| With the rapid development of semiconductor technology,high performance,lightweight and thin become the development trend of portable electronic products,but the accompanying increase in chip thermal power consumption and reduced heat dissipation space make portable electronic products face a serious heat dissipation problem,which seriously affect their stable operation.Heat pipe has the advantages of passive heat dissipation,high thermal conductivity and lightweight,which is an ideal component to solve the heat dissipation problem of electronic products.Therefore,the manufacture of ultra-thin heat pipes with efficient heat transfer performance(power transfer and thermal conductivity)has become the focus of research to solve the thermal control problems of thin portable electronic products.Thus,this paper firstly focuses on improving the heat transfer performance of ultra-thin heat pipe by studying and optimizing its inner cavity structure and wick.Then,the special ultra-thin heat pipes are designed and manufactured for the heat dissipation requirements of different thin electronic products,and their heat transfer performance are experimentally investigated.The main research contents are as follows:(1)Theoretical analysis of the inner cavity vapor-liquid flow of layered and novel spaced structure ultra-thin heat pipes.Firstly,the relational equation of the steam chamber friction coefficient of conventional layered structure heat pipe is derived and its variation rules with the thickness and width of steam cavity is obtained.It was found that the steam chamber friction coefficient increases rapidly with the decrease of thickness when the thickness <0.3 mm.In order to optimize the support structure,numerical calculation was used to analyze the effects of the shape,diameter and spacing of the support columns in the steam chamber on the vapor pressure drop.Then,a new type of flat-plate heat pipe with spaced structure inner cavity is proposed,and the relationship between the steam channel friction coefficient and steam channel size is obtained.Finally,the vapor and liquid flow properties of the layered structure and the spaced structure flat-plate heat pipe were compared under the same inner cavity size condition.The results show that the proposed spaced structure is more suitable for ultra-thin heat pipes.(2)Design and fabrication of a new grooved mesh wick and its capillary performance.Based on the wick is another important factor that affects the heat transfer performance of ultra-thin heat pipes.Firstly,numerical simulations were used to analyze the effects of the width and number of micro grooves in the wick on its permeability and flow heat transfer performance.Then,a new grooved mesh wick was designed and fabricated,and a wick capillary rise device was built for testing.The characteristics of existing hydrophilic treatment processes were analyzed,and the effects of themal oxidation and oxidation-reduction treatment processes on the capillary performance of the wick were systematically studied.(3)Design,fabrication and thermal transfer performance experimental study of new ultra-thin heat pipes oriented for different application.Based on the previous research findings,different types of ultra-thin heat pipes were designed and manufactured for the heat dissipation requirements of different electronic devices.Electronic products such as notebook computers require high transmission power,but not very thin thickness(1-2 mm)for ultra-thin heat pipes.For this reason,flattened ultra-thin heat pipes(1.2 mm)and mesh combination ultra-thin heat pipes(1.26-1.77 mm)were designed and manufactured,respectively.Heat transfer performance testing devices for the ultra-thin heat pipe were built.The heat transfer performance of four types of flattened ultra-thin heat pipes(1.2 mm)with multilayer #200 mesh wick,grooved mesh wick,and grooved mesh wick treated with oxidation-reduction at 400 ℃ and 500 ℃ were tested and compared.The effects of wire diameter and mesh number of coarse and fine mesh,as well as the working angle on the heat transfer performance of the mesh combination ultra-thin heat pipes(1.26-1.77 mm)were studied.Electronic products such as high-performance smartphone require ultra-thin heat pipes with high heat transfer capacity(>5 W)under extremetly thin condition(<0.6 mm).Therefore,a spiral woven mesh composite flat screen type extreme ultra-thin heat pipe(0.5 mm)with spaced inner cavity was designed and fabricated.Then,side-by-side spiral woven mesh composite flat screen type extreme ultra-thin heat pipe(0.5 mm)with higher heat transfer capability was produced by optimizing the wick structure.After that,the thickness of the heat pipe was further reduced by the innovation of structural design and manufacturing process,and a high performance side-by-side spiral woven mesh composite flat screen at both ends type extreme ultra-thin heat pipe(0.4 mm)was successfully fabricated.Their heat transfer performances were investigated in horizontal,gravity,anti-gravity,side and invert placements,and then compared with the advanced ultra-thin heat pipes in the existing literatures.(4)Effect of ultra-thin heat pipe structure on the condensation and evaporation of the working fluid.To further investigate the effect of ultra-thin heat pipe structure on its thermal performance,the visualized heat pipes with ultra-thin inner cavity(0.3 mm)of layered and spaced structures were fabricated and tested on the visualization experimental platform.The flow characteristics of the working fluid inside the heat pipes with different heat powers and operating angles were investigated.The effect of filling rate on the flow characteristics of the working fluid inside the spaced structure heat pipe was studied.A constrained space boiling visualization test setup was built to study the boiling heat transfer characteristics of of copper planar,2-layer #200 mesh,side-by-side spiral woven mesh composite flat mesh,and spaced wide spiral woven mesh surface structures at different confined spacings. |
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