Microgravity observations of premixed laminar flame dynamics

Twenty-Fourth Symposium (International) on Combustion/The Combustion Institute, 1992/pp. 177-187

MICROGRAVITY OBSERVATIONS OF P R E M I X E D L A M I N A R F L A M E D Y N A M I C S

COREY M. DUNSKY*

Department of Mechanical Engineering University of California, Berkeley

Berkeley, CA USA

Microgravity experiments have been performed to investigate the role of buoyancy in the structure and stability of premixed laminar flame fronts. Planar and spontaneously wrinkled (cellular) flames burning in propane-oxygen-nitrogen mixtures and propagating slowly downward through a 6 cm diameter tube against a flow of fresh gas were filmed under normal and low gravity conditions in the NASA Lewis 2.2 Second Drop Tower. Results show that downwardly-propagating flames that are planar in normal gravity become curved toward the unburned mixture in reduced gravity, forming a single cell nearly filling the tube. This behavior is consistent with theoretical predictions that fiat flames are possible only when the stabilizing effect of buoyancy in this geometry balances the destabilizing effect of the hydrodynamic instability mechanism. Thus when gravity is reduced, this mechanism is unopposed and produces a curved flame.

Behavior of cellular tube flames in microgravity is considerably more complex. Mixtures producing small cells in normal gravity produce flames with larger, irregular cells in low gravity. Often in the microgravity tests, one cell grows at the expense of the others, which move downstream and disappear. The remaining cell propagates through the tube as an oblique curved flame, while smaller cells form and disappear along the front. These observations are interpreted qualitatively in terms of the predicted effects of buoyancy on flame stability and cell sizes. It is concluded that while buoyancy is not the dominant mechanism in the formation of the cells, it contributes significantly to the overall flame stability and structure.

Introduction

Cellular flames are an example of a flame front instability in premixed combustion. They are com- monly (though not exclusively) observed near the flammability limits of systems that are stoichiomet- rically deficient in the component with lower molecular weight, e.g., rich hydrocarbon-air or lean hydrogen-air mixtures. In general, these flames appear as roughly hemispherical cups, convex toward the unburned gas and separated by dark ridges. These non-planar and often unsteady flames are viewed by some as intermediate modes in the tran- sition from laminar to turbulent flame propagation.

Current theoretical models attribute the cellular flame phenomenon to an instability resulting from the interaction of several mechanisms. Reviews of recent theoretical treatments are given in Refs. 1- 3. Identified mechanisms include molecular diffusion of mass and heat to and from the flame front,

*Current Address: Combustion Applications De- partment, 8362, Sandia National Laboratories Liv- ermore, CA 94551-0969 USA.

respectively (the so-called thermo-diffilsive instability); heat loss from the flame; the hydrodynamic (Darrieus-Landau) instability, and buoyancy (Ray- leigh-Taylor instability). These can exert either a stabilizing or destabilizing influence, depending on the mixture composition (diffusion effects), direction of flame propagation (buoyancy), and experiment configuration (heat loss). Despite several de- cades of study, however, the relative importance of these mechanisms remains unclear, and a recent review 3 has noted the possibility of resolving the controversy through microgravity experiments. This study, therefore, investigates the normal-gravity (1 g) and microgravity (;xg) structure and stability of both planar and cellular flame fronts, in order to further understanding of the relative importance of the above mechanisms.

In the current work, planar and cellular flames burning in propane-oxygen-nitrogen mixtures propagate slowly downward through a 6 cm diameter tube, against a steady flow of fresh gas. The tube configuration largely eliminates the effect of stream- wise heat loss, which has been found to exert an important stabilizing influence on burner flames. 4'5 The flames are filmed under 1 g and Ixg conditions

177

178 LAMINAR FLAM ES--DYNAMICS

g

Chamber Open at Top -->...

1

/

Fine Wire Mesh

~ , Ignitor

,ll II~

Pyrex Iube Open at Top

~Lexan Window

FIG. 1. Schematic of experimental apparatus. Fresh mixture is introduced into the tube through a porous plug, and is spark ignited near its open top. One wall of the experiment chamber is Lexan to provide optical access, and the chamber top is left open to maintain constant pressure conditions.

in the NASA Lewis 2.2 Second Drop Tower, and the observed differences in flame structure are interpreted in terms of the expected effects of buoyancy.

Experimental Apparatus and Procedure

The porous burner, the low-gravity experiment package, and the NASA Lewis 2.2-Second Drop Tower used in these experiments are discussed in detail elsewhere.l's Figure 1 describes details of the experiment configuration. For these tests, a 15 cm long Pyrex tube of 5.9 cm I.D. was mounted downstream of the burner surface. A combustible propane-oxygen-nitrogen mixture with known equiva- lence ratio, ~b and oxygen index, 6 ~- [02]/{[02] + [N2]} was introduced through the porous burner with volume flow rate Q. Ignition was accom- plished with a spark near the mouth of the tube, resulting in a flame propagating toward the burner. The tube mouth was open to the surroundings but covered with a fine-mesh screen to eliminate 1 g buoyant entrainment of ambient air near its walls.

In all tests, the flames propagated against a flow of unburned mixture. Tests were confined to slowly- propagating flames, corresponding to mixtures with higher nitrogen dilution than air, i.e. 0.140 -< 6 -< 0.180. Flames in the selected mixtures exhibit laminar burning velocities in the range 7 -< uL -< 15 cm/s, t'6 and by tuning the speed of the approach flow, flame propagation rates relative to the tube

wall as low as 1 cm/s were achieved. This allowed time to establish a slowly-propagating flame in nor- mat gravity, initiate the 2.2-second low-g interval, and allow the flame to traverse the length of the tube and settle near the burner while still in reduced gravity. Alternatively, in some tests the flame was not ignited until the package was in low gravity conditions. This procedure eliminated any residual natural convective flow in the tube generated during a 1 g burn which could affect the low-g flame.

Since changes in flame propagation speed have been observed in previous low-g laminar flame studies, 7's it was anticipated that this would occur in the present experiments as well. Consequently, it was necessary to maintain a very steady velocity in the flow of fresh mixture, so that any changes in flame propagation occurring in low g could be at- tributed solely to gravity effects. The low-gravity drop package flow system had a measured repeat- ability in the flow velocity better than 1% over the course of four to five experiment runs.

At the flow speeds used here, the wall boundary layer should grow to an appreciable fraction of the tube radius at the tube exit, resulting in transverse velocity gradients. Measurements near the porous plug and at the tube exit, however, showed no significant changes in the velocity profile over the length of the tube. l In addition, 1 g fiat and cellular flame behavior was not observably influenced by boundary layer effects, and any transverse velocity gradients should not have affected the differences between 1 g and I~g flame dynamics discussed below.

As in previous low-gravity burner experiments, 5 side views of tile flame structure were recorded at 64 frames/second with a variable-speed 16 mm camera, using flame self-illumination. The Kodak color high-speed Ektachrome EF (tungsten) film was force-processed 2 f-stops to enhance flame images.

Results

A total of 53 drop tests were performed with the downwardly-propagating tube flames. In addition, 81 normal-gravity tests were filmed to serve as ref- erence cases. Mixtures producing both planar and cellular fames were studied, as discussed below.

Flat Flames:

Initial experiments investigated the role of buoyancy in the stability of fiat flame fronts. Other in- vestigators have found a threshold value of the flame propagation speed, ut,*, above which planar 1 g propane-oxygen-nitrogen flames cannot be stabi- lized, even in lean mixtures. They report ut,* ~ 12 cm/s for flames propagating downward in a tube 11 cm in diameter. ~ The present work confirms that

MICROGRAVITY PREMIXED

planar fronts cannot be achieved in mixtures producing rapidly propagating flames, although propagation speeds were not measured here. However, estimates of ut,*(cb, 6) at the stability threshold observed in the current work yield 10 -< ut,* -< 13 cm/s.I Lean flames with uL > ut,* were observed to be smooth, but convex toward the unburned gas.

Figure 2 shows a typical normal-gravity test of a fiat flame for which (~b, 6) = (0.75, 0.15). The flame is generally planar except for an annular region ex- tending from the tube wall < 1 cm into the center. Within this region, the flame curves back toward the burned gases. It should be noted that this curvature is not the same as that described above for some lean flames, which are curved over their entire surface. The flame curvature seen near the tube wall in Fig. 2 is apparently due to heat loss effects, which locally diminish the flame speed. At 3.72 and 4.48 seconds after ignition small departures from

LAMINAR FLAME DYNAMICS 179

planar shape are evident. This was common throughout the 1 g flat flame tests, though generally the flame returned to nearly planar shape after experiencing perturbations.

Fig. 3 shows a low-gravity test of a flame with identical (~b, 8, Q) conditions. A fiat 1 g flame has been established in the first image. Within 0.10 seconds of the onset of low gravity, however, the flame has begun to depart from planar shape. The sequence from 1.77 to 2.17 seconds after ignition shows the flame front curving toward the unburned gas and "tipping" toward the camera. At 1.86 seconds, the near edge of the flame has moved upstream of the far edge and both edges are visible. In subsequent frames, the far edge moves downstream and disappears and the flame reaches a quasi- steady curved configuration. The propagation speed increases in low gravity by several cm/s, which is consistent with its increase in area.

0.93 s 2.33 s

3 .72 s 4.48 s FIG. 2. Time sequence of a normal-gravity downwardly-propagating flat flame with (~b, 6, Q) = (0.75,

0.15, 170 em3/s). The flame curves downstream in a small region near the wall, due to heat loss. Small departures from planar shape can be seen at 3.72 and 4.48 seconds, though the flame generally returned to planar after perturbation. Times are after ignition.

P

LAMINAR F L A M E S - - D Y N A M I C S

Onset of Low g

180

1.40 s 1.68 s 1.77 s

1.86 s 1.96 s 2.17 s

2.48 s 2.70 s

FIC. 3. Low-gravity test of a flame with (r 6, Q) = (0.75, 0.15, 170 cm3/s). A fiat flame is established in normal gravity, but becomes curved and tips toward the camera in low gravity. Times are after ignition.

The behavior seen in Fig. 3 was diJplicated in every drop test. In summary, the major results of the low-gravity planar flame tests are: 1) no flames remained planar in Ixg--all became curved toward the fresh mixture; and 2) flame propagation speeds increased in txg, approximately in proportion to the increase in flame area.

Cellular Flames:

Cellular flames exhibited more dramatic behavior than did planar flames when subjected to reduced gravity. Fig. 4 shows a typical normal-gravity test of a cellular flame for which (~b, 6) = (1.25, 0.15). Throughout the test, the flame is roughly planar with superimposed wrinkles. Cell size is comparable to that observed in previous studies for burner-anchored flames with identical (qS, 6), though for burner flames cell size is more regular and depends on Q.4A Cell motion is more pronounced than for

the burner flames. Cells on the burner are observed to oscillate slightly in the radial direction, generally in phase with the radial fluctuation of a diffusion flame surrounding the rich premixed flame. 4 In the tube configuration, however, no diffusion flame is formed, and no radial oscillations of the normal-gravity cells are observed. Rather, the cells exhibit incessant random motion over the entire flame surface.

Figure 5 shows a low-gravity test of a flame with (~b, 6, Q) identical to that in Fig. 4. A cellular i g flame has been established in the first image. Within 70 msec after the onset of low gravity, the flame has developed 8-10 large, irregular cells. Subsequently, (1.30 to 1.49 s) some cells grow at the expense of others, and eventually, cells on the right of the image move downstream and disappear (1.40-1.68 s), while the portion of the flame on the left grows and fills the tube (1.49-1.96 s), forming an oblique "tipped" flame. Wrinkles significantly

MICROGRAVITY PREMIXED LAMINAR FLAME DYNAMICS 181

i

4.07 s 6 .38 s

7 .73 s

FIG. 4. Normal-gravity downwardly-propagating cellular flame with (qb, & Q) = (1.2,5, 0.15, 170 cm3/s). Cell size is comparable to that observed in burner-anchored flames at identical (qb, 3) conditions.

larger than the 1 g cell size are evident in the tipped flame (1.68-1.96 s). By 2.23 seconds after ignition, the flame is settling down near the burner. Some recovery of the small, regular cellular structure is evident here. Finally, at 2.70 seconds the flame is anchored above the burner in steady state. The burner flame consists of small cells similar to those observed in the 1 g portion of the test, but with more uniformity in the cell size, which is typical of both 1 g and I, tg burner flames.

Additional drop tests conducted with this (q~, (5, Q) condition show phenomena generally similar to that seen in Fig. 5. However, when the flame is ignited in 1 g, its low g behavior appears to be sen- sitive to the "initial conditions" occurring at the onset of low g. In the test shown in Fig. 5, for ex- anaplc, the flame had not reached a completely steady, planar shape when the period of reduced gravity began. The complete film record indicates that the center of the flame was moving slightly up-

ward relative to the tube at the onset of low g, while the edges were propagating downward. This motion would eventually be damped in normal gravity, but the low gravity condition apparently allowed it to be amplified, creating the flame dynamics shown (1.30-1.96 s).

Flames in other drop tests had different motions at the beginning of low g, and consequently behaved somewhat differently. Nevertheless, they consistently exhibited .some of the same features seen in Fig. 5, including 1) larger, irregular low-g cells; 2) flame "tipping"; 3) large cells forming in the tipped flame; 4) thster propagation than the corresponding 1 g flame; and 5) formation of small cells when the flame nears the burner surface.

Due to the sensitivity of low-g cellular flames ignited in 1 g to these "'initial conditions," tests were also conducted in which ignition occurred in p,g. This eliminated disturbances to the flow field created by the 1 g burn. Fig. 6 shows such a low-

182 LAMINAR FLAMES--DYNAMICS

Onset of Low g

1.15 s 1.23 s 1.30 s

1.40 s 1.49 s 1.68 s

1.96 s 2.23 s 2.70 s

FIG. 5. Low-gravity test of a cellular flame with (~b, & Q) = (1.25, 0.15, 170 cm3/s). Within 70 msec after the onset of low gravity, the flame has developed several large, irregular cells. From 1.30 to 1.49 seconds after ignition, some cells grow at the expense of their neighbors. Subsequently, cells on the right move downstream and disappear (1.40-1.68 s), while the portion of the flame on the left grows and fills the tube (1.49-1.96 s), forming an oblique, "tipped" flame. Wrinkles with wavelength significantly larger than the lg cell size are evident in the tipped flame (1.68-1.96 s). The left side of the flame begins to interact with the burner by 1.96 s, and recovery of the small, regular cell structure typical of burner flames is evident at 2 .23-2 .70 s.


......... i ? ~ r

0.20 s 0 .43 s 0.52 s

:j '

1.18 s 1.35 s 1.43 s

1.49 s 1.55 s 1.92 s

FIG. 6. Low-gravity time sequence of downwardly-propagating cellular flame with (~b, 8, Q) = (1.15, 0.14, 240 cm3/s). Times are after ignition, which occurs in low gravity. Large cells form, �9 move erratically, and disappear along the front. The flame is within 1 cm of the burner surface at 1.43 seconds, and subsequently breaks into smaller, more uniform cells. The steady-state burner flame appears in the final image. At this flow rate it is fiat, but it becomes cellular at higher Q.

gravity flame with (~b, 8) = (1.15, 0.14) Times in- dicated are after (low-gravity) ignition. At 0.20 seconds after ignition, the flame front has formed two large cells. By 0.43 seconds, at least four large cells are visible, moving erratically and showing a broader distribution of sizes. This is also evident at 0.52

seconds. Here, the large cell at the lower left has grown at the expense of the other three, which shrink and eventually disappear. The remaining cell grows to fill the tube and then splits again into two large cells (1.18-1.35 s). By 1.43 seconds, the flame has approached to within 1 cm of the burner sur-


face. The sequence from 1.49 to 1.92 seconds clearly shows the effect of the burner on the flame, break- ing the large cells into smaller ones with sizes comparable to the 1 g cell dimensions (compare Fig. 4). By 1.92 seconds, the flame has reached steady state above the bumer. The flame is fiat, but this appears to be due to heat loss; 1 at higher Q, this mixture yields a cellular burner flame.

Most of the flames ignited in ixg were not as well- behaved as the flame shown in Fig. 6. In many of the tests, the flame initially exhibited large, irregular cells and then tipped, propagating obliquely down the remainder of the tube. Such a flame is shown in Fig. 7, with (~b, ~, Q) the same as in Fig. 5. Through 0.54 seconds after ignition, the flame front exhibits large-scale convexity and is composed of numerous cells. As in Fig. 6, the cells are larger than their 1 g counterparts, though the distribution of 'cell sizes appears to be broader. The complete film record shows cells undergoing a random "boil- ing" motion. At 0.63 seconds, one cell on the right begins to grow at the expense of the others, which disappear by about 0.8 seconds. By 0.96 seconds, the single cell has spread across the tube cross sec- tion, forming an oblique tipped flame. Smaller cells subsequently form along its surface, though these cells still have larger dimensions than their 1 g counterparts (cf. Fig. 4). The right side of the flame nears the burner by 0.97 sec, and by 1.34 sec, the flame has broken up into small cells. The final image shows the steady-state burner flame with char- acteristic small, regular cells.

The behavior seen in Fig. 7 was duplicated in numerous drop tests. In summary, the major results of the low-gravity cellular flame experiments are: 1) larger, more irregular cells than in 1 g; 2) flame "tipping"; 3) more rapid low-g propagation; and 4) verification that the burner exerts a strong influence on cell characteristics, maintaining small, regular cells in low gravity.

Discussion

Flat Flames:

Recent theoretical analyses 2'1~ predict the ex- istence of a critical value of the reciprocal Froude

1 goL number, = 3, below which a stable planar flame

F r u L

front cannot occur. Here, a is the mixture thermal diffusivity; g is the gravitational acceleration, taken positive for downward flame propagation; and instability occurs for 1/Fr < 1/Frc. In physical terms, Fro corresponds to a condition at which for long- wavelength disturbances, the stabilizing influence of buoyancy just balances the hydrodynamic instability, the strength of which is proportional to uL.

The current observations of both rapidly-propagating 1 g lean flames and slowly-propagating p,g lean flames appear to confirm this prediction. In the former case, the development of large-scale flame front curvature would correspond to 1/Fr falling below 1/Frr when UL > UL*. In the latter case, 1/Fr decreases below 1~Fro due to low values of g. Taking UL* (go) = 13 cm/s (based on experimental evidence), a = 0.53 cm2/s, and g = go = 980 cm/s 2 yields 1/Frr = 0.24, which allows es- timation of uC (gr For g = 10 -3 - 10 -4 go, one obtains uL* ~ 1.5 cms/s, well below UL at the 1 g lean flammability limit. Although there is some evidence that even leaner (hence slower) flames can propagate in I~g, 7'8 such slow flames could not be sustained in the current apparatus. Thus, the fact that no planar Ixg flames were observed is consistent with estimates that for all the flames, UL > uL*(g = 10 -3 -10 -4 go).

The above discussion assumes flame stability is based solely on a competition between the stabilizing effect of buoyancy and the destabilizing influence of hydrodynamics. The stability properties of real flames, however, are also affected by the dif- fusive and chemical properties of the reactive mixture. A more complete treatment which includes consideration of these properties yields predictions of UL* as a function of a, g, gas expansion across the flame, and the Lewis and Zerdovich num- bers. lo For reasonable values of the above param- eters, the analysis indicates that at the g-levels in the experiments, all propane-o~gen-nitrogen flames with UL > 1 cm/s are unstable. Since it is unlikely that flames with uL < 1 cm/s could propagate even in reduced gravity, this suggests that stable planar flame fronts are not achievable under low-gravity conditions.

Regardless of whether or not stable planar flames are possible in low g, the present experiments support the theoretical predictions that fiat flames are possible only when the stabilizing effect of buoyancy (for downward propagation) counters the hydrodynamic instability mechanism. In this view, when gravity is reduced, the Darrieus-Landau mechanism is unopposed and produces a curved flame. In addition, normal-gravity planar flames become curved when uL becomes great enough so that the destabilizing effects of the hydrodynamic mechanism overwhelm the stabilizing influence of buoyancy.

It should be noted, however, that caution should be exercised when interpreting the present results strictly in terms of the Darrieus-Landau instability, since this mechanism may be modified by the finite transverse extent of the flames. This has been re- ported in recent numerical simulations of the hydrodynamic instability, z2 which yielded large-scale convex fronts remarkably similar to those obso- 'ed here experimentally.


0.32 s 0.42 s 0.54 s

0.63 s 0.72 s 0.91 s

0.97 s 1.34 s 2.00 s

FIG. 7. Time sequence of cellular flame with (q~, 6, Q) = (1.25, 0.15, 170 cm3/s), ignited and propagating in low gravity. The flame initially exhibits large irregular cells (0.32-0.54 s), then tips and propagates obliquely (0.63-0.97 s). Smaller cells form and disappear along the oblique front. The flame is settling near the burner at 1.34 seconds, and has formed small, regular cells by 2.00 seconds.

Cellular Flames:

The reduction of buoyancy in low gravity appears to effect cellular flame characteristics on several different scales. On a global scale (i.e. over the entire flame surface), buoyancy clearly plays a stabilizing role for 1 g flames, causing departures from hori-

zontal to be damped for both planar (smooth) and cellular (wrinkled) fronts. When this stabilizing influence is removed, departures from horizontal are amplified. In the planar case, the flame restabilizes as a curved front with a steady shape convex toward the unburned gas.

In the cellular case, however, the low-g dynam-


ics described above are more complicated. It has been found in previous studies 9A3 that a downwardly-propagating 1 g cellular flame can induce a velocity upstream of the preheat zone as large as 50 percent of UL. The induced velocity fluctuations, moreover, can significantly effect the dynamics of the flame itself. It is possible that in normal gravity, the stabilizing influence of buoyancy is suffi- cient to damp the perturbations caused by this feedback effect, but that in low g the disturbances can grow, leading to the dynamics seen in Figs. 5- 7. Thus, in this view, the hydrodynamic instability initiates the departure of the cellular front from horizontal and the upstream induced flow perturbations cause the subsequent flame "tipping". This view is supported by the observation that the low- g dynamics of cellular fronts depends on the flame motion at the onset of reduced gravity. However, more information about the I~g upstream flow field is needed to verify this interpretation.

On a smaller scale, reduced gravity also appears to influence the characteristics of the cells them- selves. The low-g tests generally yielded larger, more irregular cells, as well as a broader distribution of cell sizes. These observations are consistent with results of a recent low-gravity investigation of polyhedral Bunsen flames. 14 In that study the number of facets in the polyhedral flames decreased in low g, suggesting that larger wavelengths are prefer- entially unstable when buoyancy is reduced. The observed behavior is also consistent with predictions that the least stable wavenumber decreases as g decreases. Finally, the broader range of cell sizes observed here in low g also suggests that a broader range of wavelengths become unstable when gravity is reduced. However, more quantitative measurements of I~g cell sizes are needed to verify this.

In summary, the low-g cellular tube flame experiments indicate that while buoyancy is not the dominant mechanism in the formation of the cells, it plays a significant role in both the global flame front stability and the local cell characteristics.

Burner Effects:

Figs. 5-7 clearly show the burner exerting a significant effect on the cellular flame characteristics. As the flame neared the burner in these tests, the front broke up into smaller, regular cells with sizes comparable to the 1 g cell dimensions. This suggests that burner effects dominate buoyancy effects in the overall stability of these flames, resulting in burner flames whose Ixg cell characteristics differ little from that of 1 g flames. The influence of the burner is believed due mainly to heat loss, which previous normal-gravity studies have shown to play a significant role in cell characteristics. 1 However, theoretical investigations 15 point to burner influ- ences on additional mechanisms besides heat loss.

Consequently, the exact mechanisms remain unclear.

Conclusions

Three major conclusions can be drawn from this work:

1. These experiments support theoretical predictions that fiat flames are only possible when the stabilizing influence of buoyancy (for 1 g downward propagation) counters the hydrodynamic instability. Flat flames were never observed under low-gravity conditions. 2. Gravity appears to play a significant role in the overall stability and structure of rich cellular propane-oxygen-nitrogen flames. In low g, the flame is destabilized "globally" across its entire surface due to the absence of buoyancy, causing it to tip. In addition, the absence of buoyancy produces "local" effects, apparently causing larger wavelengths as well as a broader range of wavelengths to become unstable. 3. These experiments establish that the burner exerts a major influence on the cellular flame structure. Comparisons between low-g burner and low- g tube flames indicate that burner effects produce a smaller range of unstable wavelengths, and that the burner effects dominate buoyancy effects.

Acknowledgments

This work was supported by NASA grant NAG- 3-861. I wish to thank Prof. A. C. Fernandez-Pello (UC Berkeley) for his guidance and Dr. H. Ross and Mr. D. Stocker (NASA Lewis) for their pa- tience and enthusiasm.

REFERENCES

1. DUNSKY, C. M.: Experimental Studies of the Effect of Gravity on the Stability of Pre- mixed Laminar Flames. Ph.D. Dissertation (1991).

2. CLAVIN, P.: Prog. Energy Combust. Sci. i1, p. 1 (1985).

3. SIVASHINSKY, G. I.: Phil. Trans. R. Soc. Lon- don, A, 332, p. 135 (1990).

4. VANTELON, J. P., PAGNI, P. J. AND DUNSKY, C. M.: Cellular Flame Structures on a Cooled Porous Burner. Dynamics of Reactive Systems, Prog. in Astro. and Aero. vol 105, J. R. Bowen et. al. (Eds.), AIAA, New York, p. 131 (1986).

5. DUNSKu C. M. AND FERNANDEZ-PELLO, A. C.: Proceedings of the Twenty-Third Symp. (Inter- national) on Combustion, The Combustion In- stitute, Pittsburgh, PA, p. 1657 (1991).


6. HERTZBERG, M.: Prog. Energy Combust. Sci. 15, 203 (1989).

7. STREHLOW, R. A., NOE, K. A. AND WHERLEY, B. L. : Twenty-First Symp. (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1899 (1986).

8. RONNEY, P. D. AND WACHMAN, H. Y.: Com- bust. Flame 62, 107 (1985).

9. QUINARD, J., SEARBY, G. AND BOYER, L.: in Dy- namics of Reactive Systems, Prog. in Astro. and Aero. vol 95, J. R. Bowen et al. (Eds.), AIAA, New York, 129 (1985).

10. PELCE, P. AND CLAVIN, P.: J. Fluid Mech. 124, 219 (1982).

11. JACKSON, T. L. AND KAPILA, A. K.: Combust. Sci. Teeh. 49, 305 (1986).

12. GUTMAN, S. AND SIVASHINSKY, G. I.: Physica D, 43, 129 (1990).

13. SE~BY, G., SABATHIER, F., MONREAL, J., CLAVIN, P. AND BOYER, L.: in Dynamics of Reac- tive Systems, Prog. in Astro. and Aero. vol 95, J. R. Bowen, et al. (Eds.), AIAA, New York, p. 103 (1985).

14. Dvaox, D. AND PRUD'HOMME, R.: Behaviour of Premixed Flames under Parabolic Flight Con- ditions. Presented at the 7th European Sym- posium on Materials and Fluid Sciences in Microgravity, Oxford, UK, September, 1989.

15. JoviaN, G.: Combust. Sci. Tech. 53, 315 (1987).

COMMENTS

Robert K. Cheng, Lawrence Berkeley Labora- tory, USA. Can the different flame structures observed under microgravity be explained by the change in the venting conditions during the drop?

Author's Reply. Assuming equal heat release in ~g as in lg, the flow in the burned gases should be the same. Then the only difference possible in the venting could be caused by the entrainment of

cold air into the lg buoyant plume downstream of the tube exit, which is absent in p,g. A fine mesh screen was placed across the tube mouth in these tests to inhibit disturbances to the flow within the tube. The pressure drop across this screen was neg- ligible. I believe that this configuration was ade- quate to ensure that any observed changes in flame dynamics from lg to ~g were due either to inher- ent changes in the flame stability, or modifications to the flow upstream of the flame.

Microgravity observations of premixed laminar flame dynamics

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