雙液滴碰撞中彈開與斷裂之研究

本研究利用實驗、數值及理論方法探討雙液滴碰撞後的彈開即斷裂行為。根據前人研究，液滴碰撞結果可依韋伯數(Weber number)及撞擊參數(impact parameter) 分為六大區域：(一)液滴輕微變型後結合、(二)彈開、(三)液滴顯著變型後結合、(四)液滴反射分離、(五)液滴拉伸分離、(六)旋轉分離。實驗研究中我們發現液滴於對稱碰撞反射分離後至少會產生一顆衛星液滴，而透過增加雙液滴的大小差異可以將此衛星液滴消除。數值模擬指出液滴破裂前，其中心附近的流場幾乎停滯，且自由液面厚度的縮減遵守普遍性的標度律 (universal scaling law)，並以非對稱型態破裂。此規則使得液滴不可能於中心破裂，並且破裂時必須有兩個鏡射的破裂點以維持對稱性，因此衛星液滴必然產生。液滴偏心碰撞後旋轉分離的部分，我們透過數值模擬得知，此分離型態下的雙液滴質量交換比其在反射及拉伸分離還多，另外數值模擬亦呈現碰撞過程中，短暫結合的液滴內存在一相互作用區及滑移區，相互作用區的質量做震盪運動，而滑移區的質量則繞著液滴中心旋轉。當此兩區的流體於二次匯合時合動量大於表面力的束縛，旋轉分離隨即產生。關於液滴碰撞後彈開，我們發現除了提高環境氣壓，加增液滴直徑亦可以有效地使發生彈開的韋伯數範圍增加;而減少液滴直徑則可使彈開區縮小，甚至使烷類液滴的彈開區在正撞的情況下消失。我們更從尺度分析中 (scaling analysis) 找到無因次參數(〖Oh〗_g,〖Oh〗_l,A*)，可以用來分辨是否可以在碰撞結果圖找到完全發展的彈開區。

關鍵字

液滴碰撞；結合；彈開；反射分離；液滴直徑；旋轉分離；表面張力

並列摘要

In this dissertation, the mechanisms of bouncing, coalescence, and breakup in binary droplet collisions are investigated. For the droplet breakup in symmetrical head-on collision, satellite droplet formation has been studied by numerical simulations and experiments. The simulations have demonstrated that the tentatively coalesced drop cannot break at the center due to a nearly-motionless zone produced around the initial impact point. On the other hand, pinching dynamics of the thinning necks follow the universal scaling theories of a thinning liquid filament, showing the nature of asymmetrical pinch-off for free surface flow and thus permits the formation of a satellite droplet upon breakup. Moreover, we have found an effective way to eliminate satellite droplets, providing a possible interpretation of the discrepancy between previous studies. Regarding the separations generated in off-center collisions, the mechanism of a newly found rotational separation (VI) has been investigated by experiments compared with numerical simulations and a theoretical model. The experiments have shown that the territory of the regime (VI) is separated from stretching separation (V), which can be influenced by (We,B,〖Oh〗_l ) and eliminated when 〖Oh〗_l≥ 0.592. The simulations have further demonstrated that in the temporarily coalesced drop, the stronger coupling of the reflective and rotational flow motions contributes to the occurrence of rotational separation. Supported by the theoretical model based on momentum theory and the present simulations, the non-monotonic transitions of coalescence and separations in the off-center collision can be further elucidated. In the last part, the key parameters governing droplet bouncing and coalescence are investigated by experiments and scaling analysis. Previous studies on bouncing and coalescence have shown that suppressing the rebound of two hydrocarbon drops can only be achieved by reducing the ambient pressure. Here, we have demonstrated that bouncing in the head-on collision can be created by increasing droplet diameter and discouraged by decreasing it. Theoretically, we have derived a thickness-based criterion and found the dimensionless groups (〖Oh〗_g,〖Oh〗_l,A*) to determine whether the bouncing regime can occur in the head-on collision for any fluid properties. It has been found that as (1.2〖Oh〗_g)/(1-2〖Oh〗_l )>∛(A*), the bouncing regime can emerge in the head-on collision, reducing the coalescence efficiency in droplet collision.

並列關鍵字

droplet collision ； coalescence ； bouncing ； surface tension ； droplet diameter

參考文獻

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雙液滴碰撞中彈開與斷裂之研究

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