單一穴蝕氣泡產生與破裂行為之試驗研究

Abstract

本研究利用旋轉U型平台產生單一穴蝕氣泡，藉由壓力震波分析氣泡破裂過程於固體邊界附近所產生之流場特性與氣泡破裂過程中形成逆向噴流之結構。單一氣泡的產生試驗係利用透明圓柱管或長方形管放置於旋轉U型平台設備上，並橫跨過轉軸中心，試管內加入適當之自來水，開始旋轉U型平台，使轉軸中心附近之試管壓力降低於蒸汽壓力產生球狀體與扁平單一穴蝕氣泡。此氣泡形成後，靜水壓力不足以使氣泡破裂。穴蝕氣泡受到氣泡周邊壓力之擠壓而破裂，為得到與實際情況相似，藉由脈衝裝置所產生之震波壓力擊破氣泡，同時以高速攝影機拍攝瞬間影像，記錄不同位置之氣泡破裂過程。此外，亦藉由質點影像測速法計算氣泡破裂過程之速度流場特性。 於扁平穴蝕氣泡破裂試驗中發現，受壓後產生液體噴流直接突破氣泡表面，產生第一次氣泡破裂，但無法產生滯流環與逆向噴流現象。此與球狀體氣泡之液體噴流突破表面後產生噴流，進而發展成蕈狀雲氣泡與Kelvin-Helmholtz渦流現象有所差異。對扁平氣泡而言，逆向噴流在氣泡中心距離固體邊界的位置介於1至3倍氣泡半徑之間無法形成。 另一方面，球狀體穴蝕氣泡當氣泡中心距離固體邊界的位置介於1至3倍氣泡半徑之間時，液體噴流或Kelvin-Helmholtz渦流碰至固體後，在固體邊界上形成滯流環，使氣泡表面經過滯流環向內擠壓形成逆向噴流，隨後氣泡破裂。在氣泡中心距離固體邊界3倍氣泡半徑之臨界值時，氣泡變形隨著震波強度而有所差異，其氣泡破裂過程可產生Kelvin–Helmholtz vortex、Richtmyer–Meshkov不穩定性或逆向噴流等現象。另一個臨界值為氣泡中心距離固體邊界等於1倍氣泡半徑之時，液體噴流碰至固體邊界後以輻射方向向外形成噴流，而沒有形成滯流環與逆向噴流。穴蝕氣泡破裂過程之各種複雜流場現象，均可清楚呈現於本研究中。

This study utilizes a U-shape platform device to generate a single cavitation bubble for a detailed analysis of the flow field characteristics and the cause of the counter jet during the process of bubble collapse caused by sending a pressure wave near the solid boundary. During the experiment of generating a single cavitation bubble, each utilized the transparent cylindrical tube or the rectangular tube on the U-shape platform is filled with tap water and cross the central axis. When angular velocity is gradually increased, the pressure at the center of the rotation in the tube is gradually decreased to a saturated vapor pressure at local water temperature. At this condition, a spherical or a flat shape single cavitation bubble near the rotating axis can be generated. After the cavitation bubble is generated, the U-shape platform is stopped to restore the pressure back to the hydrostatic pressure. This pressure difference alone is not enough to collapse the cavitation bubble. The major cause of the cavitation bubble collapse is the surrounding pressure of the fluid to squeeze the bubble and result in its collapse. To observe the flow field of the collapse of the cavitation bubble, this study uses a pulse setup to hit the piston of the tube in contact with the free water surface and instantly generates a shock wave pressure that sends a pulse to cause the collapse of the cavitation bubble. A high speed camera is used to record the flow field of the bubble collapse at different distances from a solid boundary. In addition, the study is also used the particle image velocimetry method to calculate the characters of velocity flow field during the bubble collapse. In the flat shape bubble collapse experiments detect that the bubble produce the first time collapse when a liquid jet penetrates the bubble surface after the bubble is compressed and deformed. The flat shape bubble was not to produce the stagnation ring and the counter jet. It is different from the spherical shape bubble that a mushroom shape bubble and a Kelvin–Helmholtz vortex are formed when a liquid jet penetrates the bubble surface. Therefore, for a flat shape bubble collapse process with the formation of the counter jet phenomenon cannot be found when the bubble center to the solid boundary is within one to three times the bubble’s radius. On other hand, for the spherical shape bubble with on the bubble center to the solid boundary being within one to three times the bubble’s radius, a stagnation ring will form on the boundary when impinged by the liquid jet or Kelvin–Helmholtz vortex . The fluid inside the stagnation ring will be squeezed toward the center of the ring to form a counter jet after the bubble collapses. At the critical position, where the bubble center from the solid boundary is about three times the bubble’s radius, the bubble collapse flow will vary. Depending on the strengths of the pressure waves applied, the collapse can produce a Kelvin–Helmholtz vortex, the Richtmyer–Meshkov instability, or the generation of a counter jet flow. If the bubble surface is in contact with the solid boundary, the liquid jet can only move inside-out without producing the stagnation ring and the counter jet; thus, the bubble collapses along the radial direction. The complex phenomenon of cavitation bubble collapse flows is clearly manifested in this study.