| With the improvement of the integration of electronic equipment,high performance heat transfer devices are required to accommodate increased rates of energy dissipation within these systems.Aluminum vapor chamber,which operation principle bases on two phase flow and heat transfer,is an important solution to this problem in the field of aerospace,5G mobile networks and electric vehicle.Wick structure is a deciding factor for enhancing heat-transfer capacity and heat transfer coefficient of vapor chamber.However,the traditional aluminum wick structures,e.g.extruded groove,stainless steel mesh and aluminum felt,has disadvantages of being low capillary pressure,high wall superheat and less vaporization core.The threedimensional sintered porous wick structure has excellent heat transfer performance,but it is hard to be manufactured as a result of the inert mass propagation of aluminum oxide film on the surface of the Al-particles.So,its manufacturing technology needs to be studied.In this paper,the aluminum powder sintered porous layer with high porosity is manufactured by liquid activated sintering on the surface of plate and pin-fin structure.the main research contents are summarized as follows:(1)Design of wick structure for gas and liquid phase transmission channel separationThe sintered pin-fin composite porous wick with the porous layer covering on the surface of micropillar is designed to overcome the deficiency of the traditional powder sintered wick structure in terms of nucleation area and gas-liquid separation.Using the minimum pressure drop as the objective function,a thick sintered porous layer with small pore size applied on the bottom of wick structure and a thin sintered porous layer with large pore size applied on the surface of pin-fin pillar are designed in separately.The hydraulic dynamic model derives the optimal structural parameters,e.g.pin-fin height and center distance of adjacent pillar,for the stability of steam flow.(2)Manufacture of Al powder sintered porous wick structreThe difficulties of the fabrication method and process of Al powder sintered porous wick structure may restrict the application of vapor chambers.Using the liquid activated sintering method,the sintering mechanism of Al particles was analyzed,and the effects of activator content,temperature curve and apparent density on the composite porous structures were examined.The results show that the uniform aluminum powder sintered porous wick structures with porosity exceeding 55% were obtained under the process parameters of 10 wt% Al Si10 Mg alloy addition,615 ℃ sintering and holding time for 10 minutes.Finally,the three-dimensional pin-fin composite porous sintered wick structure was fabricated by CNC milling molding,powder liquid-phase activation sintering and laser processing.(3)Evaluation of liquid transport properties of aluminum porous structuresIn order to enhance the liquid transport performance of the wick structure for the vapor chamber,chemical acid pore etching is adopted to optimize the morphology of the sintered aluminum porous wick stucture.The wettability properties of porous materials are measured with contact angle.Based on capillary wicking dynamics,a method for evaluating liquid transport in porous aluminum wick structure was established,and the capillary wick performance of porous structure was tested by infrared capillary climbing.The laws of powder diameter,porosity,surface micro-modification and composite porous structure on capillary wick performance were systematically studied.The results show that chemical etching helps to expand multi-scale pores,increase surface micro-scale concave pores,and enhance liquid wetting and spreading performance;can effectively inprove the overall performance.The capillary pressure,permeability and gas flow resistance of the treated porous wick are balanced comprehensivelythe liquid penetration resistance and further improve the liquid transport performance.(4)Pool boiling performance of sintered composite porous wick structuresA pool boiling test platform was built to test the boiling heat transfer performance of the sintered porous wick structure.The effects of structural parameters such as the thickness of the porous layer,the particle size of the aluminum powder,the porosity,the surface modification treatment and the pin-fin composite wick structure on the boiling heat transfer performance of the sintered porous structure were comparatively studied.The results show that the thickness reduction improve the critical heat flux density(CHF),while the particle diameter and porosity mainly affect the boiling superheat.The pin-fin composite wicks obvious reduce the boiling superheat and increase the CHF.The optimal parameters were as follow: porous layer on the surface of pin-fin with the thickness of 0.35 mm,the central distance between pillars of 3 mm and the pillar height of 2.95 mm.The boiling heat transfer coefficient of pin-fin composite wick structure with the optimal parameters were found to be 180% times that of the aluminum sintered porous wick and 7times that of aluninum plate.(5)Application of pin-fin composite porous structure in aluminum vapor chamberBased on the demand of the large heat consumption digital equipment for the aluminum vapor chamber,the pin-fin composite porous aluminum sintered wick structure is applied in the aluminum vapor chamber.Firstly,according to the shell mechanics analysis and the gas flow pressure drop theory of vapor chamber,the shell parameters,the height of the vapor chamber and the wick structure of the evaporation and condensation surface are designed.The manufacturing process of the pin-fin composite porous wick structure aluminum vapor chamber was formulated,and the aluminum powder sintered wick structure and the pin-fin composite porous aluminum wick structure aluminum vapor chamber were prepared.Then,the aluminum vapor chamber were tested at different anti-gravity heights(inclined angles),heat source size and operating temperature(cooling water temperature on the condensing surface).Both of them achieved good start-up and temperature uniformity performance,while the pin-fin composite porous structure can effectively reduce the axial temperature difference and thermal resistance of the vapor chamber device under the same power.Under the condition of satisfying a certain thermal resistance,the pin-fin composite wick structure vapor chamber has a higher limit heat power. |
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