Investigations On Venturi Unsteady Cavitating Flow And Round Jet Breakup Of Dilute Polymer Solution

Most of the liquid functional materials contain large molecules and usually exhibit viscoelastic non-Newtonian characteristics.Considering the military needs for efficient dispersal,safe design and stable launching of munitions in weaponization applications,two basic scientific problems of cavitation flow and jet breakup of polymer solutions are refined.Investigations on cavitation flow and jet breakup of dilute polymer solutions in Venturi are carried out systematically.Main research contents and innovative results are following.A series of cavitation flow experiments,based on filtered water,glycerol solutions of three concentrations,and polymer solutions of various molecular weights and concentrations,are carried out,recorded by high spatio-temporal resolutional camera from multi-view.Based on analysis of the cavitation flow characteristics,the structural composition of venturi cavitation flow is revealed.The mechanism the recirculation layer of venturi cavitation is proposed,and the phenomenon of cavitation return jet,which is questionable in academia,is analyzed and interpreted scientifically.The law of viscosity-enhanced shear of liquid jets in venturi cavitation flow that causes the length of attached cavitation clouds to increase is found.The stabilization mechanism of the cavitated nucleus by the microstructure of the polymer molecule with viscoelastic characteristics is proposed for the steamy supercritical cavitating bubble presented by the cavitation flow of the polymer solution.The mechanism of cavitation inhibition by polymer solutions is illuminated in terms of energy transfer and viscoelastic fluid turbulent boundary layer characteristics.The law of variation of Venturi cavitation volume oscillation and shedding is obtained by analyzing the kinematic laws of cavitation flow in polymer solutions.It is revealed that the bulk shedding of cavitation clouds in polymer solution is due to the shock wave induced by the cavitation cloud collapse propagate to the Venturi throat,and temporarily inhibit cavitation occurrence.The secondary shedding of cavitation clouds during flow is due to the violent expansion of supercritical bubble and the blocking of the jet stream by collapsing.The modal decomposition of the cavitation complex flow of viscous liquids and polymer dilute solutions by POD method is used to extract the modal and spectral characteristics that characterize the main features of the cavitation flow and to obtain the structure and characteristic size of the cavitation complex flow.The liquid viscosity makes the cavitation flow appear three-dimensional structure,and the corner vortex cavitation layer develops at the front and back walls of the flow channel,and the attached cavitation cloud along the throat gradually forms "squeeze" and inhibits effect,and the corner vortex cavitation layer shows different scale cavitation cloud shedding due to turbulent separation.Modal decomposition of the flow of five PEO solutions at two kind of ? numbers corresponding to sheet cavitation and cloud cavitation is performed to obtain the modal and frequency spectra characterizing the cavitation flow of polymer solutions of different molecular weights and concentrations.Low molecular weight polymer solutions with high number of vacuoles,small volume of bubbles and stable cavitation flow,the main flow feature is the shedding of small-scale cavitation clouds in the front and back wall surface corner vortex cavitation layer.As the molecular weight and concentration of PEO increase,the number of bubbles decreases,the unstable characteristics of cavitation flow are enhanced,the shedding of large cavitation clouds in the throat increases,and the secondary shedding of attached cavitation clouds is motivated by the steamy supercritical bubble.The viscoelastic characteristics of the polymer solution cause only a thin corner vortex cavitation layer to develop on the front and back walls of the flow channel,which has a weak squeeze effect on the attached cavitation cloud.Based on the conclusions of the flow modal analysis of viscous and viscoelastic fluid venturi cavitation,the three-dimensional flow structure and unsteady properties of fluid venturi cavitation are fully revealed.The time domain and frequency domain characterization of filtered water and five PEO solution cavitation-induced vibration signals are studied.PEO solution's unsteady cavitation flow characteristics and collapsing of supercritical bubbles could induce flow channel form high acceleration level impact vibration.Except for 5 million molecular weight 500 ppm PEO solution(cavitation is severely inhibited),the peak values,root mean square values,kurtosis values of the vibration signal are greater than that of filtered water.The characteristics of PEO solution's cavitation-induced vibration are: with the ? value decreases,the cavitation-induced vibration signal gradually shifts from 5 k Hz to 8 k Hz frequency.when the ? less than the critical value,28.7 k Hz ? 29 k Hz band range vibration signal in the frequency band of the vibration signal is motivate.It is found that dilute polymer solution has stronger jet stability,and the analysis concluded that the unrelaxed elastic stress formed in the nozzle of the liquid column is the main cause of the enhanced jet stability.The typical feature of dilute polymer solution jet fragmentation is that as the liquid jet velocity increases,the liquid film and liquid filament are continuously peeled off from the liquid column by the strong aerodynamic shear effect.The large amplitude perturbation of the liquid column due to the presence of elastic stresses is observed during fragmentation,validating the new form of jet perturbation of viscoelastic fluids predicted by Ruo's theory results.Four characteristic morphologies are defined to characterize the degree of breakup of the polymer dilute solution jet,namely,no film and filament,film type breakup,film and filament concomitant type,net like filament type.The jet breakup phase diagram is proposed in which the flow pattern is divided according to the Re and De numbers of the polymer dilute solution jet.