| Energy problem is a major problem during the development of human society,and the thermal management is one of the key factors affecting the efficiency of energy conversion of energy equipment.Due to its high heat transfer efficiency,phase transition heat transfer technology has broad application prospects in the field of thermal management of electronic equipment.As a vapor-liquid phase transition enhanced structure with excellent performance,the porous metal structure has been widely used in the field of phase transition heat transfer due to its advantages of simple manufacturing process,high thermal conductivity,and strong stability.With the development of industrial equipment to light weight and high performance,its thermal management system has an increasing demand for high-performance aluminum phase transition heat transfer devices.However,owing to the limitation of the dense oxide layer on the aluminum surface,the traditional sintering process of metal powder in the field of phase transition heat transfer is no longer applicable for the aluminum porous structure.The low-cost fabrication of aluminum porous structures has become a major problem,which restricts the development of high-performance aluminum phase transition heat transfer devices.To address these issues,an innovative method of ultrasonic cavitation is proposed to fabricate aluminum porous structure,which can realize the fabrication of porous structure on the surface of aluminum plate and aluminum micro groove.In this thesis,the formation mechanism and phase transition heat transfer performance of ultrasonic cavitation porous structure are symmetrically studied.The main research contents and conclusions are summarized as follows:(1)Ultrasonic cavitation machining process and simulation analysisThe apparatus for ultrasonic cavitation machining was designed and built,and a reasonable operation process was formulated.Based on the actual ultrasonic cavitation machining process,a cavitation bubble growth model was established to study the influence of key parameters of ultrasonic machining on the evolution of cavitation bubbles.The results show that the parameters,such as temperature,initial cavitation radius,ultrasonic frequency and amplitude,can affect the life cycle and collapse radius of cavitation bubbles,which futher affect cavitation rate and cavitation impact strength.Simultaneously,the effect of temperature on the critical radius of cavitation bubbles is slight,while the critical pressure goes up with the increase of temperature.In addition,the theoretical models of the sound field and flow field during ultrasonic machining were established,and the influence of topography of aluminum and machining clearance on the distribution of sound field and flow field during ultrasonic machining was studied through the realization of multi-physics coupling simulation by finite element software.It is indicated that the distribution of sound field in the plane model is similar with that of the micro-groove model,and the micro groove can reduce the radial attenuation of sound pressure.With the decrease of the machining clearance,the amplitude of the sound pressure under the ultrasonic horn increases and the rate of radial attenuation is also on the increase.Moreover,the maximum flow velocity increases,and the highest flow velocity area develops towards the bottom of the groove.and too small processing spacing will also deteriorate the uniformity of the flow rate distribution between the micro-grooves.Besides,too small machining clearance will also deteriorate the uniformity of the flow velocity distribution between the micro-grooves.(2)Morphology and performance characterization of ultrasonic cavitation porous structureThe effects of ultrasonic cavitation machining parameters and grades of aluminum on the formation of the porous structure on the surface of the aluminum plate were investigated experimentally,and thereby the formation mechanism of porous structure on the aluminum surface with different topographies under the action of ultrasonic cavitation was analyzed.The results show that cavitation impact is the main reason for the formation of porous structure on the aluminum surface,pure water being the best working fluid,and the pore size of the porous structure decreases with the increase of the hardness of aluminum alloy.Porous structure preferentially occurs at the bottom of micro groove where the radius of curvature is smaller.However,by appropriately increasing the ultrasonic treatment time,the porous microstructure layer can be uniformly formed over the entire groove surface.Additionally,excessive ultrasonic cavitation treatment can also destroy the main structure of the micro grooves.After ultrasonic cavitation treatment,Both the wetting properties of the aluminum plate and the capillary properties of the aluminum micro grooves were studied.It is indicated that under the interaction of porous structure and organic functional groups,the aluminum surface with ultrasonic cavitation porous structure has become hydrophobic and super-oleophilic.The porous structure in the microgrooves not only provides additional capillary pressure,but also gradually increases the permeability resistance,and its capillary performance can still be improved to a certain extent under the combined effect.(3)Pool boiling heat transfer performance of ultrasonic cavitation porous structureThree kinds of aluminum surfaces with representative ultrasonic cavitation porous structures were selected for the experimental study of pool boiling heat transfer.The influence of ultrasonic cavitation porous structure on the pool boiling heat transfer performance was compared and analyzed,and the function mechanism of different types of porous structures on the boiling heat transfer performance under subcooling conditions was revealed.The results show that both MP1 and MP5 have better enhancement effect on boiling heat transfer performance,but MP1 also shows obvious boiling hysteresis.Under subcooling conditions,the micro-convective heat transfer regime is enhanced,and the bubble nucleation requires a higher degree of wall superheat,so the phase transition heat transfer become relatively weak and the critical heat flux increases.When all the nucleation sites are fully activated,MP1 has the best comprehensive boiling heat transfer performance.The visualization study of bubble in the boiling test also shows that the ultrasonic cavitation porous structure with high nucleation site density is beneficial to realize higher density of detached bubble at low heat flux,to improve the efficiency of bubble coalescence at medium heat flux,and to increase the bubble detachment rate at high heat flux.(4)Fabrication and heat transfer performance of aluminum flat heat pipe with ultrasonic cavitation porous groove wickThe reliable and stable manufacturing process of aluminum flat heat pipe is developed,and an aluminum flat heat pipe test platform is designed and built.The effect of ultrasonic cavitation porous structure on the start-up performance and steady-state thermal performance of aluminum flat heat pipe is studied.The results show that the R5 type porous groove wick is more suitable for the aluminum flat heat pipe than the R6 type.The critical heat load of UFHP-2 with R5-type porous groove wick is 30 W at the inclination of 0° and 70 W ～ 90 W at the inclination of 30°～90°,which are about 100% and about 75%～80% higher than that of FHP,respectively.In addition,the effect of filling ratio of working liquid and cooling temperature on the thermal performance of aluminum flat heat pipe is explored.The optimum filling ratio of the aluminum flat heat pipe with ultrasonic cavitation porous groove wick is related to its working inclination.The optimum filling ratio ranges from 100% to 125%,and the larger one is more suitable at the small incline angle.Meanwhile,the cooling temperature for the best comprehensive heat transfer performance of the aluminum flat heat pipe is 40℃. |
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