不同管徑中單一穴蝕氣泡破裂行為之試驗研究

Abstract

本研究利用三組管徑不同L型透明圓管放置於U型平台上旋轉，並改變轉軸轉速，以得到不同大小單一穴蝕氣泡。藉由調整氣泡與固體邊界間的距離，利用壓力震波將試管內氣泡震破，氣泡破裂後會於氣泡與固體邊界中間形成低流速、高壓力情況，並藉此分析氣泡的破裂行為對固體邊界之影響。 本研究使用高速攝影機，以每秒8000張連續拍攝氣泡破裂過程的影像做分析，分析各種不同間距氣泡破裂產生流場變化對固體邊界之影響。並以質點影像測速法，進行流場演變過程量測與速度場分析。 根據分析結果顯示，氣泡受震波擠壓破裂，由於表面張力與固體邊界牽引而導致流場紊亂，紊亂處有密度差異引起之Rayleigh-Taylor不穩定、受震波壓力產生之Kelvin-Helmholtz不穩定渦流與液體界面剪應力不同而有Richtmyer-Meshkov不穩定現象發生。 實驗結果發現，距離參數γ值為2(即距離固體邊界正好一個氣泡大小的距離)時，壓力量測恰是該組試驗成果最低值，渦度場分析亦為該組試驗成果最大值，故若能控制管路內氣泡破裂於距離固體邊界正好一個氣泡大小的位置，則對固體邊界之破壞程度較小。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 utilized three different diameter pipes filled with water, to generat different size single cavitation bubbles by changing the U-shape platform’s angular velocity. When angular velocity was increased, the pressure at the center of the rotation in the pipe was decreased. The single bubble was collapsed by triggering a pressure pulse under different distance between bubble and solid boundary. During the experiment, the study analyzed the zone which has low velocity and high pressure after bubble collapse. A high speed camera (CCD) which can take 8000 images in one second was adopted to take sequential images of the bubble collapse process and calculated velocity fields with Particle Image Velocity (PIV) theory. According to the surface tension and attraction by the solid boundary, the study observed the Rayleigh-Taylor phenomenon (caused by gravity), Kelvn-Helmholtz vortex phenomenon (caused by shock wave), Richtmyer-Helmholtz instibility phenomenon (caused by shear stress) when bubble collapsed. Experimental results show when the distance parameter equals to 2 (which means the bubble exactly far from the solid boundary a diameter away) smallest pressure and biggest vortex value can be observed. So it would make less damage due to bubble collapse by controlling the distance between the bubble and the rigid boundary in the pipe. In addition, the complex phenomena of the collapsing flows of single cavitation bubble were presented and analyzed in this study.