Research On A Flat Micro Heat Pipe With Fiber Wick

With the rapid development of manufacturing technology for integrated circuit, theintergrated level of ultra large scale integration becomes higher, density of the line onchip becomes bigger, but heat dissipation area is smaller. so this increase the density ofthe power and the temperature of the chip rises significantly. The high temperatureseriously affect the reliability and service life of the chip. Micro heat pipe is a kind ofheat transfer device, which works by phase changing. the advantages of the micro heatpipe are small volume, light weight, high heat transfer efficiency, low cost, easyintegration, no need of extra power and so on. Therefore, the micro heat pipe caneffectively solve the problem of heat transfer for micro devices or chips. Micro heatpipe develops from a single micro heat pipe to micro heat pipe arrays, and then to theflat micro heat pipe process. Flat micro heat pipe can reduce the interfacial shear forceon the surface of the working liquid, and can improve the performance of the micro heatpipe. So most researches about heat pipe are focused on flat micro heat pipe.The principle and the heat transfer limitation of the micro heat pipe are introducedfirst. An one-dimensional steady-state analytical model is used to analyze themicro-grooved heat pipe and point out three structural problems which restricted theperformance of the micro heat pipe.(1) the problem of the interfacial shear stress,(2)the problem of sharp corner in the micro channels,(3) the number of channels in perunit area. Appropriate solutions are made in response to these problems. A new kind ofmicro heat pipe with fiber wich is probosed according to the structural solutions. Fibersare pushed closely in order to form the liquid wick, and there are charp corners in theliquid channel which is formed by two tangential circles. And the liquid and vaporchannels are seperated so that the interfacial shear stress can be reduced. Furthermorethe fibers can be arraged on several lays so that the channels can be multi-layer arrangedwhich increased the channels per unit area. For these reasons, the new micro heat pipewith fiber wick solve the structral problems and may improve the performance of themicro heat pipe.Theoretical research on flat micro heat pipe focus on micro-grooved heat pipe whichis not suitable for the micro heat pipe with fiber wick presented in this paper. So a novelmathematical model is founded for the micro heat pipe with fiber wick. In the model,not only the micro flux of the liquid is considered, but also the heat transfer in the fibersor between fibers is taken into account. The model is founded by finite volume method.The micro heat pipe divided into several small volume both along the Z direction and onXY surface. The governing equations are founded in each volume, which contains fourkinds of equations: continuous equations, momentum conservation equations, phase changing equations, and heat transfer equations. All the equations are numericallysolved in MATLAB, the results contains meniscus radius distribution in each level ofchannels, the liquid pressure distribution in each level of channels, the liquid velocitydistribution in each level of channels, the mass flux of phase changing distribution ineach level of channels, the temperature distribution in each level of fibers and in theshell. The maximum heat transport can be calculated by the temperature differencechange for different input power.The material, the parameters of the structure and the working liquid will affect theperformance of the micro heat pipe with fiber wick, so the micro heat pipe is optimizedby the mathematical model. First, the working liquid is researched. the contact angleand the latent heat of vaporization is the most important parameters for working liquid,the effect of which is analyzed on the performance of the micro heat pipe. Second, thewick fibers are researched. The effect of diameter and levels are analyzed on theperformance of the micro heat pipe. Last, the length of the adiabatic section isresearched. All the analysis provieds theoretical principles and results for the optimaldesign for the micro heat pipe with fiber wick.In order to realize the micro heat pipe, the fabrication process is designed in thispaper. A testing system is also established in order to test the micro heat pipe. Thetemperature profiles are tested for different input power, and the maximum heattransport is also can be got by the testing results. The testing results are compared withthe theoretical results in order to validate the mathematical model. Though thetemperature profile has some difference between the theoretical results and the testingresults, the testing results are in good accordance with the theoretical result on thetemperature difference between the two ends of the micro heat pipe. So themathematical model can calculate the maximum heat transport accurately. Then theeffect of three kinds of working liquid, the effect of diameter of the fibers and the effectof the length of adiabatic section are all researched experimentally. The results of theresearch are all compared with theoretical results and are all in good accordance.