Numerical Simulation And Influencing Factors’ Studies Of Cavitation Behavior In Water-jet Cavitation Peening Processing

Water-jet cavitation peening(WCP) is a new strengthening metal surface process. The process uses the shock pressure and high-speed micro-jet caused by the collapsing cavitation bubble to impact the metal surface in order to achieve the purpose of strengthening metal parts. Because the cavitation bubbles in the water have a strong mobility, WCP has an excellent adaptability when some parts with complex surface shape were strengthened, especially for some small grooves, the tiny bubbles could flow inside and strengthen it. Therefore, more and more researchers began to focus on the new surface strengthening technology. So far, most of the findings can prove that the WCP could induce residual stress in the metal surface effectively in order to improve the fatigue life of components,and the surface roughness caused by WCP is much lower. However, the research on the cavitation phenomenon in the process, especially for the nozzle cavitation performance, is relatively less, which obstructs WCP to improve further.The technology of numerical simulation is applied to study the cavitation behavior of convergent-divergent nozzle in WCP process. With selecting the Standard k-epsilon turbulence model, Mixture multiphase model and Zwart-Gerber-Belamri cavitation model in Fluent, the cavitation flow filed inside and outside convergent-divergent nozzle in WCP process was simulated. The simulation’s results show that: When the high-pressure liquid enters the convergent-divergent nozzle, the intense cavitation phenomenon is found in the divergent segment of nozzle. The speed in divergent section is suddenly dropped so that a large number of cavitation bubbles stay and inflate in the divergent section. So, the vapor fraction in the divergent section is significantly increased. Under the condition that the inlet pressure is certain: When the nozzle divergent angle is constant, the larger the diameter of the nozzle throat is, the lower pressure value of nozzle throat generates, therefore, the cavitation effect of nozzle is enhanced; When the diameter of nozzle throat is constant, the smaller the nozzle divergent angle is, the lower pressure value of nozzle throat generates, therefore, the cavitation effect of nozzle is enhanced.In this paper, Fujifilm pressure sensitive paper is applied to measure impact pressure caused by cavitation water-jet under different stand-off distances. The results show that the impact pressures measured in the stand-off distances between 90 mm and 150 mm are all maintained at a relatively large level(above 290MPa), so the stand-off distances between this range are more suitable to strengthen workpiece in WCP process. Impact pressure measured by the pressure-sensitive paper is up to 300 MPa, the fact that this value is much higher than inlet pressure 32 MPa can prove that convergent-divergent nozzle produce intense cavitation phenomenon in water-jet.