Study On The Modification And Performance Of Carbon Fiber Wick Applied On Flat Loop Heat Pipe

With the rapid development of electronic information industry,the problem of heat dissipation of electronic equipment has been paid more and more attention by researchers at home and abroad.The loop heat pipe(LHP)is a kind of highly-efficient heat transfer device that drives flowing of the working fluid by the capillary force produced by capillary wick in evaporator and transfer the heat by using the phase transformation process of the working fluid.The evaporator of the flat plate LHP belongs to the flat plate type with large heating area and small space height.Furthermore,it is beneficial to mating with the heating element.Especially,it is applicable to heat dissipation of electronic device with limited internal space,large calorific value and high density of heat flow rate.Its structural characteristics make it more closely bond with the heating element,which helps to reduce thermal resistance.The thermal load heats the LHP evaporator wall and the capillary wick inside the evaporator.The generated steam enters the vapor-line and cools into liquid in the condenser.The working fluid flows back to the evaporator via the liquid-line resulting in the completion of working fluid cycle.As a key part of LHP,capillary wick in the LHP evaporator provides sufficient capillary pumping force to circulate the working fluid.On the other hand,the wick transfer the generated vapor to the vapor-line in time to realize the heat forward transfer in LHP.Therefore,the development of the capillary wick plays a key role in improving the overall performance of LHP.At present,capillary wicks made of various materials have been developed.The conventional sintered capillary wicks exist the disadvantages of rigidity and brittleness.When used in the flat plate LHP,sintered wick contact with the evaporator less compactly,which affects effective heat transfer and reduces start-up and operation efficiency.Furthermore,the characteristics of the flat plate loop heat pipe,such as the flat plate capillary wicks and low mechanical strength,limit its application occasions.These shortcomings affect the overall performance of the LHP and become more prominent with the LHP miniaturized.As a kind of flexible material,carbon fiber has the advantages of high strength and its own porous structure.Compared with the conventional sintered capillary wicks,its high strength and good flexibility ensure the bond with evaporator more compact.In the meantime,the problem of brittle fracture is solved well by the soft characteristic which is advantageous to the development of plate-type loop heat pipe to miniaturization.This paper studies the working characteristics of the thin PAN-based carbon fiber with porous medium structure for loop heat pipe(LHP).The activated specific surface area and surface energy of carbon fiber is low before the surface treatment.As a hydrophobic material,the capillary pumping performance will be not exploited.In view of the requirement of copper-water flat-plate loop heat pipe(LHP)for capillary wicks,the copper chemical-plating method and flame spraying process metal coating method are used to modified the surface of carbon fiber.The results show that carbon fiber becomes a kind of hydrophilic material with an excellent capillary force after the modification by both methods.By means of crystallization in aqueous solution,the internal porosity and pore size structure of carbon fiber can be adjusted,so that the internal structure characteristics of carbon fiber capillary wick can be changed more flexibly,so as to optimize the working performance of loop heat pipe.The chemical plating means that a reducing agent is used to reduce metal ions in solution to metal elemental deposition and to form plating layer on the material surface based on the oxidation-reduction reaction.As the capillary wicks,the carbon fiber with copper plating on its surface can have better working performance than the copper mesh.The carbon fiber is slimmer than the copper wire so the capillary wick has less bore diameter and larger porosity and is more flexible.Its fabrication process is simpler and the fabrication cost is reduced.The results show that the surface treatment by chemical plating significantly improve the wetting ability of carbon fiber,so that its internal porous structure plays a better capillary pumping performance on water.The relative permeability is 10-11 m2,which satisfies the requirements of loop heat pipe capillary wick.Based on the study of the heat transfer characteristics of the chemical plating copper carbon fiber wick LHP,a method of preparing the composite carbon fiber capillary wick is proposed to further optimize the working performance of the carbon fiber capillary wick LHP.The main process of flame spraying is to spray metal powder into flame,and to spray metal powder onto carbon fiber surface by high pressure and temperature flame.The flame spraying process is used to spray the surface of carbon fiber with metal coating to increase its surface roughness and surface energy,so that the wetting ability of the carbon fiber to water is greatly improved,with excellent capillary pumping performance.Because the flame spraying method can apply different thermal conductivity coatings on different surface of carbon fiber,the composite carbon fiber capillary wick with more reasonable thermal conductivity distribution can be prepared.The results show that the composite capillary with prepared by flame spraying has the properties of LHP capillary wick and more reasonable thermal conductivity distribution.In order to further improve the system of carbon fiber for the preparation of loop heat pipe capillary wick,a new method was proposed to change the porosity and pore diameter of carbon fiber capillary wick by means of the internal crystallization reaction.The test results of the porosity of carbon fiber capillary wick after crystallization showed that the reaction time played an important role in controlling the porosity of carbon fiber,while the solution concentration had little effect.After observing its microstructure,the result showed that on the basis of the original carbon fiber skeleton forming holes,a new pore size formed after crystal precipitation was formed,which reduced the size of the original pore size.Based on the crystallization reaction to improve the pore size of the carbon fiber,the carbon fiber capillary wick with better pore size distribution can be obtained,which is of great significance to the preparation system of carbon fiber capillary wick.The thermal behaviors of flat LHP carried out carbon fiber wick were tested with the numerical simulation of internal heat conduction process of evaporator.The heat leak between the reservoir and vapor chamber is one of the most important negative factors which influence the startup and operation characteristic because the miniaturized evaporator.The numerical simulation results showed that the coatings on the reservoir decreased the heat leak effect.Furthermore,combined the technical process of carbon fiber fabricated by flame spraying,composite capillary wick was realized to solve the heat leak problem.Specifically,different coatings were sprayed on the two sides of carbon fiber to improving the heat transfer performance on vapor chamber side and reservoir side.After the modification of carbon fiber capillarity wick by chemical copper plating and flame spraying and the corresponding optimization measures,the heat leakage problem was well solved.The subsequent operation results of LHP showed that the heat transfer power could reach 75W,and the temperature responded quickly in the process of power change adjustment,and the operating temperature was stable without fluctuation.In conclusion,this thesis presents and improves the method of preparing loop heat pipe capillary wick based on flexible carbon fiber and the corresponding optimization measures.Also,the performance of the capillary wick and the LHP system assembled with the capillary wick is analyzed and tested.It provides a feasible technical route and theoretical guidance for the further improvement and miniaturization of the working performance of flat loop heat pipe.