PS01
滑斜面を用いた低重力場発生装置の開発と検証
Development of partial-gravity generator launching overa less-friction slope
江州晴彦 1, 中村祐二 2Haruhiko GOSHU 1 , Yuji NAKAMURA 2
豊橋技術科学大学大学院工学研究科機械工学専攻，Department of Mechanical Engineering, Toyohashi University ofTechnology
2 豊橋技術科学大学大学院工学研究科機械工学系，Department of Mechanical Engineering, Toyohashi University ofTechnology
11.

Introduction

To date, exploration of Lunar and Mars have been paid special attention in the space missions. With thisrespect, a safety management under partial gravity environment become inevitable issue. A fire is one ofsignificant potential hazards in the space missions, thus, precise understandings on burning mechanism underpartial (low)-gravity conditions (0𝑔<𝑔𝐿, 𝐿𝑜𝑤 <1𝑔) must be made with our best knowledge. As is well-known
feature of combustion in microgravtity1), candle flames (diffusion flame) tend to be spherical due to theabsence of buoyancy-induced flow, suggesting that there should be significant difference on material burning
character because the heat feedback from the flame to the burning specimen is strongly connected. Indeed,Sacksteder et al.2) showed that flammable range of thin combustible was enlarged in microgravity due to themodification of heat transfer process depending on the adopted gravity. Nakamura et al.3) numerically
predicted that spontaneous ignition of externally heated solid fuels under various partial gravity conditions,revealing that there is “optimum” ignition condition at certain partial gravity field. Most recent ISS scientificproject, code name is “FLARE”4), it has been experimentally revealed that the flammable feature was mostpronounced at low-speed incoming flow (not microgravity), as a model of partial gravity field, which is same
trend found in the prediction by Nakamura et al.3). Although there are experimental data at partial gravity
conditions, it is still not enough to receive sufficient data for comparisons and discussions. For this purpose,easy-access device to generate the various partial gravity conditions is highly demanded.
Researchers have made efforts to obtain such “abnormal” gravity field by developing/proposing specialmethodologies. For instance, controlled parabolic flight5), counter-weight free-fall method6), control theacceleration of dropping the test chamber7), using centrifuge facility together with zero-gravity hardware8).
Nevertheless, having wide range of controllable device with relatively easy-to-access (e.g., Lab-friendlydesign) feature is not available at present except a few proposals, namely, yo-yo mechanism proposed by
Akiba et al.9) and Slope-sliding Method (SSMe) proposed by Nakamura and Sekimoto10). Combining withcentrifuge facility and microgravity facilities (drop tower, parabolic flight etc) have been recently paid
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attention by several groupsex.11-12), however, the combination itself is not easy and handy (far from lab-scaletests). In the present work, we propose the extended (revised) version of SSMe which one of coauthor has beendeveloped.
A “revised slope-sliding method” proposed in this study is followed by the concept of original SSMe yet
the protocol is opposite. In the original SSMe, test chamber was set at the top of slope and the partial gravity
field is achieved during the “sliding down” operation. In the present (revised) version, the test device shall be
set at the bottom and it is launched upwardly along the slope, similarly, partial gravity field is achieved during
the “climbing up” operation. Theoretically, both operations shall give the same result, however, we employed
the reverse operation to receive high-quality of partial gravity level.

2.

Experimental design

2.1

What is SSMe?

The principle of SSMe is briefly explained as follows: SSMe involves sliding an experimental rack down/up
an inclined base. Assuming no friction between the contacting surfaces (downface of the rack and surface ofthe slope), during the sliding operation, only vertical component of the gravity against the slope will act on
the sliding rack. With varying the slope angle, ideally, various levels of partial gravity condition shall beachieved. Figure 1 shows the original conceptual diagram, referred from Ref. 10 (left) and the revised SSMe
(right: currently work). Basic design is exactly the same, however, the protocol to obtain the partial gravity
environment is different, namely, one is sliding down and the other is climbing up as described previously.

Figure 1. Overview of original SSMe10) (left) and revised SSMe (right)

2.2

Experimental setup

Figure 2 shows a schematic diagram of the presently designed experimental setup. A PTFE sheet (1800 mmx 300 mm) coated flat wooden board was used as the low-friction surface, which is the slope in this study.
Experiments were conducted at five prescribed angles; such as 50°, 60°, 70°, 80°, and 85°. The rack was released
to upward at the launching point. Sprite ramp, whose wick is 7 mm of diameter and 3 mm long, was set in therack to generate the reference flame to be studied. Considering the limited low-gravity duration
2 of 7(approximately 0.7-0.8 seconds), a steady diffusion flame, which is highly responsive to gravity changes, wassuccessfully formed. Flame dynamic behavior was monitored by handy camera (GoPro Hero 10: ISO 200,
shutter speed at 1/480 sec) with high-speed mode at 120 FPS. Camera was set 95 mm distance away from thecenter plane along the wick axis in order to capture full image of the flame. For better visibility, a blackend
aluminum sheet was placed at the backside to ensure flame visibility. The fuel for the spirit lamp was acommon fuel alcohol consisting of 85% methanol and 15% isopropyl alcohol. Figure 3 shows direct
photographs of experimental setup. To measure the precise gravitational values acting on the rack, an SRIC
G-men GR01 g-sensor (resolution 0.02 g, response frequency is 100 Hz) was mounted on the rack.

Figure 2. A Schematic diagram of the experimental setup

mesh
Wooden board

Spirit lamp

1800 mm

300 mmPMMA clinder

PTFE Sheet

Camera(GoPro)g sensor

PTFE Sheet

Figure 3. Photographs of experimental setup
3．Results and Discussion
3.1

Confirmation how to work this method

Figure 4 shows the output values of the g-sensor for each angle. At t = 0 sec is correspond to the time of thelaunch and certain acceleration is experienced for a while (~ 50 msec). Irrespective of imposed slope angles
studied in this study, it only took less than 100 ms to achieve target partial gravity conditions, although the3 of 7

value is fluctuated around its averaged. Table 1 summarizes the g measurements for each angle (duration oflow-g period, average g, variance of g, theoretical g value (cosθ)). "Average g" and "Variance g" represent theaverage and variance during the partial gravity period. This summary clearly revealed that the gravitational
value decreases proportionally with the angle and our simple device can offer the relatively-reasonable“expected” partial gravity condition. Difference between the theoretical (target) and obtained values arearound 0.04-0.05 g, which will be caused by the friction over the slope. Furthermore, although not shown inthis figure, we have recognized that the most of the jitter came from the variation in slope-direction component,
implying that the treatment of surface might be effective to obtain high-quality of partial gravity environment
in this method. By the way, this variation is within g-quality achieved in parabolic flight experiment which
naturally generate the g-jitter during the flight operation. Although we must admit this level of difference
since “no” friction surface is not available, we currently work on finding the best surface treatment. Yet, thevalue of variance is small enough against to the average, indicating that a sufficient partial gravity
environment can be provided. Important to emphasize is that the present methodology is less-cost yeteffective; this feature is nothing comparable to the other partial-g test facility available thus far.

Figure 4. Time variation of output values from the g-sensor during the sliding operation

Table 1. Summary of measured g-level by g-sensor for adopted slope angle, θ.
θ

50.0°

60.0°

70.0°

80.0°

85.0°

833

870

801

750

736

0.69

0.54

0.37

0.21

0.11

Variance g0.0068

0.00830.00540.00820.0028cosθ(ideal)

0.641

0.500

0.342

0.174

0.087

Available time ofpartial-g [ms]Average g[normalized byearth gravity]

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3.2

Flame images under partial gravity environment

Figure 5 shows the flame images at different partial gravity conditions. Note that all figures are taken bycamera which is fixed at the sliding chamber, therefore, the vertical direction of the given images are normal
to the sliding surface (perpendicular to the slope). Although still figures are shown here, we confirmed that
there is no apparent different during the sliding operation within the present test time (~ 0.7 s). Therefore, welisted the images taken at the end of climbing operation (namely, the rack is located at the maximum height
along the slope). For better comparison purpose, the brightness was increased by 80 % to improve flamevisibility. A flame image under normal gravity (1 g) taken under the same conditions is also included (leftmost)for comparison purpose.

Figure 5. Appearance of flames formed over different g-level obtained by revised-SSMe (note that brightness
was increased by 80% for better visibility)

It is clearly observed that, as the gravitational value decreases, the flame height decreases, approaching thespherical shape. Additionally, the brightness and the orange soot region decrease. This gravity response onsoot generation seems opposite to what has been considered in the past works13), where the soot accumulation
is pronounced without the presence of gravity to accelerate the fuel pyrolysis. However most of tests haveused hydrocarbon fuels, which does not contain oxygen atom in the fuel. Here we used methanol so that sootgeneration is not pronounced as expected. According to NASA’s report13), propanol flame under microgravity
shall exhibit less-soot flame during the propagation over the liquid pool. This fact may support what we haveobserved here. Of course we can adopt hydrocarbon fuel using the present methodology which may exhibit
the excess soot production as the gravity level reduced. Although this would be our future work, it is quite
sure that the developing the lab-scale partial gravity generator can bring merits for fundamental flame study
including soot generation, for instance.
Lastly, Figure 6 directly compares the flame shape at different gravitational values. Flame shape is extracted
from the bright edge shown in Figure 5. This figure clearly shows the general trend on flame shape, especially,
the flame height is almost linearly decreased as the gravity reduced. Oppositely, flame width increases as thegravidity decreases. In lowest gravity in this study (G = 0.11g), the flame shape is similar to micro-jet flames14),
which the effect of the gravity-induced flow on flame is suppressed instead the diffusion transport is relativelypronounced (Re < 1 and Fr >> 1, where Re and Fr stand for Reynolds number and Froude number, respectively).
This fact suggests that the present methodology can work for validation study of gravity-related scaling law
5 of 7such as so called Froude (Fr) or Grashof number (Gr) modeling. It is also noted that the flame height inaxisymmetric configuration is insensitive to the surrounding flow status15), suggesting that the present
response of flame height under various gravity levels tells the variation of fuel mass flux (less fuel loading
cause reduction of the flame height). We will revisit this issue in our future work.

Figure 6. Comparisons of flame shape at various imposed gravity level

4.

Remarks
Partial gravity generator using the slope was developed and its feasibility is tested. This time, the protocol

is taken as opposite as the previously reported methodology; named “Slope Sliding Method (SSMe)”; namely,
the experimental rack is launched along the slope from the bottom then climbing up operation is made. Levelof partial gravity, achieved climbing up operation, is controlled by the prescribed slope angle and shows muchbetter than what we have reported previously (original SSMe). g-quality achieved by this method is firstly
measured and discussed. It is clearly revealed that achieved partial gravity level is sufficiently close to thetarget (ideal) value. Even the g-jitter is presented, the variance is several-order of magnitude smaller than theaverage. With this, we can safely say that the present method is satisfactory to study overall combustion
response subjected to the various partial gravities. As demonstration test, diffusion flame formed over thewick has been adopted for various gravity levels (0.11g-0.69g), showing that the continuous flame height
reduction is presented as the subjected gravity becomes less. Overall, it is ensured that the presentlydeveloped methodology works well for future use in fundamental combustion researches.

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