Investigation On Mechanism Of Submerged High-pressure Cavitation Jet And Metal Strength Improvement Using Cavitation Bubble Shock Wave

The high-pressure water jet technology has the advantages of strong impact,high efficiency and cleanliness,which is widely used in the fields of ship cleaning,mineral mining and metal cutting.When the high-pressure water jet is in a submerged state,a large number of cavitation bubbles are generated at the jet boundary under the action of strong shear stress.By the reasonable use of the cavitation bubbles in the flow field,the impact performance of the jet can be significantly improved under the same working pressure.When the cavitation jet is used to impact the metal material,the shock wave and micro jet water hammer action on the surface of the material can change the crystal structure of the surface layer of the material,which threreby increase the surface hardness and form residual compressive stress,thereby improves the material surface strength.However,if the cavitation jet is not properly controlled,the strong cavitation impact may cause cavitation damage to the metal surface.Therefore,studying the mechanism of submerged cavitation water jet cavitation and mastering the cavitation degree and impact performance of the jet under different working conditions are of great significance for the development of green and efficient metal surface strengthening processes.This article is funded by the Jiangsu Provincial Key R&D Program competition project"Research and Development of High Capacity and High Parameter Hydromechanical Blade Cavitation Shock Wave Enhanced Anti-Cavitation and Abrasion Technology"?Project Number:BE2017126?.The effect of cavitation shock wave strengthening metal has been systematically studied.The main research work and innovations achieved in this article are as follows:?1?A high-pressure water cavitation jet visualization experiment system was built.Unsteady features of cavitation jets with different cavitation numbers and nozzle geometric parameters were obtained through high-speed photography experiments.The image intrinsic orthogonal decomposition?POD?algorithm was used to extract the spatial modes and time coefficients of cavitation clouds.The period of shedding in cavitation jets are obtained by analyzing the time coefficients.The change of the emission wavelength of excited fluorescent particles is used to solve the problem of the interference of bubbles in PIV shooting in multiphase flow.The PIV measurement of the gas-liquid two-phase flow velocity field under different working conditions and different nozzle jets is carried out.The distribution law of jet gas-liquid two-phase flow velocity field is obtained.It was found that the development of the submerged high-pressure water jet cavitation cloud has a periodic shedding phenomenon,and each cycle is accompanied by three stages of growth,shedding and collapse of the cavitation cluster,and the collapse of the cavitation cloud mainly occurs after shedding.When the cavitation number and the nozzle configuration is changed the periodic characteristics will become significantly different.The velocity field of submerged cavitation water jet has good self-mode characteristic,and the dimensionless velocity distribution conforms to the exponential function.Gaussian fitting can be used to establish the radial distribution function of cavitation jet velocity.By comparison,it is found that the change of the cavitation number has little effect on the velocity distribution curve in the radial direction,while the geometry of the nozzle has a greater effect.?2?The Zwart-Gerber-Belamri cavitation model is combined with a variety of turbulence models to simulate and analyze the submerged high-pressure water cavitation jet flow field.The effect of the vortex resolution scale of the turbulence model on the prediction accuracy of jet cavitation is studied The RANS model is affected by time-averaged processing and cannot obtain the vortex structure in the jet shear layer.Therefore,the calculated cavitation is only distributed at the nozzle outlet,which is far from the real situation.The RANS-LES hybrid model can obtain better results for cavitation jet.The SBES model has better prediction performance in shear layer instability than the DES model.The simulation results of the submerged jet under different cavitation numbers are in good agreement with the experimental results.Through the calculation results,the volume fraction,phase change rate,vorticity and turbulent kinetic energy in the shear layer at the outlet of the nozzle were compared and analyzed.It is found that the dominant position of vorticity is discontinuously distributed,and the phase change rate corresponding to the area with higher vorticity is also higher,indicating that the shear cavitation is mainly caused by the low pressure in the center of the vortex.The dimensionless curves of the velocity distribution in the radial distribution at different cavitation numbers basically coincide,and they are in good agreement with the fitted velocity formula using experiment data,which verifies the applicability of the formula at low cavitation numbers.?3?Based on numerical calculations,orthogonal test was carried out on the cavitation performance of the angular nozzle,the influencing factors of the cavitation performance of the angular nozzle were analyzed,and the order in which the geometric parameters affected the cavitation performance was as follows:outlet expansion angle?>Length of outlet section L₃>length of constricted section L₁>length of throat L₂.According to the main factors affecting the cavitation performance,the influence of the nozzle outlet shape and the outlet angle on the cavitation performance was studied.It was found that when the angle of the nozzle is 60°,the cavitation performance is better than 40°and 80°.It shows that the outlet angle of this type of nozzle has an optimal angle within the range of the value studied.Through the simulation analysis of the three types of organ pipe nozzles with no whistle,cylindrical whistle and expanded whistle,it is found that the nozzle with the expanded whistle has the best performance,and the nozzle without the whistle has the second performance,the performance of nozzle with the cylindrical type whistle is the worst.?4?The compressive properties and acoustic propagation properties of gases and liquids are defined by polytropic equation of state for gas and Tait equation for liquid.Based on ANSYS Fluent software,the relationship between medium density and local pressure is established through UDF.The VOF interface tracking method is also used to simulates the dynamic development process of laser-induced cavitation in the infinite domain and near the wall through a pressure-based solution algorithm.The propagation law of the shock wave generated by the bubble collapse in the fluid is analyzed.The impact load characteristics formed on the surface are simulated by the finite element algorithm to reveal the stress-strain characteristics of aluminum alloy materials under the transient nonlinear load of the cavitation shock wave.It was found that when the outermost wavefront of the shock wave reached the wall,the load on the wall increased rapidly.During this period,the strain and internal stress of the material also increased.After the impact,the elastic deformation of the material completely disappeared,and the Residual compressive stress is left in the center.?5?High-pressure water cavitation jets were used to conduct impact tests on aluminum alloys Al1060,Al6061 and 304 stainless steel.The impact performance of cavitation jets was compared by measuring the Al1060 mass loss before and after the impact.After observing the surface morphology of the material,the hardness and residual stress at different depths on the surface of the impacted material were measured,and the strengthening effect of the cavitation water jet on the material was analyzed.It is found that each type of nozzle has a corresponding optimal stand-off distance under a specific pressure.After changing the shape of the nozzle outlet,the optimal stand-off distance will not change basically,but the impact performance will change significantly.Use a nozzle with an expanded whistle to impact aluminum surface,the rate of Al1060 material mass loss is the fastest.The surface hardness of aluminum alloy Al6061 after being impacted by cavitation jet can be increased from70 HV to 82.5 HV,the maximum residual stress reaches 151 MPa.The surface begins to plastically deform when the impact time is 5 min,and the surface begins to fatigue and flake when the impact time reaches 10 min.The hardness of 304 stainless steel can be increased from 200 HV to 264 HV after cavitation jet impact.When the impact time reaches 10 min,the hardness of the surface layer of the stainless steel material does not change obviously.When the impact time reaches 20 min,the plastic deformation on the surface of the stainless steel begins.According to experimental analysis,the impingement time of cavitation water jet for aluminum alloy materials should be controlled at about 5 min,and the impingement time of cavitation water jet for stainless steel materials should be controlled at about 10 min.