於漸縮加速流場中的液滴變形與破碎反應之分析

Experimental study of droplet deformation and breakup by accelerated flow in narrowing channel

張夢帆(Mong-Fang Zhung)

指導教授 ： 王興華

國立臺灣大學/工學院/機械工程學研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.00964

摘要

本文主要在探討液滴於遭遇高流速之氣流時其破碎效應的機制，並觀察流場內的加速度對該效應的影響。故設計一具漸縮流道的實驗裝置，調整空氣流量並讓液滴通過之，觀察其接觸到流場所產生的變形及破碎情形。測量液滴於流道中的位移以求出其相對於流場的速度，並結合各項數據求出其無因次參數，以之比較不同的空氣流量下，加速流場中液滴破碎情形的差異，藉以得到液滴臨界破碎值之變化趨勢。
結果顯示，於不同組分下其相對速度差異雖不大，但其韋伯數之差異卻可高達三倍以上：例如當下流量刻度皆為180 L/Min的情況下，其出口端的相對速度皆於25-30m/s左右，但韋伯數之之差距則大上很多，水與甘油約在10~15之間，而乙醇、正庚烷、正十六烷則在25~30之間。而於漸縮加速流場下的液滴破碎情形，基本上其臨界破碎韋伯數會比利用震波管的實驗機構所做的液滴破碎研究還來的低，把韋伯數與其流量比對，發現當流量越小，其破碎點越接近出口時，臨界韋伯數會有小幅度的下降趨勢。而韋伯數與On數之關係中，也可以發現臨界液滴破碎的幅度也有隨著On數調低而小幅下降的趨勢。
此外，本實驗也觀察到液滴的破碎現象於漸縮加速流道內具有環狀結構前覆的情形，而當破碎當下距離流道下端出口越近，則該現象越明顯。

關鍵字

液滴 ； 破碎 ； 韋伯數

並列摘要

Experimental applies the breakup mechanism of droplets encounter high speed air flow, and observe how acceleration influent the effect. To match this purpose, we design the device which have narrowing channel to allow air flow passing through, then let the droplets dripping into the channel to observe drop deformation and breakup when they contact with flow. To obtain the trend of the critical fragmentation value of droplet, we measure the displacement to find the relative velocity, combine with the other data to calculate the dimensionless parameters, and compare the difference about the situation of drop breakup in acceleration flow field with several air flow rate.
In our experiment results shows not much difference in relative velocity, but the differences of Weber number can be up to more than twice. For example, when the scale of air flow rate are 180L/min, the relative velocity are separated about 25~30m/s, but the differences of Weber number are much larger: Weber number of water and glycerol solution are about 10~15, and ethanol, n-heptane, n-hexadecane are about 25~30. About drop breakup situation in narrowing acceleration flow field, basically its critical weber number are lower than the other research using shock tube. Comparing with Weber number and flow rate, we discover that the flow is smaller, the breakup point would close the outlet of the channel, and the critical weber number would be a slight downward trend. In addition, the relations between weber number and ohnesorge number can be found that the magnitude of the critical weber number has a slight downward trend with the lowering of the ohnesorge number.
Furthermore, our experiment also found that the phenomenon of drop breakup in narrowing acceleration channel has circumstances about droplets which edge structure are contract forward. The phenomenon is more obvious when breakup point are close to the flow channel export.

並列關鍵字

droplets ； breakup ； weber number

參考文獻

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