Infrared Spectra of Propellant Flames ARTHUR D. DICKSON', BRYCE 1. CRAWFORD, JR., AND DAVID L. ROTENBERG
School of Chemisfry, Universify of Minnesofa, Minneapolis 74, Minn.
Propellant strands were burned in a vessel that permitted simultaneous observation by a rapidscan spectrometer and a movie camera while the nitrogen surrounding the strand was maintained a t a pressure of 100 or 150 Ib. per sq. inch gage and a linear velocity of 100 cm. per second. The intensities of the absorption bands of carbon monoxide and carbon dioxide were followed simultaneously to a distance of 2 cm. from the surface. The absorbances of these molecules were plotted against the distance from the solid surface. The spectral region from 4000 to 600 cm.-l was surveyed, and various absorption bands were observed but were not positively identified. Special attention was given to the region close to the surface. The fragments found there contain the greatest amount of information about the mechanism of burning.
T
H E study of propellant flames and related gas-phase reactions has been carried on at Minnesota since 1950 ( 3 ) , with the purpose of determining those contributions t o the knowledge of flame kinetics which can be obtained through infrared methods. Llass spectrometric analyses and other analytical treatments of quenched products have indicated the stable molecular species t o be found in the various zones of a burning propellant ( 2 ) . It is felt that excited molecules and other unstable species which exist in the flame but m-hich cannot be observed by those sampling methods may have a significant role in the decomposition process. The feasibility of observing these, as well as the stable molecules, was investigated. It was also hoped that the spatial resolution of the observations by probing techniques could be exceeded by this method.
pneumatic detector was amplified, it produced a n oscilloscope deflection which was recorded photographically. This instrument has proved t o be adequate for all requirements. A more permanent problem was presented by the low absorption coefficient of the molecules. T o obtain a sufficiently great absorption t o permit accurate measurement, a great many molecules must be put in the light beam. This is customarily accomplished by using a long absorbing path length or high concentration of the molecules, but this is not easily done when the gases are in a flame. KO effective solution has been found. It was planned t o measure the absorption of the gases a t close intervals outward from the burning surface by using a beam passing through the gases in a direction parallel t o the surface. T h e conditions t h a t would best permit these data t o be obtained would be reached by having the burning surface remain flat and parallel and by having the gases flow away smoothly in a n inert atmosphere. With these goals in mind, the experimental work was begun. TOP VIEW
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INLET
/A
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Figure 1.
Gas cell and filling line
Figure 2.
High pressure cell
A.
The first problem was the design of an instrument capable of obtaining data in the short time a flame could be maintained. For this purpose a rapid-scan infrared spectrometer which was capable of producing a t least 20 spectra per second (3) was designed, built, and operated. Until the rate of 10 spectra per second was exceeded, there mas no appreciable decrease in the absorption at band maxima. The resolution, of course, was low because of the speed and range of the scan. The scanning was accomplished by the use of a sinusoidally oscillating Littrow Mirror in a conventional Perkin-Elmer Model 1 2 4 spectrometer. The scanning range was continuously variable and at a moderate setting would cover two drum revolutions. This corresponded to about 500 cm.-' in the carbon monoxide-carbon dioxide absorption region. After the signal from the Golay 1 Present address, E. I. du Pont de Nemours & Co., Electrochemicals Department, Wilmington, Del.
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Strand holder and feed B. Screens C. Crystal windows D. Glass windows Internal crass section = 1 3 / 4
X 4 inches
A measure of the quantities of various gases in the flames was obtained by a low temperature-low pressure decomposition of approximately 0.1 gram of a nitroglycerine-nitrocellulose type double-base propellant. The propellant was placed in the borosilicate glass tube at A in Figure 1, and the system was then evacuated. T h e tube was gently warmed with a Bunsen burner Aame until it decomposed. T h e pressure of the liberated gases was measured and a n infrared survey spectrum of the gases collected in cell B was run. The bands were identified as characteristic of carbon dioxide, nitrous oxide, carbon monoxide, and nitric oxide; a oalculation based on the data indicated t h a t a propellant
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Figure Figure 3.
7. 1 6 - m m . photographs of '/*-inch strand propellant powder at 150 Ib. per sq. inch
Atmospheric carbon dioxide
(2350cm.-l band) FLAME OF 3/8 X I
POWDER STRAND
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Figure 8 Figure 4.
Flame spectrum of carbon dioxide ( 1 1 rnm. from surface) co2 FLAME OF
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Figure
5. Flame spectrum of carbon monoxide (25mm. from surface)
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co FLAME OF 3/8X 1/2
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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
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ROCKET PROPELLANTS
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318 X 3/8 NITROCELLULOSE STRAND (20%NG) 200 p s i NITROGEN NO INHIBITOR
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strand of ‘/rinch thickness would produce a sufficiently high concentration of these four gases to permit ready observation. A vessel was designed t o hold the strand while flushing nitrogen swept by a t a pressure of 150 lb. per sq. inch and a linear velocity of approximately 1 meter per second. Limited access t o the spectrometer beam dictated t h a t the burning surface of the propellant strand be vertical on an end-burning strand. This vessel is illustrated in Figure 2. Windows were placed in this vessel in such a fashion as t o permit simultaneous observation of the strand by the spectrometer and a movie camera. (The camera was needed t o determine how nearly perpendicular t o the strand sides the surface had burned during the observation.) The observation path of the camera was perpendicular t o that of the spectrometer. The exit orifice of the vessel was adjusted to give a linear flow rate t h a t held the flame t o a compact horizontal form. This rate was also sufficient to remove all the combustion products from the vessel. The flame lifted, of course, but not sufficiently t o i iterfere with the observation. The flames of commercial solid propellants were examined first. Large scanning amplitudes were used to survey the regions where bands were expected. The bands of carbon monoxide and carbon dioxide were observed readily, that of nitric oxide was scarcely detected, and nitrous oxide was not seen. The carbon monoxide and carbon dioxide spectra of Figures 3, 4, 5 , and 6 were obtained. The 16-mm. photographs of the burning strands in Figure 7 illustrate the contour of the burning surface. The surface of the strand was cooled by the nitrogen but no other inhibition was in effect. The data obtained during these runs are illustrated in Figures 8, 9, and 10, where optical density versus distance from the burning strand surface is plotted. The data were first interpreted t o indicate a region of low chemical activity where the absorption of these molecules increased slowly. This would be a distance up t o 10 mm. from the surface, which is approximately the length of the dark zone a t this pressure. A strand of 80% nitrocellulose and 20% nitroglycerine was obtained and yielded the curve of Figure 11. Here the dark zone seemed t o be absent. Several runs under different conditions yielded essentially identical data. Examination of the spectra made several facts apparent. First, the peak optical densities of the bands were almost independent of absorbing path length: a I-inch path gave a spectrum that was indistinguishable from that of a ’/&ch path. And secondly, since the separation of the P and R branch peaks of a molecule is proportional to the square root of the temperature, it was possible t o determine that the apparent temperature of the gases yielding the spectra was approximately 400’ C. These two facts suggest that most of the observed peak ab-
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r\
COMPOSITE
Figure 12.
Temperature effect on band shape
sorption is due t o gases that have been cooled by mixing with the nitrogen. Thus, if the depth of the cooled layer is independent of the strand size, its absorption stays almost the same; the peak absorption due t o the hot gases is much smaller. The type of spectrum formed by a layer of gas covering a wide range of temperatures is demonstrated in Figure 12. This is derived from Herzberg’s book on diatomic molecules (1). The effect of the temperature is most significant in that it apparently lowers the peak absorption, and thus hot molecules are more difficult t o observe. The region of the flame that is closest to the solid surface is of major interest. Attempts t o study this region were made by reducing the effective burning rate t o allow a greater number of observations in the available time. This was accomplished by moving the strand a t some fraction of the burning rate so as partially t o compensate for it, but no attempt was made t o match the two rates. The data obtained were in no greater detail than is shown in Figures 8 to 10. There was serious interference by surface irregularities and by particles of ash that were raised during the burning. Although the authors cannot claim to have much positive data a t the present they have determined that a t least two molecules are present in sufficient concentration t o be observed. Measurements under a variety of conditions will be necessary t o determine the relation between the spatial variation of optical density and the chemical and thermal structure of the flame zones. Low pressures expand the zones and thus permit better observation. T o facilitate low pressure measurements, the apparatus is being modified t o allow the end-burning strand to be held vertically. Also, plans to study the low temperature-vapor-phase decomposition of simple nitrated hydrocarbons are being made. Literature Cited (1) Herzberg, G., “Spectra of Diatomic illolecules,” p. 126, Van Nostrand, New York, 1950. (2) Needham, D. P., Powling, J., Proc. R o y . SOC.( L o n d o n ) 232A, 337-50 (1955). (3) Wheatley, P.J., Vincent, E. R., Rotenberg, D. L., Cowan, G. R., J . O p t . SOC.Amer. 41, No. 10, 665-72 (1951); Cowan, G. R., Vincent, E. R., Crawford, Bryce, Jr., Ibid., 43, No. 8 , 710-11 (1953); Eightingale, R. E., Cowan, G. R., Crawford, Bryce, Jr., J. Chem. Phys. 21, 1398-9 (1953); White, H. F., Cowan, G. R., Rotenberg, D. L., Crawford, Bryce, Jr., Ibid., 21, 1399 (1953). RECEIWD for review October 18, 1965. ACCF~PTED February 20, 1956. This work was supported by the U. S. Navy, Bureau of Ordnance, through contract with the University of Minnesota.
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