Hydraulic cavitation(HC)is an important method of producing cavitation,and it plays an important role in industrial processes such as sewage treatment,sterilization and chemical reaction acceleration.The hydraulic cavitation device featured by Venturi-pipe is simple in structure and consumes low energy,and has been widely used.Liquid medium imposes critical effect on cavitation.Thus far,cavitation phenomena in acidic and alkaline solutions have been acknowledged,but related studies have rarely been reported.A systematic study on effects of physical properties of liquid medium on cavitation is of great significance for deepening the understanding of the mechanism of cavitation and extending the application of cavitation.In present dissertation,a hydraulic cavitation study was carried out using water,0.5%and 1.0%NaOH solutions,1.0%and 2.0%CH3COOH solutions as liquid media.Cavitation and flow characteristics of these liquid media were compared.A closed loop with a Venturi pipe was constructed for producing hydraulic cavitation.The high-speed photography(HSP)technique was used to observe cavitation and its evolution in the Venturi pipe under different flow conditions.Pressure fluctuations were measured using a high-frequency pressure measurement system.The relationship between cavitation,flow and the energy of pressure fluctuation is expected to be established.Major works and conclusions drawn from the study are as follows:(1)With the closed loop,the five liquid media were devoted to hydraulic cavitation experiments.Instantaneous cavitation patterns in the Venturi pipe were captured using a high-speed camera and then processed using image-processing codes.The effects of liquid medium and flow rate on cavitation were investigated.For the five liquid media considered,the developments of cavitation clouds are similar and periodic.The cavitation intensity gradually increases with increasing flow rate.The critical flow rate for cavitation inception in water is considerably higher than the others.Under the same flow conditions,the length of the cavitation cloud in water is the smallest.For NaOH and CH3COOH solutions,as the mass fraction increases,cavitation effect is enhanced.A re-entrant jet structure is formed immediately downstream of the cavitation cloud.Such a characteristic structure expands as the flow rate increases.At the same flow rate,compared with the other four liquid media,water is associated with the highest intensity of the re-entrant jet.(2)Pressure fluctuations at different monitored points in the Venturi pipe were measured using the LMS pressure fluctuation measurement system.The time domain data were processed using Fourier transform and then analyzed statistically.A comparison of pressure fluctuation distributions and spectrum characteristics was performed between the liquid media and under different flow conditions.The mechanism of pressure fluctuation in the liquid media flowing in the Venturi pipe was revealed.As the cavitation inception is not accomplished,the proportions of pressure fluctuation amplitudes at the monitored points are nearly equivalent,and the characteristic frequencies are gathered within the frequency range of 0–500 Hz.As cavitation occurs,the proportion of pressure fluctuation amplitudes at each monitored point rises sharply,and the characteristic frequencies gradually shift to the high-frequency zone.For the liquid media considered,the pressure fluctuation amplitudes at the monitored points vary similarly with the flow rate.As the flow rate increases,the proportion of pressure fluctuation amplitudes at the monitored points gradually increases.Meanwhile,the monitored point where the maximum pressure fluctuation amplitude is reached tends to migrate towards the outlet of the Venturi pipe.(3)A comprehensive analysis of cavitation clouds and pressure fluctuations in the liquid media and under different flow conditions was performed.The power spectral density of pressure fluctuations,total pressure fluctuation energy and its spatial distribution,RMS energy at each monitored point were obtained.The relationship between the pressure fluctuation energy and the evolution of cavitation was explained.The contribution of cavitation to the pressure fluctuation was evaluated.As the liquid flow rate increases,all monitored points share an increase in the power spectral density.Drastic variation of the power spectral density occurs at some monitored points,and the variation amplitude increases with the liquid flow rate.As the flow rate increases from 17.7 m3/h to 24.9 m3/h,the overall pressure fluctuation energy in the cavitating flow increases continuously.The energy is subjected to the effect of the liquid medium.Under the same flow condition,the energy in the liquid with relatively low mass fraction is higher than its counterpart in the liquid with high mass fraction.The position where the drastic variation of pressure fluctuation energy occurs approaches the position of cavitation collapse.This demonstrates the pressure fluctuations and the variation of pressure fluctuation energy are induced primarily by cavitation collapse. |