OR2-1
微小重力場における冷炎を考慮した液滴間の燃え広がりに関する研究
Study on the flame spreading between droplets
considering the cool flame in microgravity
原田 真作, 坂野 文菜, 三上 真人 1
Shinsaku HARADA, Ayana BANNO, and Masato MIKAMI 1
山口大学 大学院 創成科学研究科）
Graduate School of Sciences and Technology for Innovation, Yamaguchi University1

1. Introduction
Spray combustion is often used for practical combustors such as diesel engines and gas turbines. In thismethod, liquid fuel is atomized through a fuel injection nozzle and is ignited. Since multiple physical andchemical processes occur at the same time and in a short time, its mechanism is very complicated and there
are many points that have not been clarified. Elucidation of the mechanism will contribute to higher
combustion efficiency and accurate simulations. In order to research spray combustion from a fundamental
viewpoint, fuel droplet arrays with a diameter of several hundred μm are often used so as to obtain enoughspatial and temporal resolutions. With such droplet diameters, however, it is difficult to get rid of the effect ofnatural convection in normal gravity, and therefore combustion experiments are conducted in microgravity.
For example, Mikami et al.1) investigated the appearance of cool flame in flame spread over droplet-cloudelements. They used a mid-wave infrared camera and captured infrared luminescence around droplets whichis caused by combustion products of cool flame outside the flame-spread limit. Cool flame is a low temperatureoxidation around 700 K occurring for hydrocarbon fuels, and researchers have paid attention to thisphenomenon because it may affect combustion dynamics and efficiency. Mikami et al. 2) researched that Mode
3 flame spread appears in such limit of flame spreading. In this mode, the next droplet is ignited by heat fromthe diffusion fame whose leading edge does not reach the flammable-mixture layer around the next droplet.
Harada et al.3) researched redefinition of the flame-spread mode considering cool flame in liner droplet arrays.
They found there appears cool flame in Mode 3 flame spread and it turned to hot flame. Based on these results,
this research focuses on the Mode 3 flame spread and researched the effect of inter-droplet distance.
2. Experimental Apparatus and Analysis Method
This study used a droplet array model with two droplet interaction as shown in Fig. 1 (a). It consists ofone ignition droplet, one interaction part: Droplets B and A, and one observation droplet: Droplet L. Alldroplets are tethered at cross points of X-shaped 14 m SiC fibers (NIPPON CARBON, Hi-Nicalon) that are
fixed to four aluminum rods as shown in Fig. 1(b). Inter-droplet distances are determined by S/d0, where thedistance S between droplets is normalized by the initial droplet diameter d0. Droplet-array-generation system
consisting of three stepping motors which can move in X, Y and Z directions enables us to make droplets at
any position. For the droplet ignition, we used half-loop shaped Fe-Cr wire electrically heated in a short time.
For observation, we used two types of cameras: a high-speed camera (IDT, CCM3510) placed near the lateralside observation window and a mid-wave infrared camera (Vision Sensing, VIM-640ULC-WB) placed near
the bottom side observation window. These cameras are included in a two-way simultaneous shooting system
as shown in Fig. 1 (c). When the cool flame appears, very weak infrared luminescence is emitted from the1 of 4

combustion products, CO2 and H2O. They can be observed through the mid-wave infrared camera. By
comparing the mid-wave infrared image and high-speed camera image with a back-illumination, we candistinguish the hot flame and the cool flame. The experiments were conducted in room temperature andambient pressure and used n-decane as a fuel. Microgravity was realized by dropping the experimental
apparatus freely in the drop experiment facility of Yamaguchi University.

(a) Droplet-array model(b) Droplet-array supporting system(c) Two-way simultaneous shooting system with high-speed camera and mid-wave infrared camera
Fig. 1 Experimental apparatus
3. Results and Discussion
Figure.3 shows the flame-spread behavior in SBL/dL0 = 15.0, with and without flame spreading. Picturesgrouped “a” were taken by the high-speed camera and those of grouped “b” were taken by the mid-wave
infrared camera. Comparing from the two kinds of pictures, the infrared luminescence appears in larger area
than the visible luminescence from soot of diffusion flame. In flame spreading result, the interactive twodroplets, Droplets B and A, were ignited around 0.5 s/mm2 and Droplet L was heated by heat from diffusion
flame of the interactive droplets. At 1.26 s/mm2, infrared luminescence appeared around Droplet L. After that,
diffusion flame appeared at 1.51 s/mm2 accompanied with strong luminescence of SiC fiber around Droplet L.
Therefore, these results indicate that Mode 3 flame spread appeared. On the other hand, in the no-flamespreading result, Droplet L was not ignited although the infrared luminescence reached near Droplet L. Under
S/d0 = 15.0 condition, there appeared dispersion of data: both flame-spreading and no-flame-spreading resultsappeared, therefore we conclude that this condition represents the flame-spread limit.
To know the detail of the result in Fig. 3(b), we analyzed the distribution of luminescence value for flame
spreading direction as shown in the plots in Fig. 4. The vertical axis shows the infrared luminescence value
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that reflects the average of 12 pixels in droplet center and the horizontal axis shows the distance from Droplet
L normalized by its initial diameter. When diffusion flame appeared around the interactive droplets, there
were some peaks around 2400. After that at 1.26 s/mm2, another peak appeared around Droplet L and had
smaller luminescence value than the diffusion flame of interactive droplets. When Droplet L was ignited, thepeak value was increased. Comparing the result of Fig. 2 (a) and (b) at 1.26 s/mm2, the high-speed camera
image doesn’t include any phenomenon around Droplet L. On the other hand, infrared luminescence appears
around Droplet L in the infrared image. Furthermore, in Fig. 4 at 1.26 s/mm2 the infrared luminescenceappeared around Droplet L has smaller peak than diffusion flame appeared at 1.51 s/mm2. Therefore, theluminescence comes from the cool flame.

Fig. 2 Burning behavior with flame spreading to Droplet L(a) high-speed camera (b) mid-wave infrared camera

Fig. 3 Burning behavior without flame spreading to Droplet L(a) high-speed camera (b) mid-wave infrared camera

Fig. 4 Luminescence distribution at different times in the same condition as in Fig. 2 (b)3 of 44. Conclusion
This study investigated flame spread behavior near the flame-spread limit considering the appearance ofcool flame. The conclusions of this study are as follows.
1.
2.

Cool flame was observed in the flame spread over unevenly arranged droplet array consisting of Droplets
I, B, A and L. The infrared luminescence distributed around Droplet L.
Cool flame has a smaller luminescence peak than diffusion flame.

5. Acknowledgements
This research was subsidized by a JSPS KAKENHI Grant-in-Aid for Scientific Research (B) (21H01532).
We would like to express our sincere gratitude to Mr. Naito, Mr. Mori and Mr. Matsuura for their cooperationin microgravity experiments.
6. References
1)
2)
3)

M. Mikami, K. Matsumoto, Y. Chikami, M. Kikuchi, and D.L. Dietrich, Proc. Combust. Inst., 39 (2023), 2449
M. Mikami, H. Oyagi, N. Kojima, M. Kikuchi, Y. Wakashima and S. Yoda, Combust. Flame, 146(3) (2006), 391.
S. Harada, Y. Matsuura, A. Banno and M. Mikami JASMAC-35, (2023).
© 2024 by the authors. Submitted for possible open access publication under
the terms and conditions of the Creative Commons Attribution (CC BY) license(http://creativecommons.org/li censes/by/4.0/).

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