Design And Application Of Cavitation Jet Nozzle For Underwater Wall-Climbing Decontamination Robot

With its advantages of simple structure,low cost and environmental friendliness,cavitating jet is widely used in underwater cutting,tank bottom decontamination,reservoir bottom dredging and other fields.The research of cavitation jet is one of the hot spots in the field of water jet.The design of cavitation jet nozzles plays an important role in the effecting effect.Currently,the cavitation jet nozzles used in the market are usually simple Angle or contraction nozzles,and their effecting effect is still lagging behind that of Helmholtz self-excited oscillation nozzles.It is necessary to study the depth of Helmholtz self-excited oscillating nozzle.The continuous development of computer hardware and commercial computational fluid dynamics（CFD）software provides good conditions for the optimization design of cavitation jet nozzles.In this paper,the cavitation jet nozzles used for the fouling system of underwater hull climbing fouling robot are taken as the research object.The Helmholtz type self-excited oscillation pulse cavitation jet nozzles with single chamber and series-wound double chamber are analyzed theoretically,numerically and the structure optimization design is studied by using CFD method.The specific research work of this paper is as follows:（1）Research on optimization design technology of single chamber nozzle structure.Based on the large eddy model and multi-phase flow cavitation model,the Fluent software was used to conduct numerical simulation of the internal flow field of the single-chamber nozzle.The flow inside the nozzle under the boundary condition of low pressure is analyzed,and the main mechanism of the nozzle to generate the pulsed cavitation jet is as follows:Cavitation gas and bubbles are generated periodically near the shear layer and move with the main jet towards the outlet channel.Periodic cavitation gas and bubbles form periodic slit flow channels,which promote the main jet to produce pulsing velocity jet effect.At the same time,the exit diameter,cavity diameter,cavity length and other structural parameters of the single chamber nozzle were optimized by numerical simulation.The optimal ratio of structural parameters under the current boundary condition is d₂/d₁=1.3、L₁/d₁=8、D₁/d₁=10.（2）Research on structural optimization design technology of double-chamber nozzle.On the basis of the above research,the centralized parameter equivalent network model of single chamber nozzle and double chamber nozzle is established according to the fluid network theory.The natural frequency and frequency response function of single/double chamber nozzles are derived.In addition,a comparative analysis of the flow field in the single/double chamber nozzle shows that the pulse effect of the double chamber nozzle cavitation jet is more intense.More energy can be released instantly,resulting in a better cleaning effect.At the same time,the middle flow channel length,outlet diameter,cavity diameter,cavity length and other structural parameters of the double-chamber nozzle were optimized.Time domain monitoring and FFT frequency analysis of nozzle outlet are carried out.The optimal ratio of double chamber structural parameters under the current boundary conditions is l₁/d₁=4.0、d₃/d₂=1.3、L₂/d₂=6、D₂/d₂=10.（3）Research on jet application of self-excited oscillation pulse nozzle.Numerical simulation was carried out on the efflux effects of single/double chamber nozzles at 30mm,50mm and 70mm target distances in submerged environment,and the optimal target distances and characteristics of the two nozzles were analyzed.At the same time,combined with the underwater hull shell climbing fouling robot project of the research group,two kinds of multi-nozzle cavitation jet cleaning arm structures are designed,and the cavitation jet effects of the two cleaning arm structures are analyzed through numerical simulation.It provides a scheme for the subsequent practical application of cavitation jet decontamination robot.