An experimental programme on flame spreading over the surface of polymethylmetacrylate (PMMA) samples has been conducted at reduced gravity conditions in the NASA KC-135 air craft laboratory. A total of 36 experiments were performed. From the results of these experiments the flame spreading velocities over PMMA samples have been obtained, as well as their laws of variation with pressure and mixture composition. Both cylindrical and flat samples have been investigated. These results were compared with those obtained for the same conditions of pressure and composition on the ground at 1 g. In this way it was shown how gravity does influence the spreading process and how this influence was affected by pressure and mixture composition

Corchero Díaz, Gregorio

López Juste, Gregorio

Sánchez Tarifa, Carlos

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

C. Sánchez Tarifa, G. Corchero, G. L. Juste 
An Experimental Programme on 
Fíame Spreading at Reduced Gravity 
Conditions 
An experimental programme on flame spreading over the sur-
face of polymethylmetaaylate (PMMA) samples has been con-
ducted at reduced gravity conditions in the NASA KC-135 air-
craft laboratory. A total of 36 experiments were performed. 
From the residís of ¡hese experiments the fíame spreading ve-
locities over PMMA samples have been obtained, as well as 
their laws of variation with pressure and mixture composition. 
Both cylindrical and fíat samples have been imestigated. Tliese 
results were compared with those obtained for the same condi-
tions of pressure and composition on the ground at 1 g. In this 
way it was shown how gravity does influence the spreading pro-
cess and how this influence was affected by pressure and mix-
ture composition. 
1 Introduction 
The heterogeneous combustión process of flame spreading 
over the surface of a condensed (soíid or liquid) fuel and a 
reacting gaseous atmosphere it is strongly influenced by 
gravity. In the first place, the characteristics of the flame that 
spreads depend strongly on free convection, and in the se-
cond place the spreading mechanism, specially the diffusion 
process of the fuel vapors into the reacting atmosphere is 
also altered by gravity. 
On the other hand, flame spreading would be an essential 
part of the mechanism of fire in a spacecraft. 
As a consequence, flame spreading can be considered as a 
combustión process appropriated and of positive interest for 
the study of the influence exerted by gravity. In addition, 
these types of experiments are suitable to be carried out by 
means of parabolic flights. However, the information avail-
able on these processes is very scarce. 
Although several fuels were initially studied, considera-
tions of simplicity, and above all, safety dictated the selection 
of PMMA. 
2 Test Equipment 
Combustión Chambers 
The flame spreading experiments were carried out in closed 
chambers. 
Fig. 1. Combustión chambers in NASA KC-135 aircraft laboratory 
Reloading of the chambers between parabolic flights is 
very difficult. In order to avoid this problem it was decided 
to utilize several equal combustión chambers (fig. 1). 
Maintaining a constant composition of the atmosphere 
within the chambers during the combustión process would 
have implied the design of a complex system of gas extrac-
tion and mixture supply with a complicated control system. 
Mail address: Prof. C. Sánchez Tarifa, Escueia Técnica Superior 
de Ingenieros Aeronáuticos, Polytechnic University of Madrid, 
E-28040 Madrid 3, Spain. 
20 40 60 
0¡ mass fraction ( % 1 — — 
Fig. 2. Variation ofthe 02 mass fraction in the oxidizing mixtures for a 
buming time of20s at l-g 
C. Sánchez Tarifa et al.: An Experimental Programme on Fíame Spreading 
Therefore, it was decided to design the combustión cham-
bers with a mínimum volume such that the compositíon of 
the atmosphere would not change sustantially during the 
combustión process at reduced gravity conditions. 
For a chamber of 25 dm3 in volume, it is shown in fig. 2 
that the gas compositíon does not change sustantially during 
the 20 seconds combustión characteristic time of a parabolic 
ñight. This volume was selected since larger chambers were 
precluded due to problems of volume and weight. 
NASA safety regulations specify design pressures of the 
chamber as function of both, máximum measured pressure 
and adiabatic combustión conditions of the total amount of 
fuel contained in the chamber. This last conditions implied 
the utilization of low mass samples (1-2 g). Máximum re-
corded pressure during the combustión processes was of the 
orderof 135 kPa. 
According to those valúes the chambers were designed 
and tested at 800 kPa. 
Samples 
The cylindrical samples were of 60 mm in length and 4 mm 
in diameter. They were designed with a central hole of 
2.0 mm in diameter utilized to hold the sample with a 
wire. The surface of this hole was inhibited with asbestos to 
prevent combustión acting at the same time as heat transfer 
insulator of the wire. 
The fíat samples were of 60 mm in lenght, 12 mm width 
and 2 mm thickness. They were embedded in low thermal 
conductivity plaster and contained in a rectangular stainless 
steei box, leaving only a flat surface exposed to the combus-
tión spreading process. Plaster was utilized in order to pre-
vent heat transfer along the metallic box which might alter 
significantly the fíame spreading process. 
3 Test Procedu re 
Ignition presented special problems in the environment of 
the tests. Typical liquid fuels ignitions systems, normally used 
with PMMA could not be utilized at the conditions of the 
tests. 
A very effecitive ignition system utilizing a plástic double 
base propellant and an electric spark was developed and test-
ed on the ground. Unfortunately, it had to be discarded due 
to safety considerations. Therefore, ignition had to be carried 
out by means of an electric heated wire, coiled at the extrem-
ity of the sample. This type of ignition was relatively slow 
specially at low oxygen concentrations, and it has to be start-
ed slightly before the flight reaching reduced gravity condi-
tions. This ignition system originated some lack of precisión 
in the results. 
A photographic camera taking photographs every two se-
conds was utilized as well as a video camera. 
Fíame spreading velocities were measured from the time 
recorded photographs, and also on the ground, by recording 
as function of time the temperaturas given by three therrno-
couples embedded in the fuel surface. 
Pressure in the chamber was also recorded during the 
fíame spreading processes. However, it only varied slightly 
during the processes. 
4 Results 
Experiments on the ground 
An experimental programme was conducted on the ground, 
directed to the achievement of the following objectives: 
• Information of the order of magnitud of the fíame spread-
ing velocities by carrying out experiments at low pressure 
in order to reduce the Grashof number. It allowed to se-
lect appropriated dimensions of the samples, which were 
later verified in the first parabolic flights. 
• Development and testing of ignition systems, photo-
graphic equipment and recording devices. 
• Finally, a series of experiments were carried out in order 
to obtain the valúes of the fíame spreading velocities with 
the same samples and for the same range of pressures and 
mixture compositions that would be later obtained in 
flight at reduced gravity conditions. 
Downward spreading velocities were measured, taking 
average valúes of several experiments for each case. 
Results of the spreading velocities as function of mixture 
compositíon and pressure are shown in figs.3 and 4. 
Experiments at Reduced Gravity Conditions (KC-135 NASA 
Aircraft Laboralory) 
The experiments were carried out in two ñight campaigns, 
with a total of 36 experimets. 
M 60 80 
0? mass frac f ion 1%) — 
100 
Oí mass fraction (%) • 
Fig. 3 (left). Downward fíame spreading veloc-
ity over cylindrical samples at 1-g and at dif ferent pressures 
Fig. 4 (right). Downward fíame spreading ve-
locity over fíat samples al 1-g and al differení 
pressures 
C. Sánchez Tarifa et al.: An Experimental Programme on Fíame Spreading 
P = 98 kPa 
+ micro-g 
¡,0 60 80 
02 mass fraction ( % ) — » 
100 
Fig. 5. Fíame spreading velocily over cylindri-
caí samples at ]~g and al reduced gravity 
P = 50kPa 
100 
0¡ mass fracHon (% l 
Fig. 6. Fíame spreading velocily over cylindri-
caí samples at 1-g and at reduced gravity 
40 60 80 
0; mass fracHon [%) — • 
ico 
Fig. 7. Fíame spreading velocily over cylindri-
caí samples al l-g and at reduced gravity 
Fig. 8. Fíame spreading over cylindricaí samples at reduced gravity 
conditions in the NASA KC-135 aircraft laboratory. Y0 = 0.90, P = 
9SkPa 
The first flight campaign was devoted to cylindricaí 
samples and the results obtaíned are summarized in figs.5, 6 
and 7. Two representad ve photographs are shown in figs.8 
and 9, and for comparison a fíame at 1 g is also shown 
(fig. 10). 
The flames show some radial irregularities which are 
probably due to monomers formation. 
Due to the shortness of the time available at reduced grav-
ity conditions, it was not feasible to detect variations of the 
spreading velocity as function of the distance, and therefore, 
average valúes were taken. 
There were some scattering of results, but they were mod-
érate except at low pressure and low concentration. The very 
low spreading velocities at such conditions prevented to ob-
tain meaningfu! results, as shown in fig. 7. 
The second experimental flight programme was devoted 
to fíame spreading over fíat samples. 
Some representad ve photographs are shown in figs.ll, 12 
and 13 and the results obtained are summarized in figs.14,15 
and 16. 
Fig. 9. fíame spreading over cylindricaí samples at reduced gravity 
conditions in the NASA KC-135 aircraft laboratory. Y0, = 0.80, P = 
20kPa 
Fig. 10. Downward fíame spreading over cylindricaí samples at l-g 
Y0. = 0.90. P = 98 kPa 
C. Sánchez Tarifa et al.: An Experimental Programme on Fíame Spreading 
Fig. 11. Fíame spreading over fíat samples al reduced graviiy condi-
lions in ¡he NASA KC-135 aircraft laboraloiy. Y0¡ = 0.80, P = 
98kPa 
Fig. 12. Fíame spreading over fíat samples al reduced graviiy condi-
tions in the NASA KC-135 aircraft laboraiory, Y0. = 0.80. P = 
50kPa 
Scattering of results was appreciably lower for these fíat 
samples and the spreading fíame showed a remarkable bidi-
mensional symmetry, probably due to the utilization of very 
low conductivíty material surrounding the sample. 
The spreading velocity was smaller for these fíat samples 
than for cylindrical ones, as shown in the preceeding figures. 
This effect is discussed in the following paragraph. 
Finally, the influence of pressure on the spreading process 
over those samples is specificaily shown in fíg. 17. 
5 Conclusions 
1. Fíame spreading velocities are considerably lower at re-
duced gravity conditions ~ 1 0 - 2 g, but within the same or-
der of magnitude than fíame spreading velocities at 1 g. 
At this respect, it has to be considered that the Grashof 
number at 1 g is small, owing to the small size of the sam-
ples tested. 
Fig. 13. Downward jlame spreading over fíat samples at 1-g. Y0, = 
0.90,P=98kPa 
3.5 
3 
2.5 
2 
15 
1 
0.5[ 
0 
P = 98kPa 
+ micro-g 
-
 Q l
" 9 
/ 
\ / 
\ / 
i + ^ 
í.0 60 80 
Oí mass frachon (%) -
100 
21 
18 
15 
B. u 
•S 0.9 
0.3 
P = SO kPa 
• 1-g 
+ micro-g 
,y 
A i t_ 
40 60 BO 
0¡ mass fraction (%) -
100 
Fig. 14. Fíame spreading velocity over flat Fig. 15. Fíame spreading velocity over fíat 
samples at 1-g and at reduced gravity samples at 1-g and at reduced graviiy 
1.6 
U -
1.2 -
1 • 
0.6 
04 
0.2 
P = 30 tiPa 
_ n 1-g y 
+ micro-g / 
• 
/ 
: / 
• 
/ / 
/ 
D 
: / / 
/ / 
* + 
• 
/ 
/ 
+ 
/ ' 
40 60 80 
OH mass fraction 1%)-
100 
Fig. 16. Fíame spreading velocity over fíat 
samples at 1-g and at reduced gravity 
C. Sánchez Tarifa et al.: An Experimental Programme on Fíame Spreading 
50 
pressure IkPa) — • -
Fig. 17. Influence of pressure on fíame spreading velocity over fíat sam-
ples a! l-g and at reduced gravity 
2. The influence of gravity decreases when the mixture pres-
sure is reduced. This result could be expected since at 1 g 
the Grashof number is proportional to the square of the 
pressure if temperatures are constant. In ref. [2] it is shown 
that the pressure dependence of the fíame spreading ve-
locity is of the order of the \ power of the gas pressure. 
3. When the oxygen concentration decreases the difference 
between results at 1 g and at reduced gravity also de-
creases. 
This conclusión may be explained because when the oxy-
gen concentration is lowered the combustión temperature 
4. 
5. 
6, 
decreases reducing the fíame spreading velocities in both 
cases, but more significantly at 1 g since in this case the 
Grashof number also decreases. 
Fíame spreading velocities are lower for the case of bidi-
mensionai symmetry than for the case of cylindrical 
symmetry. The main reason of this result is probably the 
difference in the fuel thickness of the samples, being only 
one half in the cylindrical samples as compared with the 
flat ones. At low pressure the difference in the spreading 
velocities is higher as it could be expected, since in this 
case the thermally heated layer in the samples becomes 
thicker. 
Fíame spreading over flat samples at test conditions 
showed a good approximation to real bidimensional con-
ditions. Therefore, there are suitable to verify theoretical 
models. 
Due to the test conditions and the short time available, 
some scatter of results has not been possible to avoid. 
Acknowledgment 
This research programme has been sponsored by the Microgravily 
Department of the European Space Agency. 
Tlie authors acknowledge and thank the help given by that Depart-
ment as well as the help of the flight personnel ofESTEC and NASA, 
which made possible to carry out the experimental flight programme. 
References 
1 Sánchez Tarifa, C, Corchero, G., Juste, G. L.:"An Experimental 
Programme on Heterogeneous Combustión Processes under Míe-
rogravity Conditions. Preliminary Results". IAF-86-288 Paper. 
37th International Astronautical Congress, Innsbruck, Austria, 
October, 1986. 
2 Sánchez Tarifa, C, Liñán, A., Salva, J. J., Corchero, G., and Juste, 
G. L.: "Heterogeneous Combustión Processes under Microgravity 
Conditions. Final Report". ESA CR-6934/86/F/FL. February, 
1988. 


An Experimental Programme on Flame Spreading at Reduced Gravity Conditions.

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