Experimental Study on Generation of Single Cavitation Bubble and Its Collapse Behavior
|關鍵字:||穴蝕氣泡;Kelvin-Helmholtz渦流;滯流環;逆向噴流;液體噴流;cavitation bubble;Kelvin–Helmholtz vortex;stagnation ring;counter jet;liquid jet|
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.
|Appears in Collections:||Thesis|
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