Study On Mechanism Of Surface Topograhpy Effects On Generation Of Cavitation Erosion

The researches on the initiation mechanism and control methods of cavitation erosion is of great importance in both theoretical aspects and practical application as it is a complicated phenomenon, which is microcosmic, instantaneous, random and multiphase, and it is also one of the key damage forms of hydraulic machinery. This thesis focuses on the cavitation erosion in the incubation period. Experiments are performed and computational fluid dynamic methods are conducted to investigate the mechanism of surface topography effect on the generation of cavitation erosion.Experimental results show that surface topography and micro-particles have remarkable influences on the incubation of cavitation erosion. The dimension, shape and distribution of the topography affect the erosion degree greatly. The"rough"topographies do not always enhance the cavitation erosion and certain topography structures can mitigate the erosion. Micro-particles also act an important role in the initiation of cavitation erosion. The size of the micro-particles has significant effects on cavitation erosion. Among the particles 0.1~4μm in diameter, when that about 1μm in scale are added into the de-ionized water, the surfaces are eroded most seriously.By means of computational fluid dynamics, the pressure and velocity distributions of the near-wall flow considering the effect of surface topography are analyzed and their influences on the formation, transportation, growth and collapse of the cavities are investigated. Because of the presence of the solid surface and its topography, the motion of the cavities and micro-particles in the near-wall region are difference from that in the free stream. Numerical methods for multiphase flow simulation is adopted to track their moving trajectories and results shows that cavities and large micro-particles will move away from the surface and only the small micro-particles (about 1μm) can keep moving in the boundary layer. The cavities can be absorbed by the micro-particles and this could increase the chance that the cavities keep moving in the vicinity of the solid surface. The pressure rise caused by the topography is the key factor to induce the collapse of cavities in near-wall region. High speed micro-jet will generate while the cavity collapse and if it is close enough to the solid surface, the surface will be eroded. The velocity of the micro-jet drops down quickly in the medium, but the local pressure rise induced by the surface topography could enhance the velocity of the micro-jet and its impact intensity. Cavitation, formation process of the cavities, is affected by the surface topography. Numerical results indicate that the surface topography in millimeter scale or even larger will enhance the possibility of cavitation which will increase the amount of cavities.According to the experimental and numerical results it is reasonable to conclude that the mechanism of cavitation erosion in the incubation period is: the cavities are absorbed by the micro-particles and move into or keep transporting next to the solid surface; the pressure rise caused by the surface topography acts on the cavities and drives them to collapse, and finally induce the cavitation erosion. By analyzing the integrated effect of the topography structure, regular distributed grooves with small trailing angle are designed to prevent the cavitation erosion, and this is supported by the experimental results.