Study On Cavitation Erosion Behavior And Ultrasonic Cavitation Characteristic

Cavitation erosion is one of the main erosion manners on the hydraulic machines and buildings.When the high speed fluid impacts these hydraulic systems,it is inevitable that cavitation erosions such as needle and fish-scale pits are produced on the region surface of the transition flow parts such as the screw propeller,the hydraulic turbine and the spillway of hydraulic building.But the mechanism and discipline of cavitation is still a mysterious science problem without solution due to its complexity.This study based on computational fluid dynamics was to investigate the characteristic of a bubble which was collapsing in different states and the pressure distribution rule of the high speed water jet for exploring the mechanism of bubbles making the solid damage.In addition,the distribution characteristic of bubble and the impact factor of cavitation induced by a magnetostrictive transducer were discussed by a great deal of experiments.It was found that the appearances of cavitation erosion produced by different input power were various.The distribution of bubbles and pressure in ultrasonic cavitation flow field were obtained by simulations and it was consistent with the experiment results.These results provided a new theory and experiment evidences for knowing the cavitation erosion rule of material on the surface of solid.The following discoveries and conclusions were gained:1.The distribution and evolving process of pressure on the solid wall generated by a high speed water jet was obtained by a simulation in three dimensions.The results showed that the max pressure suffered by the rigid wall was 2.3 times or 2.5 times of the water hammer pressure when the high speed water jet impacted the wall with a round or plane head respectively.The max pressure occurred on the position where the rigid wall intersecting with the axis of the round head jet or the boundary of the plane head jet.the latter case was probably the cause of circumferential cracking on the surface of brittle material solid.2.The collapse processes of a vapor bubble under different conditions were simulated by the numerical calculation.The results showed that the lasting time and collapse pressure of a bubble is influenced by the distance between the bubble and the wall as well as the flow velocity;and the case whether or not the impact of jet was direct to act the solid wall would decide the erosion appearances.The bevel jet was generated contrary to the flow direction when the bubble collapsed in the high speed water,which may cause the ripple and fish-scale pit damages on the transition parts of many hydraulic systems.3.The distribution characteristic of bubbles on the emitting surface of ultrasonic was investigated by experiments under different conditions.The causes of the distribution of bubble affected by different impact factors were analyzed.The form mechanism of the arborization structure of bubbles was explained simply by introducing the diffusion limited aggregation model and fingering model from the fractal theory.4.The cavitation erosion appearances generated under different input powers were analyzed by three dimension appearance;and confirmed that the erosion appearance characteristic was decided by the distribution of bubbles.The distribution structure of bubble would generate the corresponding erosion structure,and the damage of material was annular according to the damage strength of bubble.The damage velocity and degree of cavitation erosion were decided by the content of bubble in local region.The changing trend of the depth of cavitation pit and the degree surface damage with time increasing was consistent but the final damage appearance which is related with the content of bubble and the thickness of the bubble layer in the corresponding region was the counteractive result between the damage capacity of interior bubble and the buffer action of the exterior bubble.5.The results showed that the content of bubble was increasing with the enhancement of the amplitude of vibration and frequency which led to more complicated flow field and more rapidly attenuation of pressure wave by the analysis of the cavitation simulation under different conditions.The interference of pressure waves and the generation of cavitation accelerated the decline of vibration amplitude of the wave and produced a great deal of unorderly frequencies which implicated the aberration of pressure waves.The pressure or bubble on the emitting surface were distributed annularly according to the magnitude of pressure or the content of bubble,and even in a same annular region,the pressure magnitude or the bubble content was not equal.The results conformed to the appearances characteristic that the cavitation erosion was dendritic on the surface and annular at the axis direction of the sample.The typical dendritic structure was formed as the bubble regions on the emitting surface over a content value were linked by beelines from the interior to the exterior,which accorded with the phenomenon observed in the experiment.