OR4-1
単一アンモニア液滴の着火方法調査と微小重力場・高圧雰
囲気における燃焼速度定数計測
Investigation of ignition methods of single ammonia
droplets and measurement of burning-rate constant athigh pressure in microgravity
松浦勇翔 1, 坂野文菜 1, 三上真人 1,
Yuto Matsuura1, Ayana Banno 1, Masato Mikami 1
1

山口大学大学院，Yamaguchi University#1,

1. Introduction
Spray combustion is widely used in liquid-fueled combustors. However, the detailed mechanism has notbeen clarified completely because various physical and chemical processes proceed simultaneously. In orderto improve understanding of its mechanism, many studies on combustion of a single droplet, which is acomponent of spray, have been conducted from a fundamental viewpoint. In addition, in recent years whenthe movement of carbon neutral is becoming popular, many studies on combustion using carbon neutral fuels
are actively conducted. Ammonia is one of them. Another merit of using liquid ammonia as a fuel is that thecost of change of the existing supply system can be minimized. Okafor et al. 1) succeeded in stabilizing
ammonia spray combustion using a high-temperature swirling air flow. Since ammonia is in gaseous state at
normal temperature and atmospheric pressure, however, it is difficult to handle it as a liquid. So, researches
using single ammonia droplets have rarely been conducted. Matsuura et al. 2) succeeded in generating single
ammonia droplets and measuring vaporization-rate constant as the first step. However, it is known thatammonia is difficult to ignite because the heating value is much lower than that of hydrocarbon fuels, and thelatent heat of vaporization is much greater, and so on.
At the second step, this study investigated methods of igniting single ammonia droplets at a pressure higher
than saturation pressure of ammonia at room temperature. Therefore, the burning-rate constant, which is an
important index for investigating the combustion characteristics of an ammonia droplet, was measured at highpressure in a microgravity.
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2. Experimental apparatus and procedure
This study used high-pressure condition to observe burning of ammonia droplets. Liquid ammonia wasfirst supplied to a vacuumed high-pressure syringe, which is a part of the ammonia-droplet-generation system.
Then the liquid ammonia supply path was pressurized by N2, so that ammonia vapor generated in the syringe
was fully liquefied and thus the syringe was filled only with liquid ammonia. The syringe with liquid ammonia
was installed in the droplet-generation system, which was inserted into the pressure vessel. The pressurevessel was pressurized at 1.5 MPa using mixed air (N2 79%, O2 21%). Figure 1 shows the ammonia droplet
generation system, which is equipped with a three-axis traverse stage that moves the glass tube and pushesout the syringe with liquid ammonia, a fiber base with crossed SiC fibers, a self-designed syringe, and asolenoid valve. An ammonia droplet was generated through the fine glass tube by supplying liquid ammonia
from the stepping-motor-driven syringe after the solenoid valve was opened. After an ammonia droplet wasgenerated, it was ignited by the igniter in microgravity. A hot wire ignition was used for the ignition method.
For measuring burning-rate constant, it is better to remove the igniter from the field of view. Therefore, wemade a device to remove the igniter using a stepping motor, as shown in Fig. 1. To achieve the purpose of thisstudy, two type shapes of igniter were used for ignition trial, as shown in Fig. 2. The main change was theshape of hot-wire. First, as shown in Fig. 2(a), the hot-wire was looped to surround the droplet to ignite easier,which is named “Ver. 1”. Second, we employed another type of igniter in Fig. 2(b), named “Ver. 2”, which ismoved away from the vicinity of the droplet. As shown in Fig. 2(b), the lower half of the droplet is surroundedby the hot-wire when viewed from the top, and in Fig. 2(c), the droplet is sandwiched from both sides whenviewed from the side. The energization time was set to 200ms for both of types. The microgravity experiments
were conducted at a drop experiment facility of Yamaguchi University. The falling distance is 4.5 m and themicrogravity duration is 0.9 s.

(a) Ver. 1 shape(b) Top view of Ver. 2 shape

Figure 1. Ammonia droplet generation systemwith an igniter removal devise.

(c) Side view of Ver. 2 shape
Figure 2. Shape types of hot-wire of igniters
2 of 53. Results and discussion
3.1 Ignition trial of single ammonia droplets by changing oxygen concentration in normal gravity
Ammonia has the characteristic of being difficult to burn. Moreover, it is much more difficult to ignite
liquid ammonia than gaseous ammonia due to its latent heat of vaporization. Matsuura et al. carried out
ignition trials of ammonia droplets in air in normal gravity. They reported that the ammonia droplets did notignite, and only the red heat of the hot-wire created an ammonia vapor cloud3). Therefore, aiming to promote
ignition by lowering the ignition temperature and raising the flame temperature, an ignition trial wasconducted by increasing the ambient oxygen concentration. The oxygen concentration was changed to 25%,30%, 35%, and 40%, where mixed air and pure oxygen were mixed. A fan was installed in the pressure vesselso that the oxygen concentration was uniform. As results, the ignition of ammonia droplets was not realizedat oxygen concentrations of 25% and 30%. At 35% oxygen concentration, ammonia droplets were sometimes
ignited. At 40%, conditions were met for ignition in normal gravity. Figure 3 shows sequential images ofammonia droplet flame at 40% oxygen concentration with the initial droplet diameter of 1.1mm using thelooped type hot-wire shown in Fig. 2(a). Here, we define 0 s as the start of camera shooting. After the flame
formed, the ammonia droplets continued to burn, and after a certain period of time, the droplets flew and fell.

3.2 Ignition trial in microgravity.
Since the effect of gravity on spray droplets is negligible, investigations in microgravity are necessary forpractical application. In addition, the natural convection becomes negligible in microgravity, so the ignition
probably becomes easier. Therefore, we tried to ignite ammonia droplets in microgravity. We used mixed air(N2 79%, O2 21%) as the ambient gas. Figure 4 shows sequential images of the flame of ammonia droplet using
Ver. 1 shape in Fig. 2(a) with the initial droplet diameter of 1.1 mm. Figure 5 shows them using a device toremove the igniter shown in Fig. 1 and Ver. 2 shape in Fig. 2(b) with the initial droplet diameter of 1.1 mm.
Figures 4 and 5 clearly show that the ignition of an ammonia droplet was realized in high-pressure air
atmosphere in microgravity. Here, we defined 0 s as the fall start. As shown in Fig. 4, a flame was observed
immediately after ignition, and the intensity of the flame brightness became weaker in the latter stage ofcombustion. The red heat of SiC fibers suggests the presence of the flame. As shown in Fig. 5, from early tomiddle stage of combustion, the tendency is similar to Fig. 4, however, the flame rapidly contracted in thelatter stage of combustion, and the droplets finally bursted. The phenomenon that the droplet expands once
and then shrinks or bursts was observed several times. We will continue to figure out this phenomenon.

3.3 Measurement of burning-rate constant in microgravity
We confirmed the appearance of the ammonia droplet flame in microgravity and therefore measured theburning-rate constant of the ammonia droplet flame. Burning-rate constant is the important index for
3 of 5investigating the combustion characteristics of droplet flames. The d2- law, in which the squared value of thedroplet diameter decreases linearly, is expressed as Eq. (1), where 𝐾𝑏 is burning-rate constant.
𝑑 2 = 𝑑0 2 − 𝐾𝑏 𝑡

(1)

Figure 6 shows the variation of squared droplet diameter over time for a single ammonia droplet of Fig. 5.
According to Fig. 6, the burning-rate constant of ammonia droplet has two steps: the first step, a constantburning-rate constant is shown from the beginning of combustion to the middle of combustion; and the secondstep, 𝐾𝑏 becomes gentle about 0.1 s before the droplet diameter becomes unmeasurable. Table 1 shows theresults of measurement, such as the initial droplet diameter 𝑑0 , the first-step burning-rate constant 𝐾𝑏𝐹 , thesecond-step burning-rate constant 𝐾𝑏𝑆 . The first step, 𝐾𝑏𝐹 is 0.89~0.98 mm2/s and the second step, 𝐾𝑏𝑆 is0.44~0.49 mm2/s. We will investigate the details of this phenomenon in the future.

Figure 3. Sequential images of ammonia droplet

flame in normal gravity

Figure 4. Sequential images of ammonia droplet

flame with Ver. 1 shape hot-wire

at 40% oxygen concentrationin air in microgravity

Figure 5. Sequential images of ammonia droplet

flame with Ver. 2 shape hot-wire andFigure 6. The variation of squared droplet diameterover time in air in microgravity

igniter removal device

Table 1. Measured burning-rate constant in microgravity 𝐾𝑏 and various parameters
Initial droplet
diameter[mm]
Droplet 1

1.3

Droplet 2

1.1

Droplet 3

0.99

Ambient
pressure[MPa]
1.5

Ambient
tempeature[K]

𝑲𝒃𝑭 [mm2/s]

𝑲𝒃𝑺 [mm2/s]

2990.970.49

300

0.89

0.47

301

0.98

0.44

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4. Conclusions
This study investigated methods of igniting single ammonia droplets at a pressure higher than saturationpressure of ammonia at room temperature and the burning-rate constant was measured at high pressure inmicrogravity. The conclusions of this study are as follows.
1.

Single ammonia droplets were ignited in an air/oxygen mixture with an oxygen concentration of 40%
at high pressure in normal gravity.

2.

Single ammonia droplets were ignited in a mixed air atmosphere at high pressure in microgravity.

3.

The burning-rate constant of a single ammonia droplet in microgravity varied in two steps: the first step
was 0.89~0.98 mm2/s and the second step were 0.44~0.49 mm2/s.

Acknowledgements
We would like to express our gratitude to Mr. Shinsaku Harada and Mr. Hayato Mori for their cooperation in
the microgravity experiments.
References1)

E.C. Okafor, H. Yamashita, A. Hayakawa, K. D. Kunkuma A. Somarathne, T. Kudo, T. Tsujimura, M. Uchida, S. Ito,
H. Kobayashi: Fuel, 287, 2021, 119433.

2)

Y. Matsuura, Y. Hara, M. Mikami, Measurement of vaporization rate of single ammonia droplets in room-temperature
high-pressure air in microgravity, JASMAC-34, 2022.

3)

Y. Matsuura, Y. Hara, Y. Chikami, T. Seo, M. Mikami, Realization of single ammonia droplet generation andcombustion attempt at high pressure, The Japan Society of Mechanical Engineers Chugoku-Shikoku Student
Association 52nd Student Member Graduation Research Presentation Meeting (in Japanese), 2022.

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