方型毛細管中單顆液滴之漂移與沸騰

本研究針對方型玻璃毛細管內乘載液中的單顆液滴進行加熱，探討不同的液體組合、管壁溫度及液滴管徑比對其漂移運動及沸騰現象的影響。實驗中有正戊烷在甘油承載液、冷凍液Novec 7100 液滴在甘油承載液與不同重量分率的乙醇水溶液液滴在矽油承載液中等三種組合。管壁溫度之溫度範圍在80∼120°C 之間，使用的液滴管徑比有0.3、0.5 及0.7 三種。方型玻璃毛細管受熱時，管內會在垂直管軸方向形成溫度梯度，使液滴與承載液間的介面張力不平衡而產生熱毛細現象，造成承載液引流將液滴往加熱面推移。隨著管壁溫度的升高，液滴達到特定溫度後開始沸騰，受到液滴與承載液間表面張力差值的影響，沸騰初始過熱度可依不同液體組合可分為極高和極低等兩種情況，前者又稱為過熱極限，該狀況下液滴之表面張力小於承載液；後者發生在液滴之表面張力大於承載液時。實驗結果發現，液滴之漂移速度隨乙醇水溶液重量分率及管壁溫度的上升而加速，但在重量分率為0.5 時液滴會靜止不動，或甚至向低溫面漂移，且液滴靠近管壁時，並不會受管壁阻力之影響而減緩速度，反而呈穩定加速直到碰觸管壁後才停止運動，這可能是靠近加熱面之溫度梯度較大的影響。在液滴相對尺寸的影響方面，當液滴管徑比為0.7 時，液滴漂移之初始速度會較高；而液滴管徑比為0.3 時雖初始速度較低，但隨時間的經過仍可加速到較高之速度。此外，乙醇水溶液液滴在毛細管中受熱沸騰時，因其表面張力大於矽油，沸騰之初始溫度為正常狀況下之沸點；反之，戊烷及冷凍液Novec 7100 之液滴由於其表面張力遠低於甘油，可觀察到液滴在承載液中達到極高的過熱溫度後瞬間沸騰，並伴隨劇烈聲響及震動。在實驗中改變液滴尺寸及毛細管尺寸對液滴之過熱極限皆沒有重大影響，此現象也印證了前人之研究，即液體之過熱極限只與其物理性質有關。

關鍵字

毛細管；液滴；漂移；熱毛細現象；過熱極限

並列摘要

In this work, we study the migration and boiling of a single droplet in a heatedsquare capillary tube. Three different fluid combinations are used: n-pentane droplet in glycerol, Novec 7100 droplet in glycerol and ethanol water mixture of different mass fraction (w = 0, 0.1, 0.5, 0.7, 0.9 and 1) in silicone oil. The wall temperature varies from 80°C to 120°C and nondimensional droplet ratios are tested: D/L = 0.3, 0.5 and 0.7. When the square capillary tube is heated, temperature gradient perpendicular to the tube axis is formed and the thermal capillary effect is induced so that the droplet experiences unbalanced force and starts to move toward the heated surface. As the wall temperature increase, the droplet begins to boil. The limit of superheat of the droplet can be observed when the surface tension of the droplet liquid is greater than the host liquid. From the results, the drift velocity increase with the increase of ethanol quality and the wall temperature. In addition, the droplet accelerates as it moves toward to the heated wall. This may ascribed to the rapid rise of temperature gradient near the heated wall. We also find that larger droplet has a higher initial velocity, while smaller droplet speeds up eventually. Nevertheless, the droplet does not move or even drift to the cold wall at a mass fraction of 0.5. For droplet composed of ethanol/water mixture, no superheat limits observed. On the other hand, explosive boiling occurs for droplets of n-pentane and Novec 7100. The limit of superheat is affected by fluid properties and is independent of droplet size or tube dimension.

並列關鍵字

capillary tube ； droplet ； migration ； thermal capillary effect ； limit of superheat

參考文獻

[1] C. Avedisian, "Bubble growth in superheated liquid droplets," Encyclopedia of

Fluid Mechanics, vol. 3, pp. 130-190, 1985

[2] J. Shepherd and B. Sturtevant, "Rapid evaporation at the superheat limit,"

Journal of Fluid Mechanics, vol. 121, pp. 379-402, 1982

[3] S. M. Chitanvis, "Explosive vaporization of small droplets by a high‐energy

國際替代計量

方型毛細管中單顆液滴之漂移與沸騰

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