| With the development of the economy and population growth,the environmental pollution caused by organic pollutants from chemical plants and textile mills is becoming increasingly serious.Therefore,it is necessary to find an efficient,economical,and green degradation method,which is very important for human health and the development of the global economy and society.In recent years,ultrasonic oxidation is regarded as one of the most promising degradation methods.The basic principle of ultrasonic degradation is ultrasonic cavitation of hydroxyl radical.When the small bubbles in the liquid are subjected to the force of ultrasonic waves,acoustic cavitation will occur.In the process of cavitation of bubbles,hydroxyl radicals will be generated,so as to achieve the purpose of degrading sewage.One of the conditions for generating acoustic cavitation is that the amplitude of ultrasonic sound pressure must be at the cavitation threshold.Therefore,the sound field distribution and sound pressure value in the ultrasonic cleaning tank are two key factors affecting the ultrasonic degradation rate of organic solutions.As a reactor for ultrasonic degradation experiments,the transducer in the ultrasonic cleaning tank is located at the bottom of the tank so that the liquid surface reflects the ultrasonic waves from the bottom of the tank,and finally,the standing wave sound field is formed in the ultrasonic cleaning tank.In a standing wave,the amplitude of sound pressure at the trough is less than the cavitation threshold,so there will be no cavitation at the trough.In order to eliminate the standing wave sound field and improve the ultrasonic degradation rate of organic solutions,it is necessary to optimize the sound field distribution in the ultrasonic cleaning tank.In this dissertation,the sound field distribution in the water tank under mechanical agitation and metal structure was simulated numerically,and the ultrasonic degradation experiment was designed to verify the simulation results.The main research contents is listed below.1.The flow field and sound field distribution caused by mechanical stirring were numerically simulated using the simulation software COMSOL.The simulation results show that mechanical stirring in the water tank can eliminate the standing wave sound field and improve the uniformity of the sound field distribution.The influence of stirring speed on the sound field distribution in the water tank was studied.Next,a point was selected from the water tank,and the sound pressure values at the point was simulated at different moments,and the sound pressure variation function with time was fitted.The fitted sound pressure function was then substituted into the Keller-Miksis bubble dynamics equation to obtain the cavitation bubble radius at the point as a function of time.Based on the radius of the cavitation bubble,the velocity of the cavitation bubble wall and the internal temperature of the cavitation bubble at the moment of rupture were calculated.The velocity of the bubble wall and the temperature inside the bubble are proportional to the intensity of acoustic cavitation.Therefore,based on the above method,the influence of the mechanical stirring speed on the acoustic cavitation intensity can be theoretically analyzed,which provides a theoretical basis for improving the degradation rate of the organic solution in the ultrasonic and mechanical stirring experiment.2.In order to verify the correctness of the simulation results,an ultrasonic radiation synergistic mechanical stirring experiment was designed to investigate the effect of stirring speed on the ultrasonic degradation rate of methylene blue solution.The experimental results showed that the ultrasonic degradation rate of methylene blue solution increases with the increase of mechanical stirring speed,reaches the maximum when the stirring speed is 600 rpm and 1000 rpm,and then begins to decline.At the stirring speed of 2000 rpm,the degradation rate of the solution was equal to that at the stirring speed of 300 rpm.The experimental results are consistent with the simulation results because the degradation rate of organic solutions is related to the cavitation intensity.The higher the cavitation intensity,the greater the ultrasonic degradation rate of the solutions.Besides,the effects of the mass of methylene blue and the volume of the solution on the ultrasonic degradation rate of the methylene blue solution were also investigated.The higher the initial concentration of the solution,the lower the ultrasonic degradation rate of the solution under the same parameters.When the quality of the drug is the same,the smaller the volume of the solution,the higher the concentration of the solution.At the same time,the greater the mass of methylene blue,the greater the change rate of absorbance of the solution.3.Based on the sound wave equation,the influence of mechanical stirring on the sound field distribution in the water tank is explained from the acoustic theory.The liquid level shape in the ultrasonic cleaning water tank at a certain stirring speed was obtained by using the fluid balance equation.According to the acoustic equation and the reflection characteristics of the incident acoustic wave on the interface of two different media,the relationship between the reflection angle and reflection coefficient of the reflected sound pressure and the gradient of the interface is derived.The relationship between the liquid surface slope and the horizontal direction change at a certain stirring speed is substituted into the expression of reflected sound pressure,and the reflected sound pressure distribution in the water tank at a certain stirring speed was obtained.Then,the scattered sound pressure at different flow rates in the sink was simulated numerically.The simulation results show that the scattered sound pressure caused by mechanical stirring is minimal,and its influence on the sound field distribution in the water tank can be neglected.The sound field after mechanical agitation is the sum of the incident sound pressure and the reflected sound pressure.Finally,the theoretical calculation results are verified by numerical simulation.Next,based on the relationship between the reflected sound pressure and the liquid surface slope,a parabolic metal body was added to the ultrasonic cleaning tank.The acoustic hard boundary of the metal body can change the reflection direction of the incident sound wave,so as to achieve the purpose of optimizing the sound field distribution in the tank.It can be seen from the simulation results that adding a parabolic metal body to the ultrasonic cleaning tank can improve the acoustic cavitation intensity at the node of the standing wave.In addition,the influence of the parabolic metal structure on the sound field distribution and acoustic cavitation intensity in the container is also analyzed.4.In order to eliminate the standing wave sound field in the ultrasonic cleaning tank and improve the uniformity of the sound field distribution in the tank,a metal disk with round holes was placed just above the four transducers at the bottom of the tank.The influence of the aperture of the metal disk on the sound field distribution in the container was studied.Based on the transmission coefficients of the plane sound wave at the interface of two different media,it was found that the plane sound wave emitted by the transducer mainly propagates to the water tank through the circular holes in the metal disk.In addition,when sound waves propagate to the edge of a circular hole,acoustic diffraction occurs.Therefore,with a metal disk with a circular hole,the total sound pressure includes the sum of the sound pressure passing through the circular hole,the diffraction sound pressure,and the reflection sound pressure.The simulation results show that placing the perforated metal disc in the water tank can eliminate the standing wave sound field in the water tank and improve the uniformity of the sound field distribution in the water tank.When the aperture of the metal disc is small,the sound pressure value in the water tank is reduced;When the aperture is too large,the standing wave sound field is formed in the water tank,and the cavitation area is reduced.In order to improve the ultrasonic degradation rate of organic solutions,three methods are put forward to improve the uniformity of the sound field distribution and acoustic cavitation intensity in the cleaning tank:mechanical stirring,placing the parabolic metal body and metal disk with a round hole in the tank.The results provide a theoretical basis for studying the ultrasonic degradation rate of solutions from the angle of the sound field and promote the development of ultrasonic degradation experiments. |
