PS37
遠心力が加わるポリエチレン被膜電線上の火炎の燃え拡がりに及ぼす回転半径の影響
The effects of rotational radius on flame spread over
LDPE-insulated wire in centrifuge
石川昇龍 1, 金野佑亮 1, 橋本望 1, 藤田修 1,
Shoryu Ishikawa1, Yusuke Konno1, Nozomu Hashimoto1 and Osamu Fujita111.

北海道大学大学院工学院，Graduate school of Engineering Hokkaido university #1

Introduction and Background
Manned space exploration programs in Moon and Mars are planned by space agencies over the world. For

instance, Artemis program led by NASA aims to send human astronauts to the Moon and Mars in 2020s and2030s respectively. The competition for deep space explorations among the countries become more intensethan ever, as it brings us state-of-the-art scientific discoveries and technologies and is related to the economy,and security. In near future, humanity will stay frequently in space, where gravity level and oxygen
concentration are different from those on Earth. Ensuring the safety of stay in space is important. Fire in thespacecrafts or in the habitation modules is a potential threat to the safety of people. Previous research suggeststhat some materials might be more flammable in microgravity than on the ground. Sacksteder et al. showedthat the minimum oxygen concentration required to burn the thin cellulose paper is lower under low-gravity
conditions than under gravity on the ground.1). Thus, flammability test of materials used in spacecrafts become
vital to ensure the fire safety in space. For example, NASA-STD-6001B examines whether the material used in
the spacecraft is adequate or not in terms of flammability. However, the NASA-STD-6001B test are conductedon the ground, which cannot evaluate the flammability of the materials in spacecraft environment.
Consequently, flammability tests under micro- or low-gravity environments are crucial for the fire safety. Theauthors started FLARE-3 project with JAXA which aims to evaluate the flammability of materials under lowgravity environments. By utilizing centrifugal force under micro-gravity environments such as International
Space Station (ISS), the pseudo low-gravity environments will be generated, and flammability tests will beconducted. The authors expect that the Coriolis force affects the combustion process of the materials in ISS
experiments. In addition to that, gas flow induced by centrifugal force and Coriolis force in the experimental
vessel would be complicated. It is therefore worth to conduct experiments on the ground to investigate theeffects of Coriolis and centrifugal force on the combustion before bringing experimental samples to ISS andrunning experiments.
The previous research by authors involves combustion experiments with low density polyethylene (LDPE)
coated copper and nichrome wires to gain the insight into the limiting oxygen concentration for both samples
1 of 8under hyper gravity conditions, flame spread rate (FSR) as a function of centrifugal force and the differences
between highly conductive copper wire and less conductive nichrome wire2). It turned out that FSR for flames
over copper wire is drastically reduced due to the flame tilt, whereas that over nichrome wire is not. Moreover,
the relative importance of Coriolis force increases with a decrease in centrifugal force. However, the previous
research has raised a question of how the rotational radius affects on the flame over the wire. There arenumerous pairs of rotational radius and rotational speed that realize a certain level of centrifugal force. Thus,in this study authors conduct combustion experiments of the wire samples changing the rotational radius
while keeping the same centrifugal force at the wire sample location.
2.

Overview of the experiments

2.1. Experimental apparatus
Figure 1 shows schematic of the centrifuge used for the experiments. Figure 1(a) indicates the utilized
sensors in the experiments and the placement method of samples. Cylindrical, transparent vessel rotates atconstant angular velocity realizing hyper-gravity environment at the sample location. The wire sample isplaced parallel to the rotational axis and hold by an upper and a lower plate. Two circle windows on the upperand lower surface of the chamber enables us to exchange the used sample with new one. The acceleration
sensor, the oxygen pressure sensor, total pressure sensor are mounted on the rotating vessel which send data
to the datalogger. Figure 1(b) shows view angle of two cameras. Flames are tilted by centrifugal and Coriolis
force; therefore, two cameras are needed to observe the bi-directional inclination of the flames. The camera
observing the flame tilt toward the circumferential direction is named “Front Camera”, while the other one
observing the flame tilt toward the radial direction is named “Side Camera”.

Figure 1 Components in the experimental apparatus, (a) and location of “Front camera” and “Sidecamera”, (b).

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2.2. Experimental procedure
First, the wire sample is fastened by the upper and lower plate and tensioned by a spring to avoid thedeflection of the wire due to thermal expansion during flame spread process. Next, a vacuum pump exhausts
the gases inside the vessel and prevent combustion products affecting on the ongoing experiment. After
repeating the vacuuming process, the vessel powered by a motor start to rotate. Coil-shaped igniter, made ofcanthal wire ignites the wire sample after gases in the vessel achieve the rigid-body rotation. Two cameras
record the flame spreading downward with 29.97 FPS. Furthermore, data from the oxygen, the total pressure,and acceleration sensor are recorded by a datalogger every 0.1 seconds.
2.3. Experimental conditions
Figure 2 is the image of how the samples are placed in the experimental vessel. The total pressure and theoxygen volume fraction in the vessel is fixed at 100 𝑘𝑃𝑎, 21% respectively. The location of the wire sample
is 𝑟 = 70 𝑚𝑚 and 𝑟 = 170 𝑚𝑚 where 0.5 𝐺, 1 𝐺, 2 𝐺 centrifugal force is applied. Centrifugal acceleration,
𝛼𝑐𝑒𝑛𝑡𝑟𝑖𝑓𝑢𝑔𝑎𝑙 is given as follow:
𝛼𝑐𝑒𝑛𝑡𝑟𝑖𝑓𝑢𝑔𝑎𝑙 = 𝑟𝜔2 ,(1)where 𝑟 is the distance between rotational axis and wire sample and 𝜔 is the rotational velocity of theexperimental vessel. Two rotational velocities were set for locations so that the magnitude of the centrifugal
force at the sample location does not change. To confirm the reproducibility of the experimental data atexperiments are repeated three times at each condition.

Figure 2 Location of the wire samples

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2.4. Analysis method
Videos captured by two cameras are analyzed by a software developed by authors. It detects the upstreamedge of the flame, traces the time-evolution of the location of the upstream edge and finally converts frompixel scale to length scale. Figure 3 shows time history of the flame front position during the flame spread andthe determination method of the flame spread rate. Using least square method, authors derived the regression
line whose inclination is FSR. In addition to the FSR analysis, the location of flame tip, the downstream edgeof the flame is analyzed. Here, a point on a contour of a luminous flame where the curvature is maximum isregarded as a flame downstream edge. Wire vector can be obtained by linking the upper end and lower endof the wire while, flame vector is a vector connecting a flame upstream and downstream end. The angle
between the wire and the flame, 𝛼 is obtained by the inner product of these vectors divided by magnitude ofboth vectors as described in equation (2).
𝛼 = arccos (𝑓⃗ ∙ 𝑤⃗⃗⃗)|𝑓⃗| ∙ |𝑤⃗⃗⃗|(2)As shown in Figure 4, 𝑓⃗ and 𝑤⃗⃗⃗ are flame vector and wire vector, respectively. The distance between theflame upstream edge and flame tip is the magnitude of 𝑓⃗. 𝛼𝑐 and 𝛼𝑟 are defined as the inclination angle byCoriolis force and centrifugal force, respectively. By using 𝑓⃗, 𝛼𝑐 and 𝛼𝑟 , the horizontal component of thelength of flame vector can be calculated. 𝑑𝑟 , 𝑑𝑐 is the horizontal component of flame vector in the radial
direction and circumferential direction as indicated in equation (3), (4). Authors performed the analysis every
30 frames, almost one second to observe the overall behavior of a flame in an experiment.
𝑑𝑟 = |𝑓⃗| ∙ sin(𝛼𝑟 )(3)𝑑𝑐 = |𝑓⃗| ∙ sin(𝛼𝑐 )(4)Figure 3. Determination of flame spread rate (FSR) by using least square method

4 of 8Figure 4. Defining the inclination angle between a wire and a flame by calculating inner product of wire
vector and flame vector divided by the magnitude of both vectors3 Result and discussion

Figure 5. Flame spread rate (FSR) as a function of centrifugal force

5 of 8Figure 6. Scattering of the displacement of downstream edge of the flames in the radial andcircumferential direction
Figure 5 shows how the FSR changes as the centrifugal force applied to the wire sample increases. The error
bar ranges from the minimum FSR to the maximum FSR of each experimental condition. It reveals that FSR
decreases with decreasing the rotational radius of the wire sample. FSRs over the wire at 𝑟 = 170 𝑚𝑚 arehigher than those over the wire at 𝑟 = 70 𝑚𝑚.
Figure 6 shows how the flame tip moves around the sample wire seen from the direction of vertical axis ofcylindrical vessel. The horizontal displacement from the origin is tilt of the flame in the circumferential
direction, which means how much the flame is bent by the Coriolis force. The vertical displacement from theorigin is tilt of the flame in the radial direction, which means how much the flame is bent by the centrifugal
force. As the value in vertical axis increases, a flame tip become more closer to the center of the cylindricalvessel. Yellow dots at origin are the location of the wire sample. The red plus sign is average displacement offlames tip. The black solid ellipse indicates confidence ellipse (3𝜎) in which 99.7 % of the flame tips are if one
assumes that the data follow normal distribution. The distance between a wire sample and the averaged
displacement of the flame tip is written in each graph. Since each experiment was repeated three times, all thedata of flame tips under the same experimental condition were combined and analyzed. The top and bottom
rows are the experimental results for 𝑟 = 70 𝑚𝑚 and 𝑟 = 170 𝑚𝑚 respectively. Experimental results at 0.5
G, 1 G, 2 G are placed from the left one.
Nakamura et al revealed that heat transportation mechanisms in the flame over the highly conductive
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copper wire is dominated by the heat conduction through the wire core3). So, it is considered that the amountof heat transported to the unburnt insulation of the wire by solid heat conduction can be estimated bymeasuring the extent to which the flame touches the burnt core for flames that is tilted away from the wire.
The much heat is transferred to the unburnt coating, the faster the FSR becomes. As shown in Figure 5, FSRs
over wire at 𝑟 = 70 𝑚𝑚 are lower than those at 𝑟 = 170 𝑚𝑚. In Figure 6, the average displacement of flames
at 𝑟 = 70 𝑚𝑚 are far away from the wire compared with that at 𝑟 = 170 𝑚𝑚 for the same centrifugal force
condition. As the distance between a wire sample and the averaged displacement of the flame tip increases, aflame over a wire makes less contact with the burnt wire. Therefore, thermal conduction though the wire coreunder 𝑟 = 70 𝑚𝑚 conditions is suppressed and FSRs for these conditions are lower than FSRs for 𝑟 =170 𝑚𝑚 conditions. As shown in Figure 6, the confidence ellipses for 𝑟 = 170 𝑚𝑚 are slenderer than thosefor 𝑟 = 70 𝑚𝑚 with exception of 2 G experiments. As a confidence ellipse becomes round, it means that themagnitude of Coriolis force becomes stronger relative to that of the centrifugal force. Since the rotational
velocity of the vessel is higher at 𝑟 = 70 𝑚𝑚 than that at 𝑟 = 170 𝑚𝑚, the magnitude of Coriolis force seems
to become more dominant at 𝑟 = 70 𝑚𝑚.
4 Conclusion
In this study, the authors conducted combustion experiments of the low-density polyethylene (LDPE)
coated wire samples under pseudo-hyper-gravity environments by using centrifugal force in a centrifuge. Two
important variables for the experiments were the rotational radius, 𝑟, the distance between the rotational axis
and wire samples and the magnitude of centrifugal forces, G.
⚫

Flame spread rates over the wire at 𝑟 = 170 𝑚𝑚 are higher than those over the wire at 𝑟 = 70 𝑚𝑚
because the flame under 𝑟 = 70 𝑚𝑚 condition makes less contact with the burnt wire, which reduces
heat transferred to the unburnt insulator.

⚫

The ratio of the magnitude of Coriolis force to that of the centrifugal force is higher for 𝑟 = 70 𝑚𝑚conditions than for 𝑟 = 170 𝑚𝑚 conditions.

Acknowledgement
This study was partly supported by JAXA “Kibo” utilization feasibility study, JKA and its promotion funds
from KEIRIN RACE, and JSPS KAKENHI Grant numbers JP23K13258 and JP24K21649.
References
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Yusuke Konno, Shoryu Ishikawa, Nozomu Hashimoto, Osamu Fujita: Downward flame spread and extinction overelectric wires placed in a ground-based centrifuge: Proceedings of the Combustion Institute Volume 40, Issues 1–4,(2024), 105199, DOI: https://doi.org/10.1016/j.proci.2024.105199
Yuji Nakamura, Nobuko Yoshimura, Hiroyuki Ito, Keisuke Azumaya, Osamu Fujita: Flame spread over electric wirein sub-atmospheric pressure: Proceedings of the Combustion Institute 32 (2009) 2559–2566, DOI:
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7 of 8© 2024 by the authors. Submitted for possible open access publication under theterms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/li
censes/by/4.0/).

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