Research On Heat Transfer Characteristics Of Ultra-thin Heat Pipes Based On Screen Mesh Wick

Heat pipe,as a phase change heat transfer component,has excellent heat transfer capability and operating stability.It has been widely used in heat dissipation of electronic devices.However,the miniaturization,integration and high performance of electronic devices are the inevitable trends of its development.How to solve the heat dissipation problem of high heat flux density in a narrow space is a key technical challenge faced by contemporary advanced microelectronic chip systems.Ultra-thin heat pipe?UTHP?is proposed as an effective solution to solve the problem of thermal control of thin and light electronic equipment.It should be noticed that the thermal performance of the UTHPs is sacrificed when they are becoming ultra-thin.The traditional sintered powder or grooved wick structure will not be suitable for UTHPs due to the defects of low capillary pressure or mutual extrusion between the wick.Screen mesh,with the characteristics of thin thickness and high permeability,has become the preferred choice as the wick structure of UTHP.It is of great urgency to systematically investigate the effect mechanism of screen mesh structure parameters on the thermal performance of UTHPs and to clarify the phase-change heat exchange mechanism in screen mesh at ultra-thin scales.In view of this,this paper takes the screen mesh type UTHP as the research object.The research was carried out from three aspects:theoretical calculation and analysis,hydrophilic and hydrophobic modification of the screen mesh wick and heat transfer performance of UTHPs.The main research works and conclusions are summarized as follows:First of all,theoretical calculations of porosity,permeability,and effective capillary radius are performed on five sets of screen mesh wicks with different meshes and wire diameters.Based on the basic theory of heat pipe,the capillary limit of the UTHPs is estimated by combining the structural parameters of the screen mesh wick and the physical properties of the working medium.Using surface modification technology,the evaporation section of the mesh wick was treated to be super-hydrophilic with contact angle less than 5^o.At the same time,the electrodeposition method was used to fabricate super-hydrophobic surface on the condensing section of the screen wick.The influence of deposition parameters on the surface wettability was explored.When the deposition time and voltage were 10 min and 20 V,the screen surface exhibited the best super-hydrophobic properties.Four groups of mesh wicks with different structures parameters were treated with optimal deposition conditions,and they all obtained super-hydrophobicity.Nanostructures on super-hydrophilic surfaces enhance its wettability and increase the number of nucleation points,which helps to improve the critical heat flux and heat transfer coefficient.Nanostructures on super-hydrophobic surfaces can transform film condensation into drops condensation,which will reduce the thickness of the condensate film and promote condensation efficiency.Secondly,five sets of UTHPs were designed and processed with different structures of wire mesh wicks.Experimental tests were conducted on their start-up characteristic,steady-state performance and multi-directional operating performance.The results show that the increase in the mesh number can effectively improve the start-up performance of the UTHP.It exhibited a shorter start-up time as well better temperature uniformity performance.A proper wire mesh structure should not only provide sufficient capillary force,but also have a large permeability.There should be an optimal choice of mesh structure to gain the best thermal performance for the UTHP.Among the samples studied in our works,the UTHP with180 in^-1 mesh wick exhibits the best thermal performance.In the multi-directional experiment,the UTHP with 200 in^-1 mesh wick achieves the optimum anti-gravity operating performance.Finally,for the structural design and development of UTHPs,the influence of the dimensions,liquid filling ratio and the height ratio of the vapor channel of the UTHPs on their heat transfer characteristics were systematically explored.The thermal performance indexes of UTHPs,such as operating temperature,thermal resistance and maximum heat transport capacity,were tested by experiments.Based on experimental data,empirical models for the evaporator temperature as a function of heat load and various design parameters were proposed,which can provide an effective guidance for the application of the UTHPs.