Construction Of Focused Ultrasound And Its Heat Transfer Enhancement Performance

Ultrasonic enhanced heat transfer is an effective active heat transfer enhancement technology.It has high application value in industrial heat exchangers,air conditioners,electronic chip heat dissipation,etc.However,ultrasonic enhanced heat transfer technology still needs to be further optimized and upgraded to meet the performance requirements of industrial applications.The local sound intensity of the heat exchange wall is a direct factor of ultrasonic heat transfer enhancement.However,no scholars have optimized the ultrasonic heat transfer performance by changing the sound intensity distribution.In order to improve the utilization rate of acoustic energy for ultrasonic enhanced heat transfer,this paper proposes the idea that using the focused ultrasound to enhance heat transfer and improve sound intensity distribution mode,which can further improve the effect of ultrasonic enhanced heat transfer.Two construction criteria for the focused ultrasonic field are proposed:the interference criterion and the standing wave criterion.The interference coefficient and the standing wave coefficient are defined,and the theoretical model of the focused ultrasonic field suitable for ultrasonic enhanced heat transfer is constructed.The theoretical model of the focused ultrasonic field is verified by finite element simulation.The acoustic focusing process and sound field distribution in the acoustic focusing cavity are simulated.The acoustic focusing tank is engineered according to the actual working conditions of ultrasonic enhanced heat transfer.The distribution and focusing effect of the focused ultrasonic field are simulated by acoustic finite element simulation with different interference coefficients and standing wave coefficients.The results show that the focused ultrasound field can be effectively realized when both the interference coefficient and the standing wave coefficient are integers.The sound field distribution in two different parameters of the elliptical acoustic focusing tanks and two rectangular tanks were measured experimentally.Both elliptical acoustic focusing tank were able to achieve acoustic focusing.The sound intensity of the elliptical sound focusing tank near the sound source area is higher.As the distance from the sound source increases,the sound intensity gradually decreases,and the focus area increases sharply.The experimental results are in good agreement with the simulation results,and the theoretical model of the focused ultrasound field is verified.The acoustic flow and ultrasonic cavitation in different chambers are studied,and the characteristics of the focused ultrasonic field under three-dimensional conditions are further explored.The results show that the acoustic flow of the elliptical acoustic focusing tank is stronger than that of the rectangular tank.A comet-like vacuole is found at the focus of the elliptical acoustic focusing tank with a standing wave coefficient of 5,while an elliptical vacuole is found at the sound source.The growth and motion mechanisms of the two are different,and the focused ultrasound field is found.Separation of effects and effects.The heat transfer coefficient and the ultrasonic enhancement rate distribution of the x-axis and y-axis in the elliptical acoustic focusing tank were measured by natural convection heat transfer experiments.The distribution of the heat transfer coefficient matches with the sound intensity distribution,where the focal point of the elliptical sound focusing tank of k₂=5 The ultrasound enhancement rate was the highest,which is up to 74.7%.The mechanism of the focused ultrasonic field under different heat transfer conditions was studied by the pool boiling experiment.The results show that the focused ultrasound has the best heat transfer enhancement rate under natural convection conditions,and the ultrasonic enhancement rate is lower under the condition of subcooled boiling heat transfer.While under the saturated boiling heat transfer conditions are negative strengthening effects.Through the observation of the boiling bubble,the ultrasonic suspension phenomenon of the bubble and the mulberry bubble group are discovered,and these bubble groups affect the enhanced heat transfer effect of the focused ultrasound.According to the experimental phenomenon,the theory of evaporative heat pump for ultrasonic cavitation is proposed,which can explain the mechanism of enhanced heat transfer by focused ultrasound.