Chemical inhibition of diffusion flames through addition of halogenated inhibitors is a problem of significant practical and scientific interest. Extensive studies on diffusion flames in microgravity have shown that these flames have significantly different characteristics than those under normal gravity. However, the mechanisms through which inhibitors reach the reaction zone to suppress combustion in diffusion flames and the effectiveness of these compounds under reduced gravity have yet to be investigated. This study reports preliminary results of investigations on the behavior of laminar jet diffusion flames upon the addition of bromotrifluoromethane (CF3Br) and trifluoromethane (CF3H) to the surroundings under normal and microgravity conditions. The results show that the flame structure in microgravity is significantly different from that under normal gravity conditions, and more importantly, that conditions for flame stability are less stringent under microgravity. Experiments show that flames that cannot be stabilized under normal gravity are quite stable under microgravity conditions. In addition, normal gravity experiments at reduced pressure (low buoyancy) did not reproduce the structure or stability limits of inhibited flames in microgravity

Bush, Michael T.

Hochgreb, Simone

Linteris, Gregory T.

Vanderwege, B. A.

English

NASA Technical Reports Server

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C
N96- 15609
EFFECT OF CF3H AND CF3Br ON LAMINAR DIFFUSION FLAMES IN NORMAL
AND MICROGRAVITY
B. A. VanDerWege, Michael T. Bush and Simone Hochgreb
Massachusetts Institute of Technology
Cambridge, MA
and
Gregory T. Linteris
National Institute of Standards and Technology
Gaithersburg, MD
Introduction
Chemical inhibition of diffusion flames through addition of halogenated inhibitors is a problem of
significant practical and scientific interest. Extensive studies on diffusion flames in microgravity
have shown that these flames have significantly different characteristics than those under normal
gravity [1,2]. However, the mechanisms through which inhibitors reach the reaction zone to
suppress combustion in diffusion flames and the effectiveness of these compounds under
reduced gravity have yet to be investigated.
This study reports preliminary results of investigations on the behavior of laminar jet diffusion
flames upon the addition of bromotrifluoromethane (CF3Br) and trifluommethane (CF3H) to the
surroundings under normal and microgravity conditions.The results show that the flame structure
in microgravity is significantly different from that under normal gravity conditions, and more
importantly, that conditions for flame stability are less stringent under microgravity. Experiments
show that flames that cannot be stabilized under normal gravity are quite stable under microgravity
conditions. In addition, normal gravity experiments at reduced pressure (low buoyancy) did not
reproduce the structure or stability limitsof inhibitedflames in microgravity.
Experiment
Experiments were conducted in the 2.2-second drop tower at the NASA-Lewis Research Center,
utilizing an existing test rig, consisting of a 27 literclosed cylindrical vessel (254 mm diameter by
533 mm height) provided with four sideview windows. A CCD camera (Panasonic WV-CL352)
provided images of the evolution of the flame duringthe drop. A single jet (1.7 mm diameter) was
used in the presently reported experiments. Flow rates of methane were controlled by a pressure
regulator and choked flow orifice, and peak flows measured using a 0-500 sccm Omega (FMA-
5606-ST) mass flow meter. The flow rate of all experiments reported in this paper was 154 sccm,
corresponding to a Reynolds number based on nozzle diameter of approximately 100. The tests
were performed in a quiescent chamber environment containing 0-2 percent volume
concentrations of CF3Br and 0-12 percent CF3H. A 2 percent methane environment was also
tested for comparison with the inhibitor-laden oxidizing environment. Mixtures in the chamber
were prepared by the partial pressure method, with initial evacuation and purging, followed by the
addition of inhibitor, oxygen and nitrogen to the desired final pressure. Both atmospheric
pressure and 0.25 atm tests were performed at normal and microgravity. The flames were ignited
(after drop package release in microgravityexperiments) by a retractable glow plug after about 20
ms of flow through the nozzle, and the images were then recorded.
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Results
Atmospheric pressure experiments
The characteristics of the flames in normal and microgravity tests performed are listed in Table 1.
The structures of the normal gravity, ambient pressure flames with and without inhibitor were
essentially indistinguishable. The inhibited flames, however, exhibited a more intense flicker, and
it was not possibleto stabilize flames at concentrations beyond 1.5% CF3Br in air at the present
fuel flow rate. The structure of the diffusion flames under microgravity conditions, however, was
extremely sensitive to the surrounding environment.
A typical methane laminar diffusion flame structure in microgravity is shown in Fig. 1, an oval, white,
closed-tipped flame with h/d~Re/2 [2], with the base extending around the nozzle. At
atmospheric pressure, addition of any amount of inhibitor or fuel to the oxidizing environment led
to the appearance of an outer flame surrounding the core fuel jet flame. The addition of 4% CF3H
(by volume) to the oxizing environment led to a somewhat more pronounced outer flame, and the
opening of the flame tip (Fig. 2). At higher concentrations, the main observable effect of the
addition of CF3H was to completely suppress the sooty region, creating a flame with a blue base
and no visible tip. This behavior (suppression of soot related luminosity) is consistent with the
decrease of oxygen concentrations resulting from replacement by a diluent (in this case, the
inhibitor itself) [3]. However, the flame which was observed was the size and shape of outer flames
recorded in other experiments with fuel or inhibitor addition, which leads us to believe that there
might be an inner flame which was however too dim to be registered by the camera.
In order to determine whether the outer flame could be attributed to the consumption of the
inhibitor on the air side, 2 percent of methane was added to the surrounding air under microgravity
conditions. The flame had approximately the same height as the flame without addition of
methane, but the tip was open and a faint light halo surrounded the main reaction zone (Fig. 4).
The addition of CF3Br to the oxidizing environment under microgravity at ambient pressure
produced a marked change in structure (Fig. 5). There is an obvious _ulip' double flame structure,
with a visible region significantly shorter than the uninhibited flame. An open-tipped inner flame is
surrounded by a wider outer flame whose visible region ends shortly before the outer flame.
Instead of the rounded base, a flat circular base slightly upstream of the nozzle tip is visible. The
visible outer flame is most likelya result of CF3Br reaction, supported thermally by the main inner
flame. Such behavior has been observed experimentally in coflowing two dimensional diffusion
flames [4], opposed jet diffusion flames [5], and surmised from numerical simulations [6]. The
study in coflowing flames identified the outer visible flame spectroscopically as greenish Br2
emission. A Br2peak would be expected also from opposed stretched flame calculations [6]. The
observed color of the outer flame was closer to yellow than green. It remains to be determined
whether the yellow luminosity is a result of soot in the outer flame, or of the camera response at
the given settings. The opening of the flame tip could be explained by the reduction in the
maximum stretch supported by the diffusion flame at the tip with the addition of inhibitor [7].
However, that would not explain the flame tip opening in the case of addition of plain methane to
the surroundings.
LOw Pressure Experiments (0.25 atm_
The effects of reduction in pressure on normal and microgravity flames with the addition of CF3Br.
were explored to understand how normal gravity, weakly buoyant low pressure diffusion flames
relate to microgravity flames. Experiments were performed both at normal and microgravity in a
0.25 atm quiescent environment (corresponding to a reduction in buoyancy by a factor of 16) with
and without the addition of CF3Br.
37O
Reductionof pressure at normal gravity leads to elimination of flicker at 0.25 atm, and the
presence of a stable oval white flame surrounded by a dim halo of glowing gases (Fig. 6). Addition
of 1% CF3Br causes the flame to liftoff the nozzle and stabilize at about 25 mm from the nozzle as
a flat blue-white flame with a circular base and an oval tip (Fig. 7). The stabilization mechanism is
probably the same as in other lifted flames, where premixing at the base aids in the establishment
of a stoichiometric region. With the addition of 2 percent CF3Br, the flame lit but was promptly
blown off.
Under microgravity conditions, however, apparently stable anchored flames (for the duration of
the test) were observed for both 1 and 2 percent CF3Br addition (Figs. 8-9). The flames were oval,
with closed tip, a wide blue base and a broad gas halo. The abilityto stabilize these flames shows
once again that both under normal and lower pressures, conditions for the extinguishment of
diffusion flames at these low pressures appear to be more stringent under microgravity than at
normal gravity. Therefore, experiments under weakly buoyant conditions may not capture the
stability limits of flames under inhibitingenvironments in microgravity.
Conclusions
The present experiments with laminarjet diffusion flames burning under normal and microgravity
conditions in a quiescent environment with addition of halogenated hydrocarbons have yielded
the following conclusions:
1. Stability limits of flames with the addition of CF3Br and CF3H to the oxidizer environment
under microgravity are wider than those under normal gravity.
2. The addition of inhibitorsto the oxidizer under microgravity conditions creates a double flame
structure. The outer flame is probably a region where the inhibitor reacts, supported by the
heat release of the inner fuel flame. This behavior is particularly clear inthe case of CF3Br.
3. Addition of halogenated inhibitorsto i_g flames leads to shorter, open tipped flames. Addition
of large amounts (>4%) of CF3H suppresses the appearance of luminous soot.
4. At low pressures (0.25 atm), normal gravity diffusion flames resemble their microgravity
counterparts more closely. However, flames at these pressures that cannot be stabilized
under normal gravity appear to be stable under microgravity.
Future Work
The goal of the project is to understand the effectiveness and action of halogenated inhibitors on
the extinction of diffusion flames at reduced gravity. Additional experimental work in microgravity
and ground based normal gravity experiments are planned. Numerical and analytical work to
interpret and explain the experimental findings is being prepared for the near future.
The effects of diluents, oxygen and inhibitor mole fractions, different halogenated inhibitors, fuel
types and jet Reynolds numbers will continue to be investigated in the 2.2-second drop tower. In
addition, the study of inhibited CO/H2 flames is also being considered, given the obvious
simplification of the fuel chemistry and the absence of soot. A normal gravity, low-pressure
diffusion flame bumer for normal and microgravityexperiments is currently under construction at
MIT. Additional diagnostics, including IR imaging, radiation measurement, and thermocouples, are
being considered. Complementary experiments on the behavior of counterflow diffusion flames
will be conducted at NIST this Spring to aid in the interpretation of the laminar jet flame
observations.
Plans for numerical modeling of the system are: a) interpret the structure and validate chemical
model for halogenated inhibitor addition using a 1D counterflow diffusion flame code; b) identify
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potential paths for reduction of the chemical mechanism and c) incorporate the reduced
mechanism into a 2D jet diffusion flame.
Acknowledgements
The invaluable expertise and generosity of Dr. David L. Urban in permitting and assisting us with
the use of an existing rig for our initial drop tower experiments is gratefully acknowledged. This
work is funded under NASA Microgravity Research Grant NAG3-160. Additional support was also
provided by NSF through Graduate Research Fellowships for B. A. VanDerWege and M. T. Bush.
References
1. Law, C. K. and Faeth, G. M., "Opportunities and Challenges of Combustion in Microgravity",
Prog. Combust. Sci. 20, pp. 65-113 (1994).
2. Bahadori, M. Y. and Edelman, R. B, "Effects of Buoyancy of Gas Jet Diffusion Flames", NASA
Contractor Report 191109 (1993).
3. Bahadori, M. Y. and Edelman, R. B., "Effects of Oxygen Concentration on Radiative Loss from
Normal Gravity and Microgravity Methane Diffusion Flames", AIAA paper 92-0243 (1992).
4. Simmons, R. F. and Wolfhard, H. G. "The Influence of Methyl Bromide on Flames, Part 2 -
Diffusion Flames", Trans. Far. Soc. 52, p. 53 (1956).
5. Trees, D., Ilincic, N., Weissweiller, T., Zamfirescu, N., Grudno, A. and Seshadri, K.,
"Experimental and Numerical Studies on Chemical Inhibition of Methane-Air Diffusion Flames
by CF3Br and CF3H", to appear, Central States/Western States/Mexican National Sections of
the Combustion Institute, April 23-26, 1995.
6. Masri, A. R. "Chemical Inhibition of Nonpremixed Flames of Hydrocarbon Fuels with CF3Br",
Combust. Sci. and Tech. 96, pp 189-212 (1994).
7. Hamins, A., Trees, D., Seshadri, K. and Chelliah, H. K. "Extinction of Nonpremixed Flames
with Halogenated Fire Supressants", Combust. Flame, 99, 2, pp. 221 (1994).
I10 mm
Figure 1. Uninhibited CH4 laminar jet flame in microgravity
and 1 atm (d = 1.7 mm, Q= 154 sccm, Re = 98). All
subsequent figures show same flow conditions.
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Table 1. Summary of experimental results
(d = 1.7 ram, Re = 98, Q = 154 sccm, fuel:
Environment a P (atm) I_g/ng
Air 1.0
Air 1.0
ng
_g
ng
I_g
1%CH4 1.0
2°/oCH4 1.0
for inhibited CH4 laminar diffusion flames
methane)
h Imm)b Characteristics
40-+10 Sooty with blue base. >15 Hz flieker.
55 Sooty, closed-tip, rounded base. Flame
transitions from yellow to red at the very tip.
40-+10 Indistinguishable from flame without CH4
added.
45 Thin, dim outer flame, core flame sooty with
open tip.
1% CF3Br 1.0 ng 57+_2.5
1.5% CF3Br 1.0 ng 75_15
2% CF3Br 1.0 ng
1% CF3Br 1.0 p.g
_tg1.5°/oCF3Br 1.0
38 (inner)
27(outer)
38 (inner)
25 (outer)
4 and 8% CF3H 1.0 ng 45_+12/
50-+15
4% CF3H 1.0 I_g 45
8 and 12% 1.0 I_g 25
(outer?.)
Blue base shrinks and flickers. Flamelets
break off from the long tip.
Rame blows off and stabilizes unsteadily at
about 25 mm above the nozzle with a circular
rim of about 10 mm in diameter.
Rame ignites but is blown off almost
immediately.
Obvious double (inner-outer) flame structure.
Inner flame is open tipped and about 20 mm in
width. The outer flame is about 30 mm wide and
sorter, with apparent flat blue base of about 10
mm diameter.
Very similar to 1% CF3Br flame.
Similar to uninhibited flame, with larger blue
base and intense flickering.
D_n but visible double flame structure. Inner
flame is about 25 mm wide, orange and more
diffuse than base flames, with a wide open tip.
Outer flame is visible only near the base.
Visible flame region is 30 mm wide, with a
conical blue base and apparently no soot,
possibly due to the reduced oxygen
concentrations under the high inhibitor mole
fractions.
Air 0.25 ng 30
1% CF3Br 0.25 ng 25
1 and 2% CF3Br 0.25 p.g 57 / 65
Oval white/blue flame surrounded by a thin
halo of dimly glowing gasses.
Rame has a wide circular base lifted about 30
mm off nozzle, blue-white with oval tip. At 2%,
flame lights and immediately blows off.
Closed tip oval blue-white flame with bottom
half blue and upper half sooty, with a fairly
broad halo of glowing gasses. At 2%, the halo
is brighter, and a larger fraction of the flame
white.
a) Balance air:.0.21% 02, 0.79% N2
b) For flickering flames, average height. For lifted flames, height is measured from flame base, not nozzle.
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E 
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Effect of CF3H and CF3Br on laminar diffusion flames in normal and microgravity

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