Research On The Bubble Behaviors In Confined Regions

Cavitation is a phase change phenomenon.When the local pressure is lower than the vapor pressure of the liquid at the current temperature,the local liquid is converted to vapor gas to form a bubble dominated by the liquid vapor.With the change of the external pressure,the bubble generates different behaviors such as expansion,contraction,collapse,etc.The behavior of the bubble causes physical phenomena such as jet and shock wave.With the variation of the confined regions,the behaviors of the bubble are different,especially the geometrical shapes and physical properties of the confined regions domain the deformation behaviors of the bubble.At present,there are many researches focus on the behaviors of the bubbles in free domain and near the wall.However,there is few results of the bubble behaviors in confined regions but it's a common phenomenon in engineering,such as hydraulic valves,pumps,etc.The study of the bubble dynamics in confined regions is of great significance to the understanding of bubble dynamics,cavitation prevention and its application.Therefore,it's urgent to carry out the research.In this thesis,based on the experimental phenomena,the bubble dynamics in different kinds of tubes are studied.The flow visualization,pressure acquisition,theoretical analysis and numerical calculation methods are used to investigate the dynamics of single bubble and double bubbles in a tube.It is carried out a systematic and in-depth research.It is found that the cross-sectional shapes,different diameter variations and different elastic modulus cause different deformations of bubbles.Based on the stepped pressure model,the numerical models of the bubble dynamics in defferent tubes are established separately.The variations and morphological evolutions of the bubble in different kinds of tubes are analyzed.The secondary cavitation phenomenon is found in experiments,which is the cavitation in other areas caused by the first cavitation.This thesis obtains the coupling relationship between the secondary cavitation and first cavitation with the help of the pressure acquisition and high-speed photography technologies.By analysis of the effects of the cross-sectional shapes,diameter variations,bifurcation forms and elastic modulus on secondary cavitation,the relationship between the secondary cavitation and first cavitation are revealed in this paper.By observing the interactions of two bubbles in a tube,the bubble deformation,collapsing jet are analyzed and this thesis reveals the influence of bubble spacing,bubble sizes and phase difference on the patterns of the bubble jet,collapsing jet.A regional division graph of cavitation deformation,collapsing jet is summarized.The outline of this thesis is as follows,In chapter 1,introduction,the research background and significance of the thesis are proposed.The reviews on the history and present situration of the cavitation bubble dynamics are introduced.The researches on the dynamics of single bubble and two bubbles in confined regions are presented.At the end,the main research contents,research methods and research line of this thesis are summarized.In chapter 2,the foundation of the bubble dynamics,numerical simulation and experimental systems,the ideal equation of the spherical bubble dynamics is expounded.Based on Rayleigh-Plesset eqution,the deformation of one spherical bubble in free domain is calculated numerically.The effect of surface tension and viscosity on bubble behaviors is discussed.A one-dimensional numerical model of bubble movement is established by using the stepped pressure model for the bubble dynamics in a tube.The deformation of single bubble in a tube is numerically calculated and the influence of the axial position on the collapsing jet is studied.Four different kinds of methods producing cavitation bubbles are described and the advantages and disadvantages of them are compared.The safe and reliable low-voltage electric spark method is adopted to produce bubbles in this thesis.The adopted experimental apparatus is described.The experiments of single bubble in free domain are perfomed.The comparison of the experimental and the numerical results verify the feasibility of the experimental scheme.In chapter 3,the behavior of single bubble in a circular tube,the high-speed photography and pressure acquisition technology are used to study the bubble behaviors in tubes with circular cross-section,including the behavior of single bubble in a rigid tube,elastic tube and the tube with a gas column.Based on the one-dimensional model of bubble deformation in a tube,the equation of the bubble behaviors in the tube is established.The deformation of bubble in a tube is numerically calculated and compared with the experimental results.The secondary cavitation phenomen induced by the collapse of the first bubble in the tube is observed.The position and period relationships of the secondary cavitation and the first cavitation are analyzed quantitatively.The shock energy transfer model in a tube is established.Based on the water-hammer pressure model,the effect of the tube parameters such as diameter,thickness and Young's modulus on the collapsing peak pressure of the bubble is revealed.The relationship between the tube parameters and the secondary cavitation is obtained.In chapter 4,the behavior of single bubble in special-shaped tubes,the high-speed photography technology is used to study the bubble behaviors in tubes with rectangular cross-section,variable diameters,elbow and bifurcations.Based on the one-dimensional model of a bubble in a tube,the models of the bubble behavior in above tubes are established respectively.The deformations of bubbles in different tubes are calculated numerically and compared with the experimental results separately.It is found that the numerical model can precict the bubble deformation and collapsing-jet accurately.The relationships between the secondary cavitation and the first cavitation are summarized by observing the secondary cavitation phenomenon in the tubes.In chapter 5,the interactions between two bubbles in a tube,the interactions between two vapor bubbles in a tube are studied.The types of two bubble behaviors are classified based on the dimensionless distance between two bubbles ?,the sizes of bubbles kL and ks and the phase difference of bubbles ??.Based on the above dimensionless parameters,a region division graph of the bubble deformation and collapsing jet is summarized.In chapter 6,summaries and prospections,the main research contents of the thesis are summarized.The further work is prospected.