Burning velocity measurements by the constant-pressure bomb method

56

B U R N I N G VELOCITY M E A S U R E M E N T S BY T H E C O N S T A N T - P R E S S U R E BOMB M E T H O D

W. A. S T R A U S S

I N T R O D U C T I O N

The effect of pressure on the burning velocity of combustible gas mixtures has been established for a number of systems by the bunsen burner method 1. To substantiate these results, measurements were made using the constant-pressure bomb method. It was felt that this investigation was needed for the following reasons: (1) to extend previous results to the maximum operating pressure of 90 atm since for some systems stable flames at pressures above 21 '4 atm could not be established at all or in some cases only on extremely small burner tubes; (2) flame fronts of high-energy flames were very difficult to see and photograph because of a halo of intense light surrounding the cone. It was also considered desirable to check burner tests by an entirely independent method.

A P P A R A T U S A N D M E T H O D

The method used to measure the burning velocities of gas mixtures was similar to the atmospheric pressure constant-pressure bomb method developed by Stevens ~. Explosions of the gas mixtures were photographed by a high-speed strip film camera built in this laboratory. Since the gas expands as it burns, the unburned gases have an initial velocity; therefore, the actual burning velocity (Su) is obtained from the photographically deter- mined spatial velocity, St, according to the following expression:

S~

where E represents the ratio of burned gas volume to unburned gas volume. The expansion ratios may be obtained from direct measurements of initial and final bubble size or from calculations of adiabatic flame temperature and combustion gas composition assuming that complete chemical and thermodynamic equilibrium prevails in the burned gas. Since it was found to be difficult to photograph the bubble wall and since any error in the determination of the diameter is cubed in calculating the volume expansion ratio, it was decided to derive E from thermodynamic calculations of adiabatic flame temperature and combustion gas composition. The results of these calculations are to be published later a. According

and R. EDSE

to Simon and Wong 4 the calculated expansion ratios are in general very close to the observed values.

The apparatus used to measure burning velocities of gas mixtures by the constant-pressure bomb method is shown in Figure 1. I t consists of: (1) a large-volume, high-pressure combustion chamber, (2) a remote control apparatus to position the constant-pressure spheres in the chamber at high pressures, (3) a device to ignite the high-pressure gases, (4) a device for producing the desirable gas mixture which forms the contents of the bubble, and (5) a high-speed strip film camera to record the explosion.

The design of the high-pressure chamber is shown schematically in Figure 2. I t was made of a 10 ft length of 12 in. pipe. I t has four 7]8 in. quartz windows which are used to project light into the chamber, to observe during bubble manufacture, and to photograph the propagation of the flame.

Two types of constant-pressure bombs were used for these studies. These were: (1) the non- aqueous soap solution bubble, and (2) the trans- parent thin-walled latex balloon. Whenever possible the non-aqueons soap solution bubble was used. However, at present, experiments with cyanogen-air and ammonia-oxygen are made with rubber balloons since these gases are readily absorbed by the soap solution. The soap bubble firing column apparatus employs two pneumati- cally operated electrodes to produce a spark. A 3.5 cm diameter bubble is positioned in such a fashion that the tips of the two electrodes make contact in the centre of the bubble. A spark is produced by passing a current through the electrodes and separating them shortly thereafter. The non-aqueous soap bubbles were blown with a flat blower tip. Various types of soap solutions were testedS; however, best results were obtained with a glycerine solution containing fi'om 10 to 15 per cent by volume of Maypon 4C Special No. 2 detergent (Maywood Chemical Works, Maywood, New Jersey). This mixture is both temperature and age sensitive.

The rubber balloon firing column apparatus produces a spark in the same manner as the soap bubble apparatus. The rubber balloons (closed

377

STRUCTURE AND PROPAGATION OF FLAMES

Figure 1. Photograph of high-pressure constant-pressure bomb apparatus

fl2.90"ODX 10.75"1D STEEL PIPE

OXIDIZER I0' EXHAUST il~

Figure 2. Sketch of constant-pressure bomb combustion chamber

at one end) are m oun t ed in a collar, the open end of which is inserted on the firing co lumn apparatus . All pressure seals are made with r u b b e r O rings. The bal loon is purged by successively pressurizing and depressurizing it wi th the desired gas mixture. To facilitate the conduct ion of the experiments, a conveyor with an asbestos belt was installed inside the high-pressure combust ion chamber . This conveyor was used to deliver the collars (with r u b b e r balloons) to a yoke which is subsequently

used to a t t ach the collars to the firing column apparatus . The thin-walled, t ransparen t latex balloons were washed in a mixture of carbon tetrachloride and alcohol to remove the ta lcum powder.

A flow d iagram of the appara tus for manufac- tur ing the desirable gas mixtures is shown in Figure 3. A cont inuous flow of the gas mixture is produced from which a small a m o u n t of gas is w i thd rawn to blow the bubble .

378

BURNING VELOCITY MEASUREMENTS

A high-speed strip film camera (designed and constructed at this laboratory) was used to photograph progression of the flame front within the bubble. The film is attached to the periphery of a l0 in. diameter drum and held in place by a cam-locking device. The drum speed was made continuously variable from a maximum of 7,200 rev/min to a minimum of 6 rev/min. The timing mark light pulses are generated by a 1B59 glow

Carbon monoxide (Matheson): 96.8 per cent carbon monoxide, 0-36 per cent carbon dioxide, 0.97 per cent hydrogen, 1.0 per cent nitrogen, and 0"8 per cent saturated hydrocarbons

Nitric oxide (Matheson): 99.61 per cent nitric oxide, 0-37 per cent carbon dioxide, 0.02 per cent water

The air compressed at this laboratory was dried by activated silica gel. Iron penta carbonyl,

MANIFOLD

~ ' X H A U S T ~

~ ~--TO REGULATOR DOMES

~ HAND LOADING ~ ~7 REGULATOR FLOW INDICATOR THROTTLEVALVE [ J RESTRICTOR (RELIEVING)

REGULATOR ~REDUCING REGULATOR [] BACK PRESSURE (NON-RELIEF) REGULATOR

Figure 3. Flow diagram of constant-pressure bomb gas mixture apparatus

modulator tube which is excited by an R C oscillator n whose frequency is variable within the range of 40 to 20,000 c/s. The frequency of the oscillator is adjusted to give reasonable spacing of the t iming marks for a given film speed, and the frequency is measured by an AN/UAM-26 frequency meter, the rated accuracy of which is :~0-1 c/s. Non-perforated 35 m m Kodak Tr i -X and Dupont SX-Pan films were used. Both were processed in Kodak D-19 which was activated by adding Kodak 'Solution A' to give maximum emulsion speed.

The gases used for the experiments were pur- chased from commercial manufacturers with the exception of the high-pressure air which was compressed at this laboratory. Typical analyses of the commercial gases as given by the manu- facturer are as follows:

Oxygen (Linde) : 99.5 per cent oxygen, 0.3 per cent argon, 0.02 per cent nitrogen

Hydrogen (Air Reduction) : 99.92 per cent hydrogen, 0.08 per cent oxygen

Methane (Matheson): 93 per cent methane, plus ethane, propane, and small amounts of nitrogen and oxygen

carbon dioxide, and the saturated hydrocarbons were removed from the carbon monoxide gas by passing it over an activated charcoal trap cooled by dry ice.

Operation of the constant-pressure bomb apparatus described above is as follows: The fuel and oxidizer gas flows are adjusted to give the desired mixture by means of the flow control apparatus (Figure 3); the constant-pressure sphere mechanism and auxiliary lines are then purged with this mixture. The purged gas is not permitted to accumulate in the chamber but is exhausted directly to the atmosphere. During the purging time, the timing light frequency and strip film camera speed are adjusted to the desired values. A constant-pressure sphere of approximately 3-1/2 cm was then blown with the gas mixture. Next, the positive electrode piston is pressurized causing this electrode tip to contact the ground electrode at the centre of the sphere. The mixture within the bubble is exploded remotely by a switch which: (1) sends current through the electrodes, (2) actuates a solenoid valve which depressurizes the positive electrode piston, and (3) energizes two solenoids which trip both the shutter

379

S T R U C T U R E AND PROPAGATION OF FLAMES

of the t iming mark lens and tha t of the taking lens of the strip film camera. T he shut ter of the taking lens is opened jus t before the electrodes separate.

R E S U L T S

Carbon monoxide-oxygen flames The burn ing velocities of the ca rbon monox ide - oxygen system were measured at 1,5.1,21.4, 52 and 90 a tm. Figure 4 contains a series of photographs showing typical records of explosions of a ca rbon

flame. Since the low values of bu rn ing velocity of the double valued exper iments (based on initial port ion of explosion trace) fall a long the same curve which is defined by experiments whose fronts d id not oscillate, i t was assumed tha t the lower values at bo th pressures are most valid. Strehlow and Stuar t 6 observed high-frequency oscillations in flames bu rn ing at a tmospher ic pressure. In the present invest igation this type of oscillation did not occur at a tmospher ic pressure

I otmol~phere 52 otmospheres 9 0 otmollpheres 8 0 - 2 4 % CO 80-76 % CO 8 0 . 2 8 % CO

FILM SPEED 4GG.Icm.sec" FILM SPEED I O I O 4 r 1 6 9 FILM SPEED 1123.9em.sec "~

BURNING VELOCITY- - BURNING VELOCITY- - BURNING V E L O C I T Y - - 103"0 cm. s e e " 142"9 �9 17"/'-2 & 114.Otto. s e e "

Figure 4. Typical record of explosion of carbon monoxide--oxygen mixtures at various pressures

monoxide-oxygen mixture of approximate ly 80.25 per cent carbon monoxide at 1, 52 and 90 atm. Explosion traces at 1 and 5.1 a t m pressure were similar. The spatial flame speed was constant as indicated by the sharp, s traight- l ine bu r n i ng trace (after a short in i t ia t ion period). However , in some of the 21.4, 52 and 90 a t m pressure tests the bu rn ing fronts oscillated. T he ampl i tude of the oscillations increased bo th with the radius of bu rn ing and also wi th increasing pressure. For very r ich and very lean mixtures no oscillations were observed. T he wavy flame traces for these experiments were evaluated at bo th the initial and the final port ions of the bu rn ing trace. The final por t ion was evaluated by drawing an average line th rough the oscillations and considering this as the front along which an average flame would advance. For abou t 30 per cent of the experiments at 52 a tm a n d for most of those a t 90 a tm two values of the bu r n i ng velocity were ob ta ined as shown in Figure 5. According to these results the bu rn ing velocity increases wi th pressure for the final stage of burning , whereas it decreases with pressure for the initial p ropaga t ion of the

250

(J

O J eo

50

40 50

I I r

I~ 52 ATM {fNITIAL TRACE) I~ ,r~. 4TM ( F/NAL TR ACE )

Q 90 ATM (INITIAL i'RACE ) ~) 90ATM (FINAL TRACE )

r \

/- . . ~ ' ~ . , , ,

sO 70 80 90

PERCENT CARBONMONOX[0C IN MIXTURE

Figure 5. Comparison of carbon monoxide-oxygen burning velocities evaluated from initial and final portions of

explosion trace

380


possibly because of the large volume of the combustion chamber. However, the low-frequency oscillations encountered in some experiments were found to be related to the time required for sound waves to move across the chamber diameter. The oscillations are therefore believed to be caused by pressure waves reflecting from the chamber wall.

The results of the measurements of the carbon monoxide-oxygen burning velocities are graphi- cally depicted in Figure 6. This graph shows

�9 I ATM x 5'1ATM

- - - - - 52 ArM . . . . . 90 ATM

- " 140 / / , ~ -- .

~20

/

,o /l

2 0

0 3 0 40 50 6 0 70 80 9 0

PERCENT CARflON MONOXIDE IN MIXTUR Ir

Figure 6. Burning velocities of carbon monoxide-oxygen flames

clearly that the burning velocity increases with rising pressure up to 52 a tm and thereafter decreases.

~o I y "-r- - - r - - -~- T r �9 - - o I ATM

�9 90 ATM

40

i , G 2 5

?

1 .)-/ 0

/ \

I " - , i !

T -

\ \

15 20 25 30 55 40 45 flO 55 60

PERCENT CARBON MONOXIDE IN MIXTURE

Figure 7. Burning velocities of carbon monoxide-air flames

the burning velocities derived by the constant- pressure bomb method are approximately 50 per cent higher than those obtained with the burner method. The dip in the curve representing the burning velocities of methane-a i r mixtures burning at 90 atm cannot be explained at this time. Simon and Wong 4 pointed out that the bubble method is not suitable for the determination of low burning

4 0

3 5

3 0

, o tn 25

Carbon monoxide-air and methane-air flames

The carbon monoxide-air and methane-a i r ,e systems were investigated at 1, 21-4 and 90 atm. ~ 2o Burning velocities of these mixtures are plotted in Figures 7 and 8, respectively. As seen from these graphs the burning velocities of these mixtures > q5

decrease with increasing pressure. This result is in _~ agreement with the observations obtained by the z bunsen burner method for similar systems. How- ~ ~o ever, for the carbon monoxide-air flames the maximum value of the burning velocity as deter- mined by the bubble method is approximately 5 75 per cent above that obtained by the burner method while the maximum values of the burning velocity for given pressures occurs at the same 0 mixture ratio. For methane-ai r mixtures the peak burning velocities also occur at approximately the same mixtures; however, at 21.4 and 90 a tm

I | l ATM

21-4 ATM | 9 0 ATM

i

I

4 6 8 IO t2 14 f6 PERCENT METHANE IN M I X T U R E

Figure 8. Burning velocities of methane-air flames

381


velocities because of the convective rise of the bu rned gases. This effect of the b u r n e d gas is par t icular ly serious a t elevated pressures. Since the flame traces were ob ta ined by pho tograph ing the horizontal componen t of the spreading flame, the buoyancy would tend to yield lower bu rn ing rates. However, for these systems the bu rn ing velocities derived f rom the bubb le me thod are above those of the b u r n e r method.

Methane-oxygen, hydrogen-oxygen, and hydrogen-nitric oxide flames

The bu rn ing velocities of m e t hane - oxygen and hydrogen-oxygen mixtures were invest igated at 1, 21.4 and 90 a tm while the hydrogen-n i t r i c oxide system was invest igated a t 5-1, 21-4 and 52 atm. The results of these investigations are shown in Figures 9, 10 and 11. These graphs indicate tha t

| i ATM

20C

I00

\

J l 0 15 EO 25 30 35 4D 45 50 55 PERCENT METHANE IN MIXTURE

Figure 9. Burning velocities of methane-oxygen flames

500

400

,

~ooi

the burn ing velocities of these mixtures increase with increasing pressure. This observat ion is in exact agreement wi th the measurements obta ined with the bunsen bu rne r method. Where- as the m a x i m u m bu r n i ng velocities ob ta ined by the constant-pressure b o m b method at a tmospher ic pressure for the methane-oxygen and hydrogen- oxygen systems are again above those obta ined by the burne r method, at 21.4 a tm the reverse is true. The burn ing velocities of hydrogen-oxygen mixtures measured by the bubb le me thod at a tmospheric pressure are not very accurate since the explosion traces on the film were very faint and, therefore, difficult to evaluate.

m

2(100

I 0 0 0

5 0 0 0 ~ - 0 ! ATM

x 14,6 ATM 90 ATM

400C

| / /

4 0 50 6 0 7 0 8 0

PERCENT HYDROGEN IN MIXTURE

Figure 10. Burning velocities of hydrogen-oxygen flames

D I S C U S S I O N O F R E S U L T S

The preceding discussion has shown tha t quali ta- tive agreement exists be tween the results obta ined for the effect of pressure on the bu rn ing velocities of various combust ible gas mixtures according to two entirely different methods of measurement . In general, it can be stated tha t the burn ing

//~l,*

7 0

" G 0

5"1 arm O ==~ ' a //,o,.

3 0

2 0 3 0 4 0 5 0 6 0 7 0

PERCENT HYDROGEN IN MIXTURE

Figure 11. Burning velocities of hydrogen-nitric oxide flames

382


velocities of low-energy systems such as fuel-a i r flames decrease with increasing pressure whereas those of most high-energy systems such as fue l - oxygen mixtures increase with rising pressure for pressures between 1 and 90 a tm. At the present only two major exceptions to this rule are known: (1) the bu rn ing velocities of carbon monox ide - oxygen mixtures reach a m a x i m u m at 52 atm, a n d (2) the bu rn ing velocities of hydrogen-n i t r i c oxide mixtures are only moderately affected by pressure.

The quant i ta t ive relationships between b u r n i n g velocity and pressure for all systems that have been invest igated by the constant-pressure b o m b method in this laboratory up to this t ime are graphical ly shown in Figure 12. In order to

"* PRI~SSIJRE la t i n } Io Poo

Figure 12. The effect of pressure on the burning velocity of various gas mixtures

explain these relationships theoretically it is necessary to consider all factors which cont r ibute to the p ropaga t ion of a flame front th rough a combust ible gas mixture. For l aminar flames these factors are : (1) composit ion of gas mixture, (2) initial t empera ture and pressure of gases, (3) t ranspor t properties of init ial gases and reactants (heat and particle transfer rates), (4) chemical reaction rates, and (5) flame radiation.

For a given gas mixture at a fixed initial t empera tu re a change in pressure will have an effect on the t ranspor t phenomena , on the chemical react ion rates and on the rad ia t ion propert ies in the flame zone. In traversing the reaction zone of a s teady state flame normal to its p lane we pass

from the initial state of the u n b u r n e d gas through regions of different composit ion until we reach the burned gas. It may be assumed tha t for most flames at a certain point this b u r n e d gas is in a state of complete the rmodynamic and chemical equi l ibr ium at a t empera ture which is only slightly below the adiabat ic flame temperature . In any zone upstream of this p lane such equi l ibr ium does not exist. Therefore, according to the laws of thermodynamics , reactions leading to eventual equi l ibr ium occur in these zones. T h e rates of these reactions depend on the gas t empera tu re and on the concentrat ions of the reactants. Because of these variations of concentrat ions of particles and tempera ture in a direction normal to the flame front, diffusion and heat transfer occur. To formulate the cont r ibu t ion of the heat transfer is r a ther difficult because the hot molecules are also chemically different from the cold molecules. Therefore, heat transfer may be considered to be part ial ly dependent on diffusion of reac tants and intermediates. In any event the rates of these t ranspor t processes will depend p redominan t ly on the gradients of t empera ture and concentra t ion. W h e n the pressure is increased, these gradients may change for two reasons: (1) the thickness of the flame zone decreases, and (2) the t empera tu re and the absolute concentrat ions of particles in the bu rned gas may increase. Unfor tunate ly , very little is known abou t the effect of pressure on the flame thickness. Only the ad iaba t ic flame tempera tu re and the equi l ib r ium composit ion of the burned gas are amenab le to exact theoretical calculations at present. According to these calculations the m a x i m u m flame tempera tures of h igh-energy fuel-oxidizer mixtures such as methane-oxygen, hydrogen--oxygen, hydrogen nitric oxide, and carbon monoxide oxygen a m o u n t to approximate ly 3,000~ at a tmospher ic pressure while at 90 arm they z approximate 3,600~ The absolute concentrat ions of radicals (ni) such as OH, O and H in all h igh-energy systems are pract ical ly of equal order of magni tude . For a given mixture rat io they depend on pressure according to the following relat ionships:

n o n ~ pO+, no ~ pO.8, nu ~ p0.8

For an estimate of the cont r ibu t ion of diffusion of particles from the b u r n e d to the u n b u r n e d gas, however, the mole fractions of these particles have to be considered. According to the the rmodynamic calculations of the equi l ibr ium composit ion of the burned flame gases the mole fractions, y, of OH, O and H decrease with increasing pressure as follows :

70H ~ p-O.09, 70 ~ p-O.06, 7H ~ p-0.17

Spectroscopic measurements made at The Genera l Electric Co. 7 have shown tha t the actual

383


radical concentrations in the flame zone are higher than those derived from the equil ibrium composition of the burned gas. Assuming that the effective flame thickness practically does not depend on pressure, it appears that on the basis of the calculations of the equilibrium conditions in the burned gas the rate of diffusion in high-energy flames is reduced only slightly while the rate of heat transfer is increased by a fair amount. Therefore, it might be expected that the burning velocities of these systems increase with increasing pressure. This trend has been observed for a number of fuel-oxidizer flames as shown in the previous section. Since the adiabatic f a m e temperature does not increase indefinitely with pressure and since at very high pressures the mole fractions of the active radicals will decrease at a rate greater than that indicated by the equations for the 1 to 90 atm pressure range, the burning velocities of these systems should eventually decrease with rising pressure. Such a trehd was observed only for the carbon monoxide-oxygen flames.

In general, for flames of low-energy systems such as fuel-air flames the adiabatic flame temperatures are practically independent of pressure while the mole fractions of active radicals decrease rather sharply with pressure s . Therefore, the burning velocities of such mixtures should decrease with rising pressure if it is again assumed that the effective flame thickness is practically independent of pressure and that the chemical reaction rates are not so low as to become the rate determining link. In the previous section it was shown that the burning velocities of all fuel-air systems decrease with rising pressures for all mixture ratios. From Figures 6and 9 it can be seen that for rich mixtures of the high-energy systems the burning velocities also decrease with increasing pressure. This observation is in agreement with the fact s that the adiabatic flame temperatures of these mixtures do not increase with rising pressure.

It is true that the effect of pressure on the burning velocities of pre-mixed combustible gases m a y be explained by suitable assumptions con- cerning the mechanism of the chemical reactions in the various regions of the flame. However, it is believed that sufficient information on these mechanisms is not available at present to justify formulation of a theory including any possible contribution of the reac.tion mechanism to the effect of pressure on the burning velocity of flames of pre-mixed gases. It might be argued that in most cases the rates of the chemical reactions occurring in flames are rapid enough to disqualify them from being the rate-determining factor for the burning velocity. In order to inves- tigate the contribution of the chemical reactions to the mechanism of flame propagation we have started a series of experiments with binary fuel

systems following a suggestion made by Karl Scheller of the Aeronautical Research Laboratory at the Wright Air Development Centre, Wright- Patterson Air Force Base, Ohio.

In the present study of the mechanism of flame propagation the influence of the chemical reactions on the rate of flame propagation became strongly apparent only in the case of hydrogen-nitr ic oxide flames. Although the adiabatic flame temperature and the radical concentrations in the burned gas of this system are of the same order of magnitude as those of corresponding hydrogen-oxygen mixtures, the burning velocities of hydrogen-nitr ic oxide mixtures are only about 1/20 of those of hydrogen-oxygen flames. Furthermore, the burning velocity of hydrogen-nitr ic oxide flames increases only very moderately with pressure. According to the bunsen burner method it was found to be independent of pressure. The most obvious reason for the great difference in the behaviour of these two systems may be seen in the large activation energy of the initiation reaction of the hydrogen-nitr ic oxide reaction which probably is represented by the homogeneous bi- molecular decomposition of nitric oxide. The activation energy for this reaction has been reported as 75 kcal by Kaufman and Kelso a. However, Roslovskii 9 finds the activation energy to be only 50.7 kcal. He derived this value from measurements of the burning velocities of hydrogen-nitric oxide mixtures burning at pressul:es between 1.8 and 19 atm in constant-volume bombs assuming a third-order reaction. For a mixture containing 55 per cent nitric oxide Roslovskii found that the burning velocity depends greatly on pressure whereas for a mixture containing 33.8 per cent nitric oxide the burning velocity was described by him as being pressure independent. His numerical values for the burning velocities are much lower than those measured in this laboratory. The very small pressure dependence of the burning velocities of hydrogen-nitr ic oxide flames found in this laboratory must be considered as evidence against the assumption of termolecular initiation reaction in the flame.

That the low burning velocity of hydrogen-nitr ic oxide mixtures is not caused by the presence of nitrogen in the reacting gas can be seen from the fact that the burning velocities of hydrogen- oxygen-nitrogen flames 1~ whose gas compositions correspond to those of hydrogen-nitr ic oxide flames are approximately 8 times faster than those of the hydrogen-nitr ic oxide flames. The possible influence of nitric oxide on the burning velocity was investigated in this laboratory by adding 1 per cent nitric oxide to a stoichiometric hydrogen- oxygen mixture. The burning velocity of this mixture was about 5 per cent lower than that of a pure hydrogen-oxygen flame.

384


In conclusion it can be stated that the effect of pressure on the burning velocities of pre-mixed gas mixtures can be explained by the effects of pressure on the rate of diffusion of active radicals, on the rate of heat transfer from burned to unburned gas, and in cases where a chemical reaction is the rate determining process it can be explained by the effect of pressure on this chemical reaction. To some extent all these factors, including probably the flame radiation which increases greatly with pressure, are inter-related. Since they depend on temperature and pressure in a very complicated fashion, the problem of finding an analytical expression for the burning velocity as a function of pressure over a large range of pressures appears to be practically insurmountable.

This research was supported in part by the United States Air Force under Contract No. AF 33(616)-2833, monitored by the Aeronautical Research Laboratory, WCLJC, Wright Air Development Centre, with The Ohio State University Research Foundation.

REFERENCES 1 STRAUSS, W. A. and EDSE, R. Investigation of

flames burning at pressures up to 100 atmo- spheres, WADC Technical Report 56-49, 1956, ASTIA No. AD 103 095

2 STEVENS, F. W. A constant pressure bomb, NACA Report 176, 1923

a STRAUSS, W. A. and EDSE, R. Adiabatic Flame Temperatures, Combustion Gas Composition and Expansion Ratios of Combustible Gas Mixtures at Various Pressures. In press, WADC

4 SIMON, D. M. and WoNo, E. L. Evaluation of the soap bubble method for burning velocity measurements using ethylene-oxygen-nitrogen and methane-oxygen-nitrogen mixtures, NACA Technical Note 3106, February 1954

5 KAUFMAN, F. and CooK, H . J . Non-aqueous soap bubbles for flame studies, B.R.L., Aberdeen Proving Grounds, Tec.hnical Note 575, January 1952

s STRgHLOW, R. A. and STUART, J. G. An improved soap bubble method of measuring flame velocities, Aberdeen Proving Grounds, Report No. 835, October 1952

7 General Electric Review 61 (1958) 8 8 KAUFMAN, F. and KELSO, J. R. Kinetics and

mechanism of high temperature reactions of nitric oxide, B.R.L., Aberdeen Proving Grounds, Report No. 923, August 1954

9 ROSLOVSKn, A. I. Combustion of Mixtures of Nitric Oxide With Hydrogen, Translation No. 656, 1957. Royal Aircraft Establishment, Ministry of Supply

10JAHN, G. Der Zi~ndvorgang in Gasgemischen. 1934. Berlin; Oldenbourg

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Burning velocity measurements by the constant-pressure bomb method