微颗粒对超声空蚀影响的试验研究

Experimental Study of the Effect of Microparticles on Ultrasonic Cavitation

为了探明微颗粒对超声空蚀影响的具体机制,使用超声振动空蚀试验装置对45钢在四种不同的微颗粒-纯水的悬浊液中的空蚀行为进行了研究,使用扫描电子显微镜和激光共聚焦显微镜观测空蚀后的样品表面,对空蚀坑的数量分布及微观结构进行分析,结果表明空蚀破坏程度与SiO_(2)悬浊液浓度有重要关联.SiO_(2)悬浊液存在1个临界浓度,小于临界浓度时,空蚀破坏程度随浓度的增加而增大;大于临界浓度时,空蚀破坏程度随浓度的增加而减弱.空蚀破坏程度随粒径的增大而减弱,但空蚀坑的坑径随微颗粒粒径的增大而增大.相比于纯水介质,添加SiC、Al_(2)O_(3)和SiO_(2)微颗粒均会加剧空蚀破坏程度,其中,SiC微颗粒对空蚀破坏的促进最为显著.相比之下,Al微颗粒添加后空蚀破坏却明显减弱,起到了抑制超声空蚀的作用,这可能与不同颗粒所负荷不同电荷类型有关.

Cavitation erosion is a common form of damage that occurs in hydraulic turbines,propellers,turbine blades and other hydraulic machinery components.It arises from the strong micro-jets and shock waves produced as the cavitation bubbles collapse,which would cause damages to the near-wall surfaces of the equipment.It is worth to be noted that micro-particles have significant impact on the cavitation erosion because the micro-particles in the liquid will reduce the threshold of vacuole nucleation and then promote heterogeneous nucleation.Though many studies on the effect of micro-particles on cavitation erosion,the present results are ambiguous,or even contradictory with each other.In order to investigate the effect of the micro-particles on ultrasonic cavitation erosion,cavitation experiments on the substrate of 45 steels were carried out in suspensions of pure water with different SiO_(2) concentrations(0.01,0.1,1,10 g/L),SiO_(2) particle sizes(500 nm,2μm,70μm)and various micro-particles(SiC,Al_(2)O_(3),SiO_(2),Al)by using an ultrasonic vibration cavitation test device.The experimental ultrasonic power was 480 W and the total experiment time was 10 min.In addition,a distance of 15 mm between the sample and the variable amplitude rod was adopted.During the experiments,the water temperature was kept at about 20℃and a peristaltic pump was employed to circulate the suspension of micro-particles in pure water to avoid precipitation of the micro-particles.The surface morphologies of the samples were observed by using a laser confocal microscope and a scanning electron microscopy.The number of cavitation pits in the acquisition area,which was used to characterize the degree of cavitation damage,was counted by using ImageJ software.The results were as follows:1)The degree of cavitation damage depended on the concentration of the SiO_(2) suspension significantly.In this work,the number of cavitation pits ranged from 103 to 396.However,there was a critical concentration of 1 g/L.At this concentration,the cavitation pits was at the maximum of 396.The degree of cavitation damage increased with increasing concentration when the concentration was less than the critical concentration.This was because the large micro-particle concentration corresponded to big micro-particle number.The number of the gas nuclei and heterogeneous nucleation sites also increased,which produced more cavitation bubbles and inevitably led to more cavitation pits.When the concentration increased to 10 g/L,the number of cavitation pits,however,reduced to302.This was attributed to the agglomeration of microparticles,the formation of"protective layer"and the combined action of the cavitation bubble-microparticle association.2)The degree of cavitation damage was closely related to the particle size of the SiO_(2) micro-particles.In this work,the number of cavitation pits decreased from 454 to 264 with the increase of particle size.However,the pit sizes of cavitation pits became bigger with increase of the micro-particle size.When the SiO_(2) microparticle size was 70μm,the cavitation pit diameter reached 20~40μm,which was far larger than those of other SiO_(2) microparticle sizes.It can be attributed to the larger diameter of the bubbles formed on the surface of the larger particle size.3)The addition of SiC,Al_(2)O_(3) and SiO_(2) micro-particles aggravated the degree of cavitation damage,and the number of cavitation pits was 257,210 and 396,respectively,which was much larger than that of 103without the addition of micro-particles.However,the addition of Al micro-particles suppressed the ultrasonic cavitation damage,and there were almost no cavitation pits on the sample surface.This provided a novel new idea for inhibiting cavitation damage.