【作者】 徐鹏；

【导师】 刘树红；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2024， 博士

【摘要】 空化及其伴随的压力脉动、空化泡射流等对水力机械系统的安全高效运行具有直接影响,因此从机理层面,提出不同情况下空化发生的判断条件、建立空化产生后的生长溃灭过程分析模型,对于空化的控制和利用十分重要。本文针对工程实际中不同场景下的两类空化,分别就局部加速度诱发的管内空化泡问题,以及质量力加速度影响下的空化泡非球形溃灭问题开展了基础研究。首先,针对瞬态过程液体局部加速度诱发的空化,本文根据量纲分析改进了刹管法实验装置,实现了关键参数之间的解耦,并结合高速摄像和水听器进行了系统实验和观测。理论分析表明空化泡的产生和最大尺寸分别由无量纲空化数(6₁和(6₂控制,结合两个空化数可以提出空化相图用于预测无空化、小空化泡和大空化泡三种空化结果。与此同时,本文建立了该过程管内柱状空化泡的动力学方程,该方程可以很好地描述实验中空化泡的生长溃灭特点,且结合水锤理论对柱状空化泡的溃灭速度和溃灭时的破坏性进行了讨论和验证。其次,在改进的刹管法实验基础上,本文通过预置微小空气泡的方式,讨论了含空气泡时局部加速度诱发的管内多位置空化问题。实验中观察到了无空化、仅空气泡位置空化、仅管底位置空化和两位置均空化的四种情况。基于理论分析,发现了各位置初始空化核尺寸和相应液柱长度决定其空化发生条件的机理,提出了预测空化发生位置的空化相图。基于壁面以及空化泡之间的相互影响,获得了Keller-Miksis修正方程,建立了局部加速度激励下的管内双空化核响应模型。此外,针对定常过程质量力加速度对空化泡非球形溃灭的影响,本文分别搭建了静止减压水箱和旋转圆桶内的两个激光空化泡实验台,研究了从常重力加速度到最大50倍超重力加速度条件下的空化泡动力学行为,发现了固壁面和质量力加速度对空化泡变形影响的差异。建立了基于无量纲开尔文冲量的分析模型,获得了空化泡非球形溃灭时各量化参数与无量纲开尔文冲量之间的标度律关系。综上所述,本文揭示了局部加速度诱发空化的条件以及相应的空化泡动力学规律;深入研究了质量力加速度对空化泡非球形溃灭特性的影响,并建立了相应的标度律关系,为预测空化的产生以及降低空化的破坏性提供了理论支撑。更多还原

【Abstract】 Cavitation involves destructive bubble collapses and the subsequent formation of micro-jets.The above process is accompanied by intense pressure fluctuation,which has a direct impact on the safe and efficient operation of hydraulic machinery systems.Therefore,from the mechanistic level,it is crucial to clarify the criteria of cavitation on-set and establish the corresponding analysis models of bubble dynamics under different conditions.This thesis focuses on fundamental research of two types of cavitation in en-gineering practice,specifically addressing the issue of cavitation bubble（s）induced by the local acceleration within a tube and the non-spherical collapse of cavitation bubble under the influence of body force acceleration.Firstly,for the cavitation induced by local acceleration in transient process,this the-sis improves the“tube arrest”experimental apparatures based on dimensional analysis,achieving decoupling among key parameters.Systematic experiments and observations are conducted using high-speed cameras and the hydrophone.Theoretical analysis indi-cates that the generation and maximum size of the cavitation bubble are controlled by dimensionless cavitation numbers(6₁and(6₂,respectively.Combining the two cavi-tation numbers,a cavitation phase diagram can be proposed to predict three kinds of cav-itation results:no cavitation,small cavitation bubble（s）and large cavitation bubble（s）.Simultaneously,the dynamic equation of the cylindrical cavitation bubble in the tube is established,which effectively describes the growth and collapse characteristics of the cavitation bubble in the experiments.Combined with water hammer theory,the collapse velocity of the cylindrical cavitation bubble and its destructive feature are discussed and verified in this thesis.Secondly,based on the improved tube arrest apparatus,this thesis discusses the multi cavitation bubbles generated at different locations in the tube,where a tiny air bubble is seeded in the liquid column.Four types of cavitation results are observed in the experi-ments:no cavitation,cavitation only occurs at the air bubble position,cavitation occurs at the tube bottom,and cavitation occurs at both positions.Theoretical analysis reveals that the initial size of the cavitation nuclei at each location and the corresponding liquid column length determine the conditions for cavitation occurrence.Here a cavitation phase diagram is proposed to predict the locations of cavitation onset according to the cavitation number and its critical value.Considering the influence of the tube wall and the other cav-itation bubble,a Keller-Miksis modified equation is obtained,and a response model for dual cavitation nuclei within the tube under local acceleration excitation is established.Additionally,to investigate the influence of body force acceleration on the non-spherical collapse of cavitation bubble in steady flow,this thesis designs and establishes two laser-induced cavitation bubble test platforms,including the static decompression wa-ter tank platform and the rotating cylinder water tank platform.The dynamic behavior of cavitation bubbles is studied under conditions ranging from normal gravity acceleration to a maximum of 50 times super-gravity acceleration.Differences in the effects of rigid boundary and body force acceleration on cavitation bubble deformation are found.An analytical model based on the dimensionless Kelvin impulse is established,and scaling laws are obtained between various quantitative parameters and the dimensionless Kelvin impulse during the non-spherical collapse of cavitation bubbles.In summary,this thesis reveals the conditions for cavitation induced by local accel-eration and the corresponding cavitation bubble dynamics.It also conducts a thorough investigation into the influence of body force acceleration on the non-spherical collapse characteristics of cavitation bubble and establishes corresponding scaling laws.These findings provide theoretical support for predicting the occurrence of cavitation and re-ducing its destructive effects.更多还原