Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11462019)
Received Date:05 August 2019
Accepted Date:23 September 2019
Available Online:26 November 2019
Published Online:05 December 2019
Abstract:In previous papers, researchers generally adopted a simplified assumption that there is a single type of bubble in liquid. In this paper, two simplified bubbles models are established based on the shape and structure of cavitation bubbles formed in ultrasonic field, i.e. linear model consisting of three bubbles and cluster model composed of five bubbles, and each model is assumed to consist of two types of bubbles. The influences of large bubbles on cavitation effect of small bubble in the middle for three- and five-bubble model are numerically studied by using the modified Keller-Miksis equation and van der Waals equation. The changes of the expansion radius of the small bubble in the middle, the temperature in the small bubble and the secondary Bjerknes force between large bubble and small bubble with the increase of the initial radius of large bubbles are mainly investigated. In the calculation, it is assumed that the locations of bubbles stay unchanged and the shape of bubbles remains spherical in the oscillation process. Meanwhile, the time-delay effect on the secondary Bjerknes force can be neglected because the sound wave speed is very fast in water and the bubbles in the liquid can oscillate synchronously. The calculation results show that the expansion of small bubble in the middle can be completely suppressed or delayed when the initial radius and the number of bubbles are appropriate on condition that the distance between large and small bubble, the driving sound pressure and frequency of the applied sound field stay unchanged. The main reason for delayed expansion is that the total pressure acting on small bubbles is delayed to form a negative pressure in the process of the change. The collapse time of delayed expansion of small bubble is basically the same as that of large bubbles, which makes small bubble collapsed rapidly and thoroughly under the strong radiative barotropic effect of large bubble's collapse, and leads the maximum temperature in the small bubble to be higher than that of the single bubble with the same initial radius and the small bubble with normal expansion. In addition, when the expansion of small bubble is delayed, the secondary Bjerknes force between the large bubble and the small bubble appears to be repulsive first and attractive then. This rule is different from the change rule of the secondary Bjerknes force between the two bubbles in normal expansion. The results in this paper have significant theoretical guidance for specific problems such as suppressing liquid cavitation or enhancing liquid cavitation effect by the manual injection of large bubbles. Keywords:multi-bubbles model/ delayed expansion/ bubble temperature/ secondary Bjerknes force
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2.1.两种多泡模型
在空化多泡声致发光试验中可观察到, 在超声场下形成的空化多泡的大小是不均等的, 且在各方面作用力的平衡下, 形成线状或团状等一些特殊稳定结构. 文献[14]研究了线状气泡链模型和球状气泡群模型, 但模型中各气泡的大小都是均等的, 即均匀气泡模型. 然而, 事实证明气泡大小的非均匀性对气泡空化特性的影响是显著而重要的, 所以文献[11,12]研究了大小不同的两个气泡组成的双泡系统. 双泡系统是最简单的多泡模型, 由于过于简单而有时候不能反映多泡系统的一些重要空化特性. 因此, 本文要建立由三个球形气泡组成的简单线状模型和五个球形气泡组成的简单团状模型(如图1所示). 在两种模型中忽略了各气泡的空间移动, 即认为气泡中心位置不变并始终保持球形, 中间小气泡的初始半径为$ R_{10} $, 周围大气泡的初始半径都为$ R_{20} $, 中间小气泡与周围大气泡之间的距离都是$ D $. 此模型虽也是较简单的多泡模型, 但对于液体中人工注入几个大气泡来抑制或增强液体空化问题[9], 以及液体中有些小气泡融合形成几个大气泡而影响周围小气泡的空化效应问题[21,22] 等一些实际问题具有重要的研究意义. 图 1 三泡和五泡模型 Figure1. Three-bubble model and five-bubble model