Research On Characteristics And Performance Of Artificially Submerged Cavitation Jet

Cavitation jet is a low-cost,high-efficiency,and clean surface treatment technology,which has been widely used in the cleaning of large-scale equipment such as ships and chemicals,and the surface strengthening process of metal materials.The artificial submerged cavitation jet can overcome the shortcoming of the conventional submerged cavitation jet's insufficient target distance.It is more flexible in dealing with large and medium-sized workpieces.However,the current understanding of the internal flow structure of the artificially submerged cavitation jet is not impressive enough,and the influence of pressure,target distance and other parameters on the jet impact performance remains to be studied in depth.Therefore,mastering the flow characteristics of artificial submerged cavitation jets and determining the optimal impact conditions of artificial submerged cavitation jet are of great significance for improving the performance of artificial submerged cavitation jet and expanding its application fields.This paper is supported by the Jiangsu Provincial Key R&D Program Competition Project "Research and Development of Cavitation Shock Wave Strengthening Anti-cavitation Erosion Technology for Large-Capacity and High Parameters Hydraulic Machinery Blades"(No.BE2017126).This paper uses a combination of experiment and numerical calculation to study the characteristics and impact performance of artificially submerged cavitation jet.The main research work and conclusions are as follows:(1)Built a visual test bench for artificially submerged cavitation jet,used high-speed cameras to capture the shape and distribution of jet cavitation clouds,and analyze the influence of different nozzle upstream pressures on the cavitation jet flow field distribution.The test results show that the cavitation cloud in jet exhibits periodic characteristics,the cavitation clouds will undergo four stages: initiation,development,shedding,and collapse in a single cycle.Changing the pump pressure of the inner and outer nozzles have a significant effect on the jet flow field distribution.When the pump pressure of the inner nozzle increases,the degree of cavitation increases,the size and density of the cavitation clouds both increase significantly,and the outer low-pressure water column has a tendency to shrink inward,which enhancing the turbulence of the downstream flow field,so that the cavitation clouds begin to collapse before it completely falls off.When the pressure of the outer nozzle increases,it slows down the trend of inward contraction,and improves the turbulent structure of the downstream flow field.However,the increase in the pressure of the outer nozzle also increases the ambient pressure conditions,which reduced the cavitation.(2)Numerical simulations of artificial submerged cavitation jet based on RANS model,LES model and Hybrid RANS/LES model were used respectively,and the prediction capabilities of different turbulent models for artificial submerged cavitation jets were compared and analyzed.The results show that,because the calculation results by the SST k-? model were timeaveraged,and it cannot capture enough transient flow field details.The obtained flow field information is quite different from the actual situation.Both the LES turbulent model and the SBES model in Hybrid RANS/LES can captured the eddy structure in the shear layer of the artificially submerged cavitation jet.Besides,the SBES model can not only capture more small-scale eddy structures in the flow field,but also has better computational economy.The cavitation shape obtained by the SBES model is close to the experimental results.Therefore,this paper chooses the SBES model as the turbulence model for this study.(3)Based on numerical calculations,the distribution of cavitation bubbles in the flow field under different pressure inlets were analyzed,and the characteristics of the jet impinging flow field under different impingement distances are studied.It was found that the high-pressure water flow entrains the low-pressure water flow,which generates shearing action near the expansion section of the inner nozzle and its outlet,and then forms vortex cavitation to produce cavitation.The cavitation was first generated in the expansion section of the inner nozzle and shears along the jet.When the pressure of the inner nozzle increases,the shearing effect between the two water streams is strengthened,and the cavitation phenomenon increases.When the pressure of the outer nozzle increases,the lowpressure water flow speed increases,the jet clustering is enhanced.However,the increase in the pressure of the outer nozzle increases the ambient pressure of the cavitation clouds,which limits the growth of the cavitation bubble,and the cavitation phenomenon is weakened.This result is consistent with the result of the visualization experiment.In the artificially submerged cavitation jet impinging flow field,the jet forms a high-speed diffusion zone near the wall,and a secondary vortex structure is generated in this area to accelerate the collapse of the cavitation bubble.When the impact distance is increased to H/d=45,the tangential velocity near the wall is reduced,and the range of the high-speed diffusion zone is expanded.More cavitation bubbles accumulate in the high-speed diffusion zone,and the cavitation distribution area on the wall increases.(4)The artificial submerged cavitation jet impact test was carried out on the pure aluminum sample,the optimal combination of internal and external nozzle pressures under different impact target distances were analyzed.The axial retraction distance between the inner and outer nozzles was optimized,and the impact time is used as a variable to analyze the evolution process of the cavitation area on the surface of the sample.The impact strengthening of 304 stainless steel material,which verified the strengthening performance.The test results indicate that appropriately increasing the inlet pressure of the outer nozzle can increase the jet impact target distance and improve the impact performance.Under the condition of low target distance,using a lower internal nozzle pressure can get a better impact effect.The best pressure combinations under each impact target distance were determined.It is found that the jet impact performance can be improved when the axial retraction distance between the inner and outer nozzles b=+3mm.The mass loss density of the samples increases with the jet impact time.The surface hardness of the specimens after the jet impact strengthening have been improved,and good smooth quality is maintained,which verified the strengthening performance of the artificial submerged cavitation jet on the metal surface.