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| Title: | Effect
of Material Properties on Wire Flammability in a Weak Ventilation of Spacecraft | | Principle
investigator: | Osamu Fujita(Hokkaido University) | | Co-investigator: |
Takashi Kashiwagi
Kenichi Itoh
Akira Umemura |
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National Institute of Standards and Technology
Hokkaido University
Nagoya University |
 | Salient
points |
The most characteristic point of combustion phenomena
is that temperature changes drastically ( from room temperature to approximately
2000 degree C) in a very short time. Also, the density difference between the
flame and the surrounding air causes a very complex flow (Thermal Convection)
in normal gravity. Therefore, it is not easy to investigate the essential mechanism
of combustion phenomena by precisely in measuring transportation of fuel, oxygen,
and heat. However, it is possible to measure these parameters in a microgravity
environment since thermal convection and induced turbulence are suppressed. Also,
in microgravity, it is possible to observe combustion phenomena much longer than
in normal gravity since diffusion is the only mechanism for oxygen transfer and
moderates combustion phenomena. Microgravity can be considered as a high-speed
camera, because it enables observation of fast phenomena with extended time. Therefore,
microgravity is extremely useful as a research tool.
Combustion phenomena
such as candle flames, burner flames, and burning fuel droplets have been studied
in microgravity by using drop towers, space shuttles, etc. In Japan, much fundamental
research on spray combustion have ever been conducted. "Spray Combustion"
is one of the most widely used combustion method. It is used in many types of
internal combustion engines, industrial boilers, etc. Single fuel droplet combustion
has been investigated, since a single droplet is the most fundamental element
of spray. These investigations successfully revealed most combustion mechanisms
for a single droplet, including the effects of environmental conditions such as
ambient oxygen concentration on droplet lifetime and flame diameters.
In actual spray combustion, however, multiple fuel droplets burn with complex
interactions. Also, it is important to understand flame propagation phenomena.
Based on the knowledge of single droplet combustion, extended and better understanding
of flame propagation in multiple droplets will lead to an essential understanding
of spray combustion mechanisms. Combustion experiments on a static spray conducted
using the TR-IA
sounding rocket have been the only space experiments conducted from the above-mentioned
viewpoint. As the next step, it is appropriate to investigate combustion mechanisms
of such well-defined and relatively simple systems, as droplet arrays. Subsequently,
the knowledge or combustion model obtained through research should be expanded
to more complex systems. However, some points, such as the effect of fuel vapor
diffusion on flame propagation in the droplet array are still unclear .
The following research theme selected as a flight candidate through the first
IAO, studies flame propagation phenomena over solid rod fuels by using an experimental
apparatus developed by NASA. This investigation seeks to determine the diffusion
effect of vaporized fuel on flame propagation phenomena. It may also provide essential
knowledge relevant to improved understanding of the flame propagation phenomena
of droplet arrays, droplet matrixs, and sprays. These results are expected to
be applied to developing next-generation combustion devices.
| Brief
summary of the theme |
A droplet array is used to conduct fundamental
combustion research. However, it is difficult to analyze the flame propagation
process, since the layer of vaporized fuel is formed dis-continuously ahead of
the flame front. However, the layer is formed continuously ahead of the flame
front when continuous rod fuel is used. It is expected that the phenomena will
be simplified, enabling precise analysis. For these reasons, this study will investigate
flame propagation phenomena over solid rod fuels in air flow parallel to the fuel
surface. The effect of the fuel vapor field on flame propagation will be studied.
According to theoretical analysis, the flame spread rate is expected to be especially
large at very low flow velocities, as a result of interaction between the concentration
field and temperature field. However, it is difficult to verify this hypothesis
in normal gravity since buoyancy-induced flow (thermal convection) and turbulence
occur (Fig. 1).
The Combustion Integrated Rack (CIR) developed by NASA
will be accommodated in the US module on the International Space Station (ISS).
This study employs the Flow Enclosure Accommodating Novel Investigations of Combustion
of Solids (FEANICS) as the experimental apparatus. It is an experimental insert
for solid fuel combustion, which is set within the CIR. In this study, rod-shaped
polyethylene with an inner core wire is used as the test sample. It is set parallel
to the air flow within the FEANICS. The test sample is ignited at the downstream
end, and subsequent flame spread toward opposed airflow is observed precisely.
As a result, the flame spread rate is measured (Fig. 2). In addition, experiments
with different airflow velocities and oxygen concentrations will be conducted
to study the effect of the fuel vapor field ahead of the flame front.
| | |
Fig. 1 Flame spreading in normal gravity
(Strong buoyancy flow occurs.) | Fig.
2 Flame spread in microgravity
(No buoyancy flow occurs.) |
In addition to these experiments, modeling of phenomena and numerical analysis
are also conducted in order to quantitatively analyze the effect of the fuel vapor
field on flame spreading over fuel surfaces . The results of this study are expected
to contribute to improved understanding of the effect of the fuel vapor field
on flame propagation in a droplet array, a dis-continuous and more complex phenomena.
In addition, the results of this study should have benefitted applications since
airflow on the ISS is very slow. It includes better evaluation of material flammability
for rod shaped materials such as wire insulations, and improved fire safety strategy
in space.
Last Updated : Aug. 19, 2003
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