| Ultrasonic cavitation is a unique phenomenon of power ultrasound used in liquids,and most of ultrasonic applications in liquids are related to ultrasonic cavitation.When ultrasonic waves propagate in a liquid,a sound field in which positive and negative pressure phases alternate occurs in the liquid.When the amplitude of the ultrasonic is large,the original cavities existed the liquid will grow under the negative pressure phases of the ultrasonic wave,and they will be compressed and collapsed under the positive pressure phases and then maybe rebound occurs.This phenomenon is called ultrasonic cavitation,and correspondingly the cavities in the liquid are called cavitation bubbles.Ultrasonic cavitation bubbles expand in the negative phase of the ultrasonic,absorb and store energy;the bubbles will be compressed and collapsed in the positive phase.Because the during of collapsing phases of the cavitation bubble is usually on the order of nanoseconds,the energy accumulated in the bubble have not enough time to release out the bubble and thus high temperature and high pressure environments form inside the cavitation bubbles,which provides a favorable condition for physics effects?chemical reactions and sonoluminescence and so on in the bubbles.Theoretical and experimental research on ultrasonic cavitation has become an important research topic in the past century.In the theoretical study of ultrasonic cavitation,some assumptions were always made due to its own complexity.Releasing one or more assumptions to establish a more accurate model is of great significance for studying the effects of ultrasonic cavitation.In the practical application of ultrasonic cavitation,the population(size and probability density)of bubbles in the sound field is an important aspect affecting cavitation efficiency,so this research is also of great significance for practical applications.In this study,we derived a new single cavitation bubble model based on the mass conservation equation,the momentum conservation equation,the continuity equation in the liquid,and the boundary conditions at the cavitation bubble wall.The model not only takes the effect of the compressibility,that of surface tension,that of liquid viscosity on the bubble wall and that of nonequilibrium evaporation and condensation of water vapor,that of heat conduction and other factors into account,but also firstly considers effect of the bulk liquid viscosity on the dynamics of a single cavitation bubble.Eighty-nine chemical reactions are also considered in this model including 25 kinds of radical particles:H,O2,O,OH,H2,HO2,H2O2,O3,N2,N,NO2,NO,N2O,NO3,HNO,HNO2,HNO3,NH3,NH2,NH,N2H4,N2H3,N2H2,N2O4,and N2O5.According to the numerical simulation results of a single cavitation bubble,it is concluded that,to a certain extent,the thickness of the uneven temperature layer inside the bubble affects the severity of cavitation bubble collapse,thus affecting the maximum temperature and maximum pressure in the bubble.The numerical simulation of the heated liquid boundary layer surrounding the bubble shows that the duration of the heated liquid boundary layer is very short(generally in a few nanoseconds)and the thickness is several to several hundred nanometers.With the increase of ultrasonic frequency,the decrease of ultrasonic amplitude,the increase of surface tension,and the increase of liquid viscosity,both the duration and thickness decrease.Finally,the influence of the bulk liquid viscosity on the dynamics of cavitation bubbles is calculated.Compared with the maximum temperature and maximum pressure inside the collapsing bubble without considering the bulk liquid viscosity,that with considering the bulk liquid viscosity are higher;and with the increase of bulk liquid viscosity,the effect becomes larger.The comparison result between maximum pressure and the maximum temperature finds that the maximum pressure inside the bubble has more sensitivity on the bulk liquid viscosity that the maximum temperature.In order to verify the effect of bulk liquid viscosity on the dynamic of a single cavitation bubble,single cavitation bubble experiments were carried out in a 30%(v/v)glycerol-water solution.The comparison result between the experimental results and theoretical results with or without considering the bulk liquid viscosity shows that the bulk liquid viscosity has little effect on the cavitation bubble radius,but the number of rebound phases of the cavitation bubble.And the sonoluminescence intensities of a single cavitation bubble were measured under different acoustic pressures(amplitudes).By comparing the coefficient of determination between the experimentally measured sonoluminescence intensities and the theoretical calculation results,it was found that the model with considering the bulk liquid viscosity may is superior to the model without considering the bulk liquid viscosity.The distribution of cavitation bubbles(size and probability density)in the ultrasonic field is one of the important factors affecting the sonochemical efficiency.Considering the range of cavitation bubble radius(rather than a single value)and the probability density of distribution of bubbles in the ultrasonic field,the bubble numbers per unit volume per unit time were calculated based on theoretical calculations and experimental results about yield rates of H2O2 under different frequencies,different powers and different dissolved gases.As the frequency increases,the radius range of the bubble in the sound field decreases,and the number of bubbles per unit volume per unit time increases;with the increase of power,the radius range of the bubble in the sound field is basically unchanged,and the number of bubbles per unit volume per unit time decreases;the effect of dissolved gases on the bubble radius range follows the order:air>argon>oxygen,and dissolved gases have little effect on the number of bubbles per unit volume per unit time. |
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