PS35
室温・高圧におけるアンモニア液滴生成精度の向上と予蒸発抑制の試み
Improvement of ammonia droplet generation accuracy
and suppression of pre-vaporization at room temperatureand high pressure
森 隼人 1, 坂野 文菜 1, 三上 真人 1,Hayato MORI1, Ayana BANNO 1, and Masato MIKAMI 1
1 山口大学 大学院 創成科学研究科
Graduate School of Sciences and Technology for Innovation, Yamaguchi University #1

1. Introduction
In recent years when the movement of carbon neutral is becoming popular around the world, many studies
on combustion using carbon neutral fuels are being conducted. Ammonia is one of them. Since ammonia isrelatively easy to operate in the liquid state and the supply infrastructure is well-developed, ammonia is apromising fuel for practical use. Among combustion research using ammonia, spray combustion using liquidammonia has been studied. The advantage of liquid ammonia spray combustion is that it requires minimal
modification of existing equipment and reduces installation costs. Okafor et al. 1) succeeded in stabilizing
ammonia spray combustion using a high-temperature swirling air flow. However, the detailed mechanism has
not been clarified completely because various physical and chemical processes proceed simultaneously. To
improve understanding of its mechanism, it is necessary to clarify the mechanism from a fundamental
perspective using droplet combustion, which is a component of spray combustion. In a study on ammonia
droplet combustion, Matsuura et al. 2) generated a single ammonia droplet at room temperature and 1.5 MPa
condition and investigated its burning behavior. However, the initial droplet diameter could not be controlled
well. In the study of droplet combustion, the initial droplet diameter is used to normalize the spatial and timescales, so it is important to control the initial droplet diameter. Therefore, this study modified the ammonia
droplet generation system devised by Matsuura et al. 2) to improve the accuracy of droplet diameter generation.
In addition, we attempted to suppress pre-evaporation of ammonia droplet so that it could maintain acontrolled diameter as much as possible.

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2. Experimental apparatus and procedure
Figure 1 shows the ammonia-droplet- generation system, which is equipped with a three-axis traverse stage,a fiber base with crossed SiC fibers, a high-pressure syringe, a glass needle for fuel supply, thermocouple,valves, and pipes. The valves controlling the fuel supply to the glass needle were either solenoid valves usedby Matsuura et al. 2) or servomotor-driven ball valve developed in this study. After evacuating the highpressure syringe and piping with the solenoid valve or ball valve closed, filling of the high-pressure syringewith liquid ammonia was performed at high pressure. In addition, N2 pressurized the same area to completelyliquefy the gaseous ammonia present in the syringe and pipes, so that the syringe was filled only with liquidammonia. The droplet generation system was inserted into the pressure chamber pressurized at 1.5MPa usingmixed air (N2 79% and O2 21%). When generating a droplet, the solenoid valve or the ball valve was opened,
and a syringe plunger was pushed out by a stepper motor. The pushed liquid ammonia reached the glass
needle in the following order: syringe, pipe, solenoid valve or ball valve, and pipe, and was suspended at theintersection of SiC fibers with a diameter of 14 μm. Images of the droplets were captured at 1000 fps using ahigh-speed camera (IDT, iN8-S2) in a back-illumination system.
For the verification of droplet diameter control, 20 pulses were input to the stepper motor for a target
diameter of 1.0 mm. Non-measuring droplet were generated prior to generation of the measuring droplet toprevent bubbles from forming in the glass needle and errors in the diameter of the generated droplets. The
droplet diameter was calculated using the MATLAB code. The calculated values were compiled into ahistogram, and the average and standard deviation of the generated droplet diameters were calculated andused to evaluate the accuracy of the droplet generation.

Figure 1 Ammonia-droplet-generation system placed in a high-pressure chamber

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3. Results and discussion
3.1 Verification results of droplet diameter control of a droplet-generation system using a solenoid valve
Figure 2 shows the droplet diameter distribution for 15 data in the droplet-generation system using asolenoid valve. Figure 2 shows that many droplets with diameters larger than 1.0 mm were generated,indicating that the amount of liquid ammonia supplied from the glass needle did not correspond to the amountof liquid ammonia pushed into the plunger of the syringe. This is considered to be due to the continuous
dripping observed after the syringe plunger was pushed out. The average and standard deviation of the
produced droplet diameters were 1.20 mm and 0.12 mm, respectively.

3.2 Consideration of the cause of uncontrolled droplet diameter and improvement of the system
According to the results of Section 3.1, the uncontrollable droplet diameter was caused by the continuous
dripping when a solenoid valve was used. We investigated the cause of the continuous dripping and confirmedthat many air bubbles entered the supply path during the syringe plunger push. The air bubbles in the supply
path were pressed by the syringe plunger push. The bubbles were expanded after the plunger push wasfinished, and this expansion pushed the liquid in the path toward the glass needle, which caused thecontinuous dripping. The solenoid valve is considered to be the cause of this bubble entry. Solenoid valves use
magnetic force to open and close the valve, so they have a coil that generates heat when energized. Whenliquid ammonia passed close to the coil, it may have been affected by the heat and vaporized. Therefore, we
substituted a ball valve without a heat source for the solenoid valve. A servo motor and coupling components
were used to rotate the shaft of the ball valve to open and close the ball valve in the pressure chamber, whichcannot be handled manually.

3.3 Consideration of the cause of uncontrolled droplet diameter and improvement of the equipment
Figure 3 shows the droplet diameter distribution for 15 data after the improvement of the system. In the caseof the improved system, the continuous drop formation was not observed after the improvement. The averageand standard deviation of droplet diameters were 1.06 mm and 0.06 mm, respectively, suggesting an

7
65
4
3
2
10

Number of droplet

Number of droplet

improvement in droplet diameter control compared to the results in Section 3.1.

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

1.30

1.35

1.40

0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4

1.45

Droplet diameter [mm]765432
10

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

1.30

1.35

1.40

1.45

0.9 0.95 1Droplet
1.05 1.1diameter
1.15 1.2 1.25
1.3 1.35 1.4[mm]Figure 2 Droplet diameter distribution for droplet-generation Figure 3 Droplet diameter distribution for droplet-generation
system with solenoid valve (target diameter: 1.0 mm)

system with ball valve (target diameter: 1.03mm)
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3.4 Vaporization-rate constant of a single ammonia droplet in normal gravity
As an indicator of pre-vaporization, the instantaneous diameter of an ammonia droplet was measured undernormal gravity. Furthermore, the vaporization-rate constant was measured based on the d2- law in which thesquared value of the droplet diameter decreases linearly, and expressed as Eq. (1) 3),
𝑑 2 = 𝑑0 2 − 𝐾𝑡

(1)

In this study, 𝐾 is the vaporization-rate constant. Figure 4 shows the change of the squared diameter of three
droplets over time for a target diameter of 1.0 mm. Table 1 shows the initial droplet diameter, the ambient
temperature around droplet, the vaporization-rate constant for three droplets. As a result, the vaporizationrate constant of a droplet with an ambient temperature around 310 K is 0.0231~ 0.0285 mm2 / s.
Next, we attempted to suppress pre-vaporization by lowering the ambient temperature near the droplet byattaching a Peltier element to the droplet generation system. However, it became clear that the ambient
temperature could not be lowered to a level that would contribute to suppressing pre-vaporization, as only
about 3 K of cooling could be achieved due to the high-pressure environment and the enclosed space. In thefuture, it will be necessary to use another cooling method.

1.4
1.2

d2 [mm2]

10.80.6● Droplet 1 _311K● Droplet 2 _311K● Droplet 3 _310K

0.40.2・・・ d 2 = -0.0285t + 1.2874
d 2 = -0.0281t + 1.2126
d 2 = -0.0231t + 0.9948

000.2

0.4

0.6

0.8

11.2

1.4

1.6

1.82t [s]Figure 4 The variation of squared droplet diameter over time in normal gravity

Table 1 Vaporization-rate constant 𝐾 in normal gravity

Initial droplet diameter[mm]𝑲Tempeature
2/s][mm[K]

Droplet 1

1.143110.0285Droplet 2

1.103110.0281

Droplet 3

1.00

310

0.02314 of 5

4. Conclusions
1.

The droplet diameter could not be controlled when a solenoid valve was used to controll the fuel supply
to the glass needle. The cause of the problem was considered to be that air bubbles got mixed in theroute due to the heat generated by the solenoid valve.

2.

Using a ball valve instead of the solenoid valve and a target droplet diameter of 1.0 mm, the mean andstandard deviation were 1.06 mm and 0.06 mm.

3.

In a normal gravity condition and an ambient temperature around 310 K, the vaporization-rate constantof a single ammonia droplet is 0.0231~ 0.0285 mm2 /s.

4.

Pre-vaporization suppression was attempted by lowering the ambient temperature using a Peltierelement, but it became clear that the ambient temperature could not be lowered to a level that would
contribute to pre-vaporization suppression. In the future, it will be necessary to use another coolingmethod.

5. Acknowledgements
This paper is based on results obtained from a project, JPNP23016, commissioned by the New Energy andIndustrial Technology Development Organization (NEDO) and a JSPS KAKENHI Grant-in-Aid for Scientific
Research (B) (24K01081). We would like to express our sincere gratitude to Mr. Harada and Mr. Naito for theircooperation in experiments.

6. References
1)

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, A. Banno, M. Mikami., Investigation of ignition methods of single ammonia droplets andmeasurement of burning-rate constant at high pressure in microgravity, JASMAC-35 (2023), OR4-1.

3)

M. Mikami: Vaporization and Combustion of Fuel Droplets, “NAGARE” 20, 2001, 106-115, in Japanese.

© 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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