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淹没空化射流具有很强的空化冲蚀能力,在石油钻井、储层改造等方面有着广泛的应用,空化射流中空泡云的动态变化与射流冲蚀过程密切相关。本文开展了高速摄影拍摄淹没空化射流实验,利用图像差分法对空泡云动态规律进行了深入分析,讨论了空泡云周期性脱落的机理,并首次研究了变量类型(空泡云长度、面积、宽度、射流中心点灰度值、灰度均值)和取值位置对确定空泡云脱落频率的影响,优化了确定空泡云脱落频率的方法。研究结果表明,淹没空化射流中的空泡云具有明显的周期性,一个周期主要包括产生—发展—脱落—溃灭4个阶段。空泡云周期性脱落与剪切层内涡的周期性脱落、喷嘴结构以及空泡云导致的喷嘴过流面积变化(“憋压效应”)有关。灰度均值是计算空泡云脱落频率的最佳变量,只有在射流中游一定区域内才能得到一致稳定的脱落频率。本研究有助于加深对淹没空化射流中空泡云动态变化规律的认识,为优化喷嘴结构建立基础。
Submerged cavitation jet has a strong ability of cavitation erosion, which has been widely used in oil drilling and reservoir reconstruction. The dynamic change of hollow bubble cloud in cavitation jet is closely related to jet erosion process. In this paper, a high-speed photographic shooting submerged cavitation jet experiment was carried out, and the image difference method was used to analyze the dynamics of the bubble cloud. The mechanism of cyclical shedding of the bubble cloud was discussed. The variable types (bubble cloud length, Area, width, jet center gray value, gray mean value) and the location of the value to determine the shedding frequency of the bubble cloud to optimize the method to determine the frequency of the bubble cloud shedding. The results show that the cavitation cloud submerged in cavitation jet has obvious periodicity, and one period mainly includes four stages of genesis-development-detachment-collapse. The cyclical shedding of bubble clouds is related to the periodic shedding of vortices in the shear layer, the nozzle structure, and the nozzle overcurrent change due to the bubble cloud ( “pressure-holding effect ”). The gray mean is the best variable to calculate the shedding frequency of the vacuole cloud. Only in a certain area of the middle reaches of the jet can a consistent and stable shedding frequency be obtained. This study is helpful to deepen the understanding of the law of dynamic change of hollow bubble cloud in submerged cavitation jet and establish the foundation for optimizing nozzle structure.