In reduced gravity, the combustion of solid fuel in low-speed flow can be studied. The flame behavior in this low-speed regime will fill a void in our understanding of the flow effect on combustion. In addition, it is important for spacecraft fire safety considerations. In this work, modeling and experimental work on low-speed forced-concurrent-flow flame spread are carried out. In addition, experiments on reduced-gravity buoyant-flow flame spread are performed

Ferkul, Paul V.

Grayson, Gary D.

Sacksteder, Kurt R.

Tien, James S.

English

NASA Technical Reports Server

COMBUSTION OF SOLID FUEL IN VERY LOW SPEED OXYGEN STREAMS
James S., T'ien," Kurt R. Sacksteder, t Paul V. Ferkul," and Gary D. Grayson"
Case Western Reserve University, Cleveland, Ohio 44106
tNASA Lewis Research Center, Cleveland, Ohio 44135
N93"20207
1. Introduction
In reduced gravity, the combustion of solid fuel in low-speed flow can be studied. The flame behavior in this
low-speed regime will f'dl a void in our understanding of the flow effect on combustion. In addition, it is
important for spacecraft fire safety considerations. In this work, modeling and experimental work on low-speed
forced-concurrent-flow flame spread are carried out. In addition, experiments on reduced-gravity buoyant-flow
flame spread are performed.
2. Low Speed Forced Flow
2.1 Concurrent-Flow Flame Spread Model [1]
The model considers two-dimensional, steady flame spread in concurrent flow over a thin solid. Because of its
thinness, the fuel bed can bum through. The flame could therefore reach a constant length provided that a.
constant (and equal) flame tip velocity, pyrolysis front velocity, and fuel burnout rate, can be obtained in the
low-speed concurrent flow environment.
The flame is assumed to be laminar because of its small size and low velocity. The gas-phase fluid mechanical
treatment is more comprehensive than most works in the past. In the flame base region (where the upstream
flow is seen firs0, sueamwise heat and mass diffusion is included. This, together with finite-rate, gas-phase
chemical kinetics, enables us to examine the question of flame stabilization and extinction. The elliptic treatment
of this flame stabilization zone entails the full Navier-Stokes equations. On the other hand, to save
computational time, a boundary layer approximation is employed in the downstream region. The two zones are
coupled at an appropriate gas-phase location. They are also coupled indirectly through the solid by energy
exchange. A simplified solid fuel model is used. The solid is assumed to be thermally thin and to pyrolyze
according to a one-step Arrhenius law with no char and tar formation. A solid-surface radiative-loss term is
included which becomes critically important in the low-speed flammability limit. Because of the coupling
between elliptic and parabolic regions and between the gas and solid phases, the numerical solutions require
many iterations and are computationaUy intensive; they are carried out using the Cray X-MP Supercomputer at
the NASA Lewis Research Center.
Although the model has been formulated for a mixed forced and buoyant flow, extensive computations have been
performed for purely forced flow only, using oxygen mole fraction and free-stream velocity as parameters. Fig.
1 gives all the points calculated and the extinction boundary. Some of the important results are summarized
below.
The extinction boundary consists of a quenching and a blowoff branch similar to those in
stagnation-point flow [2] and in opposed-flow flame spreading [3]. The quenching limit is due to the
surface radiative loss. Without radiative loss, the model predicts no low-speed limit. Fig. 2 shows the
maximum computed flame temperatures. In all cases, the maximum temperature decreases with flow
velocity, especially near the quench limiL The flame spread rate decreases monotonically with flow
velocity, as expected (Fig.3).
Flames quench at low speed by shrinking in size. Near-limit flames remain stabilized in the fuel
burnout region, as shown in Fig. 4. This fixed stabilization is in contrast to flame blowoff at higher
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speedwhere the flame cannot be stabilized in the fuel burnout region. Extinction is reached when the
flame is blown downstream. This illustrates one of the distinctions in flame extinguishment between
quenching and blowoff. This difference becomes clear in a two-dimensional analysis but is obscure in
one-dimensional stagnation-point flames. The reactivity contours shown in Fig. 4 resemble the visible
blue flame seen in experiment (take w=10 -4 g/cm3/sec for example), which will be discussed in the next
section.
- Using finite-rate kinetics enables us to examine the integrated flux of fuel vapor crossing any vertical
plane. Far downstream, the fuel-flux stops changing (because the reaction becomes frozen), providing a
measure of fuel escaping the flame. The fraction of fuel not consummed in the flame increases
dramatically at lower speeds because of the increased importance of flame-tip quenching. Additionally,
the variation of escaped fuel vapor is not monotonic with flow velocity.
2.2 Forced Concurrent Flow Flame Spread Experiments [4]
Low-speed concurrent-forced-flow experiments were conducted at Lewis Research Center's 5-second drop tower
using thin tissue paper Kimwipe) as the solid fuel. The relative flow was generated using a sample translation
device in a total of twenty-seven drop tests. In these tests, the flow velocity was varied from less than I cm/s to
5 cm/s and the oxygen concentration in nitrogen (at normal atmospheric pressure) from 30% down to extinction.
Color cine films were made of the flames.
. k -- - -
While in most tests the leading edge of the fuel receded steadily as it was burnt following the ignition transient,
the downstream flame tip propagation was unsteady. Flames grew in length (after the ignition transient) at
higher oxygen concentrations and relative flow velocities, but shrunk at lower concentrations and velocities. In
higher-speed flows, in normal gravity, flame length has always been observed to grow after ignition [5]. The
shrinking flames observed in these microgravity tests could be quenching, but the slow evolution of the flame
length in the 5 second test time did not allow enough time to observe their eventual fate. Alternatively, the
small, slowly evolving flames may be indicative of near-steady configurations of flames in very-low-speed
concurrent flows. Fig. 6 (the color photos are grouped together) shows a comparison of flames at approximately
5 cm/sec relative concurrent flow at three different oxygen concentrations. The shape of the 15% oxygen
concentration case can also be compared favorably with the computed reaction contours (e.g. 104 g/cm3/sec)
shown in Fig. 4 for the same test condition.
The measured spread rates of the flame base, the leading edge of the fuel at which burnout occurs, are similar to
the computed values shown in Fig. 3. However, spread rates at high oxygen concentrations and high
relative-flow velocities are lower experimentally at the end of the drop while the rates at low oxygen
concentrations and low relative-flow velocities are higher experimentally. These observations are consistent with
the suggestion that the flames might not have had sufficient time to reach their final configuration in the
available 5 seconds.
A flammability map, based upon the status of flames at the end of 5 seconds of microgravity, has been drawn
and is similar to Fig. i. No detailed comparison can be made because steady flames have not been achieved
experimentally. It is clear that even for a thin fuel such as Kimwipes, which provide ample steady flamespread
data in opposed-flow configurations, longer microgravity duration is needed to satisfactorily determine flame
spread rates and extinction limits in concurrent flow. -
3. Buoyant-Flow Flame Spread in Reduced-Gravity Experiments
Flame spread in buoyant flow under reduced-gravitational conditions has been observed using the NASA KC-135
aircrai't flying parabolic trajectories at approximately 1/3rd, l/tth, and 1/i0th normal earth gravity levels. The test
apparatus includes-pro--visions for controlled ignition energy, color-schlieren flame imaging and three dimensional
measurements of thelocal acceleration levels. Tests were conducted in O2/N2 mixtures from 18% oxygen down
to extinction (of the same thin Kimwipe tissues used in our other tests) at normal and reduced pressures for
upward (concurrent fl0w) flame propagation; and at normal pressure for downwaru (opposed flow) propagation.
246 • -- ' ....
3.1 Downward Flame Spread and Extinction
Schlieren images of flames spreading in buoyant, opposed flow showed steady propagation in oxygen
concentrations below the normal-gravity flammability limit. At each oxygen concentration (18, 16, and 15%
oxygen) spread rates decrease with increasing acceleration, though the difference between 1/10th g and 1/6th g is
small. Tests conducted at 14% oxygen showed flame propagation at 1/10th g, and a blowoff extinction was
observed during the aircraft pullout maneuver to approximately 2g. Flames at higher acceleration levels and
14% oxygen did not appear to survive the ignition transient. Fig. 5 shows a flammability map using acceleration
level and normal-atmospheric-pressure oxygen concentration as parameters. The solid line gives the high-g
blowoff boundary. The low-g extinction boundary, which cannot be determined accurately in the aircraft
experiment, is indicated by the shaded curve. Fig. 7 shows schlieren images of the downward spreading flames
at 15% oxygen at two reducod-gravity levels.
3.2 Upward Flame Spread
Schlieren images of flames spreading in buoyant, concurrent flows show propagation of flames down to normal-
atmospheric-pressure oxygen concentrations of 12%, in both 1/6th and 1/3rd g. A single test at 1/10th g did not
result in a propagating flame at 12% oxygen, while another at 14% did.
At normal atmospheric pressure, the upward spreading flames were characterized by thermal/flame plumes that
become unsteady downstream of the flame stabilization zone. The tissue-paper fuel tended to curl, particularly
in the normal pressure tests, disturbing the bottom of the flame. Fig. 8 a and b show two parts of an upward
propagating flame at 15% oxygen in l/6th g as they passed by the single schlieren window. In Fig. 8a, the
unsteady/unstable plume is shown, while in Fig. 8b, the bottom of the flame is shown stal_ilized near the lowest
point on the fuel surface.
Tests were conducted at reduced pressure and showed the existence of low-pressure limits. In the small number
of tests conducted, the pressure limits were not determined with precision. At reduced pressure the flame plumes
were more laminar and the fuel deformed more slowly during burning. Fig. 8c shows the downstream plume of
a 15% oxygen, 0.5 atmosphere pressure flame at l/6th g. Below, in Fig. 8d, the stabilized bottom of the same
flame is shown.
4. Future Plans
Although surface radiative loss is included in the current model and has proven to be important in low-speed
flows, gaseous radiation has been neglected. In stagnation point flow, gas radiation is found to alter the low-
speed quenching limit for one-dimensional stationary flames. In concurrent flow gaseous radiation is likely to be
more important because the flame is longer. Comparison with experiment also shows that without gas radiation
the model overpredicts the flame length. The inclusion of gaseous radiation in the model is the logical next step
and will complete our understanding of radiative effects on flame spreading over solid fuels in low speed flows.
At the same time thermally-thick solid fuel needs to be treated in the theory (presendy, we have dealt with
thermally-thin fuel only). The new elements that need to be added will include solid-phase heat transfer, fuel
geometry change and an unsteady flame growth process.
Experimentally we will also begin to test thicker fuels in low-speed flows. Because of the limitations of
microgravity time in ground-based facilities, steady state is not expected, even were burnout possible. In order
for the results to be useful, a controlled ignition process will be extended to thick fuels. The ignition transient
experiments and related modeling will lead to a rational design of a space-based experiment required to obtain
concurrent-flow flame spreading data in the very low-speed regime.
Because of the flammability limit data we have obtained for both low-speed forced flow and buoyant flow in
reduced gravity, we also hope to contribute to the reevaluation of flammmability test methods for spacecraft
materials.
247
REFERENCES
I.Ferkul,R V.:A Model of Low-Spe2xl Concurrent-FlowFlame Spread over a Thin SolidFuel,Ph.D. Thesis,
Department of Mechanical and Aerospace Engineering, Case Westem Reserve University, Cleveland, Ohio,
expected October 1992.
2. T'ien, J. S., Combustion and Flame, 65, I, pp. 31-34 (1986).
3. Olson, S. L. Ferkul, P. V., and T'ien, J. S., 22nd Sym. (Int.) on Combustion, The Combustion Inst., pp.
1213-1222 (1988).
4. Gmyson, G. D.: An Experimental Study of Low-Speed Concurrent Flow Flame Spread over a Thin Fuel,
M.S. Thesis, Department of Mechanical and Aerospace Engineering, Case Western Reserve University,
Cleveland,Ohio (1991).
5. Loh, H. T. and Fernandez-PeUo, A. C.: Flow Assisted Flame Spread over Thermally Thin Fuels, proceedings
of the First International Sym. on F'tre Science, Hemisphere (1986).
21
20
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Fig. 1
-O_O 0 _IDO 0 0 0
• I"0 0 o 0 0 Steady Flamex Extinction
0 O0 0
Ou
Branch
O O O(:D(
O _ "
O -"
-'X" _Blowolf
- - _ Branch
1.5 2 2.5 5 4 5 6 7 8 1.5 2
I 0 0 I 01
Free Slream Velocity (cm/s)
Extinction Boundary of a Thin Solid Fuel in Concurrent Flow
248
06 2 3 4 2
1 10
Relative Veloci(y (cm/s)
FiE. 2 Maximum Flame Temperature
2+7cm/,
)'0 , I
-1 4
.5 cm/$
-I 4 9
I 0 cm/,
_0 7 T [ ; ....
-1 4 9 14
15,5 ,'m/=
L
-1 4 g 14
x(cm)
FiE. 4 Reactivity Contours at 15% OxyEen
from QuenchinE to Blowoff
(Outermost contour is 10-6g/cm3/s;
a factor oflOseparates contours)
249
2 3 4
10
Relative Velocity (cm/s)
Fig. 3 Flame Spread Rate
17
16
0
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n
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>,
X
0
14
13
0.(
+- 0
I O Steady Flame lx Extinction
..t I I I I
0.2 0.4 0.6 0.8 1.0
g
Fig. 5 Extinction Boundary for
Kimwipe in Buoyant Opposed
Flows (earth: g = i)
15%
18%
25%
Fig. 6 Flame Shapes at 5cm/sec
Relative Concurrent Flow
1/i0th g 1/6th g
Fig. 7 Downward Spreading in 15% Oxygen, 1 Atm
(c)
(d)
Fig. 8 Upward Spreading in 15% Oxygen, 1 atm (a,b), 0.5 atm (c,d)
250
See Golor Plate F-3


Combustion of solid fuel in very low speed oxygen streams

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