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Flame-spreading Process over Thin Aluminum Sheets in
Oxygen-enriched Environments
a
a
a
C. L. YEH , D. K. JOHNSON , K. K. KUO & M. M. MENCH
a
a
Department of Mechanical Engineering, The Pennsylvania State University, 234 Research
Building East, University Park, PA, 16802, U.S.A
Version of record first published: 20 Jan 2011.
To cite this article: C. L. YEH, D. K. JOHNSON, K. K. KUO & M. M. MENCH (1998): Flame-spreading Process over Thin
Aluminum Sheets in Oxygen-enriched Environments, Combustion Science and Technology, 137:1-6, 195-216
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Flame-spreading Process over
Thin Aluminum Sheets
in Oxygen-enriched Environments
C. L. YEH*, D. K. JOHNSON, K. K. KUO and M. M. MENCH
Department of Mechanical Engineering, The Pennsylvania State University,
234 Research Building East, University Park, PA 16802, U.S.A.
(Received 14 January 199B; In final/arm 24 April 199B)
An experimental study of flame-spreading process over thin aluminum (99 % Al and 1 % Mn)
sheets was investigated in oxygen-enriched environments. The objective of this study was to
determine the dependency of flame-spreading rate over aluminum sheets as a function of initial
chamber pressure, sample thickness, oxygen purity, oxygen flow condition, and sample
orientation. The reaction mechanism of aluminum in oxygen was also studied by examining the
recovered partially-burned sample using a scanning electron microscope (SEM) coupled with an
energy dispersive spectrometer (EDS). The flame-spreading rate over aluminum sheets was
measured by an array of fast-response lead-selenide (Pb-Se) IR photodetectors. The initial
chamber pressure was varied from 0.1 to 6.3MPa. Two grades of oxygen gas were used with
purities of 99.996 % and 99.75 %. In terms of the effect of pressure on the flame-spreading rate,
as the initial chamber pressure was increased, the flame-spreading rate was found to increase to
a maximum, decrease to a minimum, and then increase again. Based upon the comparison of
flame-spreading rates in horizontal, upward, and downward orientation, the flame-spreading
process over aluminum sheets was found to be dominated by the solid-phase heat conduction
mechanism. The continuous oxygen flow showed a strong influence on the flame-spreading
behavior, and it was demonstrated that the flame can be blown off when the counter-current
flow velocity exceeds a critical value. The flame-spreading rates under high-purity (~99.996 %)
oxygen environments were found to be significantly greater than those in commercial grade
(~99. 75 %) oxygen. In addition, the oxygen content in the white ceramic-type nodules formed
on the burned edge of the recovered partially-burned sample is much higher than that on the
unburned surface. These imply that there exist heterogeneous reactions between aluminum and
either oxygen or gaseous aluminum sub-oxides on the burning surface.
Keywords: Aluminum; flame-spreading rate; heterogeneous reaction; photodetector
• Corresponding author. e-mail: [email protected]
195
196
C. L. YEH et al.
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INTRODUCTION
Aluminum is a one of the most widely used materials. Much of the initial
interest in aluminum combustion was associated with propulsion applications. Aluminum particles are commonly used as a fuel ingredient in solid
propellants to increase the specific impulse and to suppress the oscillatory
combustion instability in rocket motors. In recent years, high-surface-area
structured aluminum pac kings have been used extensively as the cryogenic
distillation columns in air separation plants because it can result in systems
that consume less energy than conventional distillation tray technology
(Dunbobbin et al., 1991). Among various aluminum alloys, the 3003
aluminum alloy (99 % AI and 1 % Mn) is the major material used in the
cryogenic air separation plants for producing oxygen; therefore, the flammability of 3003 aluminum alloy in oxygen is of great interest to the society of
compressed gas industry. Increased applications, both in industry and
aerospace, have prompted the continuous study on the flame propagation
over aluminum rods or sheets and fundamental combustion of isolated
aluminum particles.
In the flame-spreading study, the upward flame-spreading process along
aluminum wires or rods (I, 2, and 3 mm in diameter) has been investigated
by Kirschfeld (1961); Long and Sebald (1965); Sa to and Hirano (1986); Benz
et al. (1986) and Sato (1989) in oxygen with pressures ranged from 0.1 to
69 MPa. A peculiar variation of the flame-spreading rate with initial oxygen
pressures was observed; as the oxygen pressure was increased, the flamespreading rate was found to increase to a maximum, decrease to a minimum,
and then increase again. However, this observation has not been fully
explained.
Concerning the combustion mechanism of aluminum, two contradicting
theories have existed. Glassman (1959) offered a concept which predicts that
aluminum combustion takes place in the vapor phase because the boiling
point of aluminum oxide is greater than that of aluminum. Earlier studies
(Drew et al., 1964; Drew, 1965; Prentice, 1965 and 1970) on ignition and
combustion of aluminum particles (with diameters from 10 to 200 um)
appear to support the vapor-phase burning mechanism. Several theoretical models (Brzustowski and Glassman, 1964; Law, 1973; Brooks and
Beckstead, 1995) were developed to describe the vapor-phase combustion of
aluminum particles.
Steinberg et al. (1992), however, argued that Glassman's hypothesis
(1959) is not suitable to justify the mode of metal combustion, since the
metal oxide could decompose before reaching its boiling point. A f1ame-
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FLAME SPREADING OVER ALUMINUM
197
spreading study by Sircar et al. (1991) using cylindrical aluminum rods in
oxygen suggested that aluminum burns in both gas and liquid phases,
according to measurement of the sample weight change during combustion.
Recently, Dreizin (1996) based upon his experimental results and proposed
a combustion mechanism of aluminum particles in air following three
distinct stages: a vapor-phase combustion during the first stage, a second
stage combustion with an increase of size and density of the smoke cloud
when the dissolved oxygen content in the aluminum droplet reaches the limit
needed for liquid Al z0 3 formation, and a third stage associated with oxide
cape formation and growth in a non-symmetric manner. Dreizin (1996)
believes that the suboxides (AIO and AIO z) formed in the gaseous reaction
zone could diffuse to the particle, and then a fraction of these suboxides
dissolves into the molten aluminum. The radial profile of gaseous AIO
concentration was measured by Bucher et al. (1996) using PLIF during the
burning of isolated aluminum particles in air. AlO was found at the particle
surface and has been proposed to produce Al-O through the surface
heterogeneous reaction with molten aluminum (Bucher et al., 1996). Above
studies have shown that heterogeneous reactions should play an important
role in the aluminum combustion.
The objective of this research was to investigate the flame-spreading
characteristics of thin aluminum sheets (3003 aluminum alloy with the
composition of 99 % Al and 1 % Mn) in oxygen-enriched environments
using a non-intrusive optical diagnostic technique. This investigation
includes a study of the effects of initial chamber pressure, sample geometry,
oxygen purity, oxygen flow condition, and sample oreintation on the flamespreading rate of aluminum sheets. The data of flame-spreading rates and
threshold pressures obtained in this study represent the first experimental
measurements for the 3003 aluminum alloy. In addition, a composition
analysis was conducted with recovered partially-burned samples to facilitate
a fundamental understanding of the combustion mechanism of aluminum.
METHOD OF APPROACH
Measurement of Flame-spreading Rate
A schematic diagram of the experimental setup to measure the flamespreading rate over thin aluminum sheets is shown in Figure I. The
experimental setup and diagnostic technique have been described in detail
previously (Yeh et al., 1997a and 1997b). Briefly, the flame-spreading rate
c. L. YEH ct
198
al.
Combustion
Product
Exhaust
OrificePlate
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Stainless Steel Windowed
Test Chamber
,,,
__,""S""'
"
Mirrors
t
--,-
,, ,,
--~-\:
,, ,,
,
,
,,,
,
,
,,
,,
,
,,
,,
,
"""',,*===F=="-'i*~=9P=~P=~~,*=":I
:
I
--l-r-
..,.
Test Celt
:"",,,ss,,ss"SSSSS,,ss"SS"SSSSSssss"SS,,ss"SSSSss,,,,,Sj
I
Camrot
Area
High-Speed
Semi-lransparent
mirror
~
U
FIGURE I
Movie
Camc:I11
CCDV'k.
Camera
Schematic diagram of experimental setup to study flame-spreading behavior over
thin aluminum sheets.
was deduced from the flame front trajectory measured by an array of leadselenide (Pb-Se) infrared (IR) photodetectors, which were mounted on the
top wall of the test chamber. Typical IR photodetector responses and linear
flame-front trajectories, indicating a constant velocity process, were also
presented (Yeh et al., 1997a). The overall transient event of ignition and
flame-spreading of aluminum sheets was recorded by both a CCO video
camera and a high-speed movie camera (HYCAM). In addition, the
pressure-time history during the flame-spreading event was recorded by
three fast-response pressure transducers mounted in the upstream, midstream, and downstream of the test chamber.
FLAME SPREADING OVER ALUMINUM
199
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Aluminum Sample and Igniter
Aluminum samples used in this study were the 3003 aluminum alloy (99 %
Al and I % Mn). Plain aluminum sheets have three different thicknesses of
0.15 mm, 0.20 mm, and 0.40 mm. The thickness of corrugated aluminum
samples is 0.18 mm. The dimension of test samples was 25.4 mm wide and
432 mm long; the reason for using long sample strips is to accurately
determine the flame-spreading rate. Pyrofuze'P, an aluminum-palladium
composite wire, which generates tiny discreet particles through an intermetallic exothermic reaction when resistance hated, was used as the igniter
(Yeh et al., 1997a).
Test Conditions
In this study, the initial chamber pressure was varied from O. I to 6.3 MPa.
Selection of the pressure range was based upon the operation pressure in the
cryogenic air separation plants. Two different grades of gaseous oxygen
were used with purities of 99.75 % and 99.996 %. For stagnant tests (no
continuous oxygen flow), flame-spreading rates in horizontal, upward, and
downward orientations were measured in order to determine the dominant
heat transfer mechanism.
For flowing tests (with a continuous oxygen flow), two modes of flame
spread were studied. One is the flame-spreading process occurring with the
external oxygen gas flow moving in the same direction of flame propagation
(i.e., concurrent flow). The other is external oxygen flow directed against the
spreading flames (i.e., counter-current flow). In this study, external oxygen
flows were set to have a velocity which varied from 5 to 13.5 tn]«.
DISCUSSION OF RESULTS
Recorded Flame-front Trajectory and Pressure Trace
The flame-spreading rates (VFS) in this study were determined from
measured flame-front trajectories by both the photodetector output and
high-speed movie film. Figure 2 shows a plot of flame-front trajectories
deduced from the photodetector output in different horizontal flamespreading tests. The linearity of the time derivative of trajectories indicates
that the flame-spreading process can be treated as a constant-velocity event.
The accuracy of the flame-spreading measurement can be demonstrated by a
comparison of measured flame-front trajectories using photodetectors and
200
C. L. YEH et al.
500
--0--
I
------.-
400
Pressure
Al Thickness
4.1 MPa
0.20 mm
99.75%
1.8 MPa
O.J5mm
99.996%
5.1 MPa
0.40 mm
99.75%
COX Purity
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><:
'"
~
1P...
300
.z8'"
s:
E
200
0
.I:
...u
~
is
100
o-t-~~-r-"-r-~~~---r~~-,-~--,-~~~-r-+-
0.5
1.0
1.5
2.0
2.5
Time (s)
FIGURE 2 Typical flame-fronltrajectories and deduced flame-spreading rates in horizontal
flame-spreading tests.
high-speed movie pictures, as shown in Figure 3. In this particular test, the
HYCAM framing rate was set as 10,000 pictures per second (pps). The
pictures recorded by the high-speed camera can be found in Johnson (1997).
It should be noted that the deduced flame-spreading rates by these two
different methods are in very close agreement.
A typical pressure-time trace recorded from a test with stagnant oxygen
is shown in Figure 4. The observed increase of chamber pressure was the
result of gaseous mass and heat generation due to the combustion of the
aluminum sample. The decrease of pressure beyond the peak value at
the end of combustion was caused by the cooling of combustion products by
the ambient gases and chamber walls. Figure 5 shows a typical pressure-time
trace obtained from a test which maintained a continuous oxygen flow. As
can be seen in Figure 5, the chamber pressure reaches a maximum value at
the completion of combustion, decreases to the initial value due to heat loss,
and then drops rapidly after the oxygen supply is shut off. It is useful to note
that measured pressure traces from three pressure transducers mounted
in the upstream, midstream, and downstream of the chamber were almost
Flame-Front Trajectories Determined/rom
- - - HYCAM Picture; VfS = 464 rnm/s
400
- - Cl - •
Photodetector: V FS = 466 mmls
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300
200
100
o
200
400
600
800
1000
Time (ms)
FIGURE 3 A comparison of flame-spreading rates deduced from high-speed camera films and
photodetectors,
1000
Test #39: 0.15 mm AI Sheet, 99.996% Oxygen
andInitial Chamber Pressure = 3.1 MPa(430 psig)
6.5
900
Completion of Combustion
.ee,
6.0
/
'CO
.;;;
800
".
5.5
1f
"1:!
~
~
5.0
700
~
~
"
.c
"3
[
-e
il
~
~
_"il
0..
E
()
4.5
."
~
-e
600
U
4.0
500
e
3.5
3.0
400
2.0
3.0
4.0
5.0
6.0
7.0
Time (5)
FIGURE 4
A typical pressure-time trace recorded from a test with stagnant oxygen.
202
C. L. YEH et al.
8.0
Completion of
Combustion
7.2
TeSl#51:
0.15 nun AI Sheer,99.75%Oxygen
1000
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Initial Pressure = 4.2 MPa(600 psig)
GOX Flow Velocity =5.4 m/s
6.4
800
5.6
4.8
oe-
3
~
""Cl
'"
e
'"
!l
600
4.0
Onsetof Ignition
3.2
i::""-e
.::.
400
2.4
200 +~~-,-.....-r-r-,-~-r--,-,~~""~--,--,,,,~--,--1 1.6
2.0
4.0
6.0
8.0
10.0
12.0
14.0
Time (s)
FIGURE 5
A typical pressure-time trace recorded from a test with flowing oxygen.
identical, showing that there was no pressure wave generated during the
flame-spreading event.
Stagnant Tests
Horizontal Flame Spreading
Figure 6 shows the measured flame-spreading rate (VFS) versus initial
chamber pressure (P;) for different samples in the horizontal orientation. As
the initial pressure is increased, VFS increases to a maximum, decreases to a
minimum, and then increases again. At a constant initial pressure, the V FS is
higher for the thinner aluminum sheets. The threshold pressure for selfsustained flame propagation increases with increasing sample thickness.
Limited data for the corrugated sheet (0.18 mm) were presented and the V FS
of corrugated sheets were between those of 0.15 mm and 0.20 mm thick
plain sheets.
The observed reverse trend of VFS with Pi can be explained by three
possible mechanisms. First, as Pi increases there is more accumulation of
203
FLAME SPREADING OVER ALUMINUM
700
Al Sample Thickness
o
0.15 rnm (plainsheet)
•
0.20 mm (plain sheet)
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600
I
500
>
~
400
ee
300
'"E
200
Oxygen Purity = 99.75%
0.40 mm (plain sheet)
0.18 mm (corrugated sheet)
~
'"~..
e0.
.."
ti:
100
o -j-,-.oro<>r~-,--p-p-r-~<J,---r-r<O;-T-J~~-,--r~-,,-~~-I­
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Initial Chamber Pressure, Pi (MPa)
FIGURE 6 Effects of initial chamber pressure and sample thickness on measured flamespreading rates of horizontal orientation.
gaseous aluminum sub-oxides (e.g., AI20 and AlO) or small AI203 droplets
near the flame front. These combustion products could form a diffusion
barrier which reduces the collision frequency between aluminum vapor and
oxygen, thereby resulting in a lower V FS' Second, a transition of the ratelimiting step occurs from one gas-phase reaction to another, which shows a
lower reaction rate in spite of the higher pressure (Kirschfeld, 1961). Third,
as initial pressure increases the size of the enveloping diffusion flame around
the burned edge of the aluminum sheet decreases; therefore, it results in a
smaller view factor of the flame by the unburned aluminum surface, causing
a decrease in radiation heat fluxes and flame-spreading rates. As shown in
Figure 6, further increases in pressure lead to a higher VFS' This increase is
believed to be caused by the increase of convective heat-transfer flux from
the flame zone to the unburned material. Since the gas density increases
when pressure is higher, it leads to larger Reynolds and Nusselt numbers of
the induced flow near the combustion zone. Therefore, the convective heat
transfer coefficient is increased with the increase of the Nusselt number.
In comparison with the previous study (Sato, 1989) using aluminum
rods or wires, the flame-spreading rates of thin aluminum sheets were
C. L. YEH et 01.
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204
substantially higher than those of rods, even though a similar trend of flamespreading rate versus pressure was observed. This is believed to be caused by
the larger surface area-to-volume ratio (A/V) of thin aluminum sheets than
that of small rods or wires. For thin sheets, A/V"" 2/ I where I is the thickness
of sheets. For rods or wires, A/V = 4/d where d is the diameter. Table I
shows a comparison of measured flame-spreading rates between aluminum
sheets and rods at initial chamber pressures of about 5 MPa of commercialgrade oxygen.
The effect of oxygen purity on the flame-spreading rate is presented in
Figure 7. For 0.15 mm thick aluminum sheets, results indicated that the
threshold pressure (0.65 MPa) for self-sustained flame propagation in highpurity (99.96 %) oxygen is much lower than that (2.86 MPa) in commercial
grade oxygen (99.75 % purity). The substantial increase of threshold
pressure in commercial grade oxygen is because of the presence of argon
(Ar), the major impurity in gaseous oxygen cylinders. The accumulation
of argon near the combustion zone could create a diffusion barrier to
the inward diffusion of oxygen, resulting in a higher threshold pressure.
The strong influence caused by a small amount of Ar in oxygen on the
flammability of aluminum was also observed by Benning et al. (1988). The
difference in threshold pressures between 3003 (99 % Al and 1% Mn) and
6061 (99 % Al and I % Mg) aluminum alloys can be seen in Table II. Due to
the different alloy composition, it was found that the threshold pressures of
3003 aluminum alloy are much higher than those of 6061 aluminum alloy
in similar oxygen-purity environments. This result indicates that a slight
difference in the alloy composition can lead to a significant difference in the
flammability.
Figure 7 also shows that, under the same initial chamber pressure, the
flame-spreading rate measured in high purity oxygen (99.996 %) environments is substantially higher than that in oxygen with a purity of 99.75 %.
TABLE I Comparison of measured flame-spreading rates between aluminum sheets and rods
at initial chamber pressures of about 5 MPa
Data source
sample geometry
The present study
SOia, 1989
Sheet (25.4 mm wide and 432 mm long)
Rod (120mm long)
Sheet thickness (I, mm)
or rod diameter (d, mm)
0.15
0.20
0.40
Surface area-tovolume ratio (A/V, rnm"')
13.33
10
5
Flame-spreading rate
(VFS, mm/s)
441
360
236
2
3
4
2
1.33
68
60
48
FLAME SPREADING OVER ALUMINUM
205
Oxygen Purity
•
99.996%
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99.7S%
o
800
600
400
200
AISampleThickness:
0.15 mm (Plain Sheet)
o +r--,<>;l"«'r~-,-p;>r-~""~--r-r~~--,r~.,,~~+­
0.0
1.0
2.0
4.0
3.0
6.0
5.0
7.0
Initial Chamber Pressure, Pi (MPa)
FIGURE 7
Effects of oxygen purity and initial chamber pressure on measured flame-spread-
ing rates of horizontal orientation.
TABLE II
alloys
Comparison of measured threshold pressures between 3003 and 6061 aluminum
Data source
Aluminum alloy
Oxygen purity
Threshold pressure (MPa)
The present study I
3003 AI
99.75%
2.86
3003 AI
99.996%
0.65
Benning et al., 1988'
6061 AI
99.793 %
0.83
6061 AI
99.993%
0.21
1 The lest sample geometry is an aluminum sheet with a thickness of O. )5 mm, a width of 25.4 mm, and a
length of 432mm.
2
The test sample geometry is an aluminum rod with a diameter of 6.4 mm, and a length of 76 mm.
The flame-spreading rate changes by several tens of a percent, while the
oxygen concentration is only changing by a couple thousandths of a percent.
The high sensitivity of aluminum consumption rate to the oxygen purity
indicates that heterogeneous reactions on the aluminum sample surface
could play an important role in the combustion of aluminum. It is known
that the mass consumption rate by heterogeneous reactions on the burning
metal surface is directly proportional to the local oxygen concentration,
which is relatively small (Glassman and Law, 1991). Any small, or even
C. L. YEH et al.
206
trace, quantities of impurities (such as argon) could effectively reduce the
oxygen concentration at the surface, thus significantly decreasing the aluminum consumption rate and flame-spreading rate.
Downloaded by [University of Tennessee, Knoxville] at 17:12 07 November 2012
Upward Flame Spreading
In order to obtain a better understanding of the heat transfer mechanism in
flame-spreading processes, the influence of sample orientation on the flamespreading rate has received considerable attention. Excluding external
sources, the three principal mechanisms by which heat may be transferred to
the unburned material are: radiation from the flame, conduction or convection through the gas from the flame, and conduction through the solid
(Fernandez-Pello and Hirano, 1983). As shown in Figure 8, the vertically
upward flame-spreading rates are nearly the same as those in horizontal orientation at similar initial chamber pressures. This implies that
the heat conduction through the aluminum sheet is the dominant heat
transfer mode.
I
1000.0
I
Sample Orientation
.
I
800.0
'V'V
Horizontal Harne Spreading
Upward FlameSpreading
DownwardFlame Spreading
0
'V
~
>~
~
l>:
AI Sample: 0, J5 mm Plain Sheet
Oxygen Purity= 99.75%
600.0
I-
bll
~"
~
. .
..'to--.
400.0 -
"'~"
0
~
til
f-
§
fi:
200.0 -
0.0
I-
,
,
,
4.0
5.0
-\-,-,o-r--.<rr-~-<>?~-,--j>"~~,,...~,,~-r-,--r~~+­
0.0
1.0
2.0
3.0
6.0
7.0
Initial Chamber Pressure, Pi (MPa)
FIGURE 8
Effects of sample orienlation on measured flame-spreading rates.
FLAME SPREADING OVER ALUMINUM
207
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Downward Flame Spreading
As also shown in Figure 8, vertically downward flame-spreading rates were
much higher than those measured from horizontal and upward orientations
under the same initial chamber pressures. It should be noted that, unlike
horizontal and upward flame spread, the downward flame-spreading rate is
not constant and it accelerates along the length of the sample. As shown
in Figure 9, the downward flame-spreading rate increases from 670mm/s
(which is based upon the output of first three photodetectors) to 1120mm/s
from the last three photodetectors. The data of downward flame spread
plotted in Figure 8 are based upon the linear fit of all six photodetector
outputs. This substantially increase in downward flame-spreading rate is
believed to be caused by the fact that the dripping of molten aluminum or
oxide from the reaction zone to the unburned sample enhances the rate of
heat transfer and then the flame-spreading rate. The dripping and flowing of
molten aluminum along the sample sheet could increase the reacting surface
area of aluminum, thereby resulting in the acceleration of flame-spreading
rates. It is useful to note that a significant amount of molten materials was
Downward FlameSpreading
Initial Chamber Pressure = 5.06 MPa
Al SampleThickness = 0.15 nun(PlainSheel)
Oxygen Purity = 99.75%
300
250
E
V FS = 873.4 mmls
200
~"
.§.
c
u
s
"« = 1120.7 mrnIs
150
~
is
100
v" = 669.4 mrnIs
50
0
3.4
3.5
3.6
3.7
3.8
3.9
Time (sec)
FIGURE 9 A typical flame-front trajectory of downward flame-spreading tests.
C. L. YEH et al.
208
found at the bottom of the test chamber after downward flame-spreading
tests, when compared to upward and horizontal flame-spreading tests.
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Flowing Tests
Concurrent Oxygen Flow
All of the tests with continuous oxygen flow were conducted under the
horizontal orientation in this study. Effects of both concurrent and countercurrent oxygen flows on the flame-spreading rate were shown in Figure 10.
Under concurrent oxygen flows, the V FS increases with the flow velocity,
reaches a maximum value and then starts to decrease with a further increase
in the oxygen velocity. It is anticipated that for a given initial chamber
pressure there exists an optimum oxygen velocity beyond which the V FS
decreases with the increase of oxygen velocity. For the series of tests at
pressures of 4.24 MPa shown in Figure 10, the optimum oxygen velocity is
Oxygen Flow Rate (SCFM)
0.0
800
5.0
10.0
15.0
20.0
,
25.0
30.0
35.0
,
I
+~-'--.l~~-LL~J...L~~'--'-~'-'-~........J'--'-'~"""",.-
- - .. - . Concurrent oxygen flow
- 9 - - Counter-current oxygen flow
•
Stagnant oxygen
700 -
I-
600
--'-- .•... ~
/---l-"
500 -
400
300
200
Initial Chamber Pressure = 4.24 MPa
Oxygen Purity = 99.75%
AI Sample Thickness e 0.15 nun (Plain Sheet)
100
....~~rr~..,....,~
....I ~~rr~-,-~
...I ..,....,~tI
I
I
o-h~
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
Oxygen Flow Velocity (mls)
FIGURE 10 Effect of oxygen flow condition on measured flame-spreading rates of horizontal
orientation.
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FLAME SPREADING OVER ALUMINUM
209
about 5 tn]«, which corresponds to a Reynolds number of 4.43 x 105 in a
2.7 ern by 4.5 em flow channel (the cavity dimension of the test chamber).
In the concurrent mode of flame spread, the hot, still reacting or post
combustion gases generated at the burning region of the material are driven
ahead of the pyrolysis front close to the unburned combustible surface
(Fernandez-Pelle and Hirano, 1983). The proximity of the hot gases or
combustion products (e.g., Alz0 3 fine particles) to the unburned surface
favors the heat transfer (convection and radiation) to the unburned material, thus enhancing the flame-spreading rate. When the oxygen flow rate
is larger than the optimum value, the heat loss to the cold oxygen gas
stream dominates the enhanced heat transfer by the flow; therefore, a
decrease in V FS is observed.
Counter-current Oxygen Flow
For experiments with counter-current oxygen flow (also shown in Figure
10), there exists an initial region at flow velocities less than about 10 mls
where the flame-spreading rate remains practically independent of the
oxygen velocity. After this initial region, the flame-spreading rate increases
with the oxygen flow velocity, reaches a maximum and then decreases
drastically as the opposed flow velocity increases. Once the opposed oxygen
velocity reaches a threshold value, the flame plume is blown off by the flow
and the propagation rate drops abruptly from a measurable value to zero. It
was found that extinction or non-flame propagation occurs in a very narrow
range of flow velocities. Above flame-spreading phenomena are the
characteristic of thermally-thick fuels, which are dominated by conduction
heat transfer through the solid phase during the flame-spreading process
(Fernandez-Pello et al., 1981).
The existence of an initial region at low opposed velocities where the
flame-spreading rate remains almost independent of the oxygen velocity is
because the opposed flow obviously affects the gas-phase heat transfer more
than the solid phase by convective cooling. Therefore, the net amount of
heat transferred through the solid-phase conduction still remains nearly
constant, resulting in a constant flame-spreading rate. The second regime
shows a near linear increase of V FS with the opposed flow velocity and is
mainly dominated by thermal processes (Fernandez-Pello et al., 1981). As
the opposed oxygen velocity is increased, the diffusion flame moves closer to
the aluminum surface and the rate of mass burning and the rate of heat
transfer through the solid increases. It is useful to note that the character of
Downloaded by [University of Tennessee, Knoxville] at 17:12 07 November 2012
210
C. L. YEH el at.
the leading edge flame can be described by the ratio of the residence time
(time required for a fuel particle to travel to the gas preheated region) to the
chemical time (time required for a fuel particle to react with the oxidizer),
i.e., the Damkohler number (Fernandez-Pello et af., 1981 and FernandezPello and Hirano, 1983). For large Damkohler numbers (low opposed flow
velocities), the reaction rate is very large. Gas phase chemistry becomes
unimportant and the flame-spreading process is controlled primarily by heat
transfer to the unburned material.
For small Damkohler numbers (high opposed flow velocities), the
reaction rate is slow. Finite rate chemical kinetics becomes important and
flame-spreading rate depends strongly on the rate at which the fuel is
consumed. If the Darnkohler number becomes smaller than a critical value,
the extinction of the reaction occurs at the flame leading edge (FernandezPello and Hirano, 1983). Therefore, the sudden decline in VFS shown in the
last regime of Figure 10 is mainly caused by the gas-phase chemical kinetic
effect (Fernandez-Pelle and Hirano, 1983 and Ray and Glassman, 1983). As
the opposed velocity is increased further, the flame front must propagate
against a velocity greater than the lean limit flame speed. Consequently, the
name is blown off downstream.
Figure II shows the relative velocity (V'e1) between oxygen gas velocity
(Vgas) and flame-spreading rate (V FS) versus the flame-spreading rate. The
relative velocities for different test conditions are defined below.
For concurrent flow tests, V,cl = V gas- V FS'
For counter-current flow tests, V,cl = - Vgas- VFS'
For stagnant tests, V,cl = - VFS'
As shown in the Figure II, the flame was blown off in a counter-current
flow test when the relative velocity was - 13.5 m/s. However, when the
relative velocity was about 13 mls in the concurrent flow test, the flame was
not extinguished by the flow. This difference may be due to the effect of
concurrent flow on the shape of molten region and thus its reactive surface.
Even though there is more heat loss to the external flow when the concurrent
flow rate is higher, the flow-induced drag force can increase the reacting
surface of the molten aluminum to continue the flame propagation. It is still
possible for the blown-off phenomenon to occur in the concurrent flow test,
as long as the heat loss from the combustion zone to the external flow
exceeds a critical value, in which the heat flux transferred to the unburned
material is not sufficient to pyrolyze the aluminum and to maintain the flame
propagation process.
FLAME SPREADING OVER ALUMINUM
,
800
Initial Chamber Pressure = 4.24 MPa
Oxygen Purity = 99.75%
AI SampleThickness = 0.15 nun (PlainSheet)
700
Downloaded by [University of Tennessee, Knoxville] at 17:12 07 November 2012
~
g
:2
>
"
:a"
"ea.
fI-
600
-.
500
B
0::
ea
211
400
-----.
,
,
,
,','~
I-
I-
300
Vl
""
e
[i:
200 -
Counter-current Flow
100
Concurrent Flow Tests
Tests
-
-<
o
-15.0
-10.0
-5.0
0.0
5.0
10.0
15.0
Relative Velocity, V~I (mls)
FIGURE II
Effect of relative velocity on flame-spreading rates of thin aluminum sheets.
Another interesting point is that with continuous oxygen flow (both
concurrent and counter-current flow), fresh oxygen is constantly available in
the reaction zone and takes part in combustion during the flame-spreading
event. Therefore, the diffusion is not the only gas transport mechanism in
flowing tests. As shown in the figure, when the relative velocity (Vrel ) is zero,
a minimum value of the flame-spreading rate is expected. This is because the
diffusion is the only mechanism to transport oxygen to the reaction zone.
Analysis of Recovered Samples
The partially-burned aluminum sheets were quenched by a rapid depressurization of the test chamber to interrupt the combustion event. The burned
edge shows nearly one-dimensional flame propagation over the sample. The
molten portion of the sample (from the edge to the unburned material) is
about 0.8 mm. It was found that a number of ceramic-type white nodules
was sporadically deposited on the edge.
The recovered partially burned aluminum sheets were examined under
a scanning electron microscope (SEM) coupled with an energy dispersive
Downloaded by [University of Tennessee, Knoxville] at 17:12 07 November 2012
212
C. L. YEH et al.
spectrometer (EDS). The EDS spectrum measured from the unburned
sample is shown in Figure 12. The atomic oxygen concentration on the
unburned sample is about 2.2 %, which is believed to be associated with an
oxide film formed on a clean metal surface exposed to room air. The SEM
image, shown in Figure 13, represents a typical structure of the burned edge
(perpendicular to the sample surface) of a recovered aluminum sheet. White
ceramic-type nod ules were formed on the burned edge of the recovered
sample. As shown in Figure 14, the atomic oxygen concentration (about
25.5 %) on those white nodules "is much higher than that on the surface of
unburned samples. It is speculated that the formation of white nodules
could be caused by either growth of oxide caps or products of heterogeneous
reactions on the molten aluminum surface.
The substantial increase in oxygen content on the burned edge of
recovered samples provides an evidence that the heterogeneous reactions of
molten aluminum with either oxygen or gaseous aluminum sub-oxides are
important. It is believed that the gas-phase intermediate product, AIO,
Coun'ts
(XIO' )
,
I
16
14
-
12 Al
-
10
B-
-
6
-
4
-
2-
-
0
\
0
~
)
\
I
I
1
2
Ran •• (keV)
FIGURE 12 EDS spectrum measured from the surface of an unburned aluminum sample.
Downloaded by [University of Tennessee, Knoxville] at 17:12 07 November 2012
FLAME SPREADING OVER ALUMINUM
213
FIGURE 13 Scanning electron micrograph showing white ceramic-type nodules formed on
the burned edge of a recovered aluminum sample.
Count ..
(XIO')
I
I
3-
Al
I-
2-
I -
0
},
I
I
I
\.
I
2
Ran •• (k.V)
FIGURE 14 EDS spectrum measured from white ceramic-type nodules formed on the burned
edge of a recovered aluminum sample.
Downloaded by [University of Tennessee, Knoxville] at 17:12 07 November 2012
214
C. L. YEH et al.
diffuses from the gas phase to the sample surface, resulting in the increase in
the oxygen content on the burning edge (Bucher et al., 1996 and Dreizin,
1996). The heterogeneous reaction of A10 and molten Al on the surface has
been proposed as a means to produce AlzO (Bucher et al., 1996 and Yuasa
et al., 1997). Based upon the measured AIO profile of isolated burning
aluminum particles (Bucher et al., 1996), the non zero value of AIO at the
burning surface suggests that the rate of this heterogeneous reaction is
kinetically and not diffusively limited. This also explains that the higher
flame-spreading rates measured in 99.996 %-purity oxygen than those in
99.75 %-purity oxygen (see Fig. 7).
CONCLUSIONS
Based upon the experimental measurements and observations of this study,
several major findings are listed below.
I. In terms of pressure effect on the flame-spreading rates, as the initial
chamber pressure was increased the flame-spreading rate was found to
increase to a maximum, decrease to a minimum, and then increase again.
2. The flame-spreading rates under high-purity (~99.996 %) oxygen
environments were found to be significantly greater than those in
commercial grade (~99.75 %) oxygen. This is believed to be caused by
higher exothermic heat release from heterogeneous reactions on the
molten sample surface. Moreover, the threshold pressure for selfsustained flame propagation of aluminum sheets was considerably lower
under a high-purity oxygen environment.
3. For vertically oriented samples in upward flame-spreading tests, the
flame-spreading rates were found to be nearly the same as those of
horizontal tests with the same initial chamber pressure. In downward
flame-spreading tests, flame acceleration along the length of the sample
was observed. This is believed to be caused by the dripping and flowing
of molten aluminum and oxide onto the unburned sample, resulting in an
increase of reacting surface area and heat transfer rate.
4. Under the concurrent oxygen flow test conditions, the oxygen flow
enhanced the flame-spreading rate. It is anticipated that there exists an
optimum oxygen velocity (about 5 mls at 4.24 MPa) beyond which the
flame-spreading rate decreases with the increase of oxygen velocity due to
convective heat loss. Under the counter-current oxygen flow test
conditions, it was demonstrated that the flame can be blown off when
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FLAME SPREADING OVER ALUMINUM
215
the counter-current flow velocity exceeds a critical value of about
13.5m/s.
5. White ceramic-type nodules were found on the burned edge of the
recovered partially-burned sample. The atomic oxygen concentration
(25.5 %) in those white nodules is much higher than that (2.2 %) on the
unburned surface. This implies that there may exist heterogeneous
reactions between aluminum and either oxygen or gaseous aluminum
sub-oxides on the burning surface.
Acknowledgements
This work presents the results obtained from the research project sponsored
by Praxair, Inc. and Air Products and Chemicals, Inc. The authors are
grateful to R. A. Van Slooten, R. Zawierucha, and J. F. Million of Praxair,
Inc. and J. G. Hansel, P. A. Houghton, and E. C. Rogusky of Air Products
and Chemicals, Inc. for their input and support of this project.
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