Study On The Effect Of Ultrasound Cavitation And Its Application In Enhancing Heat Transfer

At the present,in the fields of energy,chemical industry,machinery,aerospace,information and so on,the heat transfer load of the heat exchange system is becoming higher and higher,and the use environment of heat exchange equipment is becoming more and more stringent.Modern highly integrated electronic devices have more and more integrated transistors per unit area,resulting in increasing heat flux density of electronic devices.However,the traditional heat dissipation and cooling technology cannot meet the heat dissipation requirements of these high-performance equipment.Therefore,it is urgent to research and develop excellent heat transfer technology to solve the problem of energy shortage and meet the heat transfer requirements of high performance equipment.As the latest enhanced heat transfer technology,ultrasound has attracted much attention from the industry and is expected to have a very broad application prospect in the future.Therefore,it is necessary to study the ultrasonic heat transfer enhancement technology.In this paper,the effect of ultrasonic waves on the boiling heat transfer of pure water and on the heat transfer of nanofluids in natural convection,subcooled boiling and saturated boiling has been discussed through experiments.The main work of this paper is as follows.(1)An ultrasonic enhanced fluid heat transfer experiment table was designed and established.Rohsenow's empirical formula was used to verify the reliability of the experimental system and the data obtained by the experimental system were reasonable.(2)The influence of different ultrasonic power,liquid subcooling on boiling heat transfer of pure water is analyzed.The experimental results show that under the conditions of higher sound power,lower supercooling and closer distance,the heat transfer enhancement effect of ultrasonic wave is obvious,on the contrary,the enhancement effect is relatively weak,in addition,the principle of enhanced heat transfer is explained and analyzed from a theoretical point of view.(3)When the experimental medium is Al2O3 nanofluid with volume fraction of 0.01%,the ultrasonic wave shows the enhancement of fluid heat transfer under natural convection,but the enhancement of heat transfer shows a decreasing trend.At the same heat flux,the heat transfer enhancement ability becomes weaker and weaker with the increase of fluid temperature.When the liquid temperature is 50?,70?and 80?,the heat transfer of nano-fluid by ultrasound is enhanced.Its strengthening efficiency can reach up to 14%,27%and 22%respectively.However,when the liquid temperature is 60?,the heat transfer is inhibited,which is related to the strong pressure pulse formed when the ultrasonic cavitation bubble collapses.In saturated boiling state,ultrasound will enhance the boiling heat transfer of nanofluids,and with the increase of heat flux density,the heat transfer coefficient will increase,but the heat transfer enhancement capability will also decrease gradually.However,compared with subcooled boiling,nano-fluid has stronger heat transfer capability in the initial stage of boiling with low heat flux density,while in the saturated boiling stage with high heat flux density,ultrasonic enhanced heat transfer capability has decreased.(4)When there is ultrasonic action,the nano-fluid has a lower critical heat flux density.After this point,the heat flux density drops rapidly in a smaller wall superheat.When the wall superheat is raised again,the heat flux density will increase again,but the growth rate is slower.And in the presence of ultrasound,the entire boiling curve of nanofluids is to the left of the boiling curve in the absence of ultrasound,indicating that the superheat required to cause boiling is small.(5)The surface characteristics of the metal platinum wire after boiling with or without ultrasonic wave concentration of 0.01%were analyzed by scanning electron microscopy.It was found that a layer of nano-particle adsorption layer was sintered on the surface of the platinum wire after the boiling of the nano-fluid without ultrasonic action,and the surface roughness and hole density were significantly increased compared with those when ultrasonic action was applied.On the one hand,when there is ultrasonic action,Brownian motion in the nanofluid is more intense and nanoparticles are less likely to deposit.On the other hand,even if there are particles deposited on the surface of the platinum wire,the sound flow due to ultrasonic vibration and the strong impact force on the surface of the platinum wire when cavitation bubbles collapse also prevent particles from being deposited on the surface of the platinum wire for a long time.