December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
Heumann found, by applying Langmuir probe theory, that the electron temperatures were approximately 3200’ K. while the flame temperature was only 2000” K. His experimental evidence is in turn further verification of the proposal that the electrons may acquire sufficiently high energies t o excite the gases present. A third argument is obtained from Tufts (16)in 1906 who found t h a t for voltages between 100 and 400 volts per em. applied between electrodes in a flame, the current increase was greater than linear; for still higher potentials there was unmistakable evidence of ionization by impact, such as a sudden increase-in the current, a decrease in the potential gradient, and a marked increase in the temperature of the electrodes. Such considerations as these would probably explain much of the previously reported results of electric fields on flames and particularly those that allege to prove the important role played by the negative ion in flame propagation. SUMMARY AND CONCLUSIONS
For the experimental arrangement of a mantled burner flame with a field applied between the metal burner and an exterior ring on the mantle several centimeters above the burner port, the following have been demonstrated: For a positive field (ring positive) up to 16 kv: 1. The composition limits of flame stability can be increased by as much as 200%. 2. The composition limits of flame stability reach a saturation point a t very low voltages, of the order of 2 kv. 3. The range of flow rates over which a burner flame is stable can be increased by roughly 100%. 4. The dead space can be decreased by roughly 30%. 5 . The flame pressure can be increased by more than 100%.
For a negative field (ring negative) up to 16 kv: 1. The composition limits of flame stability can be increased a t high field strengths by ap roximately the same amount as with positive fields, although for Power fields there is an indication of a decrease in limits. 2. The range of flow rates over which a burner flame is stable can be increased, although in a rather capricious manner. 3. The dead space is increased and then suddenly decreased with increasing electric field; a t the highest fields the flame vibrates rapidly and then becomes derby shaped. 4. The flame pressure can be decreased by approximately loo%, and then as the field is increased to the point where the decrease in dead space is observed, the flame pressure is enhanced.
2731
By assuming a mechanical picture for the effect of the electric field on the positive ions, a calculation from the pressure change gives a value of NL-101O ( N is the number of ions in the inner cone and L is the thickness of the inner cone) in agreement with N L calculated for transverse fields. The influence of a high voltage negative electric field is explained by assuming that the electrons acquire sufficient energy t o raise the gas to an excited state. LITERATURE CITED
(1) Calcote, H. F., review article, “Electrical Properties of Flames,” unpublished. (2) Calcote, H. F., ”Third Symposium on Combustion, Flame, and Explosion Phenomena,” p. 245, Baltimore, hld , Williams & Wilkins Co., 1949. (3) Davidson, J. F., Physik Z.,7, 108 (1906). (4) Garside, J. E., Forsyth, J. E., and Townend, D. T. A., Inst. FueE (London),Bull. 175 (1945). (5) Gaydon, A. G., and Wolfhard, W. G., Proc. Roy. Soc. ( L o n d o n ) , A201,561,570 (1950); A205, 118 (1951). (6) Guenault, E. N., and Wheeler, R. V., J . Chpm. SOC., 1931, 195; 1932,2788. (7) Haber, F., Sitzber. preuss, Akad. Wiss., P h y s i k . Math., 11, 162 (1929). (8) Haber, F;, and Lacy, B. S.,2.Phzlsik Chem., 68,726 (1909). (9) Heumann, T., Spectrochim. Acta, 1,293 (1940). (10) Jost, W. (trans by H. 0. Croft), “Explosions and Combustion Processes in Gases,’’ P. 80, New York, McGraw-Hill Book Go., 1946. (11) Laidler,K. J., and Shuler, K. E., Chem. Revs.,48, 153 (1951). (12) Lewis, B., and von Elbe, G., J . Chem. Phys., 11,75 (1943). (13) Loeb, L. B., “Fundamental Processes of Electrical Discharges in Gases,” New York, John Wiley & Sons, 1939. (14) Malinovskii, A. E., and coworkers, Physik 2. Sowjetunion (1932-36). (15) Tufts, F. L., Phys. Rev., 22, 193 (1906). (16) Vogt, K., Ann Physik, 12,433 (1932). (17) van Elbe, G., and Menteer, M., J . Chem. P h y s . , 13,89 (1948). (18) Wilson, H. A., Rec. M o d . Phys., 3, 156 (1931). RECEIVED June 21,1951. The work described in this paper was done in connection with Contract NOrd 7920 for the U. S. Navy Bureau of Ordnance 88 part of Project Bumblebee, Applied Physics Laboratory, The Johns Hopkins University, and Contract X6-ori-105 with the Office of Naval Research, U. S. Navy, as part of Project Squid, Princeton University. Acknowledgment is due Dean H. 9. Taylor, who has general supervision of this work. This report was abstracted in part from a thesis by H. F. Calcote submitted in partial fulfillment of the requirements for the degree of doctor of philosophy a t Princeton University.
COMBUSTION IN BUNSENFLAMES FRANK R. CALDWELL, HERBERT P. BROIDA, AND JEROME J. DOVER National Bureau of Standards, Washington, D. C.
Fj
Apparatus has been assembled, as a part of a continuing program of research on combustion chambers, for investigating mechanisms of the combustion process. Studies are being made of effects of fuel-oxidant ratio, pressure, and temperature upon burning velocity of laminar flames above a nozzle, temperatures attained in the flames, and the nature and concentrations of free radicals that play important roles in the reactions. Apparent flame speeds and line-reversal temperatures of methane-air mixtures have been measured at atmospheric pressure, and effects of water vapor, gas velocity, and nozzle temperature on flame speeds have been determined. A brief description is given of spectroscopic equipment for determination of rotational temperatures and concentration of radicals, and some preliminary observations on an acetylene flame are presented.
Preliminary results with this apparatus are sufficiently promising to encourage continued exploration of its potentialities in the search for fundamental information on flames. Suph information may aid in a better understanding of the mechanism by which flame travels through a combustible mixture, and in the solution of some of the practical problems of combustion.
A
PART of a continuing program of research on combustion chambers, sponsored at the Xational Bureau of Standards by the Bureau of Aeronautics, apparatus has been constructed for the purpose of obtaining detailed information on the combustion process as it occurs in laminar Bunsen-type flames burning above nozzles. Sufficient preliminary results have been obtained to warrant prrsentation a t this time of a description of the new equipment and its apparent capabilities and limitations.
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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. 43, No. 12
tional, and translational temperatures of various molecules and the temperature gradients in the flame. Looking toward the future upward extension of the international tetnperature scalc, a complete understanding of the significance of these various temperatures and the spectroscopic methods by Fvhich they may be observed are matters of prime necessity. Translational temperatures are now being measured by the line-rewrsal method 011 the same flames used for studies of flame speed. At the same time, independent spectroscopic equipment is being used for the identification of radicals and for the meaburement of the intensity of individual spectral lines emitted by these radicals. The latter equipment is also being used for investigations of temperatures in flames, and it shows promise of being applicable in studies of the establishment of equipartit,ion (relaxation times) and possibly oithe concentration gradients of certain radicals. Obviously the ambitious program of F i g u r e 1. A p p a r a t u s for Investigating Combustion Process research outlined here must be of a conB = Burner tube M = Schlieren mirror tinuing nature, and its future course rvill N = Accelerating nozzle 0 = Blowout diaphragm depend on what can be accomplished by A = Pressure chamber I = Manometer connection P = Pipe to vacuum pump WI = Plate glass window refinement of equipment and techniques. V = Valve Wz = Vvcor window E = Ignition electrode F = Framework for Schlieren apparatus The preliminary results are sufficiently promising to encourage a continued exploration of the potentialities of the Basically, the apparatus was designcd to yield information perapparatus. The ultimate goal of all research of this type, ae tinent to the mechanism of the combustion process. This inforstated aptly by Coward and Payman (I),is t o ascertain the mechmation includes effects of composition of the fuel, fuel-oxidant anism by which flame travels through a flammable gas mixtureratio, pressure, and temperature on burning velocity, temperathat is, t.o correlate the speed of movement of the flame with the tures attained in the flame, and the nat'ure and concentrations of various chemical and physical factors involved. The attainment radicals which have an important role in the reactions. Since a of this goa,l will be the result of a large number of investigations, steady and reproducible flame is required for these studies, a among which it is hoped the present program will find a useful burner providing a Bunsen-type flame was selected for the present place. purpose. BACKGROUND A critical review of the literature, which contains many report,s on the use of Bunsen burners for determinations of flame epeed, True burning velocity is t'he velocity with which a flame adreveals striking discrepancies in numerical values. These diffcrvances into and transforms a combustible mixture, measured ences arise because the values obta'ined are apparent flame speeds rttlative to the unburned gas and in a direction normal t o the surthat have resulted from limitations of the apparatus. These face of the flame. I n this sense it is a physical property of the values approximate but, are not identical with true burning veloccombustible mixture, dependent only on mixture composition, itjy. Kumerous procedures are reported for obtaining traces of temperature, and pressure. It should never be confused with flame cones and for interpretation of the cone images in terms of apparent flame speed, which may vary n-idely with the condition8 flame speed. Hotvever, it has not been demonstrated which, if under which burning occurs. any, of these methods leads to reliable values of burning velocity, *4typical example of apparcnt flame speed is the speed with independent of the apparatus and the method of interpretation. which a flame moves in space through an explosive mixturcl. One purpose of the present program, as yet unachieved, is t o Burning' velocity can be calculated from apparent flame spced evolve a burner method capable of yielding values of burning only when the component of unburned gas velocity normal and relative to the flame is known. Although apparent flanie speeds velocity that are characteristic only of the coniposition and state of the combustible mixture. The apparatus is sufficiently flexible can be measured directly, it is often difficult to evaluate the normal component, of gas velocity. This latter difficult'g is one of that it can be used t o compare results obtained by different techniques. It is planned, after a satisfactory procedure has been the primary reasons why most, if not all, determinations of burning rate with Bunsen burners only approximate the true burning developed, to undertake a systematic survey of effects of fuel structure on burning velocity and flame stabilit'y. The primary ve1ocit.y. Although the burning velocity is an over-all rate of transformacriterion of such a procedure will be the independence of derived values of flame speed from all effects of the apparatus itself. tion of unburntd gas into products of reaction, it is knov-n that, It is desirable for several reasons to study both temperatures the process of combustion involves many physical and chemical and burning velocity in the same flame.' Both of these properties steps, the slowest of which will control the over-all rate of reacof the flame are required for evaluation of t,heories of flame proption, When less was known of the mechanism of the process, it was logical to assume, as did Mallard and Le Chatelier (16), that agation. Much information on the niolecular mechanism of combustion can be obtained by studying the vibrational, rotathe over-all rate was determined by the rate a t which heat n'as
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951
conducted from the burned to the unburned gas, raising the temperature of the latter until active burning began. Although neither this theory, nor any of the numerous subsequent modifications, is in accord with experimental facts, the basic concept of the importance of transfer of translational energy by diffusion from the flame was certainly sound. However, it is now apparent that the concept of transfer of heat alone must be extended t o include other forms of energy, including that which may be transported in latent form by short-lived active species such as atoms and radicals. Technical advances in reaction kinetics and in the identification of intermediate products by spectroscopic means, indicating that the combustion process is extremely complex, have stimulated the evolution of more comprehensive theories of flame propagation. These include, among others, the suggestion of Lewis and von Elbe ( 2 4 ) that the rate might be controlled by the diffusion of active particles from the burned to the unburned gas; of Tanford and Pease ($I), who calculated burning velocities of certain reactions on the assumption that the diffusion of hydrogen atoms into the unburned gas is the controlling factor; and of Hirschfelder (11) who has considered the combined effects of particle diffusion, thermal conductivity, and chemical kinetics. Although each of these theories has been extremely useful in stimulating thought and further experimentation, none yet can be considered fully adequate t o account for the complete molecular
AIR
f
BOTTLED FUEL
CITY GAS
VALVES @SOLENOID @GLOBE NEEDLE
I
Figure 2.
3 TI
0 Mm
-
Schematic V i e w of Metering Systems
MW= Weter manometer = Storase tank Ms = Mlcromanometer Pressure resulator = Dryer or ralurator RI Flow regulator = Thermocouple No = Sodium chloride vaporizer = Flat-edge orifice B = Burner tube = Mercury manometer Tz = Thermocouple N = Accelerating nozzle
-
2133
mechanism by which the chemical energy latent in a fuel is transformed during the procese of combustion. One of the need6 for further extension of the theory is more reliable experimental data, and the equipment described here was designed for the purpose of obtaining some of these data. APPARATUS
Despite the difficulties of interpreting the shape of Bunsen cones in terms of burning velocity, it was decided that the Bunsen burner had more promise than any other type equipment for the simultaneous observation of flame speed and temperature and for studies of intermediate products. If the flame shape can be interpreted properly, the burner method is rapid and convenient for studying effects of fuel structure, mixture ratio, diluents, initial temperature, and pressure. Even if interpretation in terms of true burning velocity proves impracticable, results on a relative basis will be useful, as are the many values obtained previously by related methods. Two types of Bunsen burners are capable of providing unburned mixture in symmetrical, laminar flow a t the burner port. The conventional type consists of a smooth-bore cylindrical tube whose length is many times its internal diameter. In this type ( I @ , which has been used more than any other device for studying the phenomena of combustion, the velocity a t the port has a parabolic distribution and remains laminar up, to Reynolds numbers of about 2300. It therefore has the disadvantages arising from the velocity gradient and cannot be used for faster-burning mixtures because of flash back. I n the second type, the port is the throat of a properly shaped nozzle, which forms the outlet from an appropriate calming chamber. Except for a relatively thin annular boundary layer, the velocity is uniform over the area of the port and can be increased to high values without introducing turbulence. Laminar flames of such fast-burning mixtures as hydrogen-oxygen and acetylene-oxygen are stable above sucb nozzles. Because flames on nozzles are more nearly geometric conp and because of the added freedom from turbulence and flash back, the nozzle-type burner was selected for the present equipment. The burner itself and much of the auxiliary measuring and control equipment were patterned after the apparatus of Johnston (12). The equipment, part of which is shown photographically in Figure 1,may be considered to consist of five majorparts-namely, the control and metering system, the burner, the pressure chamher, the schlieren system, and the line-reversal system. These are described in the order mentioned Parts of the control and metering equipment, mounted on the panel board, are shown in the background of Figure 1. This equipment is best described by the schematic diagram, Figure 2. The systems for fuel and oxidant are identical, except that the ranges are different, so that only one need be discussed. The pressure of the incoming gas is controlled a t a preselected value by a pressure regulator, E,. From the regulator, the gas passes through a chamber, D, which may be either a water saturator or a dryer. As the gas approaches a calibrated orifice, 0, its temperature is indicated by a thermocouple, TI, and its absolute pressure by a mercury manometer, M , (in conjunction with a barometer). The drap in pressure across orifice 0 is indicated by water manometer M , or by micromanometer M,. In all, six orifices, each calibrated at four flow rates by the Capacity, Density, and Fluid Meters Section of this bureau, are available for covering the range of gas flows of interest. These orifices were calibrated and are being used in accordance with accepted practice, as outlined in reference (1). Valves and R flow regulator, E / , on the downstream side of the orifice complete the control and metering system. When city gas is used, its pressure is increased to a convenient working level by a diaphragm-type compressor, C . Referring t o both Figures 1 and 2, the burner tube, B, has a n inside diameter of 1.5 inches and a length of about 38 inches.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2734
Except for about 6 inches within the pressure chamber, this tube is wound with Nichrome ribbon and insulated thermally. Fitted to the upper end of the burner tube is a n accelerating nozzle, N , having the interior shape suggested by Kretzschmer ( I S ) and defined by the following offsets: Diameter of Nozzle, Inches
Axial Distance, Inch
Diameter of Nozzle, Inch
Axial Distance, Inches
1.500 1.389 1.299 1,162
0.000 0.031 0,062 0.125 0.188 0.250 0.312 0.375 0.438 0.500 0.662 0.625 0.68s
0.671 0.660 0.630 0.612
0.750 0 812 0.875 0 938 1,000
1.o m 0,982 0,919
0.866 0.822 0,783 0,750 0.721 0,694
0.596
0.581 0.567 0.554 0.542 0.520
0.500 0,500
1.062 1.325
1.188 1.250 1.375 1.500 1,625
G
Vol. 43, No. 12
able adjustments, is mounted above and in line with the tube. The beam of light emerging from the prism passes in turn by knife-edge, IC,, t'hrough the plate-glass window, W 1 , and through the flame to the spherical mirror, M (radius of curvature, 46 inches). This mirror was coated Kith aluminum and a protective coating of silicon monoxide-silicon dioxide by the Radiation Laboratory at Fort Belvoir, Va. The light is reflected back through the flame and the window, paPt knife-edge K2 located a t the center of curvature of the mirror. Lens L2 then focuses the schlieren image of the flame on a viewing screen, a film, or a goniometer, G. By means of thip goniometer the angle between the flame traces can be measured to the nearest 3 minutes, without taking a photograph. A magnificat'ion of slightly over two is obtained with the present system. In order t o measure t.emperatures by the line-reversal method, a trace of sodium chloride is added to the mixture before it enters burner tube B. This is done in a vaporizer, N , (Figure 2), by passing part of the air through a chamber in which the salt is heated by an incandescent filament. The amount of salt introduced can be controlled by adjusting valves in the air by-pass line and by regulating the current through the filament. Flames are colored in this way only during the observation of tempeiatures. As shown in Figure 3, the line-reversal equipment consists of a spectroscope, S,focusing lcnses Z3 and ZI, a calibrated tungsten strip lamp, L, storage batteries to operate the latter, and means for controlling and measuring the filamrnt current. The entire syst,em is mounted on a rigid channel-iron framework (not shown in the figures), which is in t,urn supported on a solid 2-inch shaft passing through a long bearing mounted in the top of the table carrying the pressure chamber. The shaft' rests on a jack Jvhich has both fine and coarse adjustments. The entire system can be adjusted vertically by means of the jack and can be rot,ated o n the shaft, Thus any desired portion of the flame can be brought readily within the line of sight, without readjusting t'he optic:d s>,t>em. TQ
Figure 3. A 0
Schematic V i e w of Schlieren System and Combustion Chamber
Combustion chamber Mercury vapor lamp P Prism I Lens K = Knife edse = = = =
S
W = Window M = Schlieren mirror
I
"
'
"
"
"
I
G = Goniometer L = Calibrated tunssten strip lamp DC = 120-volt storage batteries = Spectroscope
At present the upper end of the nozzle is a flat annulus, 718 inch in external diameter. The shape of this portion of the nozzle may not be particularly favorable and ivill be invest,igated later. Provision is made for cooling the nozzle by an ext,ernal jacket through which water is circulated. Pressure chamber A is a welded st>eelbox, about 17 X 12 X 12 inches in internal dimensions. The near side is removed in Figure 1. Pipe P at the top of the chamber connects to a vacuum pump through valve IT, which provides control of pressure in the box from 0.1 to 3 atmospheres absolute. On chamber A are mounted the rotatable ignition elect,rode,E, the schlieren mirror, M ,a blow out diaphragm, D, and a tube, I , leading to a mercury manometer for measuring operat'ing pressure. A 7-inch plateglass window, W,, forms a part of the left end of t,he chamber, and a 2 X 4 inch Vycor windon-, TV2,is mounted in each side, in line with the nozzle. It n-as found necessary t,o heat the windows and schlieren mirror electrically to prevent condensation of Jvater on their surfaces. Even this treatment did not keep the large windom-, W ] ,clear, and it was necessary t o direct a stream of dry air against its inside surface. The schlieren system, except for mirror, M , is mounted on framework F and is partially visible at the left in Figure 1. It is also shown schematically in Figure 3. An air-cooled BH-6 mercury vapor lamp, &, is mounted at the lower end of a telescoping tube, which also support,s t,he lens, 2,. A 90" prism, p , with suit-
i
'1 IO0 0 I00 DISTANCE FROM CENTER OF NOZZLE -INCHES
Figure 4.
Velocit Profiles at Different Distances above Accelerating Nozzle
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951
EXPERIMENTAL
Performance of Nozzle. The velocity profile at and above the outlet of the nozzle was measured by means of traverses across various diameters with a tiny Pitot tube (0.033 inch outside diameter X 0.017 inch inside diameter), connected to a micromanometer. Typical results are shown in Figure 4,in which the reference measurement was the velocity a t a height of 0.004 inch above the exit of the nozzle. Here the velocity was uniform over the entire port, until the outer wall of the Pitot tube began t o overlap the nozzle. Other curves in Figure 4 show the velocity profiles, a t intervals of 0.5 inch, up t o 2 inches above the nozzle. All velocities are expressed as fractions of the velocity a t the 0.004-inch level. These results show that, except at the extreme base of the cone, the interaction of the jet with the surrounding gas does not extend far enough inward from the burner port to influence any flame having an inner cone less than 1.5 inches tall. I t was hoped that such a flat velocity profile would ensure that traces of the inner flame cones would be straight, between the base and the tip. Nevertheless they were actually curved in the initial trials. It was reasoned that the curvature might result from temperature gradients produced in the unburned gas because the nozzle was heated above the gas tempgrature by the flame. To check this hypothesis, a temperature traverse was made across a diameter slightly above the port with a 28-gage thermocouple, Using an inlet mixture heated t o 55” C., as indicated by thermocouple T z of Figure 2, a traverse was made immediately after extinguishing a flame which had been burning for 3 hours to ensure that a steady state had been reached. Typical results obtained a t exit velocities of 90 and 120 cm. per second are shown in Figure 5. The upper curves indicate that the mixture near the wall was about 50” C. hotter than that a t the axis, and the bottom curve shows that this temperature gradient was reduced to negligible proportions by water-jacketing the nozzle. However, operation of the present cooling system is time-consuming, because the provisions for preheating the gas and cooling the nozzle require completely independent control. It is believed that a n improvement in the apparatus can he made by circulating a single fluid, such as oil from a thermostated bath, through coils attached to both the nozzle and its approach tube. Characteristics of Schlieren Image. The schlieren method is considered by some ( 8 ) to be too sensitive for the present purpose, and by others (IO) to be better suited for measurements of
unity, by keeping the upstream pressure high; it could therefore be neglected The drop in pressure, Ah, across the orifice is measured with a water manometer or with a micromanometer at low flow rates. Both the temperature, TIand the pressure, P, of the gas approaching the orifice are measured, RO that its density, p, can be calculated from the gas law
P/RT
(2) The quantities P, R, and T must be in units which give p in pounds per cubic foot, and the value of gas constant, R, must apply for moist mixtures when the saturator is used. p =
WATER COOLED NOZZLE 90, 120 t M / SEC
s,
I
1.
%
- -
2735
I
I
I
- -
in the essentially monochromatic light of the Cz, CH, and OH bands, in both focused and unfocused shadow, and in schlieren. The results are not sufficiently advanced to report a t this time. Although the present schlieren traces are about 0.5 mm wide, the angle between the slightly blurred images of the traces can he measured accurately. However, with such a trace it is difficult t o pick a point that can be related to a particular point in the flame front. This is a more serious disadvantage in determining flame area than in measuring the angle of the cone. Determination of Mixture Velocity. The calibration of each metering orifice consisted of the evaluation of the coefficient of discharge, K , in the equation
W = 0.0997KDZ(A h / p ) l Iz
(1)
where W is the rate of flow in pounds per second; D is the diameter of the orifice, in inches; Ah is the drop in pressure across the orifice, in inches of water; and p is the density in pounds per cubic foot on the upstream side of the orifice. In more exact form, the right-hand member of Equation 1contains an expansion factor. I n the present work this factor was made to approach
Thus all values needed for substitution in Equation 1are available, and the weight flow rates of air, W,, and fuel, W J ,may be calculated. Again using the gas law and the observed values of temperature, T,, and pressure, P,, prevailing a t the burner port, the total volume rate of flow, V,, through the port is the sum of the volume rates of t h r air, V,, and the fuel, V / ,or
Vo =,Va
+ V’
=
(WaRa
+ WrR,)(To/Pc)
(3)
The average velocity of the mixture through the port, U,, is Vodivided by the area of the port, A, which is 0.1963 square inch for the present 0.5-inch nozzle. Thus
Li,
=
V c / A = (W&
+ W / R / )(T,/P,A)
(4)
The greatest error in the calculated values of U , is believed t o be in the metering of the gases-that is, in the values of Faand W j . In view of the care with which the orifices were calibrated and are used, it seems unlikely that this error, and hence also the error in U,,exceeds &I%. Determination of Flame Speed. To date, values of apparent flame speed, Sa, have been computed from the known values of
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
2736
U,,and from the half angle, a,between the traces of the schlieren image, as measured directly with tjhe goniometer, through the well-known relation Sa = U,, sin CY (5)
Vol. 43, No. 12
On a given steady flame, different, observers usually read values for CY such that the difference from the mean sin Q: st~ldoni exceeds 1%. The precision of repeated readings by a singlt~ observer is better than 1%of sin N. Thus the derived va1uc.r of Sashould be of this same accuracy, provided the errors in Ti, and Ein 01 are accidental. Flame speeds have been deterniiiwd in mixtures of mot1i:rnc (b) (C) (99.65% pure) and air a t an initial temperature of 24' C. over a CM /SEC range of fuel-air ratios and at several mixture velocities, undw thv following condit,ions: ( a ) with the nozzle uncoolcd and with t,he gases containing about 1.3% of water vapor by volume; (ti) with 107 the same moisture content, but with the nozzle cooled to 24' ( 2 . ; -61 CM/ and (e) with the nozzle cooled to 24" C. and the mixture drird so 134 134 that it contained less than 0.08% of water. The apparent, 94 flame speeds for these mixtures, obtained in the closed chamber Y > 122 at a pressure of 1 atmosphere, are shown in t,he upper half of Figure 6 as a function of the fuel-air ratio by weight. Rtoichio35 metric fuel-air ratio is 0.058. The values obtained with the uncoolod nozzle (Figure ea) art: m of value t80shorn-, by comparison with thoae obtaincad with the 1 . 3 % H20 0.08% H20 1.3% HO , cooled nozzle (Figure 6 b ) , that such cooling is extremely imporCOOLED NOZZL UNCOOLED NOZZLE COOLED NOZZLE tant. Errors of from 6 to more than 12%~~ereintroducedbecause of temperature gradient's established in the gas stream by the hot nozzle. As might be expected, this effec-tdecreases with incrraeing gas velocity. Comparing Figure 6b with 6c, it is apparent that 1.3 volume % of water vapor in the mixture decreases the maximum flame speed of methane-air by between 6 and 77,. More important, from the standpoint of the performance of the apparatus, is the variation of flame speed with gas veloc-ity. This effect, also observed by Johnston (Table I),is evidence that true burning velocities are not being determined by the method t,hat has been described. Further experiments are in progress in an attempt tjo find suitable means for correcting for or eliminating the effects of -61 gas velocity which appear in the present results. Values of apparent flame speeds of methane-air mixtures at I I 6 I 1 I I I ,050 060 ,070 ,050 ,060 .070 ,050 ,060 .07( atmospheric pressure obtained at various laboratories, as listed in FUEL AIR RATIO Table I, show a range of over two t o one. Although flame speeds Figure 6. Apparent flame Speeds and Sodium-Line Reversal measured with the apparatus described here lie within the range Temperatures of Mixtures of Methane and Air as Function of of values reported by others, it is impossible t o determine which Fuel-to-Air Ratio b y Weight at Various Mixture Velocities approach the true burning velocity. Determination of D-Line-Reversal Temperatures. In measValues of apparent flame speed so derived are identical with the urements of the translational temperature, made thus far by the burning velocity only if the unburned gas entering the foremost line-reversal method, the entire flame has been colored with part of the flame front i s moving vertically with a velocity, U,, vaporized sodium chloride. Many questions concerning this over that part of the cone used .n measuring 01. Since this fact method have not been fully answered, and one of the long-range has not yet been established, the numerical values reported here goals of the present program is a better understanding of the cannot be called true burning velocities. limitations of the method. One of these limitations involves temperature gradients, since these must always exist in even the emallest volume of flame to which the method can be applied. Strong and Bundy ($0) Table I . made extensive studies which show the effects of Apparent Flame Speeds at Atmospheric Pressure of Methane-Air Mixtures gradients in flames on measured temperatures. Apparent Barret (5)made a careful evaluation of the method Flame Speed Unburned Gas ,Inlet Moisture and determined the temperature distribution in a Literature (Max.), Velocity, lemp., Content, Reference Cm./Beo. Cm.iSec. c. % hletliod flame by introducing salt locally. Possible efleetr; (18) 40.8 51 Saturated Tribe-type of even t'races of inorganic salts on the combustion at room buiner temp. process it,self require further study. Despite thew 45.1 209 58 Saturated Sozzle-type uncertaint'ies, line-reversal temperatures of the burner same flames used in the measuremrnt of flltnie ... speed are being determined in the hope that thvsc 21 0.2 Tube-type may be more fully interpreted in the future. burner 75.0 Tube I n this method, the light emitted by the h ( $ a t d 33.8 Tube 36.2 40 Tube-type tungsten strip of a calibrated comparison. lamp burner normally is focused on a vertical diamet,ral plane 42.7 122 24 Nozzle-type 0 08 burner of t'he flame, and then on the slit of a spectroscope. 44.8 SI1 24 0 OR 40.5 122 24 1 3 The strip produces a continuous spect'rum, but 42.0 II4 24 1 3 when a flame colored with sodium is interposed 44.0 61 24 1 3 between the lamp and the apectroscopp, sodium
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6
4oyr
P
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9
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2737
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
December 1951
Figure 7.
Spectrometer and Recorder Rack
I) lines appear in the spectrum. These lines are bright or dark, according to whether the flame or the strip has the higher temperature. By adjusting the current through the lamp, the D lines are made to disappear in the continuum, and a t this disappearance the temperatures of strip and flame are equal. Once the strip lamp has been calibrated in terms of filament current and brightness temperature, the only quantity that must be measured for a determination of flame temperature is the filament current a t reversal However, there are precautions that must be taken and corrections to be applied if the measured temperature is to be accurately representative of the flame. Enough sodium vapor must be introduced so that Kirchoff’s law holds for the flame. It appears that minute quantities are sufficient and that there is small danger of adding too little, but the sensitivity of the method increases with the concentration of sodium. There secms to be no conclusive evidence that sodium chloride, in proportions normally used, has any >. appreciable effect on either the measured temperaturc or the burning velocity, but this mag not hold for all flames In the present apparatus the spectroscope “sees” both the flame and comparison lamp through the lenses and Vycor windows on either side of the combustion chamber. The optical parts between the flame and spectroscope transmit light from both sources, and because their effects on both are the same, their presence can be ignored. Since the light from the comparison lamp alone passes through the parts of the sysFigure tem between the flame and the lamp, a correction must be applied for the absorption and reflection of t,his window and lens. Another correction is necessary because the lamp was calibrated using red light (wave length = 0.6524~) whereas the comparison between the lamp and flame is made with yellow light (wave length = 0.589~). This latter correction must be used because the tungsten strip is not a black body, and its spectral emissivity varies with wave length. Both of these corrections, which are appreciable in magnitude, have been calculated from the laws of black-body radiation.
Corrected line-reversal temperatures, measured by sighting through the entirely colored flames a t a distance of about l / 8 inGh above the tip of the inner cone, are shown in the lower curves of Figure 6. These results are particularly confusing. For example, cooling the nozzle increased the flame temperature when the gas velocity was high and decreased it when the gas velocity was low With the cooled nozzle and with mixtures containing 1.3% of moisture, increasing the gas velocity decreases flame speed and increases flame temperature. For the dry mixtures, on the other hand, both flame speed and temperature decrease with the gas velocity. Drying the mixture causes a decrease in temperature a t high velocity and an increase a t low velocity. These seemingly random effects have not been explained. It is possible that the location of the zone of maximum temperature may shift with respect to the tip of the cone as mixture velocity or moisture content is changed. I n the future this region of maximum temperature will be located experimentally, instead of assuming that it always inch above the tip of the cone. exists about Rotational Temperature of OR. Exploratory temperature measurements of the upper electronic state of OH using the emission band of the 0-0 vibration transition, with a head a t 3064 A. units, have been made. Radiation from the OH radical was selected for initial study because it occurs strongly in hydrocarbon flames, it lies in an easily accessible spectroscopic region, and there is considerable information (6) available concerning the transition probabilities and the exact spectral position of the various branches of the hands. If there is thermal equilibrium, the temperature of the flame can be calculated from a knowledge of transition probabilities and measured Intensities. When there is nonequilibrium, the measured rotational temperatures may give information about relaxation times BAND
OF
OH
(0-0 TRANSITION)
CITY GAS-OXYGEN FLAME RECORDED BY LEEOS 0 NORTHRUP NATIONAL BUREAU OF STANDARDS
CeSodK, SPECTROMETER
IV2W50 SLOW SCANNING RATE
WAVE LENGTH, ANGSTROM UNITS
8.
Band H e a d of
OH Radical
from City Gas-Oxygen Flame
For these radiation studies, an experimental, high-resolution grating spectrometer (Figure 7), constructed and loaned by the Research Department of Leeds and Northrup Co. (7), is being used. This is a compact and rugged instrument havingdimensions of about 30 X 14 X 12 inches. Although it is still in the process of development, it has shown a resolving power, x/Ax, of 45,000 with a 90,000 line grating. As a n example of its usefulness in studies of flames, Figure 8 shows a portion of an OH band from a
2738
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
stoichiometric city gas-oxygen flame, in which lines only 0.07 A. apart are separated clearly. The resolution is distinct and the noise level is low, so that quantitative comparison of intensities is relatively simple. A survey of a n acetylene-oxygen flame a t atmospheric pressure has confirmed the measurements of Gaydon ( 8 ) ,that showed that OH in the inner cone has a n excitation temperature of many hundreds of degrees greater than the equilibrium temperature. I n addition, there appears to be a region in the outer cone, about 1.2 mm. above the tip of the inner cone, R here the temperature iE 200 O K. above the equilibrium temperature prevailing throughout the remainder of the outer cone. If the gas velocity can be determined, measurements of this kind will lead to the time required for distribution, t o other forms, of the energy corresponding t o the abnormal rotational temperature. The experiments made t o date are merely preliminary t o a comprehensive series required for obtaining quantitative information.
Figure 9.
Schlieren Photographs of Methane-Air Flames at Different Gas Velocities
Abnormal Flames. In the course of evaluating the present equipment, many flames of abnormal shape and queer behavior have been encountered. Stable flames have been maintained with mixtures of Washington, D. C., city gas and air a t pressures from 0.1 to 2 atmospheres. As is well knoTYn, the reaction zone thickens and the tip of the cone becomes more rounded as the pressure is reduced. Descriptions of some of the flames which have been observed with the present burner operated at atmospheric pressure may be of general interest. When the velocity of the combustible mixture is decreased from a value at which straight cone traces are observed, the first effect is a n increase of the apex angle of the cone as will be seen in Figures Sa and 9b. Further reduction in velocity leads first to a cone, which is concave inward (Qc), and then t o the polyhedral pattern sh0Lv-n in Figure 9d and reported by Smith and Pickering (19). At some still lower and apparently critical velocity, a n approximately flat flame, somewhat dimpled dovnv ard (9e), is
observed. A furt'her decrease of the velocity causes the flame to move donm into the nozzle without flash back. If the mixture velocity is then increased slowly, a flame resembling the frustum of a cone, but Ji-ith a slight depression in the top surface, forms above the nozzle (Sf). The altitude of the frustum inctreases with velocity (Sg), unt,il a t some crit.ical velocit,y the depression suddenly moves upward and the usual cone is again formed. Occasional unusual flames were formed, often for no known reasons. Examples of these are shown in the sketch and photographs of Figure 10. The line drawing illust,rates a flame with five separate zones, instead of the usual two. The first or inner zone was narrow and a dense bright blue. Its boundaries were sharp, and it looked somewhat, like the normal Bunsen inner cone. The next tn'o zones were also a bright blue and decreased progressively in luminosity. The boundary between zones 2 and 3 was somenhat diffuse, but the out,er boundary of zone 3 was very sharp. The color in these zones depended on the fuel-air ratio and became somewhat more green as the mixture was enriched. This is t o be expected, because more C2 is formed with fuelrich mixtures, whereas the blue is caused by emission from CH. This change in concentrat,ion of intermediate radicals was checked qualitat,ively with a small direct-vision spectroscope. Most of the color of the dim orange-red zone 4 was caused by sodium. Surrounding the entire flame was zone 5, consisting of a faint,, gray-blue mantle. I n addition t o the zones mentioned, thwe xas, a t the tips of the inner cones, a nonluminous region about 1 mm. in diameter extending through the ent'ire blue region. These multizoned flames were caused by fluctuationsoriginating in the supply system. Apparently the flow cont?oller in t'he air system amplified small periodic pressure difference8 caused by operation of the pressure regulator; overhauling the regulator and controller eliminated the effect. Although no high speed pictures of such flames were taken, it is quite evident, that the phenomenon is similar t o the acoustic effects on flames deswihrd by Rlarkst'ein ( 1 6 ) . CONCLUSION
Although the preliminary values of flame speed and temperature obtained with the qquipnient described fall within the range reported by other observera, there are still discrepancies that need to be explained. These preliminary measurements indicate that continued exploration of possible effects of apparatus and techniques on measured values is necessary before the nozzle-tj pe burner can meet the desired objectiw of measuring true burning velocities and euqilibrium flame temperatures. Adequate control
m Figure 10.
Vol. 43, No. 12
Abnormal Flames Caused b y Oscillations i n Unburned Gas
December 1951
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INDUSTRIAL AND ENGINEERING CHEMISTRY
and measurement of flow, pressure, and temperature of the mixture have been achieved; however, the cause of variation of apparent flame speed with inlet gas velocity must be determined. In addition, further experiments are needed t o learn whether the variations of measured flame temperature are due to some defect in the experimental approach or whether they are due t o a lack of equilibrium. The spectroscopic methods of measuring the relative light intensities from various intermediate molecules such as OH, CH, and Cp,as well as the equipment for determination of flame speeds, show sufficient promise t o warrant future detailed studies in the attempt t o gain relevant information about the mechanism of the combustion process. ACKNOWLEDGMENT
The assistance of A. F. Baillie and L. P. Parker of these laboratories and W. C. Johnston of the Westinghouse Research Laboratories in the design and construction of apparatus and instruments is gratefully acknowledged. The cooperation of G. T. Lalos of the Combustion Section of the National Bureau of Standards in the development of the metering systems and in photographing the flames also was most helpful. LITERATURE CITED
(1) Am. Soc. Mech. Engrs., Special Research Committee on Fluid Meters. “Fluid Meters. Their Theory and Amlication.” .. 2nd ed.: Part 1, 1927. (2) Anderson, J. W., and Fein, R. S., J. Chem. Phys., 18, 441 (1950).
Barret, P., Pubs. sei. et tech., direction inds. aeronaut. (France), Notes tech. No. 33 (1950). Coward, H. F., and Payman, W., Chem. Revs., 21, 359 (1937). Denues. .A. R. T.. and Huff, W. J., J . Am. Chem. SOC.,62, 3045 (1940). Diecke, G. H., and Crosswhite, H. M., Johns Hopkins University Applied Physics Laboratorv, Bumblebee Rept. No. 87 ( S o vember 1948). Fastie, W. G., J . Optical SOC.Am., 40, 800 (a)(1950). Gaydon, A. G., Nature, 165, 170 (1950). Gerstein, M., Levine, O., and Wong, E. L., Natl. Advisory Comm. Aeronautics. RM E50G24 (SeDt. 28. 1950). Grove, J. R., Hoare, M. F., and Linnett, J. W.,T r a n s . Faradau Soc., 46, 745 (1950). Hirschfelder, J. O., and Curtiss, C. F., J . Ch.em. Phys., 17, 1076 (1949). Johnston, W. C., S . A . E . J o u r n a l , 55, 62 (December 1947). Kretzschmer, F., Forschungsheff 381B,7 (1936). Lewis, B., and von Elbe, G., J . Chem. Phus., 2,537 (1934). Mallard, E., and Le Chatelier, H., Ann. M i n e s , [8] 4, 274 (1883). Markstein, G. H., “Third Symposium on Combustion,” p. 162, Baltimore, Md., Williams & Wilkins, 1948. Payman, W., and Wheeler, R. V., Fuel, 8,204 (1929). Smith, F. A,, Chem. Reus., 21, 389 (1937). Smith, F. A., and Pickering, S. F., J . Research N a t l . B u r . Standards, 3, 65 (1929). Strong, H. M., and Bundy, F. P., Third Symposium on Combustion. KID. _ - 641. 647, Baltimore, Md.. Williams & Wilkins. 1948. Tanford, C., and Pease, R. N., J. Chem. Phys., 15, 431, 861 (1947). ,- - . ,. (22) Wohl, K., and Kapp, N. M., Project Meteor, University of Delaware, Report UAC-42 (October 1949). I
RECEIVED June 7,1951.
HYDROCARBON FLAME SPECTRA. GEORGE A. HORNBECK
AND
ROBERT C. HERMAN
Applied Physics Laboratory, The John Hopkins University, Silver Spring,
Hydrocarbon flame spectra have been surveyed in the range -2000 to -9000 A. in order to establish the identity of band systems obtained under a variety of flame conditions. The sources and techniques required to enhance the spectra of specific molecular species by means of variations in oxygen-fuel ratio are described. The experimental results are presented chiefly in a selected series of densitometer tracings of bands obtained under a wide variety of oxygen-fuel ratios. The spectra of diatomic and polyatomic molecules, particularly the “hydrocarbon” and
“deuterocarbon” flame bands, are discussed and some comments are made concerning the kinetic mechanisms TI) OH bands are reinvolved. Several electronic (2Z ported. The results of this work are significant principally in that they provide the spectroscopic investigator of flames with a pictorial means of rapid identification. This was made possible by the fact that this survey was carried out at fairly high dispersion and with systematic variations of the oxygen-fuel ratio which enhanced specific molecular bands and made their identification relatively simple.
H E combustion of hydrocarbons has been studied intensively for many years because of its theoretical and practical in; terest. As a matter of fact flame spectroscopy in general is a subject which has held the attention of both physicists and chemists for many years. Flames have been used as sources of particular band systems for the study of molecular spectra but perhaps more generally by the chemist or investigator of combustion reactions for obtaining spectra or spectral variations in the hope that these might be useful in understanding kinetic mechanisms. A rather complete survey of the field of spectroscopy as applied to the theory of combustion has been given by Gaydon (8). The spectroscopic technique is a useful one for studying flame sources in that it clearly does not in any way affect the conditions under which the reaction occurs. It allows the identification of a t least some of the radicals and molecules that exist in the flame and a determination of their ewrgy states. Furthermore, while difficult, the examination of relative intensities affords information concerning the relative concentrations of the various species
as well as indicating the degree of thermal equilibrium. On the other hand, it is not clear in many cases whether or not all the molecular species detected in flames by spectroscopic means have significance with respect to the chemical mechanisms involved in flame reactions. Another difficulty is that frequently one readily obtains spectra which are not so easily identified because of their complexity. Also, many flame sources are so weak that adequate spectra are indeed difficult to obtain and, as will be mentioned later, one of the problems in this field is the development of suitable flame sources which yield spectra of high intensity and good contrast. An examination of the literature concerning hydrocarbon flame emission spectra in the photographic regions reveals the almost complete absence of high dispersion work. Most of the spectroscopic studies have been either a t low or moderate dispersion because of the speed of prism instruments, the paucity 6f grating instruments in the past, and because a t lower dispersion it has been felt that band heads are more easily observed. However, it is found under these conditions that while many of the most in-
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