－ －5Published PaperMicrogravity Experiments of Fuel Droplet Evaporation in Sub- and Supercritical EnvironmentsHiroshi NOMURA, Takahiro MURAKOSHI, Yusuke SUGANUMA, Yasushige UJIIE, Nozomu HASHIMOTO, and Hiroyuki NISHIDAThis research was performed as a fundamental study on spray combustion, which is widely applied in various industrial fields. This paper reports the experimental results on droplet evaporation at high pressure and high temperature. Droplet evaporation in sub- and supercritical environments was observed under microgravity conditions in order to suppress the natural convection effect. A single suspended droplet of n-hexadecane was employed for the experiments. The initial droplet diameter was 0.4 mm. A droplet was suspended at the intersecting point of a pair of alumina/silica fibers of 7 µm in diameter. The ambient pressure was varied in the range of 1.0–3.0 MPa, and the ambient temperature was set at 773 K. An evaporating droplet was recorded with a high-speed digital video camera by the backlit method. Temporal variations in the droplet diameter were measured using a self-made computer-aided image analyzer. Microgravity condi-tions were produced by a 50 m drop tower (COSMOTORRE) in Akabira, Hokkaido. Temporal variations in the droplet diameter were successfully obtained for droplet evaporations in the supercritical environments. Based on the histories of the droplet diameter, we measured the normalized droplet lifetime, the evaporation rate coefficient, and the ratio of the initial heat-up period to the droplet lifetime. It was found that the normalized droplet lifetime increased with the ambient pressure. The evaporation rate constant increased with the ambient pressure, reached the maximum value at an ambient pressure slightly above the critical pressure of the fuel, and then decreased. The initial heat-up period increased linearly with the ambient pressure, reached the maximum value at an ambient pressure of 2.0 MPa, and then decreased. We concluded that in the case of fuels with a high critical temperature, the initial heat-up period determines the ambient pressure dependence of the droplet evaporation lifetime in the environments around the critical point of the fuel. This paper received an Outstanding Paper award from the Combustion Institute of Japan.Keywords: Droplet, Evaporation, Supercritical environment, Microgravity experiment, Spray combustion亜臨界および超臨界雰囲気における燃料液滴蒸発に関する微小重力実験 この研究は，様々な産業分野で広く利用されている噴霧燃焼の基礎研究である。論文では，高圧・高温雰囲気中の液滴蒸発に関する実験結果を報告した。自然対流の影響を抑制するため，微小重力環境下で亜臨界および超臨界雰囲気中の液滴蒸発を観察した。実験には，正ヘキサデカンの単一の懸垂液滴を使用した。初期液滴直径は0.4 mmとした。直径7 µmのアルミナ/シリカファイバを交差させ，その交点に液滴を懸垂した。雰囲気温度を773 Kに設定し，雰囲気圧力を1.0～3.0 MPaの範囲で変化させた。蒸発する液滴を、バックリット法で高速デジタルビデオカメラに記録した。液滴直径の時間変化は，自作の画像分析プログラムを使用して測定した。微小重力環境は，北海道赤平市にある50 mの落下塔（COSMOTORRE）によって実現した。超臨界雰囲気中で蒸発する液滴の直径履歴を取得するのに成功した。液滴直径の履歴から，正規化液滴寿命，蒸発速度係数，および液滴寿命に対する初期加熱期間の比率を測定した。正規化液滴寿命は，雰囲気圧力の増大に伴って増大した。蒸発速度係数は雰囲気圧力の増大に伴って増大し，燃料の臨界圧力をわずかに超える雰囲気圧力で最大値に達し，その後減少した。初期加熱期間は，雰囲気圧力の増大に伴って直線的に増大し，雰囲気圧力2.0 MPaで最大値に達した後，減少した。臨界温度が高い燃料の場合，初期加熱期間が燃料の臨界点付近の雰囲気における液滴蒸発寿命の雰囲気圧力依存性を決定すると結論付けた。日本燃焼学会から，この論文で優秀論文賞を受賞した。キーワード：液滴，蒸発，超臨界雰囲気，微小重力実験，噴霧燃焼Journal（掲載誌）Proc. Combust. Inst., 36(2017), 2425-2432, DOI: 10.1016/j.proci.2016.08.046.
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