Flame spreading tests have been conducted using thin fuels in microgravity where buoyant convection is suppressed. In spacecraft experiments flames were ignited in quiescent atmospheres with an elevated oxygen content, demonstrating that diffusional mechanisms can be sufficient alone to sustain flame spreading. In ground-based facilities (i.e. drop towers and parabolic aircraft) low-speed convection sustains flames at much lower concentrations of atmospheric oxygen than in quiescent microgravity. Ground-based experiments are limited to very thin fuels (e.g., tissue paper); practical fuels, which are thicker, require more test time than is available. The Glovebox Facility provided for the USML 1 mission provided an opportunity to obtain flame spreading data for thicker fuel Herein we report the results from the Wire Insulation Flammability (WIF) Experiment performed in the Glovebox Facility. This experiment explored the heating, ignition and burning of 0.65 mm thick polyethylene wire insulation in low-speed flows in a reduced gravity environment. Four tests were conducted, two each in concurrent flow (WIF A and C) and opposed flow (WIF B and D), providing the first demonstration of flame spreading in controlled forced convection conducted in space

Greenberg, Paul S.

Kashiwagi, Takashi

Sacksteder, Kurt R.

English

NASA Technical Reports Server

N96- 15555
The USML-1 Wire Insulation Flammability Glovebox Experiment
Paul S. Greenberg_ASA Lewis Research Center
Kurt R. Sacksteder/NASA Lewis Reseaw_ Center
Takashi Kashiwagi/Nmional Institute of Standards and Technology
Introduction
Flame spreading tests have been conducted using thin fuels in microgravity where buoy'_ convection is
suppressed. In spacecraft experiments Ill flames were ignited in quiescent aimosphetes with an elevated oxygen
content, demon.mating that diffusional mechanisms can be sufficient alone to sustain flame spreading. In
ground-based facilities (ie. drop towers and parabolic aircraft) low-spe_ convection sustains flames at much
loWer ¢onc,entratio_ of atmosphericoxygen[2,3,4,51than in quiescentmicrogravity. Ground-basedexperimentsare
limited to very thin fuels (eg. tissue paper); practical fuels, which are thicker, require more test time than is
available.
The Glovebox Facility provided for the USML-1 mis_on provided an opix_.mity to obtain flame spreading data
for thicker fueL Herein we report the results from the Wtre Insulation Flammability (WIF) Experiment
performed in the Glovebox Facility. This experiment explored the heating, ignition and burning of 0.65mm thick
polyethylene wire insulation in low-slx_ flows in a reduced gravity environment. Four tests were conducted,
two each in concun_t flow (WIF A and C) and opposed flow (WIF B and D), providing the first demonsW_on
of flame spreading in comrolled forced convection conducted in space.
Experiments
The Vine Insulation Flammability Experiment (WIF) was designed to observe: Joule heating of electrical whe in
quiescent and low-speed forced-flows, and the ignition and spreading of a flame over the insulation of
overheated electrical wire in very-low-speed flows. Four nearly-identical WIF test modules, designated WIF-
A,B,C, and D were flown. Each module consisted of a miniature wind tunnel with flow provided by the
glovebox air circulation system. A small spring-loaded anemometer indicated airflow velocity. A metal screen
covering the exit port cooled combustion products and contained particulates released during combustion.
In each module a single polyethylene-insulated nichrome wire sample, 1_5 mm diameter, 110 mm long, was
mounted parallel to the airflow direction. All tests were performed using Spacelab cabin air at approximately
21% oxygen at 1 aim. pressure. The polyethylene insulation was approximately 0.65 mm thick. Samples were
ignited using Joule healed Kanthol wire wound around the sample end: near the flow-duct exit for opposed-flow
burning or the entrance for concurrent flow. Each sample was instrumented with six type K thennocouples to
measure wire, wire insnlation and gas phase temperatures. Two windows were inco_ into the tunnel for
photographic access. One window provided simultaneoes video imaging of the sample and the thennoconple
displays, the second window was used for 35 mm still photographs. Two 10 cc vacuum bottles with solenoid
valves accompanied each lest module to collect samples of off-gassing and combustion products. Two
transmission electron microscope grids were attached to each exit screen to collect emitted particles.
The nominal test procedures consisted of healing the wire and insulation samples first in quiescent then in
flowing air;,thereafter the samples were ignited with the insulation at a temperature of approximately 373IC
Video recordings were w_le of the entire sequence, motor-driven 35 mm still photographs were exposed during
the flame spreading portion. Complementmy flame spreading tests of dupficate but unheated WIF fuel samples
were performed in normal gravity in ambient air. The fuel samples were oriented to burn either vertically
upward, vertically downward, or horizontally.
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https://ntrs.nasa.gov/search.jsp?R=19960008389 2020-06-16T05:23:17+00:00Z
Test Results and Discussion
Sample Preheating
Measurements of the sample preheating temlgtatu_ were obtained for each WIF module, each configured for
diffezextt heating rates, in both quiescent and flowing conditions. The results, described in reference 6, show a
similarity between quiescent microgravity heating in air and nmmal-gravity heating in a vacuum.
Ignition and Flame Spreading
Ignition of the insulafice material was achieved in all four tests. The flames were quite bright and produced
surprising amounts of soot. Soot and particles of condensed unbumt fuel vapor collected on the module exit
screens. As the flames spread the fuel melted before vaporizing, and the molten fuel flowed into a quasi-
spherical bulb moving with the flame.
Concurrent Flow Flame Spreading
WIF.A. The flame was quite bright, saturated the video imaging device, and appeared to pulsate. Analysis of
the anemometer behavior indicates the pulsations could not be attributed to the forced flow but were inlrinsic to
the flame and attributed to bursting fuel vapor bubbles. Figure 1 is a monochrmne reproduction of a 35mm
color still photograph representing the concurrent flow flames. The flame was blue where the flow first met the
flame, and became yellow further downsueam. The tip of the flame was open, changing from yellow to a dark
red, and soot visibly escaped from the flame, often in large lhread-llke stmctme_
As the flame propagated, the molten f_el accumulated, forming a growing, quasi-spherical bead 2-3 times the
initial diameter of the insulatien and not always symmetric with respect to the wire. Near the end of the test the
gas-phase thermocouple, 1._ mm from the virgin fuel surface, was occluded by the molten fuel. The surface of
the fuel near the flame in WIF-A, as seen in the still photographs, was discolored and opaque, perhaps due to
oxidative degradation of the polyethylene (this discoloration does not occur in normal gravity heating tests
conducted in a nitrogen environment[_].) In a few of the still photographs, the opaque fuel surface appears to
have been fractmed or chipped, with an irregularly shaped gap in an otherwise nnifotmly brown surface. We
speculate that a part of a rigid surface layer may have been ejected by a bursting vapor bubble.
WIF-C. In the third test, WIF-C, ignition was attempted without flow. A tormidal cloud of vapor or con_
pyrolysis products formed around the igniter and was rendered vim'ble by the scattering of light emitted by the
hot ignitor. The cloud ignited suddenly, and hot gas expansion was felt quickly by the anemometer 75 mm
do_suemn. The flame burned weakly in quiescent air until 17 seconds after ignition when the flow was
initiated. The flame immediately brightened, but ultimately failed to wopagate. The downslream fuel ignited
then extinguished under the combined effect, we speculate, of oxygen depletion by the upslream flame and
retarded fuel vaporization suppressed by the layer of degraded polyethylene and soot on the fuel surface.
Opposed Flow Flame Spreading
WIF-B. The flames in all the WIF tests were quite bright. The mote sensitive monochrome camera was
selected before the flight to capture the expected dim flames, but could not accommodate both the bright flames
and the dimmer temperature displays. In the second test, WIF-B, the crew member took the initiative, in real
time, to adjust the video camera exposure to de-saturate the flame image. In so doing, the tem_ displays
were brought below the detection threshold of the video camera, but some imaging of the flame slructure was
recovered.
As in the concurrent flow test, the molten fuel accumulated into a bead under the flame, growing as the flame
spread into the flow until reaching the steady size shown in fig. 2 in about 15-18 seconds. A dark surface layer,
about 1-2 mm in length, appears near the flame stabilization point, again suggesting oxidative degradation. The
still photographs show vapor bubbles in the molten fuel. Fluctuations in the flame and observed ejection of
26
smallbuntingparticlesuggesttheburs_gof thesebubbles.Astheflamere.bed theend of the fuel sample,
the flame shape altered, diverging downsu'eam and more completely enveloping the molten fuel bead. As the
stationary ball of fuel burned, more frequent flame perturbations oecun'ed, suggesting higher vaporization rates.
The fuel burned until it was completely consumed.
WIF-D. For the fourth test the monochrome video camera was replaced with a color camera. Real-time
exposure adjuslments provided balance between the flame and tem_ display images. Acting on a
suggestion of J.S. T'len, the air flow was switched off after the flame reached the end of the smnple. The flame
quenched rapidly, leaving a nearly spherical fuel bead, 4-5 nun in diameter. This result provides a dramatic
demonstration of the effect of low-speed flow on the flammability of materials in microgravity.
Normal.Gravity Flame Spreading
The normal-gravity experiments were _ by significant dripping of the molten fuel away from the
spreadingfiame. The mass of dripped fuel was as much as l/2 of the toml initial fuel mass. Flames in the
normal gravity tests all had closed tips in contrast to the open downsvream flame tips observed in all the WIF
tests.
Flame Spread Rates and Lengths
The 35 mm photographic sequences provided bert= reproductions of the flame sm_ua'e, particularly the blue
leading edge of the flame (not visible in the video record). Thus while lacking the time resolution of the video
record, the still photographs were used to obtain the flame spread rates of tests WIF-A,B, and D. Similar
measuwxnents weze made from images of the normal gravity flames. The spread rate results are summarized in
Table 1, where the normal gravity reflect average values from several tests.
Flame Slxeading
Direction
Flame Spread Rate
Horizontal (If,)
Fuel Burnout-Front
Speed
0.14 cm/sec
FlameLength
Conctwrent (WIF-A) 0.16 cm/sec 0.12 cm/sec 1.8-2.6 cm
Opposed (WIF-B) 0.070 cm/sec - 1.8 cm
Opposed (WIF-D) 0.066 cm/sec - 1.9 cm
Upward (lg) 0.13 cm/sec -
Downward (lg) 024 cm/sec 0.18 m/sec
0.13 cm/sec
Table 1. Flame Spread Rates and Flame Lengths
Concurrent Flow. In concurrent flow, WIF-A, the visible downstream tip of the flame spread at an appsrendy
steady 0.16 cm/sec; while the base of the flame (wh=e the flame is stab_ and the fuel burnout occurs)
spread at 0.12 cm/sec. The length of the flame grew slowly throughout the test at about 0.04 cm/sec. Thus even
in the extended test time provided by rite glove,box, concurrent flow flames clearly did not reach an equih_orium
length. Though the possibility of steady propagation has been predicted for non-melting thin fuel[5], the WIF
experiment did not provide a steady result.
The spreading of the flame tips in the normal-gravity upward burning tests wee too rapid to meastae with the
35mm still camera, ie. the flame length exceeded the length of the fuel sample within about three seconds.
Propagation speeds of the fuel burnout front in the upward-burning lest and the WIF-A concurrent-flow test wee
(0.13 and 0.12 cm/sec, respectively). Fuel consumption via melting and vaporization in the case of the
27
WIFoAexperin_at is quite different than the dripping observed in the upward bunting experiment, however, so
the similar burnout rates therefore appear to be coincidental.
Opposed Flow. The measured steady spread rates for the WIF-B and WIF-D, opposed-flow cases differ by only
6%, having values of 0.070 and 0.066 cm/sec respectively. The fuel in WIF-B was heated to 353K at ignition
compared to 343K for WIF-D. Higher belk-fucl temperatures have been _xelated with increased flame
rates in normal gravity.Is]
The normal-gravity downward burning flame spread rate of 0.24 cm/sec is much higher than the microgravity
forced flow result. The open shape of the downstream flame tips in microgravity suggests under-ventilation.
However, the overall equivalence ratio in the microgravity tests were very fuel-lean at about 1/45, and the bright
leading edge of the flames suggest oxygen deprivation cannot account for the spread rate difference. In the
several nonnal gravity tests, spread rates varied inversely with the measured fuel mass lost to dripping. Molten
fuel in downward burning might enhance forward heat transfer (and the spread rate) if it flows ahead of the
flame but does not drip corapletely from the fuel sample. Thennocapillary effects re--re molten fuel
differently in microgravity, but rnc_ measurements would be needed to be quantitative. It is reasonable to infer
v-t __[x_ microgravity additionalthatradiative losses, as in other microgra "tyw. s, contn'buteto lower spread rates;
testswithvariedflowvelocitieswouldhelptoclarifythisissue.
Flame Temperatures
Temperatm_ histories were obtained from the video record, and the data for the WIF-A and WIF-D experiments
(Fig. 3). In WIF-A (concun_t flow) the first profile shows two gas-phase temperature peaks, reigesenting first
the flame tip and then the flame leading edge. The second profile shows the effect of the tbenn(gouple being
occluded by the molten fuel. The highest unconected temperatme measured in the concunent flow flame was
1333K. In normal gravity upward burning, the peak temperature of 1287K occurred near the flame bottom.
In WIF-D (opposed flow) the gas-phase temperatnre shows two peaks, represents first the flame leading edge
then another peak near the trailing edge. The peak temperatm_ in opposed flow was about 1273IC The
temperature of the insulation rises rapidly to the pyrolysis temperature as the flame aPlxoaches, remains flat as
the fuel vaporizes, then at burnout increases briefly to a high gas-phase tempemtnre. Peak flame temperatmes in
normal gravity downward burning were found dewnslream, away from the leading edge and peaked at 1402K.
Soot and Particulate Production
Soot escaped from each WIF flame, most prolifically in the opposed-flow tests. Soot left the flame in strand-like
stngtures aplxoximately 10cm long and accumulated on the exit screens. In normal-gravity tests some soot also
escapes and can be collected from the plume above the flames.
Transmission elecaon mk:roscopic GEM) analysis of the primary soot particles revealed mean diameters of 31
nanome,te_ in the opposed-flow tests and 27 nanometers in concmreut flow, less than one standard deviation
difference. Soot primaries collected in normal gravity showed mean diameters of 13 nanumeters. The "gEM
analysis also showed transparent particles, 100-200 m in diameter, on the exit screen of module WIF-C, perhaps
condensed vatxx from the quiescent ignition described above.
Many small bubbles were visible within the molten region, particularly in the opposed flow cases. Since the
boiling temperatures of some polyethylene degradation products (lxoducts range from C1 to CIeo or higher) [_ are
much less than the degradation temperature of polyethylene, when the degradation products are formed inside the
sample they are immediately supe_eated and form bubbles. These bobbles grow by accumulation of degradation
products through diffusion in the molten polyethylene. When they become sufl]ciendy large and close to the
surface, the pressure in the bubbles is sufficienttocausea sudden mpvae, ejecting fragments of the molten
polyethylene into the gas phase. The ejection of small burning polymer fragments were often observed during
the flame spread Im3ce_.
28
Conclusions
The WIF experimentresultsI_ovidesome introductorydataforelectricalwireinsnlationverheatingand burning
ina low-speedcouvectingenvironment,includingflamesize,structure,spreadratesand temperatures.Some
quantitativeresultswereobtained:opposedflowflameswerecool_,propagatedslow_ and producedmoce soot
than concmrent flow flames. These data arc not sufficient to carry out extensive comlzwiso_ with normal
gravity results mainly because of the limited number of tests, but provide quantitative information useful to the
formulation of theoretical modeling and focussed additional tests.
Additionally, the WIF results provide stimulating demonstrations of some unique microgravity phenomenon. The
behavior of the molten fuel,including the bead formationandbubble-bursting phmmnena, the
prodigious Im3dnctkm and size of the soot agglomerates, the Wansient quenching in an abrupdy-crealed quiescent
environment, and the accumulation and ignition of vapor from overheated fuel a_e all phenomena seen first in
these tests. Each of these phenomena have important implications in spacecraft fire scenarios: the quiescent
quenching demonslratinn has been used to convince the STS program to adopt ventilation flow cutoff as a fh-e
fighting procedure.
References
h) Bhattacharjee, S., Altenkix_, R.A., and Sacksteder, ILR., "Implications of Spread Rate and Temperature
Measurements in Flame Spread Over a Thin Fuel in a Quiescent, Microgravity, Space-Based
Environment," Combust. Sd. Teck, 91, pp.225-242, (1993).
2.) Olson, S.L., Ferknl, P.V., and T'ien J.S., "Near-Limit Flame Spread Over a Thin Solid Fuel in
Microgravity," Twenty-Second Symposium (International) on Combustion, the Combustion Institute, pp.
1213-1222, (1988).
3.) Oison, S.L., "Mechanisms of Micmgravity Flame Spread Over a Thin Solid Fuel: Oxygen and Opposed
Flow Effects," Combust. Sci. and Tech., 76, pp. 233-249, (1991).
4.) Sacksteder, K.R., and T'ien, J.S., "Buoyant Downward Diffusion Flame Spread and Extinction in
Partial-Gravity Accelerations," presented at the Twenty-Fifth Symposium (International) on Combustion,
The Combustion Institute, in press (1995).
5.) Grayson, G.D., Sacksteder, ICR., Feriml, P.V., and T'ien, J.S., "Flame Spreading Over a Thin Solid in
Low Speed Concurrent Flow: Drop Tower Experimental Results and Comparison with Theory,"
Micrograviry Sci. Teclmol., Vol.7, no2, (1994).
6.) Greenberg, RS., Sacksteder, K.R., Kashawagi, T., W/'LreInsulation Flammability Experiment: USML-1
One Year Post Mission Summary," Proceedings of the Joint Launch Plus One Year Science Review of
USML-1 and USMP-1 with the Microgravity Measurement Group, NASA CP 3272, Vol.2, (1994).
7.) Madorsk_, SJ._, Thermal Degrada6on of Organic Polymers, John W'dey and Sons, Chapter 4, 1964.
8.) Borgeson, R.A., and T'ien, J.S., "Modellingthe Fuel Temperature Effect on Flame Spread Limits in
Opposed How," Combust. Sci. and Tee&, 32, pp. 125-136, (1983).
29
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The USML-1 wire insulation flammability glovebox experiment

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