An experimental programme on flame spreading over the surface of PMMA (plexiglass) samples has been conducted under microgravity conditions in the NASA KC-135 aircraft laboratory.

A few experiments (three) were conducted in 1986 under the preceding contract no. 6284/85/F/FL, but the largest part of the experimental programme has been carried out under the present contract in two parabolic flights campaigns.

A total of 36 experiments were performed, most of them successful. 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 (axial symmetry) and flat (bidimensional symmetry) samples have been investigated

Corchero Díaz, Gregorio

Esteban, F.

Liñán Martínez, Amable

López Juste, Gregorio

Salvá Monfort, José Juan

Sánchez Tarifa, Carlos

Archivo Digital UPM

Heterogeneous combustion processes under microgravity conditions

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

HETEROGENEOUS COMBUSTIÓN PROCESSES 
UNDER MICROGRAVITY CONDITIONS 
Prof. C. Sánchez Tarifa, Prof. A. Liñan, Prof. J.J. Salva, 
Ass. Prof. G. Corchero, Ass. Prof. G.L. Juste, Mr. F. Esteban 
Laboratorio de Propulsión E.T.S.I. Aeronáuticos, Madrid, Spain 
1. E X P E R I M E N T A L P R O G R A M M E * 1.2. Selection of the experimenta 
1.1. Introduction 
An experimental programme on fíame spreading over 
the surface of PMMA (plexiglass) samples has been 
conducted under microgravity conditions in the NASA 
KC-135 aircraft laboratory. 
A few experiments (three) were conducted in 1986 
under the preceding contract no. 6284/85/F/FL, but 
the largest part of the experimental programme has 
been carried out under the present contract in two 
parabolic flights campaigns. 
A total of 36 experiments were performed, most of 
them successful. 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 (axial symmetry) and flat (bidimensional 
symmetry) samples have been investigated. 
These results were compared with those obtained 
under the same conditions of pressure and composition 
on the ground at 1 g, and it was shown how gravity 
does influence the spreading process and how this 
influence was affected by pressure and mixture 
composition. 
It must be pointed out that ñame spreading under 
microgravity conditions has been observed for the first 
time and that this ESA research programme has 
therefore been of a pioneering nature. 
The heterogeneous combustión process of a fíame 
spreading over the surface of a condensed (solid or 
liquid) fuel and a reacting gaseous atmosphere is 
strongly influenced by gravity. Firstly, the 
characteristics of the spreading fíame depend strongly 
on free convection, and secondly the spreading 
mechanism, especially the diffusion process of the fuel 
vapours into the reacting atmosphere is also altered by 
gravity. 
On the other hand, fíame spreading would constitute 
the basic mechanism of fire propagation in a 
spacecraft. 
As a consequence, fíame spreading can be considered 
as a combustión process highly appropriate and of 
great interest for the study of the influence exerted by 
gravity. In addition these types of experiments were 
considered suitable to be carried out during parabolic 
flights. 
Although several fuels were initially studied**, 
considerations of simplicity, and above all, safety 
dictated the selection of PMMA. 
As has already been mentioned, cylindrical and 
rectangular samples were investigated. 
* Contract no. 6934/86/F/FL, Final Report 
** Contract no. 6284/85/F/FL, Final Report 
54 C. Sánchez Tarifa, A. Linan, J. J. Salva, G. Corchero, G. L. Juste, F. Esteban 
1.3. Test equipment 
1.3.1. Combustión chambers 
The flame-spreading experiments were carried out in 
closed chambers. 
Reloading of the chambers in flight is very difficult, 
and in order to avoid this reloading procedure before 
each parabolic flight it was decided to use several 
identical combustión chambers; three in the first 
campaign and six in the second and third campaigns 
(see Figs. 1.1 and 1.2). 
Figure 1.1. Combustión chambers in the NASA 
KC-135 aircrqft laboratory. First campaign. 
Figure 1.2. Combustión chambers in the NASA 
KC-135 aircrqft laboratory. Second and third 
campaigns. 
Maintaining a constant composition of the atmosphere 
in the chambers during the combustión process would 
have implied the design of a complex system of gas 
extraction and mixture supply with a complicated 
control system. 
Therefore it was decided to design the combustión 
chamber with a mínimum volume in such a way that 
the composition of the atmosphere would not change 
substantially during the combustión process under 
microgravity conditions. Parabolic flights were of the 
order of 15—20 s. The amount of PMMA burnt 
during ignition and the 20 seconds of spreading 
combustión was measured on the ground by 
quenching the fíame with nitrogen and by weighing 
the sample. 
20 40 M 80 100 
02 misa fraction ( % ) 
Figure 1.3. Variation of 02 mass fraction for a 
burning time of 20 s at 1 g. 
Fig. 1.3. shows that for a chamber of 25 dm in 
volume the gas composition does not change 
substantially during the process. This volume was 
selected since larger chambers were excluded 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 (Fig. 
1.4). This last condition implied the use of low mass 
samples (1—2 g). Máximum recorded pressure during 
the combustión processes was of the order of 140 kPa 
The chambers were designed and tested at 800 kPa 
according to those valúes. 
HETEROGENEOUS COMBUSTIÓN PROCESSES UNDER MICROGRAVITY CONDITIONS 55 
• a « m • 
Figure 1.4. Máximum pressure for adiabatic and 
complete combustión. 
70rm 
2 m • 
Sonóle longitudinal sectlon Sanóle cross sectlon 
Figure 1.5. View of longitudinal and cross sections of 
a cylindrical sample. 
12rf>-
ilm 
V-
60r» 70nn 7t)nn £ 
jSuppor-tme wlr» 
Sanple frontal view 
12rm 
' 2nn 
lnn 
! Supporttng 
Sanple cross sectlon 
Figure 1.6. Frontal and cross-section view of a fíat 
sample. 
1.3.2. Samples 
The cylindrical samples had a length of 60 mm and a 
diameter of 4 mm for the first and second campaign. 
They were designed with a central hole of 2.0 mm in 
diameter which was used for holding the sample with 
a wire. The surface of this hole was inhibited with 
asbestos to prevent combustión (Fig. 1.5), acting at 
the same time as heat transfer insulator of the wire. 
The fíat samples had a length of 60 mm, a width of 
12 mm and a thickness of 2 mm. They were 
embedded in low thermal conductivity plaster* and 
contained in a rectangular stainless steel box (Fig. 
1.6), leaving only a fíat surface exposed to the 
combustion-spreading process. Plaster ^ was used in 
order to prevent heat transfer along the metallic box 
which might have significantly altered the flame-
spreading process. 
1.4. Test procedure 
Ignition presented special problems in the environment 
of the tests. Typical liquid-fuels ignition systems, 
normally used with PMMA, were unsuitable due to 
the conditions of the tests. 
A very effective ignition system utilising a plástic 
double-base propellant and an electric spark was 
developed and tested on the ground. Unfortunately, it 
* Thermal conductivity of plaster is 2—3 orders of 
magnitude smaller than that of metallic materials. 
56 C. Sánchez Tarifa, A. Liñan, J. J. Salva, G. Corchero, G. L. Juste, F. Esteban 
had to be discarded due to safety considerations. 
Therefore ignition had to be carried out by means of 
an electric heated wire, coiled around the extremity of 
the sample. This type of ignition was relatively slow, 
especially at low oxygen concentrations, and it had to 
be started slightly before the light reaching 
microgravity conditions. This ignition system was 
responsible for some imprecisión in the results. 
A camera taking photographs every two seconds was 
used as well as a video camera. 
Flame-spreading velocities were measured from the 
time-recorded photographs and also on the ground by 
recording as fiínction of time the temperature given by 
three thermocouples embedded in the fuel surface. 
Pressure in the chamber was also recorded during the 
flame-spreading processes. However, it only varied 
slightly, and therefore results are not shown since they 
were practically not influenced by it. 
1.5. Results 
1.5.7. Experiments on the ground 
An experimental programme was conducted on the 
ground, aimed at achieving the following objectives: 
— Obtaining information on the order of magnitude of 
the flame-spreading velocities by carrying out 
experiments at low pressure in order to reduce the 
Grashof number. It allowed to select appropriate 
dimensions of the samples. These preliminary data 
were later to be verified during the first parabolic-
flights campaigns. 
— Development and testing of ignition systems, 
photographic equipment and recording devices. 
— Obtaining the valúes of the flame-spreading 
velocities with the same samples and for the same 
range of pressures and mixture compositions that 
would later be obtained in flight under micro-
gravity conditions. 
Downward-spreading velocities were measured, taking 
average valúes of several experiments for each case. 
Photographs of the flames are shown in Figs. 1.7, 
1.8, 1.9 and 1.10 and the results of the spreading 
velocities as a function of mixture composition are 
shown in Figs. 1.11 and 1.12. 
Figure 1.7. Downward flame spreading over 
cylindrical samples at 1 g. Y02 = 0.9, P = 98 kPa. 
Figure 1.8. Downward flame spreading over 
cylindrical samples at 1 g. Y02 = 0.9, P = 20 kPa. 
H E T E R O G E N E O U S C O M B U S T I Ó N PROCESSES U N D E R M I C R O G R A V I T Y C O N D I T I O N S 57 
Diam.:eit=4nun.,int.=2mffl.Leii{th=t0min. 
Figure 1.9. Downward fíame spreading over fíat 
samples at 1 g. Y^ = 0.90, P = 98 kPa. 
o 
o 
I 
A 
u 
i 
< 
1-5 
0 ?- 98 kPa 
t P=50 kPa 
o P= 20 kPa 
40 80 
02 mass fraction (*) 
Figure 1.11. Results of measurements offlame-
spreading velocities for cylindrical sample. 
100 
Les(th=<0mm. tidth=t2nun. 1hidmea=2mm. 
Figure 1.10. Downward fíame spreading over fíat 
samples at 1 g. Y02 = 0.90, P = 30 kPa. 
t 
i 
u 
A 
6 
15 
2¿ 
Oi 
1-í 
- DP=MkPi 
+ P=50kPt 
«P=30kPt 
-
-
-
/ y 
j . + 
^\^-i> 
1» - " " " ^ 
i 
/ 
/ 
+ 
^» 
1 
• 
0^ 
1 
0 
/ / 
» ^ 
i 
a 
+ 
t 
i 
40 M U 
02 mu» friction (i) 
Figure 1.12. Results of measurements offlame-
spreading velocities for fíat sample. 
100 
58 C. Sánchez Tarifa, A. Liñán, J. J. Salva, G. Corchero, G. L. Juste, F. Esteban 
1.5.2. Experiments under microgravity conditions 
(parabolic flights in the NASA K-135 aircraft 
laboratory) 
In the April 1987 parabolic-flights campaign the 
experimental programme was devoted to fíame 
spreading over cylindrical samples. 
Some representative photographs are shown in Figs. 
1.13, 1.14 and 1.15 and the results obtained of the 
flame-spreading velocities as a function of mixture 
composition and pressure are shown in Figs. 1.16, 
1.17 and 1.18. Results of the 1986 parabolic-flights 
campaign are also included. 
The flames show some radial irregularities which are 
probably due to monomer formation. They were also 
observed during the short experiments carried out in 
the free-fall chamber test facility of the Instituto 
Nacional de Técnica Aeroespacial*. 
Due to the shortness of the time available under 
microgravity conditions, it was not feasible to detect 
variations of the spreading velocity as function of the 
distance, and therefore, average valúes were taken. 
There was some discrepancy between the results, but 
it was modérate except at low pressure and low 
concentrations. The very low spreading velocities at 
such conditions made obtaining meaningful results 
impossible, as shown in Fig. 1.18. 
In October 1987 the parabolic-flights campaign was 
devoted to fíame spreading over fíat samples. 
Some representative photographs are shown in Figs. 
1.19, 1.20 and 1.21 and the results obtained are 
shown in Figs. 1.22, 1.23 and 1.24. 
Scattering of results was appreciably lower in this case 
and the spreading fíame showed a remarkable 
bidimensional symmetry, probably due to the 
utilisation of very low conductivity material sur-
rounding the sample. 
The spreading velocity was smaller for these fíat 
samples than for cylindrical ones, as shown in Figs. 
1.25 and 1.26 and discussed in the following 
paragraph. 
Finally, the influence of pressure on the spreading 
process over those samples is specifically shown in 
Fig. 1.27. 
* Contract no. 6284/85/F/FL, Final Report 
t = to +0.80 s 
Fig. 1.13 Fíame spreading over cylindrical samples at 
microgravity conditions in the NASA KC-135 aircraft 
laboratory. Y02 = 0.90, P = 98 kPa. 
HETEROGENEOUS COMBUSTIÓN PROCESSES UNDER MICROGRAVITY CONDITIONS 59 
t = to +0.40 s 
t = to +0.80 s 
Fig. 1.14 Fíame spreading over cylindrical samples at 
microgravity conditions in the NASA KC-135 aircraft 
laboratory. Y^ = 0.90, P = 50 kPa. 
Fig. 1.15 Fíame spreading over cylindrical samples at 
microgravity conditions in the NASA KC-135 aircraft 
laboratory. Ym = 0.80, P = 20 kPa. 
6 0 C. Sánchez Tarifa, A. Linan, J. J. Salva, G. Corchero, G. L. Juste, F. Esteban 
CYLINDRICAL SAMPLE 
DianL:ert=4min.,inL=2miiLLeii|th-60nim. 
Fig. 1.16 
60 80 
02 mus fraction (I) 
Diam.:eit=4mm.,int.-2iam.Leii£Üi=60mm. 
100 
P=50 kPi 
0 1-J 
+ micro-j 
60 80 
02 mus fraction (!) 
100 
*-
3.5-
/"« 
1
 1 -1 
2 
v
 2.5-
•> 
0 5 2 -
1 
> 
1 
5 1.5-í 
i 
e 
k 
ft l -
11 
0.5-
CYLINDRICAL SAMPLE 
Diam.:eit-4mm.,int=2mm.Leníth=60mm. 
c 
P= 20 icPa / 
DI"! / 
+ micro-j / 
/ + 
D 
/ + 
D 
vr 
u
 1 ! 1 1 1 1 1 
20 40 60 80 100 
02 mus fraction {%) 
Fig. 1.17 
Fig. 1.18 
(Figures 1.16, 1.17, 1.18): Results of measurements of 
the flame-spreading velocities as a function of mixture 
composition and pressure. 
1.6. Conclusions 
1. The fíame does spread under reduced gravity 
conditions (g = 10-2go). This has to be pointed 
out since there had been doubts about the 
possibility that a fíame might or might not spread 
under microgravity conditions. 
2. Flame-spreading velocities are considerably lower 
under microgravity conditions, but within the same 
order of magnitude, than flame-spreading velocities 
at 1 g. Therefore it has to be considered that the 
Grashof number at 1 g is small owing to the small 
size of the samples tested. 
3. The influence of gravity decreases when the 
mixture pressure is reduced, a conclusión explained 
by the fact that at 1 g the Grashof number is 
proportional to the square of the pressure if 
temperatures are constant. On the other hand, 
under microgravity conditions the influence of 
pressure is small, since it is only exerted through 
its direct influence on the combustión process. 
HETEROGENEOUS COMBUSTIÓN PROCESSES UNDER MICROGRAVITY CONDITIONS 61 
t - t o t = to + 2 s 
Figure 1.19. Fíame spreading over fíat samples at microgravity conditions in the NASA KC-135 aircraft laboratory. 
Y^ = 0.80, P = 98kPa. 
t = to +4 s t = t0 +6 s 
Figure 1.19. (cont.) Fíame spreading over fíat samples at microgravity conditions in the NASA KC-135 aircraft 
laboratory. Y^ = 0.80, P = 98 kPa. 
t = to s t = to + 2 s 
Fig.1.20. Fíame spreading over fíat samples at microgravity conditions in the NASA KC-135 aircraft laboratory. 
Y^ = 0.80, P = 50kPa. 
62 C. Sánchez Tarifa, A. Linan, J. J. Salva, G. Corchero, G. L. Juste, F. Esteban 
t = to +4 S t = t„ +6 S 
Figure 1.20 (cont.). Fíame spreading over fíat samples at microgravity conditions in the NASA KC-135 aircraft 
laboratory. Y02 = 0.80, P = 50 kP. 
a) P = 98 kPa, Y02 = 0.80 b) P = 98 kPa, Y02 = 0.50 
Figure 1.21. Fíame spreading over fíat samples at microgravity conditions and dijferent 02 mass fractions. 
NASA KC-135 aircraft laboratory. 
4. When the oxygen concentration decreases the 
difference between results at 1 g and under 
microgravity also decreases, since the combustión 
temperature decreases at lowered oxygen 
concentration, reducing the fíame-spreading 
velocities in both cases, but more significantly at 
1 g as in this case the Grashof number also 
decreases. 
heat transferred from the fíame to the sample is 
higher per unit of volume in the case of the 
cylindrical fíame, due to the thin diameter of the 
sample compared with a characteristic length of the 
fíame. Therefore the sample temperature anead of 
the fíame is higher in the cylindrical case, resulting 
in a higher flame-spreading velocity. In addition, 
sample thickness was higher in the fíat samples. 
5. Flame-spreading velocities are lower in the case of 
bidimensional symmetry than in the case of 
cylindrical symmetry. 
This may be explained when considering that the 
6. Fíame spreading over fíat samples at test conditions 
showed a good approximation to real bidimensional 
conditions. Therefore they are suitable for verifying 
theoretical models. 
HETEROGENEOUS COMBUSTIÓN PROCESSES UNDER MICROGRAVITY CONDITIONS 63 
FLAT SAMPLE 
Lea|th=60nun. 1ilth=i2oun. Uückneas=2min 
15-
3-
15-
2-
1-5 -
1-
U¡ 
0-
P=96kPt 
+ micro-f 
D l - t 
. / 
' 
» . — * • " ' 
^ . t ^ 
1 
/ 
/ 
t 
n 
/ 
/ 
/ + 
i 
D 
+ 
1 
40 
Fig. 1.22 
1.5 
100 
02 mus fnction (I) 
FLAT SAMPLE 
Lenfth=40nun. lidth=12mnL Thicknm=2nim. 
100 
02 m u fnction (I) 
2.1 
2-1 
1.1 
1 J -
1.7-
L í -
U 
1.1 
U 
1.2 
1.H 
1 
0.1 
0.0 
0.7-1 
0.0 
0.5 H 
0.4 
0.3 
0.2 
0.1 
0 
FLAT SAMPLE 
len(th=Mmm. lidth=12mm. Thickne»=2mm. 
P=50kft 
o l - t 
+ micro-j 
40 
Fig. 1.23 
i r 
N 100 
02 mío fnction (I) 
Fig. 1.24 
(Figures 1.22, 1.23 and 1.24): Results of 
measurements of flame-spreading velocities over fíat 
samples during parabolic flights in October 1987. 
64 C. Sánchez Tarifa, A. Linan, J. J. Salva, G. Corchero, G. L. Juste, F. Esteban 
2.1-
Flame Spreading Velocity 
orar cyiindrical and flat s imple) 
2.4 
u-\ 
2 
1.1 
LH 
1.4 
u-
1 -
0 . 5 -
0.1 
».H 
u 
o 
P= 50 kf a. Uero-( 
• Cjtindrical sample 
t FUt ampie 
40 10 SO 
02 m w (nction (J) 
100 
i _ 
15-
r\ 
1
 1 -
i 
8 
v
 1 5 -
«1 
0 
i 2-i 
í 1.5-
< 
• 
Ü i -
II 
II -
4 
Fíame Spreading Velocity 
«rer cjiindricil tai flat samples 
P= 90 kfa. Kcro-í 
+ Hat «ampie 
0 CTÜndrical simple 
* - " ^ 
i i i i i 
0 M M 100 
02 m a n fraction (I) 
Figures 1.25 and 1.26: Results of measurements of flame-spreading velocities over cylindrical and flat samples. 
Acknowledgment 
This programme has been sponsored by the 
Microgravity Department of ESA. The great assistance 
given by its leading personnel as well as that given by 
ESTEC staff is gratefully acknowledged. 
Figure 1.27. Results of measurements of the influence 
of pressure on the flame-spreading process over 
cylindrical and flat samples. 


Heterogeneous combustion processes under microgravity conditions

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