P08
微小重力場における冷炎を考慮した液滴列の燃え広がり形態に関する研究
Study of mode of flame spread over droplet arrays
considering cool flame in microgravity
原田真作 1, 松浦勇翔 1, 坂野文菜 1,三上真人 1
Shinsaku HARADA1, Yuto MATSUURA1, Ayana BANNO1 and Masato MIKAMI1
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 used for diesel engines and gas turbines. In this method, liquid fuel is atomized througha fuel injection nozzle and is ignited. Since multiple physical and chemical processes occur at the same timeand in a short time, its mechanism is very complicated and there are many points that have not been clarified.
Clarification of the mechanism will contribute to an improvement of combustion efficiency and simulation
accuracy. In order to research spray combustion from a fundamental viewpoint, we use fuel droplet arrayswith a diameter of several hundred μm so that we can obtain enough spatial and temporal resolutions. With
such droplet diameters, however, it is difficult to get rid of the effect of natural convection, and therefore wehave conducted experiments in microgravity.
Mikami et al. 1) experimentally investigated the flame-spread mode using liner droplet arrays inmicrogravity. They focused the flame spread behavior between droplets and classified it as several flamespread modes. However, they didn’t consider the cool flame, which is a low-temperature oxidation reaction
from 580K to 750K occurring for hydrocarbon fuels. Researchers have paid attention to this phenomenon
because it may affect combustion dynamics and efficiency. Tanabe et al. 2) investigated the self-ignition
behavior of n-heptane, n-dodecane, and iso-octane for different ambient gas temperatures and pressures innormal gravity. They found four types of self-ignition modes: single-stage ignition only with the cool flame;
single-stage ignition only with the hot flame; two-stage ignition, the cool flame first appears and then transits
to the hot flame; and no-ignition. These results showed that the cool flame can be observed not only inpremixed combustion but also in droplet combustion. Mikami et al. 3) investigated the appearance of cool
flame during flame spread between droplets using droplet-cloud elements. A mid-wave infrared camera
captured the infrared luminescence around droplets without the hot-flame appearance, the luminance whichis caused by combustion products of cool flame.
Based on these results, this study redefines the flame-spread mode considering cool flame and clarifies thecondition of cool flame appearance.

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2.

Experimental Apparatus and Analysis Method
Figure 1 (a) shows a droplet-array model used in this experiment. It consists of three n-decane droplets. The

left one is used for ignition and the other two are used for observation of flame spread. As shown in Fig. 1 (b),all droplets are tethered at a cross point of X-shaped 14m SiC fibers (NIPPON CARBON, Hi-Nicalon) thatare fixed to four aluminum rods. Droplet distances are determined by S/d0, where the distance S between
droplets is normalized by the initial droplet diameter d0. The droplet-array-generation system consists of three
stepping motors which move in X, Y and Z directions and makes droplets at any position. The droplet diameteris also controlled by a stepping motor that pushes out a plunger of a micro syringe in certain length. Figure 2
shows the initial droplet diameter of Droplet 2 and dimensionless inter droplet distance between Droplets 1and 2. We supposed d0 of about 0.45 mm and observed the condition near the flame-spread limit. We ignore
the flame spread behavior between Droplets I and 1 because that would be disturbed by a thermal effect of
hot-wire igniter.
To ignite droplets, we used half-loop shaped Fe-Cr wire electronically heated in a short time as shown inFigs. 1 (a) and (b). For observation, we used a two-way simultaneous shooting system with a high-speedcamera (IDT, CCM3510) placed near the lateral-side observation window and a mid-wave infrared camera(NIT, TACHYON) placed under the bottom-side observation window as shown in Fig. 1 (c). Microgravity wasrealized by dropping the experimental apparatus freely in the drop experiment facility of Yamaguchi
University, which has about 4.5 m height and gives microgravity of 0.95 s.

(a) Droplet-array(b) Droplet-array supporting system(c) Two-way simultaneous shooting system with high-speed camera and mid-wave infrared camera
Fig. 1 Experimental apparatus
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(a) with Droplet 2 (b) without Droplet 2Fig. 2 Droplet-arrays with and without Droplet 23.

Results and Discussion
Figure 3 shows sequential images of the flame spread behavior between Droplets 1 and 2, taken by a high-speed camera with a back-illumination displayed in Fig. 3 (a) and by a mid-wave infrared camera displayedin Fig. 3 (b). We defined t = 0 s/mm2 as the moment of Droplet I ignition. Comparing the two types of images,
the area of infrared luminescence is much larger than that of the diffusion flame because the former includes
infrared emission from all combustion products surrounding the diffusion flame while the latter includes
visible light emission from soot inside the diffusion flame. Droplet 1 ignites around 1.0 s/mm2, then the flame
spreads toward Droplet 2, and Droplet 2 ignites around 1.69 s/mm2 with a hot flame. At 1.66 s/mm2, Droplet 2
is not ignited yet but a weak infrared luminescence appears around the SiC fibers tethering Droplet 2 andanother weak infrared luminance appears between the infrared luminance surrounding the diffusion flame ofDroplet 1 and unburned Droplet 2. When 1.69 s/mm2 ~ 2.20 s/mm2, the luminance from the diffusion flame ofDroplet 1 becomes weaker. Figure 4 shows the flame-spread behavior without Droplet 2. As in the case in Fig.
3 (b), a weak infrared luminescence appears around SiC fibers tethering Droplet 2 after 1.66 s/mm2. This isbecause the SiC fibers are heated by the diffusion flame of Droplet 1. Comparison of Figs. 3 (b) and 4 (b)suggests that another weak infrared luminance appearing between the infrared luminance surrounding thediffusion flame of Droplet 1 and unburned Droplet 2 exists only with Droplet 2 and thus conceivably comesfrom a cool flame.

ab0.87 s/mm2

1.00 s/mm2

1.34 s/mm2

1.66 s/mm2

1.69 s/mm2

1.88 s/mm22.20 s/mm2

Fig. 3. Flame-spread behavior from Droplet I to Droplet 2,(a) high-speed camera, (b) mid-wave infrared camera

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ab0.87 s/mm2

1.00 s/mm2

1.34 s/mm2

1.66 s/mm2

1.69 s/mm2

1.88 s/mm22.20 s/mm2

Fig.4. Combustion behavior without Droplet 2,(a) high-speed camera, (b) mid-wave infrared camera

4.

Conclusions
This study investigated the mode of flame spread including the appearance of cool flame. The conclusionsof this study are as follows.
1.

A cool flame appeared before the appearance of a hot flame in so-called Mode 3 flame spread.

2.

The cool flame appeared between a diffusion flame and an unburned droplet heated by the diffusion
flame.

References
1)
2)
3)

M. Mikami, H. Oyagi, N. Kojima, M. Kikuchi, Y. Wakashima and S. Yoda, Combust. Flame, 146(3) (2006), 391.
M. Tanabe, T. Bolik, C. Eigenbrod and H.J. Rath, J. Sato, M. Kono, Proc. Combust. Inst., 26 (1996), 1637
M. Mikami, K. Matsumoto, Y. Chikami, M. Kikuchi, and D.L. Dietrich, Proc. Combust. Inst., 39 (2023), 2449

© 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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