液滴撞擊有邊界液面所產生大氣泡之研究

本研究探討液滴撞擊在有邊界限制下的水面，大氣泡型態(Large Bubble)的形成與發展此邊界限制指的是液滴滴入深液時，其撞擊點周圍在動量傳遞的過程有無法忽略之環型壁面限制，透過找出有/無邊界限制下水漥發展的不同特性差異，嘗試找出影響大氣泡型態生成機制的關鍵參數。本文的第一部分首先將水漥上半部分(頸部產生區)加上固定的邊界限制，比較此條件下生成的大氣泡與無邊界狀態者所對應的液滴振盪形狀參數和發生速度範圍之關係，由此得知大氣泡生成所對應液滴撞擊速度範圍確有明顯的差異，而在不同撞擊速度下，所產生的水窪尺寸與氣泡大小也不一樣。第二部分則將限制環的位置從包圍限制水窪的上半部分一直持續降至其下半部分(底部擴張區)，甚至將限制環上緣完全浸沒於水平面下，藉此歸納出不同限制邊界位置的，對於大氣泡產生的助益程度影響。本文將有邊界限制下大氣泡產生的速度範圍大於無邊界狀態者，稱為建設區，反之稱為謂破壞區；在不同的相對位置會形成三大特性區域: 建設區、顯著建設區和破壞區。在本文第三部分所進行的定量分析，則對有/無邊界限制下的大氣泡生成差異性及三大特性區域的成因提出可能的物理解釋。最後，本文發現限制環上緣深度為零的深度值，可產生最大的氣泡產生範圍，且打破過往間大氣泡間斷式分布的生成規則 : 可以連續式的生成。而在限制環長度的效應中可發現 l/d ≈ 2 以後，所產生的大氣泡增益效果以似乎最大。而以時間尺度來看，產生大氣泡過程，水漥頸縮的時間水平時間尺度所占水漥向下發展的時間(垂直時間尺度)的比例會隨著速度的增加而縮小(從 50%縮至 20 %)。

關鍵字

限制環；頸部產生區；底部擴張；上緣深度；建設區；顯著建設區；破壞；液滴形狀參數

並列摘要

The study focused on the investigation of large bubble entrainment induced by a drop impacting upon the bounded liquid surface. We used a ring-shaped wall as the restricted boundary of the target liquid in which the impact point the drop touch down is in the center. Comparing the cavity development for bounded liquid surface with that for free surface, the key parameters of large bubble entrainment can be revealed. First of all, the ring-shaped wall was set at the position with respect to the upper part of cavity. The occurrence range of large bubble entrainment can be varied with the phases of drop shape and the target liquids with / without being bounded. In addition, the size of the bubbles can be also different with various impact velocities. Secondly, the ring-shaped wall has been vertically moved along the impact line to affect the development of cavity. It has been found that there are major differences of cavity evolution between the cases by utilizing different positions, lengths, and inner diameters of the ring-shaped wall. Three main effects of rings have been found, “constructive influence”, ”significant constructive influence” as well as “destructive influence”, respectively. Thirdly, in order to gain the insight into the generation mechanism of large bubble entrainment, we used high-speed camera to capture the cavity profiles and to analyze the data quantitatively with the techniques of image processing. Consequently, we have determined an optimal position and geometry of the ring-shaped wall, exploring the occurrence distribution of large bubble is incredibly large and ever continuous instead of discontinuous as previously thought. It has been concluded that effects of wall at certain positions could be benefial to large bubble formation and reduce the requirement of specific drop shapes.

並列關鍵字

restriction ring ； neck ； lower cavity ； constructive influence ； significant constructive influence ； destructive influence ； droplet shape parameter