Cavitation Inception And Erosive Potential For Self-Resonating Cavitating Jets Under Ambient Pressure Conditions

Self-resonating cavitating jet technology modulates jet flow by imposing structured pressure pulsations on the exit of the nozzle.The resultant formation and strengthening of coherent structures promote larger pressure drop inside the core of the vortices,thus enabling stronger cavitation inception in the jet flow under ambient pressure conditions.However,the detailed cavitation enhancement process is still unclear.Cavitation collapse intensity in the jet flow and the induced material erosion are also poorly understood.In this dissertation,we attempt to address these issues using theoretical analysis,numerical simulation,and lab tests.The investigations conducted are described and the principle conclusions are summarized as follows:?1?Largy Eddy Simulation is performed to investigate the jet flow field under the influence of self-resonating effect.The pressure pulsations resulted from the self-resonating effect leads to stronger coherent structures with higher stablility.The pressure drop inside the vortice is increased by 38%compared with the naturally formed vortices.Cavitation inception is thus facilitated within ambient pressure environment.?2?Based on the classical Keller–Miksis equation,a bubble dynamic model is constructed considering the heat and mass exchange between the bubble and ambient fluid.The model is sovled with an in-house MATLAB code using Runge-Kutta algorithm and variable step size technique.The calculation result reveals the inertia feature of cavitation bubbles when responding to the surrounding pressure change.It also gives quantitative estimate on the magnitude of the collapse pressure and temperature:¹0⁴ MPa and¹0⁴ K with jet velocity of 140 m/s and ambient pressure of 5 MPa,respectively.Self-resonating enhances the cavitation collapse pressure by 9.7%and elongates the pressure pulse duration by 28.8%.A parametric study demonstates the existence of the optimum ambient pressure given certain jet velocity.In the present study,this optimum value is calculated to be about 7 MPa with the jet velocity of 140 m/s and self-resonating parameter of 1.24.Generally,the hydraulic parameters influence the collapse intensity more singnificalty than the fluid properties.?3?An inverse finite element method based on pitting analysis is constructed to give a cavitation impact map for the cavitating jet.The results show the cavitation impacts distribute in a ring region and circular region at small and large standoff distances,repectively.The cavitation impact rate decreases with standoff distances.For most of the impacts,the peak impact pressure?_H is in the range of 1.0–1.8 GPa,impact size d_H is from 20 to 40?m,collective impact force F is from 0.5 to 1.5 N,and impact energy W_total is in the order of 10^-4 mJ.The stagnation region and the large-scale recirculation zone are two important fluid features in the impinging jet determining the impact distribution patterns.?4?The outlet of organ pipe nozzle is optimized as follows:the divergent angle is20°,the divergent length is 4 times the throat diameter,the throat length equals the diameter,and the convergent angle is 13.5°.The investigation of bubble transportation reveals a threshold flow rate given certain ambient pressure for efficient bubble transportation.The effect of of heat treatment on the cavitation rerosion resistance of stainless steel is studied by cavitation erosion test.The results show that the high aging temperature impairs the material's resistance.The resistance correlates with material's yield stress,elastic work ratio,and hardness in power law relationship.These results benefit selecting nozzle material with high cavitation erosion resistance.