OR4-4
微小重力場における高温雰囲気中での燃料液滴列の冷炎燃え広がり
Cool flame spread of a fuel droplet array in ahigh-temperature atmosphere in microgravity
藤枝佑毅 1, 菅沼祐介 2, 齊藤允教 3, 田辺光昭 3, 野村浩司 2
Yuki Fujieda1, Yusuke Suganuma2, Masanori Saito3, Mitsuaki Tanabe3 and Hiroshi Nomura2
日本大学大学院生産工学研究科，Graduate school Industrial Technology, Nihon University
日本大学生産工学部，College of Industrial Technology, Nihon University
3 日本大学理工学部，College of Science and Technology, Nihon University12

1. Introduction
Hydrocarbon fuels used in diesel engines and gas turbines may undergo a two-stage combustion process inhigh-temperature, high-pressure environments in which a low-temperature oxidation reaction, the cool flame,is followed by a high-temperature oxidation reaction, the hot flame. This cool flame reaction is known to affectthe subsequent hot flame. Therefore, it is important to clarify the combustion characteristics of the cool flame
to understand the details of combustion phenomena.
Tanabe et al. confirmed the occurrence of a cool flame in a single droplet experiment 1). Moriue et al. studied
the interference effect of the cool flame reaction between two droplets 2). However, no experiment has been
performed to observe the spread of a cool flame by forced ignition of a droplet array. In this study, a cool flame
spreading experiment was conducted by inserting a fuel droplet array into a high-temperature atmosphere in
microgravity and forcibly igniting the droplets. In this report, we present the results obtained from the cool
flame spreading experiments.

2. Experimental apparatus and conditions
2.1. Experimental apparatus
The experimental apparatus used in this study consists of a cool flame igniter, a combustion chamber, adroplet array holder, a droplet generator, a droplet array mover, a temperature controller, an optical system,a wireless LAN interface, and a power supply system. The cool flame igniter consists of an ignition wire andan ignition circuit with automatic power adjustment. 0.1 mm diameter nickel wire was used for the ignition
wire. In this study, the circuit is based on the principle of a hot wire anemometer, and consists of a Wheatstonebridge, a differential amplifier, and a transistor to forcibly ignite a fuel droplet array and generate a cool flameonly. This circuit allows the temperature of the ignition line to be adjusted to an arbitrary value by adjusting
1 of 6the variable resistor in the bridge. As a mechanism, unbalanced voltage from the Wheatstone bridge is input
to the differential amplifier. The current is then amplified by a transistor to produce the current required forheating. A feedback circuit is incorporated to set the unbalanced voltage from the bridge to 0 V by means of adifferential amplifier, which instantly makes the heating wire red hot and maintains it at a constanttemperature. When a cool flame is generated, the differential amplifier in the ignition circuit lowers its output
to return to the set value as the ignition wire is heated by the cool flame, which causes a voltage drop. Therefore,
the occurrence of a cool flame can be confirmed by the voltage drop in the circuit.
The droplet array holder consisted of SUS tubes with diameters of 1.0 and 1.6 mm and was fixed by silver
brazing at the intersection points. The plates for fixing the support frame were made of stainless steel toprevent the temperature drop in the combustion chamber and to withstand the shock during the dropexperiment. The droplets are held in place by SiC fibers stretched in an X-shape with glass beads attached at
the intersections.

Figure 1. Experimental apparatus.

Figure 2. Droplet array holder.

2.2. Experimental conditions
The ambient temperature in the combustion chamber was 523 K. The droplet spacing S was set to 2 mm and4 mm. 6 droplets were suspended at S = 2 mm and 4 droplets at S = 4 mm. The ignition line temperature wasabout 1015 K. The ignition device was activated 1.0 second after the droplet array holder had fully risen to themeasurement position in the combustion chamber. n-decane was used as the fuel to allow comparison withthe experimental results of other researchers. The initial droplet diameter d₀ for the first droplet as the ignition
source was 1.0 mm, and d₀ for the second through sixth droplets was 0.75 mm. The experiment was conducted
at Cosmotorre, a 50-meter class microgravity experimental facility. This experimental facility can produce a
microgravity time of approximately 2.5 seconds. The voltage waveforms of the ignition system were recorded
by a sequencer to confirm the generation of the cool flame. The droplet diameters were measured using thebacklit method, in which the droplet is backlit from behind, and the outline of the droplet was photographed
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by a CMOS camera. The droplet diameter was analyzed from the photographed droplet images using a selfmade program.

Table 1. Experimental conditions.
Fueln-decane

Atmosphere pressure P a (MPa)0.10Gravitynormal gravity

microgravity

Droplet spacing S (mm)2.0

2.0, 4.0

Initial droplet diameter d 0 (mm)1st1.0

2nd~6th0.75

Distance between droplet center and igniter (mm)1.0

Igniter temperature T ig (K)1015

Atmosphere temperature in combustion chamber T a (K)523

Ignition device operating time t igon/off (s)1.0

Ignition device operation waiting time t igw (s)1.0

3. Result and Discussion
Experiments were conducted at an ambient temperature of 523 K in a combustion chamber with a waitingtime tigw of 1.0 seconds for ignition activation. Figure 3 shows the image of a droplet taken by the backlit
method. From the obtained droplet images, droplet diameters were analyzed using a self-made program tocalculate the droplet diameter history. The diameter history of a droplet in microgravity is shown in Figure 4.
The first droplet, which is the ignition source, and the sixth droplet in normal gravity, which was difficult tomeasure, were excluded from the results. The droplet diameter histories in microgravity at an ambient
temperature of 523 K and a droplet spacing of 2 mm show that the slope of the graph for the first droplet
changes significantly at about 1.37 seconds. This indicates that the cool flame ignition occurred at about 0.37
seconds after the ignition device was activated. In addition, the large change in the slope of the graph after thefirst droplet suggests that the cool flame spread to the sixth droplet. The graph also shows a significant change
in slope from the first to the fourth droplet at a droplet diameter of 4 mm, which indicates the spread of thecool flame.

Figure 3. Backlit image.
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Figure 4. Droplet diameter history.

Figure 5 shows the relationship between the cool flame ignition time and droplet position obtained from thedroplet diameter history. The cool flame spread speed, Vcf, was calculated by approximating the plot with astraight line using the least-squares method and calculating the slope. Comparing the results of normal gravity
and microgravity with a droplet spacing of 2 mm, the cool flame spread speed was about 15.6 mm/s in normalgravity and about 17.9 mm/s in microgravity, which was faster than that in normal gravity. In the case of
normal gravity, the fuel vapor evaporated from the droplet is affected by gravity and flows down the droplet.
On the other hand, in microgravity, sufficient combustible mixture can stay around the droplet, which
promotes the flame spreading, thus accelerating the flame spread speed.

Figure 5. Relationship between cool flame ignition time and droplet position (S = 2 mm).
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Figure 6 shows the relationship between cool flame ignition time and droplet position obtained from thedroplet diameter history at a droplet spacing of S = 4 mm. Figure 7 shows the relationship between thenondimensional droplet spacing and the normalized cool flame spread speed in microgravity and normalgravity. The cool flame spread speed at S = 4 mm droplet spacing was 20.8 mm/s, indicating that the cool flame
spread speed at S = 4 mm was faster than that at S = 2 mm in microgravity. In addition, Figure 7 suggests thatthe normalized cool flame spread speed tends to increase as the nondimensional droplet spacing increases.
The above results suggest that the cool flame spread speed increases or decreases depending on the length ofthe droplet spacing in microgravity. This is thought to be due to interference effects caused by the coarsenessand density of the droplets. Therefore, we plan to investigate the effect of droplet spacing on the spread speedof the cool flame by changing the value of S/d0 in future experiments.

Figure 6. Relationship between cool flame ignition
time and droplet position (S = 4 mm).

Figure 7. Relationship between nondimensional droplet
spacing and normalized cool flame spread speed.

4. Conclusion
A forced ignition system was used to observe the cool flame spreading of fuel droplets in microgravity. The
findings are summarized as follows.
The droplet diameter history was measured by backlit images, and the cool flame ignition time wascalculated by the diameter history.

The cool flame spread speed was found to be faster in microgravity than in normal gravity.

The cool flame spread speed was found to increase or decrease depending on the length of the droplet
spacing in microgravity.

Acknowledgment
This study is supported by JSPS KAKENHI Grant #JP19K04843.

5 of 6References
1)
2)

M.Tanabe, M. Kono, J. Sato, J. Koenig, C. Eigenbrod and H. J. Rath: Effects of natural convection on two stage ignitionof an n-dodecane droplet. Symposium (International) on Combustion/The Combustion Institute, 25 (1994) 455
O. Moriue, Y. Nishiyama, Y. Yamaguchi, H. Hashimoto, E. Murase: Effects of droplet interaction on spontaneousignition of an n-decane droplet pair. The Combustion Institute, 34 (2013) 1585

© 2023 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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