Study On The Dynamics Of Ultrasonic Cavitation Bubble In Abnormal Fluids

The interaction between ultrasonic and medium includes thermal mechanism,mechanical mechanism and cavitation mechanism.Power ultrasound means the ultrasonic works as a form of energy through the medium,and can makes changes of it.Common applications include ultrasonic cleaning,ultrasonic welding and so on.The medium usually are normal fluids.Recent years,With the continuous improvement of people’s environmental awareness and efficiency demand,ultrasonic coupling abnormal fluid has become a trend.Such as ultrasonic enhanced supercritical carbon dioxide to realize higher cleaning rate,ultrasonic coupled subcritical water can be used to extract the effective components from plants and improve the yield,etc.At present,the study of coupling between ultrasonic and abnormal fluid is mainly by experiments,and the theoretical research on its mechanism is still rare.Based on the fluid governing equation and the theory of ultrasonic cavitation dynamics,this paper modeled and explored the dynamic behavior of an ultrasonic cavitation bubble in abnormal fluids,which include subcritical water and supercritical carbon dioxide.The model of an ultrasonic cavitation bubble in abnormal fluid was built by Fluent,which is a kind of computational fluid dynamics software.The geometric model and meshing works were made by the preprocessor: Gambit.The grid validity was verified.The interface of cavitation bubble and fluid was construct by VOF model and fluid volume fraction method.By analyzing the fluid properties of subcritical water and supercritical carbon dioxide,boundary conditions and initial parameters can be determined.The dynamic simulation of a single ultrasonic cavitation bubble was carried out and compared with the normal fluid under the same conditions.The cavitation mechanism in the abnormal fluid was analyzed from the changes of morphology,pressure and velocity.The results show that :(1)when pressure of the ultrasonic sound is 5 MPa,the frequency is 20 k Hz,and the hydrostatic pressure is 5 MPa,the cavitation effect of cold water is better than that of the subcritical water.(2)When the pressure of ultrasonic sound is 10 MPa,the frequency is 20 k Hz and the hydrostatic pressure is 8MPa,the cavitation effect in supercritical carbon dioxide is obviously more severe than that in cold water under the same condition.In order to further explore the influencing factors of ultrasonic cavitation in abnormal fluid,numerical simulation was carried out by control variable method.The effects of fluid parameters and ultrasonic parameters on the dynamic behavior of cavitation bubble were compared.The results show that:(1)the increase of hydrostatic pressure will suppress the cavitation effect,which is reflected in the cavitation bubble can maintain its shape for a long time without collapsing,and the velocity at the center of the empty bubble is small.(2)Higher temperature can promote cavitation.When the fluid medium is subcritical water,the promotion effect of temperature is not as obvious as that in supercritical carbon dioxide.(3)On the premise of transient cavitation,increasing ultrasonic frequency can make the cavitation effect more severe.