| As a new type of jet technology namely cavitation jet has been used in this investigation.It has many advantages as high energy concentrated,strong impact,environmentally friendly with high efficiency.This new Jet technique can widely be used in engineering fields such as oil exploitation,ship cleaning,and metal material strengthening.During the explosion of the cavitation jet bubble,extremely strong shock waves and micro-jets are happing.When it is controlled and used properly,cavitation can effectively improve the efficiency of jet cutting,descaling and shot peening.In contrast,when using it improperly,it will affect and damage the material matter.In the center point of cavitation jet core,the nozzle not only converts hydrostatic energy into dynamic pressure energy but ensures good flow characteristics and cavitation performance as well.Therefore,it is important to study nozzle cavitation performance and jet bubble morphology distribution law to improve nozzle jet performance and cavitation performance.This work was performed under the auspices of Jiangsu Province Key R&D Program Competition Project “Development of High-capacity High-parameter Hydraulic Machinery Blade Cavitation Shock Wave Enhanced Anti-Cavitation Abrasive Technology(Project No.: BE2017126)”.Based on the coupling of numerical calculation and experimental research,the influence of the structure parameters of the conversion and diversion nozzles is studied with the consideration of the jet cavitation performance.The structure mechanism of the jet shear cavitation is analyzed.The experiment captures the evolution process of the cavitation morphology of the submerged jet.(1)A conversion and diversion nozzle are designed to compare and analyze the application of different turbulence models in submerged cavitation jets.By selecting the multiphase flow model and cavitation model,the cavity morphology formed by submerged cavitation jets under different turbulence models is captured.In addition,the obtained data compared with the experimental work of vacuole cloud morphology.The results show that the cavitation clouds formed by the submerged cavitation jets calculated by the standard and turbulence models are far from the experimental results which are mainly concentrated in the vicinity of the nozzle outlet.The transition SST turbulence model calculates the cloud cavitation formed downstream of the nozzle.The data captured by this model was closest to the cloud cavitation results obtained by high-speed photography experiments.When the evaporation and coagulation coefficients of the Zwart-Gerber-Belamri cavitation model are 10 and 0.01 respectively,the cavitation performance of the jet is moderate,and the bubble has a good agreement with the experimental results.(2)Based on the established numerical calculation method,the numerical calculations of the conversion and diversion nozzles with different structural parameters are carried out.In addition,the performance distribution and cavity morphology of the nozzle flow field under different parameters are studied.The trend of the nozzle structure parameters and the bubble characteristics are analyzed.The influence of the law of morphological change has obtained the best value of the corresponding parameters.The results show that with the increase of the nozzle outlet diffusion angle,the lowest negative pressure value in the nozzle flow field first decreases and then increases.Whilst,the local low-pressure region formed in the nozzle diffusion section decreases with the increase of the diffusion angle.The axial length and area of the bubble increase first and then decrease.The optimal value of the nozzle diffusion angle makes the jet cavitation performance optimal.With the increase of the diameter of the cylindrical section of the nozzle,the maximum speed of the nozzle flow outlet increases continuously.Also,the local low-pressure zone formed in the nozzle diffusion section increases first and then decreases with the increase of the nozzle diameter.Furthermore,the jet cavitation area formed by the nozzle first increases and then decreases.When the length of the nozzle cylinder increases,the nozzle flow field become a lowest negative pressure.First,the pressure value decreases then gradually increases,and the maximum nozzle exit velocity gradually decreases.The axial length and area of the jet cavity formed by the nozzle first increase and then decrease.At the same time,when the inlet pressure increases,the maximum velocity of the flow in the nozzle outlet increases continuously and the lowest negative pressure value in the flow field gradually decreases,while the jet cavitation performance increases.(3)The unsteady cavitation characteristics in the nozzle were obtained by numerical calculation.The formation mechanism of the shear cavity was analyzed,and the variation of the cavity shape and performance of the nozzle flow field was obtained.It is found that the cavitation in the nozzle shaft section is located near the inlet of the outlet section of the nozzle cylinder section and is symmetrically distributed along the jet axis.In the corresponding low-pressure zone where cavitation is formed,there is a vortex symmetrically distributed along the jet axis.And,the variation law of the cavity shape with time is found,which is consistent with the changing trend of the vortex in the nozzle.The vortex core corresponds to the higher position of the bubble volume fraction.At the same time,the morphology of the cavitation in the radial section of the nozzle is analyzed.It is found that the cavitation is distributed in the circumferential direction along the radial section,and the velocity fraction of the cavitation is found to be higher,corresponding to the velocity gradient with larger turbulent kinetic energy(boundary layer).Therefore,combining the bubble variation law and the flow field distribution characteristic in the axial section and the radial section,a vortex generated by shearing in the boundary layer where the jet velocity gradient is attenuated rapidly,where the cavitation is formed due to the pressure drop caused by the vortex.(4)The high-pressure cavitation jet test rig was established.The high-speed camera of OLYMPUS Company of Japan was used to capture the cavitation form formed by the submerged jet.The variation of the bubble shape of the nozzle jet under different diffusion angles and different inlet pressures was analyzed.The test results show that the nozzle diffusion angle has a great influence on jet cavitation performance.When the diffusion angle is too small,it will limit the development of the bubble and reduce the cavitation performance.If the diffusion angle is too high value,the outlet segment will have the insufficient binding force to the shear layer and the cavitation performance is degraded.When the nozzle diffusion angle is 60°,the jet cavitation performance is optimized.The shape of cavitation clouds under different inlet pressures is quite different.With the increase of inlet pressure,the degree of cavitation and the size of cavitation clouds are gradually increased.In addition,the position of cavitation shedding occurs downstream,and the shedding period is slightly extended. |
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