OR2-4
TEXUS ロケットによる微⼩重⼒環境での正デカン液滴列に沿った冷炎燃え広がり挙動
Cool Flame Spread Behavior along n-Decane Droplet
Array under Microgravity by a TEXUS Rocket⿑藤允教 1, 菅沼祐介 1, 家村和輝 1, 髙畑優星 1, Martínez Figueira Noelia 2, Meyer Florian 2,
Eigenbrod Christian2, 野村浩司 1, ⽥辺光昭 1
Masanori SAITO1, Yusuke SUGANUMA1, Kazuki IEMURA1, Yusei TAKAHATA1,Noelia MARTÍNEZ FIGUEIRA2, Florian MEYER2, Christian EIGENBROD2,Hiroshi NOMURA1 and Mitsuaki TANABE112

⽇本⼤学, Nihon University,ZARM, Bremen University

1. Introduction
We conducted the microgravity experiment by the TEXUS#60 sounding rocket to elucidate the cool flamedynamics. Cool flame appears as a result of the low temperature oxidization mechanism preceding the hotflame from the normal alkane that is the major component of jet fuel including Sustainable Aviation Fuel (SAF)
and plays a key role in a whole ignition process. In the case of the low temperature reaction, the balance of thedissipation of heat/species (physical characteristics) and the reaction (chemical characteristics) determines theignition. Therefore, to fundamentally model the cool flame combustion, the experiment of the microgravity
environment that achieve the simple system without natural convection is necessary. The Japan and Germany
science group have been proceeding the “Phoenix-2” project1-3). The scope of the project is to understand thecool flame behavior such as spontaneous ignition and cool flame spreading near the ignition limit which isuseful to clarify the influence of the dissipation sensitively because the reaction speed is very slow. In thispaper, the cool flame spread behavior is reported. Mikami et al., studied the cool flame can spreads amongthe multi-fuel droplets even though they are apart exceeding the distance of hot flame spread limit4). Saito etal. conducted the cool flame spread experiment by the forced ignition with the ambient temperature of 293 K
to 298 K which is the lower condition of the cool flame spontaneous ignition limit5). Because cool flame affects
the hot flame behavior, the spread behavior such as the speed, spread limit, etc. need to be elucidated. In thePhoenix-2 experiment, we expect the cool flame propagation in two experimental conditions; one is a dense
array consisting of nine droplets in 8 mm spacing, the other is a sparse array consisting of five droplets in 16mm spacing. Mikami’s and Saito’s experiment, the cool flame may spread by the species and the heat transfer
to the neighbor droplet because their ambient temperatures are much lower than the spontaneous ignition
limit. On the other hand, our experimental condition is near the spontaneous ignition limit, the cool flame
spread may be driven by not only the heat and the species transportation but also chemical reaction. In this1 of 5

report, we evaluate the flame spread behavior focusing on the flame spread speed.
2.

Experimental Configuration
The Droplet array Combustion Unit 2 (DCU2) is integrated on the payload of the TEXUS#60. For the detailsystem configuration, please refer to the past paper2). The emission from the cool flame was captured by anintensified high-speed camera (Lambert, HICAM 500ST) through a lens (SIGMA, 135 mm, F1.8) and a band
pass filter (CWL: 400 nm, FWHM: 50 nm). The droplet diameters are measured by two CCD cameras (Imagesource, DMK33UJ003) with backlit. The DCU2 has two combustion furnaces whose ambient temperatures are570 K (unit 1) and 590 K (unit 2). Two cool flame images from both furnaces are able to be captured by a prismmirror. Since the unit 2 of 590 K did not work, only the result of 570 K is reported in this paper. The spatial
resolutions of the high-speed camera and the CCD camera of the unit 1 are 14.9 pix/mm and 49.5 pix/mm
respectively. The detail configuration is shown in the left of Fig. 1. The droplets and thermocouple locations
are shown in the right of Fig. 1. For the cool flame spread, we employed two droplet arrangement patterns;
one is the nine droplets (dense array pattern) with inter-droplet spacing S of 8 mm, and five droplets (sparsearray pattern) with inter-droplet spacing of 16 mm. The furnace has five K-type thermocouples whose
diameters of 1.0 mm (TC1 thru TC5).

Mirror
Re119(barLens)(cntFro8UV filter

TC2

CCD camera
Highspeedcamera

idetsghil
ack

5
d1_3
154
d1_4TC315
d1_54155 15
d1_6 4TC4
d1_7
TC157
TC515Prism mirror

4rameesid)

ad1_2

d1_1

d1_8
d1_9

Droplets
Combustion Furnaces
LED for backlight

42Figure 1. Schematic of the component layout (left) / the position of the fuel droplet (d1_1 thru d1_9) andthe tips of thermocouples (TC1 thru TC5) with inner dimension of the combustion furnace (right).

3.

Results and Discussion

Figure 2 shows the measured temperature distribution in the combustion furnace for S = 8 mm condition(left) and S = 16mm condition (right) respectively. The time zero corresponds to the droplet array stop time atthe combustion position. The target temperature was 570 K, however, the actual initial ambient temperature
was approximate 565 K. After settling the droplets at the combustion position, temperatures are slightly
decrease by fuel evaporation, then it turns to increase after about 7 s past due to ignition. The temperaturerises were approximate 10 K.
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Figure 2. Measured temperature in the furnace using TC1 thru TC5 of S = 8 mm (left) and 16 mm (right) conditions.

t = 4.71 st = 5.01 st = 4.71 s

t = 5.01 st = 4.76 st = 5.06 st = 4.76 st = 5.06 st = 4.81 st = 5.11 st = 4.81 st = 5.11 st = 4.86 st = 5.16 st = 4.86 st = 5.16 st = 4.91 st = 5.21 st = 4.91 st = 5.21 st = 4.96 st = 5.26 st = 4.96 st = 5.26 s

Figure 3. Raw (left) and processed images (right) of the high-speed intensified images for cool flame spread of S = 8mm condition.

t = 4.72 st = 4.96 st = 4.72 st = 4.96 st = 4.76 s

t = 5.00 s

t = 4.76 s

t = 5.00 st = 4.80 s

t = 5.04 s

t = 4.80 s

t = 5.04 s

t = 4.84 s

t = 5.08 s

t = 4.84 st = 5.08 s

t = 4.88 s

t = 5.12 s

t = 4.88 st = 5.12 st = 4.92 s

t = 5.16 st = 4.92 st = 5.16 s

Figure 4. Raw (left) and processed images (right) of the high-speed intensified images for cool flame spread of S =16 mm condition.

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Figures 3 and 4 show the high-speed sequential images of cool flame spread of S = 8 and 16 mm
respectively. The left two columns are the raw images. The backgrounds of these images were subtracted byaveraged images over 100 frames without the cool flame emission, and then were applied by the Bilateral filter
whose kernel size of 41 x 41 pixels, the smoothing parameter for the pixel distance of 150, and for theluminosity difference of 150 to reduce the noise with keeping the leading edge structure of the cool flame. In
the case of S = 8 mm condition, the generation for droplet d1_4 was not successful.
The cool flames appeared after 4.71 s for S = 8 mm and 4.72 s for S = 16 mm respectively. For both cases,
the flames appeared near the top wall of the furnace where the axial position corresponds with the d1_2droplet. If the initial ambient temperature in the furnace is homogeneous, the cool flame is expected to occur
from both ends of the array from the past numerical simulations6). Therefore, the temperature inhomogeneity
may have existed in the furnace. During the spread, the leading edge traveled along slightly apart from thedroplet. So, it is thought that the fuel mixture well established around the droplet and the flame spread along
this mixture layer. After the spread, all droplets established the steady cool flames in S = 16 mm, whereas d1_4
which was not successful, and d1_5 did not establish them in S = 8 mm.
To evaluate the spread speed, the leading edges were tracked. The filtered images were converted into
the binary images by Otsu’s method7). To avoid the tracking of individual outlier pixels, the most left 255 value
pixel whose right four pixels are also 255 value is defined as the leading edge as shown in Fig. 5.
Figure 6 shows the cool flame spread speed calculated by the tracking the leading edge in two ways; oneis the lateral tracking method which the speed is calculated by the only axial coordination, and the point
tracking method by both axial and vertical coordination. Comparing the two results, there are few differences,
so the spread speed is discussed by the lateral track result.
For both cases, the spread speeds changed following the same trend. The first stage just after thespontaneous ignition, spread speeds decreased from approximate 250 mm/s to 100 mm/s within the firstsecond after ignition. After the steady stage after 1-2 s, the speeds turned to increase. The speed of S = 16 mm
reached around 600 mm/s. This acceleration stage may be driven by the sequential spontaneous ignition along
the high reactive mixture. Because the cool flames of S = 8 mm and 16 mm finish to spread by t = 5.26 s and5.16 s respectively as seen in Fig. 3 and Fig. 4, the large deviation around t = 5.2 s caused by the backgroundnoise.

Leading edge

Figure 5. Definition of the leading edge.

Figure 6. Cool flame spread speed for S = 8 mm condition (left) and S = 16 mm condition (right).
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4.

Conclusions

The microgravity experiment ‘’Phoenix-2” using the TEXUS#60 sounding rocket to elucidate the cool
flame dynamics near the spontaneous ignition limit succeeded. The cool flame spread behavior focusing on
its spread speed was analyzed and the followings are found.
1. Both S = 8 mm and 16 mm cases, the cool flame occurred almost the same position after 4.7 s pastfrom the droplet inserted and settling the combustion position.
2. The cool flame spread speed firstly decreased to around 100 mm/s, then turned to increase, and finallyreached around 600 mm/s in S = 16 mm condition. It is thought that the acceleration may be driven
by the sequential spontaneous ignition.
Acknowledgements
This study was supported by ISAS-JAXA as The Front-Loading Project and Small-Scale Project, by JSPS
KAKENHI Grant Number JP19K04843 and JP21K14347, and by Nihon University President Grant Initiative.
The DCU2 development and its integration for the TEXUS rocket were supported by IHI Inspection &Instrument Co., Ltd. and Airbus Defense and Space. On the German side, this experiment was funded by theGerman Space Agency at DLR.
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