Non-wetting Wall Ultrasonic Cavitation Micro-destruction Mechanism

Cavitation cavitation problem seriously affects the operational efficiency and stability of flood dissipation facilities and hydraulic machinery,and is the frontier and difficult point of research in water engineering and fluid mechanics.The current cavitation theory at the fine scale is not quantitative,comprehensive and mature enough.The use of wall wettability to guide cavitation bubble collapse and weaken cavitation bubble collapse strength to achieve corrosion reduction is a hot spot in the research and development of anti-cavitation materials.In this thesis,the effect of cavitation source to wall distance and wall wettability on cavitation cavitation intensity is investigated mainly by physical model experimental study.The main research contents and conclusions are as follows.1.The vertical distance between the ultrasonic source probe and the specimen was adjusted,and the surfaces of different metallic materials(bronze and aluminum)after cavitation at different ultrasonic source distances were photographed and observed with the aid of a scanning electron microscope.It was found that the perpendicular action distance between the ultrasonic source probe and the material surface had a significant effect on the cavitation effect of both specimens.After experiencing the maximum cavitation time,when the distance of the ultrasonic source was 5 mm,there were still many unaffected areas on the surface of the specimen by cavitation,and the degree of cavitation damage was relatively weak.When the distance of the cavitation source increased,the cavitation effect on the specimen surface intensified and the roughness increased.The ultrasonic source distance of 2 mm is the best cavitation distance for this experiment,and the cavitation effect is the best at this time.In terms of the number of cavitation pits,when the cavitation source distance is larger,the peak of the number of pits all appear at the maximum cavitation time,while when the cavitation source distance is smaller,the peak of the number of pits all appear earlier.2.By adjusting different cavitation times,it was found that the time of ultrasonic cavitation also had a great influence on the cavitation cavitation results.At a fixed distance from the ultrasonic source,the area of bronze specimens subjected to cavitation damage increased nearly linearly with increasing cavitation time,and the growth rate of cavitation area of pure aluminum specimens became faster and faster with increasing cavitation time due to the protection of alumina film on the surface.3.Using trichloro(1H,1H,2H,2H-tridecafluoro-n-octyl)silane solvent to form hydrophobic coatings on the surfaces of bronze and pure aluminum specimens,it was found that the results of ultrasonic cavitation on different wettability surfaces were different.At the same cavitation time,the degree of surface damage obeyed the following order: bronze(sparse)> bronze > pure aluminum(sparse)> pure aluminum.Although different degrees of cavitation damage were observed on different wettability surfaces,the above results confirm that higher contact angles lead to lower energy barriers,and lower energy barriers lead to an increased probability of cavitation occurrence.4.According to the results of electron scanning microscopy,the development of cavitation can be divided into four stages.In the first stage,the bubbles break up randomly and scattered needle-like craters appear on the surface of the specimen.In the second and third stages,the bubbles collapse at the existing craters,and then the craters appear interconnected on the surface.In the fourth stage,cavitation develops vertically and horizontally under the influence of bubble rupture,and the specimen surface suffers severe cavitation damage.5.Based on the LBM thermodynamic model,a double distribution function thermal Boltzmann method(DDF)is used to analyze the forces at the contact point between a single droplet and the wall,and it is found that the local pressure difference and the unbalanced Young’s force are the two main controlling factors of the contact point dynamics.In addition,the dimensionless temperature η is introduced to describe the heat transfer efficiency at different wall wettability and wall temperatures.Throughout the evolution,the unbalanced Young’s force on the hydrophilic wall is directed to the interior of the bubble,which leads to a smaller contact radius and a stronger collapse strength.In addition,significant pressure differences are observed at the contact points due to the local concentration of momentum.On the contrary,hydrophobic wall surfaces are more suitable for wall heating processes because of the smaller collapse strength and larger contact radius.The research results of this thesis deepen the understanding of the cavitation mechanism and can provide theoretical and technical support for the optimization of corrosion reduction measures.