激光空泡与油膜-水界面相互作用的实验研究

Experimental Study on Interaction Between Laser-induced Cavitation Bubble and Oil Film-Water Interface

采用激光诱导方法在油膜-水的液-液界面附近生成单个空泡,利用两台高速相机双视角同步记录空泡溃灭过程和油膜上下界面的演化过程,探究无量纲距离γ、油膜厚度D对空泡溃灭特性和油膜形态演化的影响。结果表明,油膜-水界面附近生成的空泡在溃灭过程中会不断远离界面,并且产生背向界面的高速射流,空泡质心位移和回弹半径与各向异性参数ζ呈对数函数关系。在空泡溃灭影响下,油膜下界面受到扰动发生不同程度的形态演化,而油膜上界面则在空泡溃灭结束后产生变形。油膜上界面存在驼峰射流、细射流、阶梯射流三种典型模式,油膜下界面则存在微扰动、倒丘型变形、锥状变形三种典型模式。在驼峰射流模式中,射流无量纲最大高度hm和γ呈幂函数关系。

Objective Cavitation near a liquid-liquid interface is a well-known and an important phenomenon owing to the wide application of cavitation bubbles in emulsion preparation,wastewater treatment,petroleum refining,and other processes.However,very few studies have been conducted to observe bubble dynamics near a liquid-liquid interface,and the interaction between a bubble and the liquid-liquid interface of oil films and water has not been considered in the literature.To gain a better understanding of bubble dynamics and offer an effective way to improve cavitation techniques,this study investigates the dynamics of a cavitation bubble near an oil film-water interface.Methods Dimethyl silicone oil and deionized water are utilized to produce an oil film-water interface,and the pulsed laser-induced method is applied to generate a single cavitation bubble near the interface.To nucleate a single bubble,a laser pulse with a 532-nm wavelength is expanded and collimated by a beam expander,and then focused into a glass tank.A time delay trigger connected to the pulsed laser and two high-speed cameras is used to realize the synchronized recording of the two cameras after firing the laser pulse.Cavitation bubble dynamics near the oil film-water interface and the evolution of the lower interface(liquid-liquid interface) are captured using the high-speed camera with a frame rate of 105 frame/s,whereas the evolution of the upper interface(gas-liquid interface) on a much longer time scale is recorded using a high-speed camera with a frame rate of 2000 frame/s.Results and Discussions The effects of the dimensionless distance γ and oil film thickness D on the cavitation bubble collapse characteristics and interface evolutions are explored.Owing to the existence of the oil film-water interface,the cavitation bubble collapses nonspherically,moves away continuously,and produces a high-speed jet away from the interface under different experimental conditions.The displacement of the cavitation bubble centroid increases logarithmically with an increase in the anisotropic parameter ζ(i.e.,the decrease in γ),resulting from the enhancement of the interaction between the cavitation bubble and liquid-liquid interface.As the total collapse shock wave energy decreases with increasing ζ,more energy is needed to drive the bubble rebound to a larger volume in the secondary period of bubble oscillation;therefore,the relative rebound radius also increases logarithmically with the increase in ζ.The lower interface of the oil film evolves during the bubble collapse,which is mainly caused by the inertia of the bubble oscillation.The upper interface evolves on a much longer time scale owing to the competing effects of interfacial tension,gravity,and viscosity.Three typical modes of liquid jet are observed for the upper interface:hump jet(Fig.6),thin jet(Fig.7),and two-tier jet(Fig.8).Three typical modes of motion are also observed for the lower interface:slight disturbance(Fig.9),inverted hill deformation(Fig.10),and cone deformation(Fig.11).Conclusions In this study,a single cavitation bubble is generated near the liquid-liquid interface of the oil film and water using the pulsed laser-induced method.Two high-speed cameras are used to simultaneously record the cavitation bubble collapse and evolutions of the upper and lower interfaces of the oil film.The results show that the cavitation bubble generated near the oil-water interface continuously moves away from the interface during the process of nonspherical collapse and develops a high-speed jet away from the interface.The relation between the displacement of the cavitation bubble centroid and anisotropic parameter ζ and the relation between the relative rebound radius and ζ are both logarithmic.The lower interface motion of the oil film is observed during the bubble collapse,whereas the upper interface starts to deform after the bubble collapse is over.Three typical liquid jet modes(hump jet,thin jet,and two-tier jet) are observed for the upper interface,and three typical motion modes(slight disturbance,inverted hill deformation,and cone deformation) are observed for the lower interface.In the case of a hump jet,the dimensionless maximum height hm follows the power law,hm ∝ γθ.The observations herein offer a deeper understanding of the cavitation dynamics and provide theoretical guidance for cavitation techniques.