Optimizing Working Liquids For Realizing Large Scale Cavitation

When the negative sound pressure of the sound wave transmitted in liquids reach-es a certain value,visible bubbles emerge.This phenomenon is named as acoustic cavitation and these bubbles are called cavitation bubbles.Cavitation bubbles will expand and contract under the driving of sound wave.When it collapse violently,ex-treme high temperature and pressure are developed therein,meanwhile the jetting and shockwave are formed.Cavitation is utilized in all kinds of fields,such as sonochem-istry,cleaning,extraction,emulsification,chemical synthesis and degradation and so on.Although cavitation has broad application prospects,it has not been widely used in production and life.One of the most important reasons is that if the input power of ultrasound is small,cavitation is not obvious and the efficiency is low.But if we improve the input power to a certain extent,large number of cavitation bubbles will emerge at the area around the radiation surface of the transducer.These bubbles ob-struct the propagation of ultrasound as a screen so that the sound energy is restricted in the region adjacent to the transducer.In short,localization of sound energy happens.This thesis discusses the effect of cavitation screening on the sound field,and finds the sound energy density on far field decreases with driving power if the driving power is high enough,which results in strong attenuation for a propagating wave.Considering cavitation is related to both the sound field and the cavitation threshold,for realizing the uniform distribution of cavitation bubbles,we propose to use varying cavitation threshold liquids.Besides,we find that we can take advantage of cavitation to realize the asymmetric transmission of sound wave in double-layer liquids.1.Negative effect of driving power on far field sound energy density.Un-derstanding the sound field is a precondition to realize large-scale cavitation.During our investigation on the sound field distribution in cavitating liquids,we find that with the increasing of the driving power,the sound energy density of far field undergoes a procedure of increasing firstly and then decreases.The decreasing stage means the negative effect of driving power happens.The main reason of this phenomenon is the energy transferring from the fundamental to harmonics caused by the nonlinear oscil-lation of cavitation bubbles.Under high driving,nonlinear oscillation of cavitation bubbles is strong,which results in that higher proportion of harmonics exists in the ultrasound comparing with that under low driving.Considering the attenuation is pos-itively correlated to frequency,harmonics will decay faster than fundamental during the propagation of sound wave.So under high driving,the total sound energy reach-ing far field is less than that under low driving.Then we discuss the effect of driving and cavitation threshold on the length of negative effect region.We find that raising the cavitation threshold or reducing the driving power is beneficial for shrinking the negative effect region,these results help us further understand the cavitation screening.2.Propagation of sound wave in double-layer cavitating liquids.Considering that the sound energy decreases with the propagation of sound wave,on far field the sound energy is not high enough to induce any cavitation,we propose to use host liquid-s whose cavitation thresholds decrease with the distance to the transducer to optimize the homogeneity of cavitation bubbles.We pay our attenuation on the double-layer liquids composed of two kinds of liquids whose cavitation thresholds are different with each other.Experiments show using double-layer liquids is helpful in optimizing the homogeneity of the distribution of cavitation bubbles.During our investigation,we find the asymmetric transmission of sound energy in this system.When the sound wave propagates from the high cavitation threshold liquid to the low cavitation thresh-old liquid,the sound pressure keeps to be lower than the cavitation threshold,nearly no cavitation happens,the output sound pressure is relatively high.When the sound wave propagates from the low cavitation threshold liquid to the high cavitation threshold liq-uid,because the input pressure is higher than the cavitation threshold of the liquid with low cavitation threshold,cavitation appears and the output sound pressure is relatively low due to the screening effect.Besides,the direction of rectification of sound energy can be reversed if the driving is high enough.Both of the two liquids will be cavitat-ed when the acoustic wave propagates from the high cavitation threshold liquid to the low cavitation threshold liquid,but only the low cavitation threshold liquid is cavitated when the acoustic wave propagates from the low cavitation threshold liquid to the high cavitation threshold liquid.We think that large cavitation region may corresponds to high attenuation,which makes the direction of rectification reversed.Linear and non-linear theory is taken to reproduce this observation and the effects of more parameters on the asymmetry,such as the maximum bubble number density and ambient radius of bubbles are discussed.3.Propagation of sound wave in multi-layer cavitating liquids.Three-layers liquids is used to realize the homogeneity of cavitation.Linear models are taken to simulate the sound fields and the distributions of cavitation bubbles in the cavitating liquids.The results show that when the amplitude of driving sound pressure is fixed,the differences between the cavitation thresholds of the three layers which makes the cavitation bubbles most uniformly distributed in the liquid increase with the ambient radius of these bubbles.When the liquid is unchanged,with the increasing of the amplitude of driving sound pressure,the asymmetry of the system decreases firstly and then increase.That implies for a certain three-layer liquid,there exists the most appropriate driving which makes the distribution of cavitation bubbles to be the most uniform.The experiments are in preparation.