Although research studies on gaseous flames in microgravity in Japan have not been one-sided, they have been limited, for the most part, to comparatively fundamental studies. At present it is only possible to achieve a microgravity field by the use of drop towers, as far as gaseous flames are concerned. Compared with experiments on droplets, including droplet arrays, which have been vigorously performed in Japan, studies on gaseous flames have just begun. Experiments on ignition of gaseous fuel, flammability limits, flame stability, effect of magnetic field on flames, and carbon formation from gaseous flames are currently being carried out in microgravity. Seven subjects related to these topics are introduced and discussed herein

Niioka, Takashi

English

NASA Technical Reports Server

N96- 15618
JAPAN'S RESEARCH ON GASEOUS FLAMES
Takashi Niioka
Institute of Fluid Science, Tohoku University
2-1-1 Katahira, Aoba-ku, Sendal 980-77, JAPAN
Introduction
Although research studies on gaseous flames in microgravity in Japan have not been
one-sided, they have been hmited, for the most part, to comparatively fundamental
studies. At present it is only possible to achieve a microgravity field by the use of
drop towers, as far as gaseous flames are concerned.
Compared with experiments on droplets, including droplet arrays, which have been
vigorously performed in Japan, studies on gaseous flames have just begun. Ex-
periments on ignition of gaseous fuel, flammability limits, flame stability, effect of
magnetic field on flames, and carbon formation from gaseous flames are currently
being carried out in microgravity. Seven subjects related to these topics are intro-
duced and discussed herein.
Ignition of Gaseous Fuel (PI : M. Kono)
A methane- or hydrogen-air mixture in a cylindrical vessel with a diameter of 60
mm and a length of 16 mm was ignited by a heated metal wire, 100 pm in diameter,
the temperature of which was measured by an optical pyrometer[l]. Since yields of
the upward flow are due to the buoyancy before ignition in normal gravity, ignition
times should increase in comparison with ignition times in microgravity. A numerical
approach was also conducted for microgravity conditions.
Ignition delays due to the use of a tungsten wire for the stoichiometric methane-
oxygen mixture are plotted in Fig.l, showing, that ignition delays in normal gravity
are quite scattered and longer than those m microgravity. For the methane-air
mixture, however, there are no differences between normal gravity and microgravity.
Numerical calculation showed that the ignition point in methane-oxygen is some
distance from the hot wire, while ignition in methane-air occurs near the wire. This
means that the buoyancy effect is stronger in the case of methane-oxygen.
Figure 2 shows the temperature histories of platinum wire for the hydrogen-air
mixture. In the case of the equivalence ratio _ = 0.3, there are abundant oxygen
molecules around the wire, and so ignition times are shorter than those for ¢ = 1 and
the discrepancy between normal gravity and microgravity is not recognized. In the
case of _b = 1, however, the effect of the natural convection which supplies oxygen to
the wire surface and promotes the surface reaction is quite large. Therefore, ignition
takes place faster in this case.
Burning Velocity (PI: S. Okajima)
To measure the flammability limits and the burning velocities near the limits, a
427
https://ntrs.nasa.gov/search.jsp?R=19960008452 2020-06-16T05:20:22+00:00Z
closed cylindrical vessel with a diameter of 116 mm and a length of 120 mm was
installed in a freely falling chamber [2,3]. The burning velocities were obtained from
schlieren streak photographs.
Figure 3 shows the burning velocities plotted against the equivalence ratio for
methane and propane. The burning velocities were compared with those obtained
by other methods, and it was concluded that the burning velocities in this exper-
iment agreed well with those obtained by the counterflow twin-flame method [4],
which will be explained in the next section.
By use of the same apparatus, cellular flames were also observed for methane and
propane [5]. The authors concluded that there was an equivalence ratio which
indicated the maximum probability of the appearance of a cellular flame, and that
the cell structure of both fuels became fine as the initial pressure in the vessel
increased.
Flammability Limit (PI: T. Niioka)
The objective of this experiment was to obtain the flammability limit and to ob-
serve the flame behavior near the limit by use of the technique of counterflow twin
premixed flames, because flames in extremely low velocity fields can be stabilized
in microgravity, as shown in Fig.4.
Usually, keeping the concentration constant, the counterflow velocity is increased
and extinction is observed. Here, fuel concentration was decreased according to a
computer-programmed control, keeping the low flow velocity constant. The most
difficult point in the study of this very low speed flow is whether the flame is followed
by slight flow control. Figure 5 shows that there was little difference between the
equivalence ratio controlled arbitrarily and the flame interval.
Figure 6 is a plot of the equivalence ratio and the distance between twin flames
versus time. In the case of methane, the twin flames approached monotonously and
were extinguished after they becoming united. This proved to be true as expected.
In the case of propane, however, twin flames were united and extinguished as well,
although it was expected that extinction would occur when the flames were still
separate, owing to the Lewis number effect.
Since sudden changes of the flame interval were always recognized in the case of
propane, the location of the symbol (*) in Fig.6(b) was defined as the point of
flammability limit. This is because, after this point, flames cannot maintain their
self-propagation characteristics and only unstable flames can exist. The variation
of the nominal stretch rate with the equivalence ratio at extinction showed that the
flammability limit of the propane/air mixture was 0.50.
Blowoff Limit (PI: J. Sato)
The flow velocity gradient at the burner rim has a great effect on the blowoff con-
dition of premixed flames. When the flow velocity of the mixture is small, however,
the effect of buoyancy may also be significant, and so it is expected that there is a
discrepancy between the case for that in normal gravity and for that in microgravity
for a very lean or a very rich mixture. This experiment is now being conducted using
methane as a fuel.
428
Diffusion Flame (PI : J. Sato)
Methane is issued from a tube with an inner diameter of 5 mm, and an airflow
nozzle with a diameter of 54 mm is constructed in a concentric configuration. The
thickness of the fuel burner rim is 0.1 mm to avoid the effect of turbulence at the
rim. In normal gravity, flickering of the diffusion flame with a frequency of about 10
Hz was observed at a fuel velocity of 25.5 cm/s when the outer airflow is stopped.
Switching from this condition in normal gravity (Fig.7(a)) to the same condition
in microgravity, the flame shape changed as shown in Fig.7(b), (c) and (d), in that
order, within one second. Since air is not supplied by natural airflow convection and
is transferred only by diffusion, the flame becomes weak and only blue flame exists
around the burner port, as shown in Fig.7(d). It can be seen in Fig.7(d) that the
diffusion flame developed below the burner port owing to the downward diffusion of
methane.
In the present study, only the diffusion flame was observed, but in the next step, the
turbulent flame will be observed to determine the contribution of natural convection
to the turbulent diffusion flame. In addition, we plan to study the effect of air
temperature on the diffusion flame.
The Effect of Magnetic Field (PI : K. Ito)
Oxygen molecules are paramagnetic, and therefore the transport of oxygen should
be influenced by a magnetic field. The objectives are not only to investigate the
combustion mechanism in a magnetic field but also to promote combustion, for
......... 1 _ _f _tl_ _ Jn ...... 1- _ _ _
example, by utilizing the change o_ _ne n_me _a_pe.
A butane diffusion flame was established on a circular tube with an inner diameter
of 0.6 era. The tube was made of copper so as to be independent of magnetic field.
Figure 8 shows the arrangement of the burner, the flame(R) and the magnet (A),
the magnetic force (H) of which had a certain distribution. Butane was chosen as
the fuel.
A comparison of the flame shapes is shown in Fig. 9. Standardizing flame (A) in
normal gravity and without magnetic field, totally, flames (B, C) without magnetic
field changed more drastically, while flames (A t, B t, C t) with magnetic field did not
change much. Actually, it was not possible to establish a flame in microgravity (C),
although it was possible to sustain a flame if the field was magnetized (Ct). This
means that the effect of the magnetic field on oxygen transport is very strong and
that the flame shape is not greatly affected by natural convection.
Carbon Formation (PI" K. Ito)
From a circular tube with a diameter of 0.6 cm, butane was flowed out and ignited
in a stagnant oxidant atmosphere. The fuel flow rate was 0.3 through 1.2 cmS/s.
Figure 10 is a schematic representation of what the flame looked like from above.
Large particles of soot with a diameter of over 100/_m were formed in 50% oxygen
air, which was 200 to 500 times greater than the case in normal gravity. The carbon
zone is located in a ring-shaped region on the top of a spherical diffusion flame, as
seen in Fig.10(b).
429
REFERENCES
1. Nagata, It., Kim, H.M., Sato, J. and Kono, M.: Twenty-fifth Syrup. (Int.) on
Combustion, (to be printed).
2. Okajima, S., Iinuma, K., Kawaguchi, S. and Kumagai, S.: Twentieth Symp.
(Int.) on Combustion, The Combustion Institute, pp.1951-1956(1984).
3. Okajima, S.: archivum combustionis, vol.13, pp.7-14(1993).
4. Yamaoka, I. and Tsuji, H.: Twentieth Syrup. (Int.) on Combustion, The Com-
bustion Institute, pp.1883-1892(1994).
5. Kawakami, T. and Okajima, S.: archivum combustionis, vol.13, pp.15-20(1993).
6. Wakayama, N.I.: Combustion and Flame, Vol.93, pp.207-213(1193).
7. Ito, H., Fujita, O. and Ito, K.: Combustion and Flame, Vol.99, pp.363-370 (1994).
i
C<_l I _ | I ! ! ! I I
-- • : Normal gravity _/"_
z_ : Micro gravity L0.8 : _ : calculation
0.6 •
= • •
.. • •
•_ 0.4 O • • ,
"_ • • I o
0.2 _ w___LL_
0 ' 1 I I I I I : 411300 1350 i400 1 50"
Wire temperature [K]
F_ure 1...Comparison of ignition delay.t'unes
of a stoichiometric methane-oxygen, mixture
in normal _avity with such times m micro-
g_avity.
Ct,
E
.=_
1500
100£
i i i i i
C_qD •
0 o
• DD
(3
• []
(3
\
¢=0.31 o- #g, •" lg
¢=1.0, n: pg, ..'lg
I
Initial temperature
I | I I
0.2 0.4
Time [sec]
Figure 2. Histories of platinum wire temper-
ature for a hydrogen-air mixture.
0.6
43O
LO 
10 
0 
0.5 1 .o 1.5 2.0 25 
Equinlencc ratio g 
Figure 3. Burning velocities versus equivalence ratio for 
propane- and methaneair. 
Figure 4. Direct photo of twin flames in a low velocity 
counterflow field in microgravity. 
20 
15 
10 
5 
l ime,  sec 
Figure 5. Variation of twin flame distance with time 
the equivalence ratio was changed arbitrarily. 
S 
when 
Figure 
Figure 8. Arrangement of ex erimental apparatus for a 
flame in a magnetic f ied  
43 1 
6. The variation of twin flame distance with time for 
niethane (a) and propane (b). 
1 G  
(3) 
Soot Particles e / GeneratirigZone 
c ~ o - ~ G ,  0.76 sec after the drop 
Fuel Gas 
< 1 0 - 4 ~ ,  7.43sec after the drop 
Figure 9. Flame shape changes in a magnetic field. 
Figure 10. Picture of a flame with luminous spots taken 
from above the flame under microgravity (a) and 
schematic representation of the formation region. 
4 3 2  


Japan's research on gaseous flames

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server