Effect of mechanical agitation on ultrasonic cavitation dynamics

In order to further investigate the effect of the vortex induced by mechanical agitation on the ultrasonic degradation rate of organic solution, with water used as a medium, the acoustic field distributions at different stirring speeds are simulated by using the simulation software COMSOL. The simulation of acoustic field distribution is divided into two steps. First, the flow field distribution in the cleaning tank is obtained by using the Navier-Stokes equation and the continuity equation under the corresponding boundary conditions. Next, the velocity and pressure in the flow field are substituted into the acoustic wave equation to obtain the acoustic field distribution. In addition, the instantaneous acoustic pressure obtained by simulation is fitted by Origin, and the fitting curve shows a good sinusoidal shape. Then, substituting the fitting function into the Keller-Miksis equation, the variations of radius of the cavitation bubble with time at different stirring speeds are obtained. Finally, the temperature of the cavitation bubble is calculated from the obtained radius. The results show that mechanical agitation increases the uniformity of acoustic field distribution and the amplitude of acoustic pressure, and that the bubble temperature is greatly enhanced due to the agitation. At the same time, it is also found that the internal temperature of the bubble first increases with the stirring speed increasing. When the stirring speed reaches 1500 r/min, the temperature begins to decrease with the stirring speed increasing. The temperature inside the cavitation bubble reflects the intensity of acoustic cavitation. The higher the temperature, the greater the intensity of acoustic cavitation will be. Therefore, it can be concluded that the acoustic cavitation intensity will decrease when the stirring speed is too high. Therefore, though mechanical agitation can improve the acoustic cavitation intensity, too high stirring speed can reduce the acoustic cavitation intensity. In order to verify the simulation results, the degradation of methylene blue is performed by ultrasound coupled with mechanical agitation, and the experimental results show that the degradation rate of the solution without mechanical stirring is lowest. The degradation rate of the solution increases with the stirring speed increasing. When the stirring speed reaches 1000 r/min, the degradation rate of the solution is the same as that at 600 r/min, and then decreases with the stirring speed increasing. It can be found that the experimental results are consistent with the simulation results. The simulation results not only theoretically explain why mechanical agitation can improve the ultrasonic degradation rate of organic solution, but also indicate that too high stirring speed can reduce the acoustic cavitation intensity, thus reducing the sonochemical reaction rate. Therefore, the results obtained in this work provide a new idea for further improving the ultrasonic degradation rate by mechanical agitation.