THE MECHANISMS OF LEAN LIMIT EXTINGUISHMENT OF AN …ronney.usc.edu/AME513b/Lecture5/Papers/Jarosins... · Nineteenth Symposium (International) on Combustion/The Combustion Institute,

Nineteenth Symposium (International) on Combustion/The Combustion Institute, 1982/pp. 1549-1557

THE MECHANISMS OF LEAN LIMIT EXTINGUISHMENT OF AN UPWARD AND DOWNWARD PROPAGATING FLAME IN A

STANDARD FLAMMABILITY TUBE

J. JAROSINSKI,* R. A. STREHLOW AND A. AZARBARZINt Department of Aeronautical and Astronautical Engineering

University of Illinois at Urbana-Champaign**

The mechanisms by which lean limit upward and downward propagating flames extinguish in a standard flammability tube (51mm ID, 1.8m long) were deduced using schlieren and direct light photography and temperature measurements and by observing the behavior of the flame under transient loading. The lean methane-air system was studied. The upward propagating flame takes the shape of a stable hemispherical cap attached to a trailing skirt. This flame always extinguishes first at the holding region on the axis of the tube and a failure wave subsequently washes down the flame until extinguishment is complete. The extinguishment occurs because the flame is stretched at the tip and heat loss to the walls is not important to this extinguishment. On the other hand the extinguishment of a downward propagating flame is a multistep process. First heat loss to the walls causes the flame to extinguish near the walls and a smaller residual flame is left in the center of the tube. Once this happens, differential buoyancy of the hotter central gases and the cooler surrounding gases forces the cooler gases to move ahead of the flame. Finally, the downward propagating flame is observed to rise slightly. At this point the flame extinguishes completely.

Introduction

Flammability limits were first recognized by Humbolt and Gay Lussac (1) and first determined by Davy (2) over 160 years ago. The empirical observation that flammability limits are apparatus de- pendent was well summarized by Coward and Jones (3) almost 30 years ago. At that time they proposed that the standard apparatus for determining flammability limits be a 51mm diameter, 1.8m long vertical tube closed at the upper end and open to the atmosphere at the bottom. Using this tube, if a mixture, when ignited at the bottom propagates all the way to the top the mixture is said to be flammable. However, if the flame extinguishes part way up the tube the mixture is said to be nonflammable.

Prior to 25 years ago there was very little theoretical effort directed toward extinction mechanisms and the effect of boundary and initial con-

*Current address: Dr. Jozef Jarosinski, Institute of Aeronautics, Warsaw, Poland.

tCurrent address: Ardeshir Azarbarzin, Sun- strand Aviation, Department 715, Rockford, Illinois 61101.

**A portion of the work reported herein was performed at the Institute of Aeronautics, Warsaw, Poland.

ditions on the actual extinction process. In the intervening years a number of simple theories for limits have been proposed. Each of these has focused on a single mechanism of extinction such as heat loss from a one dimensional flame (4-8) convection and/or buoyancy (9-10) or chemical kinetics (11). In more empirical approaches, flame stretch (12) has been invoked for extinction of an upward propagating flame (10). It has also been postulated that preferential diffusion is operative at extinction (13).

Unfortunately, there has been very little effort directed toward examining the extinction process itself or determining the effect of boundary or initial conditions on flames which are propagating very near the lean limit composition. Thus there is no real experimental understanding of the extinction process itself. This, coupled with the sim- plistic theoretical approaches, means that we cur- rently have no real understanding of the mechanism of flame extinction.

This paper describes an experimental study of the effect of various initial and boundary conditions on near lean limit flames, using schlieren optics, to determine transient response characteristics, and thus gain some insight into extinction mechanisms. Additionally, temperature profiles for an upward propagating flame were measured and direct photographic observations of the extinction process were obtained.

1549

1550 IGNITION AND EXTINCTION

Apparatus and Experimental Procedure

The experiments were conducted in a vertical, 51mm, square flammability tube, about 1.8m long. The tube was constructed to match the geometry of a standard flammability tube (3) in all aspects except for the round cross section. The square cross section is necessary to allow good quality schlieren photography. The direct photographic observations of extinction were made in a standard cylindrical tube.

The square tube was constructed from two alu- minum and two plexiglass plates. Sehlieren quality glass windows replaced the plexiglass walls at the level of the schlieren beam. The ends of the tube were fitted with two rotating plate valves. One plate valve was fitted with a mixing and plenum chamber which was used to fill the tube. The other valve could only seal the tube or open it to the atmosphere. Either valve could be mounted on either end of the tube. In this way filling could be effected from either end of the tube and either end could be placed in the "open" or "closed" poisition just prior to ignition.

Both tubes were filled by displacement; about 10 tube volumes were passed through the tube before ignition. The steadiness of the flow and sensitivity of the readings were checked with great care, especially for experiments in which the flame was expected to extinguish in the tube. In spite of the fact that great care was taken in adjusting the flow to control the composition when attempting to study extinction, flame extinction was found to take place in the test section on only about 10% of those runs.

During all experiments one end of the tube was always open. For those experiments in which the mixture was ignited from the open end, ignition was usually effected using a match flame. The tube was also equipped with spark and hot wire ignition systems to allow ignition of the mixture at different locations inside the tube,

A parallel beam schlieren system for photographing the flame used two parabolic mirrors of 0.3m diameter and 2.44m focal length and a horizontal knife-edge. The 35ram drum camera was focused on the tube axis and a half silvered prism located just behind the knife-edge directed the reflected beam to the camera and the undeflected beam to a screen, making visual observation of the schlieren image possible while the photographs were being taken. Repetitive photographs were obtained using a stroboscopic light source.

Gas phase temperature profiles were measured using a horizontally positioned unshielded DISA 10 Ixm Pt - Pt 10% Ir probe with a 2.2mm wire length. The response time of this probe is suffi- ciently rapid that i t followed the local temperature of the gas. No corrections for losses were made so that only the qualitative characteristics of the tern-

perature profiles can be considered. Photographs of the extinction process using the

luminosity of the flame were obtained in a standard cylindrical tube using a Bolex 64 frame/sec 16 mm movie camera and a Javelin Electronics image intensifier.

Results

Standard upward or downward propagation and extinction

Direct visual observations of flame propagation showed that the behaviour of the flame close to the flammability limit composition in the square tube is very similar to that of a flame in a "standard" 51ram diameter round tube. Also, the flammability limits obtained for upward and downward propagation were the same as those measured using a "standard" 51mm diameter, round tube and close to those reported in the literature. It was found that the lean limits for methane-air mixtures are 5.25% and 5.85% methane for upward and downward propagation respectively. The results were highly reproducible. Schlieren images of an upward propagating flame front are shown in Fig. la. As in Levy's experiments (14), schlieren records showed that after extinction the rising bubble of hot product gases continued to move at the same speed as that of the flame without any significant change in shape for a distance of approximately 0.20m, as shown in Fig. lb. At that point the hot gases continued to propagate further, but after some time the top of the bubble became flat. Finally the schlieren image disappeared.

The visual observations of flame extinction are confirmed by the visible light movies taken with the Bolex camera and image intensifier. Figure 2 shows a typical sequence. Extinction for this up-

FIc. 1. The upward propagating flame a) (upper sequence) just prior to extinction; b) (lower sequence) just after extinction. Framing rate; 55 frames/ see. Time is increasing from left to right.

UPWARD AND DOWNWARD PROPAGATING FLAME 1551

FIC. 2. The extinction of an upward propagating flame. Photographs of the light emission during extinction. Framing rate; 64 frames per second. Time is increasing from top to bottom and then from left to right.

ward propagating flame always started at the tip and the extinction wave washed down the sides of the flame towards the skirt. Ths behavior is shown graphically in Fig. 3. Once extinguishment had started, the apparent upward velocity of the skirt of the flame increased somewhat. The long conical shaped flame shown in the last frame of Fig. 2 disappeared completely in the next frame. This is shown as a vertical dotted line in Fig. 3. The fact that the schlieren image is known to continue to rise at a constant velocity after extinguishment is shown schematically by a dash dot line in Fig. 3.

Temperature measurements were made at sta- tions 5, 10, 15, 20 and 25mm from the wall for an upward propagating flame for mixtures which .contained 5.25 and 5.86% methane. The results of these measurements are summarized in Fig. 4. The temperatures in this figure are not absolute temperatures but only indicate relative temperatures.

Two interesting features that this figure shows are: 1) the temperature drops close to room temperature very rapidly behind the flame and, 2) temperature profiles near the center of the flame are markedly different for a flame that is very close to extinction than for a flame which is quite far removed from the extinction limit.

The downward propagating flame exhibited two distinctly different behaviours. In a small fraction of the runs the flame shape was stable and con- sisted of an essentially flat flame that contained 4 to 6 cells, which retained their identity throughout the full length of the tube. More often, however, the flame propagated with a low frequency "slosh- ing" motion.

In every case, before the extinction of a downward propagating flame, the flame front decreased its speed, stopped, or sometimes rose in the tube and a small tongue of low density gas moved down-


250 I I I

Trajectory of Schlieren --...,~ .~. Image .~ , ' - Upper Edge I " "

200 / y i T i p of ,Flame of Fbme

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0 I I I 0 I0 20 30 40 Frame Number (64 Fromes/sec)

FIG. 3. The extinguishment process as recorded by visible light extinguishment. The vertical dashed line represents the fact that no luminosity was pho- tographed on the very next frame. Note that the trailing edge of the flame skirt speeds up once the flame starts to extinguish at the tip,

ward into a portion of the fresh mixture. This is shown in Fig. 5. After that happened the sharp gradient between the hot gases and cold mixture disappeared slowly.

Image intensifier photographs of the downward propagating flame confirmed that the flame did not occupy the full area of the tube for some time prior to extinction (Fig. 6). Visual observations made while photographing these flames also confirmed that the downward propagating residual flame would rise in the tube just before final extinction.

a) Propagation from the closed end of the tube

Ignition of the mixture at the bottom closed end of the tube caused upward propagation of flame with a speed which depended on the mixture composition. Because the flame speed in this case was augmented by expansion of hot gases, the flame surface was initially much larger than for conven- tional upward propagating flames and the shape of the top part of the flame had a smaller radius of curvature.

Experiments with very lean mixtures, between 5.25% and 5.58% methane in which ignition was at the upper closed end of the tube led to a down-

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FIG. 4. Temperature measurements in an upward propagating flame taken very near the limit composition and at a composition well removed from the limit composition. Notice the striking differ- ences in the thermal structure of the flame near the flame tip. The much lower temperatures in the limit flame are undoubtedly caused by excessive flame stretch in this region of this flame.


and 7.25mm respectively. It was found that 9.00mm is the quenching distance for both upward and downward propagation at the limit compositions (5.25% and 5.85% methane, respectively). Chan- nels of 7.25mm width quenched flames which were leaner than 5.55% methane for upward propagation and 6.05% for downward propagation. Reverse flows through some of the spaces were observed, but in all cases extinction or even partial extinction was not observed when the mixture composition was richer then the lean limit composition and the distance between plates in the grid was larger than the quenching distance.

Fic. 5. Schlieren photographs of two downward propagating flames just prior to complete extinction. Notice the tongue of hot gas ahead of the main flame. This is a typical observation of the extinction process for a downward propagating flame.

ward propagating flame which eventually extinguished. Ignition led to flame propagation above 5.4% methane and the distance that the flame propagated before extinction was a function of methane concentration and increased as the mixture was enriched. Before downward extinction the flame was found to experience several oscillations in its propagation speed (Fig. 7). The flame always extinguished at the end of one of these oscillations, when its overall downward propagation speed was near zero. As for the case of downward propagation from an open end, the final flame was observed to rise in the tube during the extinguishment process (note frame 36 in Fig. 7). When the mixture composition is above 5.85% methane, the flame after several oscillations, becomes stable and propagates downward toward the open end of the tube with a speed that is equal to that of a flame propagating downward from the open end of the tube.

b) Propagation through grids

In another transient response study, a number of equally spaced vertical plane plastic sheets were placed inside the flammability tube. The plexiglass plates were 1.5mm thick and 103mm long and were spaced to divide the tube into 2,3,4,5 and 6 chan- nels of equal width of 24.75, 16.00, 11.63, 9.00

c) Upward propagation with a second flame producing an impulsive motion of the gas

An upward propagating flame, when subjected to a variable acceleration of the flow ahead of the flame was found to be very resistant to changing its shape. To investigate this case, two sources of ignition were prepared. The first one was located in the lower open part of the tube and the second one in the upper part of the tube, near the closed end. Ignition of the mixture at the lower open end of the tube caused upward flame propagation. The mixture then was ignited at the top when the upward propagating flame was close to the lower edge of the test section. The second source of combustion caused expansion of the gases above the first flame and therefore caused an impulsive motion of combustible gas toward the lower open end of the tube. This caused the first flame to slow in its upward rise, stop, and eventually appear to be propagating downward without change of shape (see Fig. 8).

Discussion

a) General comments

Two observations made during this study point to the fact that heat loss to the wall is very important downstream of the flame in a standard flammability tube. Temperature measurements taken in an upward propagating flame show that the gas temperture drops to 200~ approximately 250 mm behind the leading edge of the flame front. Fur- thermore, the experiments in which the flame was propagated from the closed end of the tube showed that after initially propagating at a very high velocity because of the expansion of the hot product gases, the flame quite quickly settled down to a propagation velocity which was equal to that of a flame propagating from the open end of the tube towards the closed end. This implies that heat transfer is so complete that there is no net expansion of the gases behind the flame after the flame


q v

FIG. 6. Visible light photography of the extinction of a downward propagating flame. Notice that the final flame occupies only a small central portion of the tube. Even though the photos do not show it directly, this downward propagating flame was observed to rise in the tube just prior to extinction. Fram- ing rate: 64 frames/second. Time is increasing from top to bottom and then from left to right.

has propagated about 0.5m. Experiments with grids in the tube show that

the quenching distance for the flame is approximately 9 mm at the lean limit for both upward and downward propagation. This is reasonable for upward propagation because the thickness of this lean limit flame is approximately 5 mm. It is interesting to note, however, that in these very weak flames the composition that has a specified quenching distance is determined by the direction of propaga- finn. In other words, since buoyancy influences the speed of propagation through the grid, it also changes the relative rate of heat transfer when compared to the relative rate of heat production by the flame and this has an influence on the quenching distance.

b) Extinction of an upward propagating flame

It is well known (12) that an upward propagating flame is being "held" at the centerline and is a stretched flame, in that each elemental area of the flame is increasing its area with time as it is being

washed from the tip towards the skirt of the flame. The fact that the flame shape does not change when it is impulsively pushed from above by another flame in the tube indicates that the flow ahead of the flame is irrotational and is not influ- enced by the boundary layer growth at the walls. von Lavante and Strehlow in a recent publication (15) have used this information to calculate the flame stretch and find it to be maximum at the tip of the flame. In another piece of work, Strehlow and Reuss (16) observed that under zero gravity conditions at 5.3% methane the flame cap in the tube was very similar to that of an upward propagating flame in a one g field, but that the overall propagation velocity is much less and the flame skirt is very short when compared to a one g flame. The implication here is that the shape of the flames holding region is determined by an interaction with the flow ahead of the flame (which can be treated as an irrotational flow) (15,17) and that the length of the flame skirt is simply a function of the ratio of the speed that the flame is progressing through the tube divided by the normal burning velocity.


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FIG. 7. The behavior of a flame in a 5.7% methane-air mixture ignited at the top closed end of the tube. On the left the shape of the sehlieren image of the flame at successive times (at 55 frames per second) is shown. The downward velocity measured as the maximum downward displacement per frame is shown on the right. The flame started to extinguish at frame 29. Note that the last flame image was observed on frame 36 and that the flame then occupied only a small portion of the tube and had risen from its position at frame 29.

This behavior is also confirmed by the observations of upward propagation away from the closed end of the tube reported in this paper. Initially, a very high propagation speed and long skirt is observed, with only a small decrease in the radius of curvature of the flame cap.

FIG. 8. Downward movement of an upward propagating flame due to secondary ignition of the mixture at the top closed end. Note that even though this entire flame system is being translated downward the shape of the flame remains idential to the ordinary upward propagating flame (see Fig. 1). The gray image on the wall of the tube is caused by moisture that condensed on the wall when the flame was at a higher location in the tube. Framing rate: 55 frames/second. Time is increasing from left to right

the tube yielded an oscillating flame that extinguished only when the overall propagation speed of the flame was very low. Figure 7 also shows that the last schlieren image of the flame (note Frame 36) was ~sing at the moment of extinction of the flame. The implication of these observations is that the extinction process for the downward propagating flame consists first of heat loss to walls which cools the gas in the region near the walls to the point where the flame can no longer propagate near the wall. After this happens, buoyancy effects carry "product" (burned) gas ahead of the combustion wave which is then extinguished because the flame is now propagating into a partially diluted (nonflammable) mixture. This mechanism would seem to be the result of an instability growth gen- erated by the experimental configuration and buoyancy. It therefore is not a true limit for the propagation of the combustion wave.

c) Extinction of a downward propagating flame

The schlieren observation that some weak thermal disturbance (not a flame)propagates ahead of the downward propagating flame just prior to extinction is confirmed by the fact that visual and photographic observations of the flames luminosity show that just prior to extinction the flame exists in only a small portion of the tube and is rising even though it is still a downward propagating flame. The transient experiment in which the flame was propagated downward from the closed end of

Conclusion

The primary conclusion of this paper is that the extinction mechanism of an upward propagating flame is markedly different than that of a downward propagating flame under one g conditions. The upward propagating flame quite obviously extinguishes by a stretch mechanism in which the holding region is stretched to extinction and the extinction wave then washes down the skirt of the flame while the leading edge of the hot product


gases still retain their constant upward speed. Heat transfer to the wall is of no importance during this extinction process. On the other hand, extinction of a downward propagating flame is triggered by heat loss to the wall and the flame is finally driven to extinction by differential buoyancy which forces cooler product gases ahead of the flame.

Acknowledgment

The work reported herein was supported by U.S. Army Research Office, Durham, North Carolina and NASA Lewis Research Center, Cleveland, Ohio. Dr. JaroSiflski performed some of these studies at the University of Illinois as a visiting scholar under the Mutual Educational Exchange Program. The remainder of his work was performed at the Insti- tute of Aeronautics, Warsaw, Poland.

REFERENCES

1. HUMBOLT, A. AND GAY TUSSAC, J. F., J. de Phy- sique, 60, 129-168 (1804).

2. DAVY, J., Phil. Trans. Roy. Soc., 106, 1 (1816). 3. COWARD, H. F. AND JONES G, W., "'Limits of

Flammability of Gases and Vapors", Bureau of Mines Report #503, 1952.

4. SPALDING, D. B., Proc. Roy. Soc. London, A- 240, 83 (1957).

5. MAYER, E., Combustion and Flame, 1, 438 (1957).

6. GERSTEIN, M., AND STINE, W. B., "Analytical Criteria for Flammability Limits", 14th Sym- posium (Int.) on Combustion, pp. 1109-1118 (1973).

7. BUCKMASTER, J., Combustion and Flame, 26 pp. 151-162, (1976).

8. BUCKMASTER, J., Combustion and Flame, 28 pp. 225-239, (1977).

9. LOVACnEV, L. A., Combustion and Flame, 17 pp. 275-278, (1971).

10. HERTZRERG, M., "'The Theory of Flammability Limits, Natural Convection", Bureau of Mines Report #8127, 1976.

11. MACEK, A., "'Flammability Limits: Thermody- namics and Kinetics" Center for Fire Research, National Bureau of Standards Paper #75-28, 1975,

12. STREHLOW, R. A., AND SAVAGE, L. D., Com- bustion and Flame, 31, pp. 209-211, (1978).

13. BREGEON, B., GORDON, A. S., AND WILLIAMS, F. H., Combustion and Flame, 33, 33 (1978).

14. LEVY, A., Proc. Roy. Soc., London A-283, 134 (1965).

15. VON LAVANTE, E. AND STREHLOW, R. A., Comb. and Flame, in press (1982).

16. STREHLOW, R. A., AND REUSS, D. L., Flamma- bility Limits in a Standard Tube Chapter III, "Combustion Experiments in a Zero-Gravity Laboratory ," Progress in Aeronautics and Astronautics, VoL 73, Ed. T. H. Cochran AIAA, New York, pp. 61-89 (1981).

17. MAXWORTHY, T. A., Phys. Fluids, 5, pp. 407-417 (1962).

COMMENTS

M. Hertzberg, Bureau of Mines, USA. Lean limits for methane-in air in the Bureau's 12 ft diameter sphere are measured to be near 4.9% in upward propagation and near 5.4% in downward propagation. For upward propagation the fireball rapidly rises by buoyancy and is elevated well above its original, central ignition point. It reaches the top of the sphere without propagating into the mixture below. The rising fireball's shape is also distorted by buoyancy. While I agree that the upward flame is extinguished because it is "stretched at the tip" and that "'heat loss to the walls is not important," it should be understood that it is the buoyancy process that is generating the flow field that stretches the tip. In our essentially, unconfined case, it is the flow field ahead of the upward-rising fireball that stretches the flame, and that flow field is induced by buoyancy or gravity. In the absence of gravity, propagation would be perfectly spherical

and there would be negligible stretch once the fireball propagated beyond the initial curvature cor- responding to the ignition kernel requirements.

Author's Reply. I agree with your comments. However, we have observed that the shape of an upward propagating flame in a tube near the center is controlled primarily by airodynamics and the rate of rise up the tube is controlled by buoyancy. Thus zero g flames have almost the same radius of curvature near the tip but have a shorter "skirt" because they travel through the tubes at a much lower speed. (~1

REFERENCE

1. R. A. STREHLOW AND D. REUSS, 'Flammability limits in a Standard Tube' in Combustion Ex-


periments in a Zero Gravity Laboratory, Ed. T. H. Cochran vol. 71 Progress in Aeronautics and Astronautics pp. 61-90 (1981).

M. Sibulkin, Brown University, USA. On the well known Bureau of Mines Bulletin 503 Ill on flammability limits a 5 cm tube was used because it was believed that larger tubes would not give signifi- cantly different flammability limits. However, if flame stretch is the determining factor in upward flame propagation, would not the decreasing flame stretch with increasing tube radius cause a contin- uing decrease in the flammability limit?

REFERENCE

1. H. F. COWARD AND G. W. JONES, "Limits of Flammability of Gases and Vapors" U.S. Bureau of Mines Bulletin 503, (1952).

Author's Reply. In lean methane air mixtures aerodynamically induced s t re tch in an upward propagating flow increases the local methane concentration or the centerline just ahead of the flow. Since a flame in a larger tube has a larger radius of curvature the limit % methane actually increases slightly with radius. However, for tubes larger than about 5 cm cellular instability appears on the flame and destroys the simple steady and axisymetric propagation that is observed in smaller tubes. We have not investigated what happens when the fuel is heavier than air.

T. Takeno, University of Tokyo, Japan. I con- gratulate you for a fine piece of work which has elucidated essential natures of the flammabili ty limits, especially for the upward propagating flame.

Is there any possibility to predict the limit in terms of the kinetic and thermodynamic properties of the mixture?

Author's Reply. Thank you for your kind com- ment. I feel that three Other papers at this symposium (1, 2, 3) clearly show the effect of preferential diffusion on the lean and rich extinction process for different fuels and that those results agree well with our now limited experimental observations.

In answer to your question, it is clear to me that even though we have furthered our understanding of the extinction process, we still do not know how to predict this behavior using fundamental kinetic and thermodynamic principles. I am convinced, however, that limit behavior must be ultimately related to reactions kinetic mechanism changes that occur in the neighborhood of 1000~ K due to the competition between chain branching and chain terminating reactions.

REFERENCES

1. S. ISHIZUKA AND C. K. LAW, This symposium, pp. 327-335.

2. H. Tsu]I AND I. YAMAOKA, This symposium, pp. 1533-1540.

3. J. SATO, This symposium, pp. 1541-1548.

Documents

THE MECHANISMS OF LEAN LIMIT EXTINGUISHMENT OF AN …ronney.usc.edu/AME513b/Lecture5/Papers/Jarosins... · Nineteenth Symposium (International) on Combustion/The Combustion Institute,