不同表面張力水溶液之高速液滴與薄膜碰撞研究

Abstract

本文首先突破過去文獻中僅能使用低速液滴撞擊液膜，從而建立一套高速且穩定的液滴產生系統，同時在實驗中，使用界面活性劑，去改變純水的表面張力，控制液滴尺寸及液滴瞬時速度，來觀察在不同的韋伯數下，液滴碰撞液膜之物理現象。由本文之結果證實這些努力大幅提高此種實驗結果的重複性及精度，並觀察到液滴在高速下撞擊液膜之物理現象-閉合現象(Closed)。在此吾人使用高速攝影機去拍攝撞擊過程，來了解當中之物理機制及物理意義；本實驗著重於兩個部份，一為高速液滴的生成，如何切截出最佳長度液柱，來拉伸為液滴；另一為工作流體之表面張力之變化對撞擊後之影響；此文為首度將撞擊結果整理成一套數據，來繪出其邊界圖。 由本文之實驗結果發現：高速液滴產生系統，在切截過程中，液柱長度及半徑將影響液滴之生成時間，使得在落下後速度減慢，造成韋伯數下降，因此最佳長度將極為重要；另外對於實驗結果，繪出一分界圖，將所有物理現象隨著韋伯數與無因次薄膜厚度之改變劃分為五個區域(Ⅰ) Merging (Ⅱ) C-J (Ⅲ) C-J and 2nd (Ⅳ) Multiple (Ⅴ) Closed，並特別繪出飛濺現象發生時之分界線，最後改變兩次表面張力，進而討論兩種溶液在撞擊後會否產生與水不同之現象，以及探討三者數據圖之分界趨勢，來了解改變表面張力，對現象之影響。

The main objective of this study is experimental investigation of the droplet-film collision under the conditions of different surface tension and different Weber number. In the experiment, we control the size and the colliding velocity of droplet. To change the surface tension of water, the surfactant was used. In present experimental study, the special attention is given to the generation of high-speed droplets, including which is the best cut-off length of liquid jet in order to form droplets. Secondly, the phenomena of impingement after droplet impacts the liquid film with different surface tension is also in the focus of this investigation. The experimental results of droplet-film impact are presented in this thesis. The dependencies of splashing threshold on different surface tension are obtained. It was found that both the length and the cross-sectional radius of liquid jet significantly affect the forming time of droplets during the cut-off process under high speed conditions. They cause the decrease in the droplet velocity and therefore, the decrease in the Weber number. Base on it, one can conclude that the optimal cut-off length is consequently important. Depend on the changes of Weber number and non-dimensional thickness (H), the five characteristic regimes may be singled out: (I) Merging, (II) C-J, which implies center-jet, (III) C-J and 2nd droplet, (IV) Multiple droplets, and (V) Closed. The influence of surface tension on impinging phenomena is discussed in this thesis.