw-was plotted against the fuel-air ratio. The maximum flame velocity, which were independent of the method used. Therefore, the flame occurred a t mixtures slightly richer than stoichiometric proporvelocities obtained by the angle method and a total area method tions, was determined as the peak of the resulting curve. The are presented. Twenty-five fuels were covered in the investiflame velocities reported herein are all maximum values obgation to determine the effect of burner variables, hydrocarbon tained for the particular operating conditions studied. structure, and chain length on flame velocity. Development of Measuring Experimental Method Technique. The methods discussed herein for calculating flame Description of Apparatus. The method employed in evaluatvelocities from the photographs of ing the velocity of flame propagation of liquid fuels required a Bunsen flames are based on the vaporizing system as well as a measuring device for small liquid volume rate of flow either through flows. Basically the apparatus was designed to produce air-fuel a small element of the flame survapor mixture temperatures from 212' to 360' F., air flows from 1 face or through the entire flame to 5 pounds per hour, fuel flows stoichiometrically comparable to surface. Since an equilibrium the air flows, and mixture pressures from 0.33 to 1 atmosphere. flame exists above a Bunsen burner The fuel metering device and vaporizing chamber were adapted port, the mean velocity of flame from a similar apparatus developed by Calcote (3). propagation is equal and opposite A diagram of the equipment and a detailed sketch of the to the velocity of mixture apmixing nozzle are shown in Figures 1and 2, respectively. proach to the flame front, The Plant air was regulated through a surge drum into an ice tower velocity of mixture flow may be wherein constant humidity to the vaporizer w&s obtained. The air was then metered, controlled, and mixed with the fuel in the expressed as the volumetric rate M,xturr F,ou steam-jacketed vaporizer. Fuel from a supply chamber was of mixture flow divided by the fed into a true bore glass tube where a rising column of mercury flame area. Various investigaforced the fuel into a nozzle arrangement located in the mixing tors (4, 13, I T , 18) have atand vaporizing chamber. Fuel entered the nozzle head through a borosilicate glass capillary tube drawn to a fine point which was tempted to determine flame areas centered in the 3/s2-inch hole of a standard fuel nozzle. Air enfrom photographs by the graphtered the body of the nozzle section from the side and passed ical integration of flame surfaces through the nozzle hea'd and across the fuel tube, breaking the F~~~~~3. crorrSection and by the construction of cones liquid into fine particles which were vaporized quickly in the heated chamber. of Mixture Flow and assumed to have areas equal The resultant mixture from the vaporizer was heated in an Flame Configuration to the flame areas. The latter extended length of coiled copper tubing to obtain a uniform fuel methods may be developed by vapor-air mixture. This mixture passed to a jacketed, seamless measurements of cone heights, apex angles, or other propsteel, smooth bored burner tube held a t the same temperature as the heater section by a countercurrent steam system. A flasherties. The following two methods were used in this investiback screen a t the base of the tube was included for safety purgation: poses, and the tube and steam jacket were held in lace by packing glands to eliminate any distortion of the tuge that might ' 1. An angle method based on the general Michelson relahave occurred if welded joints were used. The dimensions of the tionship, V p = V sin CY, where V Fand V are the flame velocity burner, 36 inches long with an inside diameter of 0.62 inch, were sufficient to produce laminar flow, having a minimum of wall efand mixture velocity, respectively, a t the chosen radius for
carbons, and it was desirable to obtain a consistent set of data a wide range of hydrocarbon structures. several ods for determining flame velocities were investigated and the well-known angle method which required the measurement of the slope angle of an element of the cone at various burner radii was used. Relative flame velocities have been used for correlation with other fuel properties and in general the relative values
Id
2492
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table 1. Test Conditions Pressure, Air inches flow of mer- rate, Temp., cury Ib./ Fuel-air ratio hour Fuel O F. abs. 3.15 0.077 360 30 n-Pentane 0.077 360 30 3.15 n-Hexane 0.073 30 2.61 220 0.074 30 3.15 220 0.074 30 3.61 220 0.071 3.15 30 310 0.069 3.15 260 30 0.074 25 2.63 360 0.089 2.10 20 360 0.084 3.15 25 360 0.086 3.15 20 360 0.072 30 3.15 360 n-Heptane 0.077 30 3.15 310 3.15 0.070 30 260 3.15 30 0,068 220 2.63 25 0.081 360 20 2.10 0.088 360 3.15 25 0,098 360 0.099 3.15 20 360 0.074 30 3.15 360 n-Octane 3.15 0,074 30 n-Decane 360 0.075 3.15 30 360 1-Hexene 0.071 3.15 360 30 1-Heptene 0.073 3.15 30 360 1-Octene 0.076 3.15 30 360 1-Deoene 30 3.15 0.076 360 Isopentane 3.15 0.075 30 360 Iso-octane 0 ,076 30 3 . 1 5 310 0.076 30 3.15 260 0.083 25 2.63 360 2.10 0.078 20 360 3.15 0.085 25 360 3.15 0.093 20 360 3.16 0,081 30 Diisobutylene 360 2.61 0.085 30 220 0,077 30 3.15 220 0.076 30 3.61 220 0 ,081 30 3.15 310 0,071 30 3.15 260
Summary of Results
Luminous Cone Maximum Flame Velocity, Feet/Sec. Angle Method Tangent Total Position mea 0.707R 0.48R method 2.48 2.16 2.40 2.34 2.18 2.29 1.81 1.57 1.81 1.62 1.76 1.82 1.85 1.66 l.,SZ 2.06 1.85 1.UI 1.81 1.81 1.73 2.22 1.92 2.06 1.90 2.06 2.03 2.32 2.15 2.26 2.22 2.08 2.16 2.29 2.05 2.29 2.12 1.92 2.04 1.95 2.05 1,82 1.93 1.63 l.?? 2.47 2.21 2.45 2,28 2.34 2.15 2.41 2.30 2.62 2.50 2.35 2.20 z.*a 2.49 2.30 2.77 2.68 2.49 2.70 2.50 2.68 2.44 2.37 2.50 2.47 2.31 2.48 00 2.24 1.11 2.09 1.90 2.00 1.77 1.83 1.70 1.63 1.73 1.60 ?j: 2.26 1,92 2.01 2.37 1.58 1.74 1.37 1.64 1.58 1.97 1.97 1.69 1.73 1 . 50 1.86 1.50 1,72 1.64 1.44 1.92 1 95 1.63 1.77 1,84 1.56
$::
2.49
Temp Fuel Benzene
Toluene
Ethylbenzene Cumene (isopropylbenzene) Cyclohexane
0
:7
illethylcyclohexane Acetone Methanol Ethanol Octanol Decanol Isobutylalcohol Thiophene
::;: ::!:
which LY, the angle between the flow line and the flame surface, is determined. 2. A total-area method, wherein the mean flame velocity is equal to the volume rate of flow of the mixture divided by the surface area of the cone formed by the combustion zone. Smith and Pickering (IO,18), and Garner, Long, and Ashforth (8,6,8) have determined flame velocities using the angle measurement a t points along the luminous and shadow cone surfaces. Figure 3, the analytical development of this method, shows that the mean mixture flow velocity, V M ,for laminar flow occurB a t a distance from the burner center line of 0.707 times the radius
Luminous Cone
Figure 4.
Vol. 44, No. 10
F."
360 360 360 360 360 360 220 220 220 310 260 360 360 360 360 360 360 220 220 220 360 360 360 360 360 360 360 360 360 360 360 360 360
Test Conditions Pressure, Air inches flow of mer- rate cury lb./' Fuel-air abs. hour ratio 30 3.15 0.082 25 2.63 0.096 20 2.10 0,083 25 3.15 0.084 20 3.15 0.080 30 3.15 0.081 2.61 0.085 30 3.15 0.087 30 3.61 0.076 30 3.15 0.082 30 3.15 0.076 30 2.63 0.076 25 2.10 0.082 20 3.15 0.109 25 3.15 20 0.100 30 3.15 0.081 30 3.15 0.092 30 2.61 0.082 30 3.15 0.086 30 3.61 0.086 30 3.15 0.092 25 2.63 0.096 0.092 20 2.10 0.089 25 3.15 20 3.15 0,085 30 3.15 0.085
Luminous Cone Maximum Flame Velocity, Feet/Sec. Angle Method Tangent Total Position area 0.707R 1.48R method 2.13 2.42 2.34 2.34 2.52 2.58 2.69 2.96 2.91 2.28 2.61 2.58 2.47 2.56 2.42 1.74 2.03 2.04 1.52 1.78 1.70 1.60 1.43 1.69 1.47 1.60 1.70 1.56 1.92 1.75 1.48 1.81 1.82 2.01 2.32 2.26 2.10 2.22 2.30 2.00 2.18 2.17 2.14 2.32 2.48 2.16 2.44 2.56 1.85 2.20 2.02 1.85 1.61 1.84 1.86 1.63 1.86 1.60 1.38 1.56 2.31 2.20 2.26 2.41 2.18 2.47 2.41 2.21 2.51 2.21 2.16 2.21 2.19 2.42 2.44
__
2.13
2.32
2.27
30 30 30 30 30 30
3.15 3.15 3.15 3.15 3.15 3.15
0.101 0.224 0.141 0.099 0,099 0,096
2.23 2.38 2.44 2.46 2.06 2.21
2.54 2.54 2.66 2.56 2.17 2.44
2.44 2.52 2.60 2.52 2.15 2.36
30
3.15
0.100
2.16
2.26
2.26
of the burner. Assuming vertical flow of the mixture flow lines, angle measurements were made at the 0.707R point on the surface of luminous and shadow conee. Figure 4 presents prints of the luminous and shadow cones used. The shadow cone surface considered was the boundary between the light strip and the dark zone, outer edge of the light strip, which represents the inner edge of the shadow cone. The outer edge of the luminous cone was used for the angle measurements. Angle measurements at other points along the luminous cone surface gave flame velocity values that increased with distance above the burner port. The V.V determinations from the shadow
Shadowsreph
Photographs of n-Heptane-Air Flame
2493
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1952
cone decreased from 0.707R to about 0.5R and then increased as the cone tip was approached similar to that obtained for theluminous cone. Curves of flame velocity obtained from anglemeasurements a t different points on the cone surface are presented in Figure 5 for a n-heptane-air flame, The curve of the values from the shadow cone was appreciably different from that obtained by Garner, Long, and Ashforth (6) who reported lower V F values from the shadow cone than from the luminous cone from about 0.4R to the cone tip. The angle measurement near the tip of the shadow cone was difficult to obtain and this probably accounts for the spread in values shown for the higher cone positions in Figure 5. Considering the similarity in cone shape (Figure 4), values of flame velocity from both cones would be expected t o increase as the tip was approached. Although the shadow curve deviates from the luminous curve there exists a region, approximately 0.4R t o 0.5R, where the same flame velocity value is obtained from both cones. Garner et al. (6) found approximately equal Vp values could be obtained from either cone a t the 0.4R position for a benzene-air flame while similar results were obtained in the present investigation by using the cone angle at the 0.48Rposition. The errors involved in measuring the angle on the surface of either the shadow or luminous cones, assuming the mixture flow lines do not bend at the flame surface, were recognized by the authors. Previous investigators-Wohl (19), Grove et a1. (11), Linnett and Hoare ( I 4 ) , and Anderson and Fein (1)-have discussed the use of the shadow cone and the errors involved in using the inner edge of the cone surface for angle or area measurements. Other work, particularly that of Lewis and von Elbe (1.3) and Ashforth (a), showed that inaccurate results are obtained for flame velocities calculated from angles measured along the luminous cone surface when the bending of the mixture flow lines is neglected. The flow patterns observed by Lewis and von Elbe (1.3) as a result of photographing stroboscopically illuminated dust particles carried in the gas stream showed that the lines deviated from the vertical before reaching the luminous cone surface. It is obvious therefore from the previous studies that the bending of the flow lines-i.e., the angle at which each line passes through the cone surface-must be determined t o obtain a true flame velocity by the angle method at points along the cone surface, Even though the flame velocities determined a t the 0.48R position, because of the bending of flow lines, are lower than the true flame velocities, such measurements make it possible to compare the results of investigators using either the luminous or shadow cones, Also, these results check very well with the mean flame velocities determined by the total area method. The computation of flame velocity by the total area method from luminous cones was based on a method similar to that used by Corsiglia (4). The surface area, 8, of the flame, assuming a conical surface of revolution, was obtained from the relation S =
?r
A l/h
The cross-sectional cone area, A , was determined by measuring the luminous cone surface with a planimeter. 1/11 is the ratio of the cone slant height to cone height. The mean flame velocity was calculated by dividing the volume rate of flow of the fuel-air mixture by the surface area of the flame. Determination of the surface area was difficult for the longer and distorted flames obtained at the lean or rich fuel-air mixture limits. However, in the region of the stoichiometric fuel-air ratio the flames become very sharp and very nearly conical in shape. Table I presents the maximum flame velocities obtained under the various operating conditions calculated by the two angle methods (0.707R and 0.48R) and the total area method The maximum flame velocities by the area method were, on the average, 0.06 foot per second lower than the values obtained by the angle method at the 0.48R position. Of the 72 runs listed, six gave total area values more than 0.14 foot per second lower than the
angle method. The precision of the flame velocity values by either the angle or total area method was approximately h0.05 foot per second. The values determined a t the 0.707R position are included in Table I as a matter of interest, although measurements at this position are subject to considerable criticism (IS). Inasmuch as there are very few data in the literature on the flame velocities of liquid fuels and those that are available have been obtained under widely different operating conditions and measuring techniques, it is desirable to include the data at 0.707R for comparison with other work that may be published later 3.4
SHADOW CONE @A0 '
d 2.P
I' I I I
L U M l N OUS
'YNE I
I I I 0.6 0.8 1.o 0 0.2 0.4 Ratio of Cone Radius to Tube Radius, r / R F i g u r e 5. Variation of F l a m e Velocity over Cone Surface b y the Ansle Method n-Heptane-air flame, 360' F. mixture temperature, 1 atmosphere pressure 1.8
1
I
Reproducible results were obtained using the total area and 0.48R angle methods, and the mean flame velocity obtained by the area method checked very well the angle method value. The angle method (at 0.48R) therefore provides a short cut for the area method in obtaining mean flame velocity values of liquid fuels.
Discussion of Results For the comparison of various fuels, only the maximum V R values obtained by the 0.48R angle method were considered. Six to eight determinations of V p were necessary to establish each curve of V F vs. fuel-air ratio. The maximum V F occurred at mixtures slightly richer than stoichiometric proportions; however, there were indications that this mixture ratio varied according to the temperature and pressure conditions of burner operation. Effect of Air Flow Rate. Since the combustion characteristics of fuels are dependent upon burner design, preliminary experiments were made to evaluate the effects of burner variables. All conditions were such that laminar flow theoretically existed a t the burner port. For these conditions, Smith and Pickering (fa) found that V F was independent of air flow, and other investigators (18 ) noted that V F increased materially with air flow in the border line range of laminar flow (2100 to 2300 Re). Maximum V p for the fuels cumene, n-hexane, toluene, and diisobutylene are plotted in Figure 6 against air flow. Although the reproducibility of the angle method values was zt0.05 foot per second curved lines were drawn through the data for the sake of clarity. The curves for n-hexane, toluene, and possibly diisobutylene could be represented as horizontal lines, but the irregularities observed for cumene are beyond the experimental error and believed to be real. Based on theories of flame propagation in the laminar flow region a basic fuel property such as flame velocity would be expected to be independent of mass flow rate.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 10
7I
I 0
-v A
CUMENE
n-HEXANE TOLUENE
Dl ISOBUTYLENE I
,
3.0 3.5 Air Flow Rate, Lb./Hour Figure 6. Effect of Air Flow on Maximum Flame Velocity In 1.58-cm. tube at 920' F. and 1 atmosphere pressure for liquid hydrocarbons 0.48R angle method 2.5
However, the data obtained in this investigation, including the results of tests at lower pressures, Figure 8, show that while the flame velocity of some hydrocarbons such as n-hexane and toluene varied only slightly with changes in mass air flow rate others, particularly iso-octane, shon-ed large variations. Effect of Mixture Temperature. An air flow of 3.15 pounds per hour was used for investigating the effect of mixture temperature, and the results are given in Figure 7 . I t is apparent from the data that flame velocities increased with temperature and that the rate of increase was not the same for all hydrocarbons. Increasing the temperature increases the flow velocity for constant mass rate conditions, and if flow velocity has an effect on flame velocity, this effect is included. I t should also be noted that a change of order among the fuels was produced by changes in mixture temperature. This is one of the principal difficulties encountered when attempting to compare flame velocity data from several sources obtained under different operating conditions. KO general VF-temperature relationship is available for a group of fuels, and it becomes necessary to evaluate each fuel separately. The rate of change in the flame velocities with increased temperature were, in decreasing order, n-hexane, nheptane, iso-octane, toluene, and diisobutylene. Effect of Pressure. Gaydon and Wolfhard (8)have found that for stationary premixed flames of hydrocarbons, at pressures between l atmosphere and a few millimeters of mercury, the flame velocity is independent of pressure. Linnett and Wheatley (15) concluded that burning velocities are affected by pressure and that, for some hydrocarbons, as the pressure is lowered the burning velocity increases. Garner, Long, and Ashforth ( 7 ) found the flame velocity of benzene-air flames varied inversely with the fourth root of the pressure. For the present investigation six fuels were evaluated at reduced pressures under conditions of both constant mass air flow and constant linear air velocity. Some of the past confusion in the literature on the effect of pressure may be explained on the basis of possible mixture flow variations and the particular fuel under study. The data presented in Figure 8 emphasize the effect of pressure on flame velocity for constant air flow rate and constant linear velocity of the fuelair mixture. Each fueI was tested at reduced pressure under two fuel-air mixture flow conditions. The mass air flow rate, pounds per hour, used for the tests a t 30 inches of mercury pressure was also used at the two reduced pressure aonditions. Similarly, in a second series of tests the linear flow velocity a t the higher pressure was held constant as the burner pressure was reduced. For the fuel, iso-octane, the flame velocity increased
under constant air velocity conditions while a definite decrease was obtained a t reduced pressures when the mass flow rate, pounds per hour, was held constant. While the other fuels investigated had essentially equal flame velocities a t the intermediate pressure condition, with the exception of cyclohexane and toluene, real differences were obtained a t the lower pressure. The reversal of the trends shown for iso-octane m-as obtained for n-heptane, n-hexane, and toluene, especially at the lower pressure. On the basis of these data no conclusions are warranted concerning the effect of pressure on V p for any one group of hydrocarbons. However, several observations can be made. Changes in air flow rate show a definite effect at lorn pressures on the flame velocity of some fuels. There may be maximum or minimum values of V F with respect to pressure, and the variations of Via with air flow rate may be minimized a t pressures substantially below 20 inches of mercury absolute. The variations of T'p with pressure and air flow rate were obtained with measurements made by both the angle and total area methods. Effect of Hydrocarbon Type and Chain Length. Although sufficient data w e not available in the literature to compare flame velocity values obtained at the temperature conditions used in this investigation the same general trends-Le., effect of molecular weight, unsaturation, and the addition of side chains to the normal alkanes-were obtained as reported by other investigators (9, 16). Figure 9 presents a comparison of the maximum flame velocities obtained at 360' F. mixture temperature and I atmosphere pressure (0.48R angle method) with data reported by Gerstein, Levine, and Wong ( 9 ) . These investigators used the horizontal tube method for their study and determined the fundamental flame velocity by correcting the rate of flame travel through the tube for the mean velocity of the unburned gas set 9.4
r
I
I
I
i 2s!
ln
5. e
LL
22.0 U
i 2 1.8
i ._
v n-HEXANE o n-HEPTANE
1.6
4 TOLUENE
ISOOCTANE 0 DI ISOBUTYLENE
1.4 900
300
400
Mixture Temperature, F.
Figure 7.
Effect of Mixture Temperature on Maximum Flame Velocity
In 1.58-em. tube at 1 atmosphere pressure and 3.1 5 Ib. per hour air rate lor li uid hydrocarbons 0.48R a n g e method
in motion ahead of and away from the advancing flame. Although the authors' experiments were carried out a t the higher mixture temperatures, the data show that the effect of structure and chain length on V Pis similar a t the two temperatures. I n general the alkanes gave approximately the same flame velocities with the exceptions of methane and decane, and the alkenes below eight carbon atoms were faster burning than their normal counterparts. Gerstein et al. (9) have reported a decreasing effect on flame velocities with the addition of methyl groups to the straightchain hydrocarbons. These data along with the data from Table
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1952
n-HEXANE
n-HEPTANE
2495 0
BENZENE /
I
I
I
I
I TOLUENE
I
Figure 8.
I
I
I 30
30 25 PO Burner Proswe. Inches of Mercurv Absolute
PO
30
I
1
CYCLOHEXANE
I SOOCTAHE
I /
I 20
25
Effect of Pressure on Maximum Flame Velocity
In 1.58-em. tube at 360" F. lor liquid hydrocarbons 0.48R angle method
I on pentane, 2-methylbutane, and 2,2,4-trimethylpentane are presented in Figure 10. In addition, Figure 11 shows the effect of adding methyl groups to saturated and unsaturated cyclic hydrocarbons. Although the addition of a methyl group to benzene lowered the flame velocity, no appreciable change was obtained when an ethyl group was substituted. However, the addition of another side group (isopropylbenxene) showed a lower flame velocity than was obtained for ethylbenzene. Cyclohexane and methylcyclohexane gave approximately the same flame velocity. Several alcohols were also investigated and the data in Table I show that the fuels of low molecular weight gave the highest flame velocities. Butyl alcohol is expected to have a higher flame velocity than obtained for branched-chain isobutyl alcohol. The same general trends in the variation'of flame velocity with hydrocarbon structure were obtained for the values calculated at the 0.707R position and by the total area method. Irregularities in the variations were usually within experimental error.
mixture temperature increased; however, the rate of change in flame velocity with temperature was not the same for all fuels. Although decreasing the air flow rate did not appreciably affect the flame velocity of three fuels, there was an increase in flame velocity for four other fuels a t reduced pressure and one fuel showed a decrease. 2. Although flame velocities generally increased as the pressure was reduced, this increase depended on the burner flow conditions. One fuel showed an increase in flame velocity when the
.\*-
Conclusions Flame velocities were determined from luminous cones at a point, 0.48 of the burner radius from the axis, using the wellknown relation, Vp = V sin a. At this point, the values of flame velocities determined from the luminous and shadowgraph cones were identical, and were also in good agreement with the values obtained by the total area method, VF = volume rate of flow divided by cone surface area. The same relative differences between the flame velocities of fuels obtained by these methods were also obtained from luminous cones at a point of 0.707 of the burner radius from the axis. The following general conclusions were drawn from the investigation of 25 pure liquid hydrocarbons: 1. Flame velocity values showed a marked dependence on temperature and a relatively slight and irregular dependence on pressure and air flow rate. The flame velocities increased as the
\
0 , 0
- n- P A R A F F I N
A,A
-OLEFIN
AI/:
BUNSEN TUBE =VSlN (0.48 R POSITION)
v,
REF. 9 H O R I Z O N T A L TUBE FUNDAMENTAL F L A M E VELOCITY U,=(U,-US)A,",/AFL,
A-A-A-A
/-o--6-o-o-o
1
Figure 9.
3
5
7 Carbon Atoms
9
11
Comparison of Maximum Flame Velocities Bunsen tube and horizontal tube
3.0
.-BUNSEN
--
2.6
Comm. Aeronaut. Repts., Tech. M e m . Notes, RM E50624 (1950).
TUBE
(10) Griswold, John, “Fuel, Com-
o,oaA-HORI ZONTAL TUBE REF. 9
--
3 8.9.
f 2E“ 1.8.-
3
c-c-c
f:
1.4..c-c-c-c
c-c-c-c-c
c-c-c-c-c
7
c
c- c - c - c
c-c-c-c
-====5 c-t-c D % &
A------4
1.0. 0
1
Figure I O .
J . Chem, Phys., 11, 875 (1943). (14) Linnett, J. W., and Hoare, M. F., Trans. Faraday Soc., 47, 179 (1951). (15) Linnett, J. W., and Wheatley, P. J., iJTature, 164, 403 (1949.) (16) R e y n o l d s , T. W., a n d Ebersole, E. R., Natl. Adv i s o r y Comm. Aeronaut.
Ff
c-c-c-c-c
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T i 1609, 1948. (17) Smith, F. A., Chem. 389 (1937).
Comparison of Maximum Flame Velocities Effect of branchins
pressure was reduced and the fuel-air mixture velocity was held constant, while a decrease in flame velocity was obtained when the mass air flow rate was held constant for the same pressure variations. 3. Alkenes burned faster than alkanes, while branching of the Etraight-chain hydrocarbons tended t o reduce the flame velocity, The addition of alkyl groups t o saturated and unsaturated cyclic hydrocarbons showed no definite trend, in that the flame velocity was reduced in two instances while no appreciable change was noted in a third case
Go., 1946. (11) Grove, J. R., Hoare, M. F,,and Linnett, J. W., Trans. Farad a y Soc., 46, 746 (1960). (12) Jost, Wilhelm, “Explosion and
Combustion Processes ;in Gases,” New York, MeGrawHill Book Co., 1946. (13) Lewis, B., and von Elbe, G.,
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bustion and Furnaces,” Kew York, McGraw-Hill Book
Revs.,21,
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Acknowledgment The authors xish to acknowledge the contributions made by J. M. Godsey, who carried out a large amount of the development and experimental work presented in this report. Acknowledgment is also made t o J. E. Ullrich for operation of the burner and assistance in the measurement of cone angles and flame areas.
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Literature Cited
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(1) Anderson, J. W., and Fein, R. S., J . Chem. Phys., 18, 441
(1950).
Ashforth, G. K., Fuel, 29, 284 (1950). Calcote, H. F., Anal. Chem., 22, 1058 (1950). Corsiglia, John, Am. Gas Assoc. Monthly, 13, 437 (1931). Garner, F. H., Long, R., and Ashforth, G. K., Fuel, 28, 272-6 (Dec. 1949). (6) Garner, F. H., Long, R., and Ashforth, G. K., J . Chem. Phys.,
(2) (3) (4) (5)
18, 1112 (1950). (7) Garner, F. H., Long, R., and Ashforth, G. K., Nature, 164, 884 (1949). (8) Gaydon, A. G., and Wolfhard, H. G., Proc. Roy. Soc. (London), A196, 105 (1949). (9) Gerstein, M., Levine, O., and Wong, E. L., Natl. Advisory
Figure 11.
1 Methyl Group Substitution
9
Effect of Addition of Alkanes to Cyclic Hydrocarbons
(18) Smith, F. A., and Pickering, S. F., J . Research Natl. Bur. Standards, 17,7 (1946); Research Paper No. 900. (19) Wohl, K., “Third Symposium on Combustion,” Baltimore, Williams & Wilkins, 1949. ACOEPTBD June 2, 1952. RECEIVEDfor review April 26, 1951. Presented before t h e Divisions of Petroleum Chemistry and Gas a n d Fuel Chemistry Joint Symposium on Combustion Chemistry a t the 119th MeetCHEMICAL SOCIETY, Cleveland, Ohio. ing of the AMERICAN
