静止ナトリウム液滴燃焼実験(III)

An Experimental Study on Suspended Sodium Droplet Combustion (III)

佐藤 研二*

Sato, Kenji*

ナトリウム液滴燃焼挙動の現象論的解明に向けた研究の一環として、これまでにナトリウム静止液滴を対象に常温空気流中での着火燃焼実験を実施し、高速度カメラを用いた着火挙動の観察、着火遅れ時間、液滴温度の時間変化の測定を行ってきた。本研究では、ナトリウム静止液滴の乾燥空気中での着火燃焼実験を、まず液滴初期直径4mm、液滴初期温度300、350、400Cにおいて空気流速を400cm/sまで拡張して、次に液滴初期温度350、400C、空気流速100cm/sのもとに液滴初期直径を2.3mmから4.4mmまで変化させて行い、空気流速、液滴初期温度、液滴初期直径が着火挙動と着火遅れ時間に与える影響を調べた。主な結果は以下のとおりである。(1)液滴初期温度が300、350、400Cのいずれにおいても、実験を行った上限の400cm/sまで着火が観察された。着火(本研究では着火を液滴表面に接する気相部での橙色の発光の出現で定義)は高流速側においてもこれまで同様液滴上端付近を除く全領域にわたってほぼ同時に生じる。(2)着火遅れ時間の流速依存性は温度による違いがほとんどなく、流速が300cm/s付近までは流速依存性が小さくなりながら着火遅れ時間が減少する変化が生じ、その後いったん減少率が増え着火遅れ時間が流速とともに小さくなる傾向が続く。(3)液滴直径の増加により、着火遅れ時間は増加する。このとき、直径が大きくなるほど増加率がやや大きくなる傾向がある。実験範囲の傾向をもとに液滴初期直径をゼロに近づけたときの着火遅れ時間を外挿するとゼロに近づく。

As part of studies for phenomenological investigation of sodium droplet burning behavior, in our previous experimental studies for suspended single sodium droplet, behavior of ignition process and succeeding combustion, ignition delay time, and droplet temperature history had been investigated. In this study, combustion experiments of suspended sodium droplet were performed in upward dry air flow by expanding the range of free-stream velocity U of air flow into 400 cm/s with initial droplet temperature Ti = 300, 350, and 400C and initial droplet diameter 4 mm at first. Then, the combustion experiments were also performed by changing the initial droplet diameter from 2.3 to 4.4 mm with Ti = 350 and 400C and U = 100 cm/s. From the experimental results, the effects of free-stream velocity, initial droplet temperature, and initial droplet diameter on the ignition/burning behavior and ignition delay time were examined. The obtained results are as follows: (1) Ignition phenomena of suspended droplet were observed for all examined experimental conditions up to 400 cm/s. The orange emission observed at the moment of ignition occurs simultaneously over whole droplet surface except the top region of it. (2) The feature of the dependence of ignition delay time on the free-stream velocity is independent of the initial droplet temperature. With the increase of the free-stream velocity, up to 300 cm/s the ignition delay time decreases with decreasing dependency, and then the dependency increases more. (3) The ignition delay time increases with the increase of initial droplet diameter. The dependency increases as the initial droplet diameter increases. The ignition delay time extrapolated toward zero diameters from the obtained results becomes to be essentially zero.