Study On Cavitation Characteristics And Enhancement Of Submerged Cavitation Jet

The booming sea transportation has promoted the development of international trade,but it will inevitably have a negative impact on the global ecological environment.Ship fouling is one of the hazards that cannot be ignored,not only exacerbating greenhouse gas emissions,but also leading to biological invasion.In recent years,while the harm of ship fouling has been paid more attention,underwater cleaning has become more and more popular in the ship fouling cleaning market because of its advantages of flexibility,little time-space constraint and low cost.Submerged cavitation jet is one of the main means of underwater cleaning,and its performance and characteristics directly affect the cleaning efficiency.Therefore,in order to accurately evaluate the performance and characteristics of submerged cavitation jets,three methods,including numerical simulation,visualization experiment and erosion experiment,are used in this paper to systematically study the characteristics of cavitation intensity,behavior development and evolution,and near-target surface erosion.The main contents include:Firstly,an organ nozzle for cleaning purpose with a minimum throat diameter of 1mm was designed,and the three-dimensional numerical simulation of the organ nozzle was carried out by Fluent,a computational fluid dynamics software.The results show that the cavitation of the nozzle of organ pipe mainly originates from the throat of the nozzle and the shear layer of the outflow field.The larger the working pressure,the larger the gas phase volume fraction and velocity of the organ nozzle submerged cavitation jet.When the working pressure is 10 MPa,15 MPa and 20 MPa,the maximum gas phase volume fraction is 0.45,0.65 and 0.7,and the maximum speed is 140 m/s,170 m/s and 200 m/s,respectively.At the same time,with the increase of working pressure,the shear action between the jet and the surrounding fluid is enhanced,and the vortex ring and low pressure zone of the shear layer are increased,and the cavitation effect is enhanced.Compared with the numerical simulation results of traditional nozzles,the cavitation ability of organ nozzles is much better than that of traditional nozzles.Secondly,the visualization and erosion experiment platform of submerged cavitation jet is built to study the cavitation characteristics and performance of submerged cavitation jet.The frame difference method is used to analyze the evolution and periodicity of the cavitation cloud.It is found that the change of the cavitation intensity is the most drastic where the cavitation cloud falls off,and the formation and disappearance of the cavitation cloud appear alternately periodically.Furthermore,the dimensionless cavitation intensity image evaluation method is used to quantitatively and precisely evaluate the performance of submerged cavitation jets.When the image sample space is 200 frames,the prediction error of the maximum mass loss under 20 MPa working pressure is 1.26%.When the sample space of the image ranges from 10-5000 frames,the prediction error is within 4.24%.At the same time,the quasi-steady-state image evaluation method is used to evaluate the performance of submerged cavitation jet quantitatively and precisely.When the image sample space is 300 frames,the prediction errors of the maximum mass loss under 15 MPa and 20 MPa working pressure are 1.74% and 2.41%,respectively.When the sample space of the image is 10-5000 frames,the prediction error is within 3.67% and 4.10%,respectively.Then,the motion variation characteristics of the cavitation cloud near the target surface are studied from two angles of vertical and parallel flow direction of submerged cavitation jet,and the qualitative and quantitative analysis are carried out by using image processing method.The results show that the target surface will be covered with dense and dynamic cavitation bubbles after the impact of the cavitation cloud.In addition,only a small part of cavitation bubbles will burst during the impact,and most of them will form irregular rings and move diffusionally along the target surface and burst,with the overall periodic change of aggregation into rings,diameter enlargement and annihilation.When the working pressure is 10 MPa,15 MPa and 20 MPa,the thickness of the near-target cavitation bubble layer is 5 – 10 mm,10 – 15 mm and 15 – 20 mm,and the maximum diameter of the near-target cavitation bubble ring is about 40 mm,50 mm and 75 mm,respectively.The dimensionless time mean gray intensity and cavitation ring characteristics uniformly distributed along the same radius of the near-target surface confirm that the unsteady near-target cloud has the same time mean cavitation effect on the location of the same radius of the target surface.Finally,based on the theory of acoustic-flow coupling cavitation,an idea of ultrasonic enhanced submerged cavitation jets cleaning ship’s dirty bottom was proposed,and a joint experimental platform of submerged cavitation jets and ultrasonic waves was built to study their joint erosion effects.The erosion capacity was evaluated by the mass loss of type 1060 aluminum sheet.The experimental results showed that: When the working pressure is 10 MPa and 20 MPa,the mass loss of aluminum sheet caused by the combined action of ultrasonic and submerged cavitation jet with different frequencies of power increases obviously,with the maximum growth rate reaching 12.9% and 9.5% respectively.It is preliminarily proved that the ultrasonic enhanced submerged cavitation jet cleaning method of ship fouling bottom is theoretically feasible.