Theoretical Analysis And Experimental Study On Loop Heat Pipe With Flat Evaporator For Multiple Heat Source Heat Dissipation

With the development of integration and thermal packaging technology,number of transistors per unit area increase exponentially.Thermal management has become a key issue for heat flux with the increase in electronic perfomanc together.Thus,a high efficiency heat transfer device is urgently developed.Loop heat pipe(LHP)works as a two-phase heat transfer device,and is expected to one of the most promising ways to address heat dissipation issue of electronics with high heat flux.LHP utilizes the latent of the working fluid to transfer heat,and capillary force produced by the wick in the evaporator drives the circulation of the working fluid in the loop.The evaporator and condenser is joined by smooth tube,which possesses advantages of high efficiency,heat transfer over long distance,low thermal resistance and flexible arrangement.Based on previous work,manufacturing process of capillary wicks,the evaporator structure improvement and LHP systems with dual flat evaporators are experimentally and therectically investigated in two terms of LHP components and system stucture.Capillary wick provides the main driving force of the circulation of the working fluid in the loop,and is also an important component to form the vapor-liquid isolation in the evaporator.Good capillary wick should be equipped with large capillary force and small hydraulic resistance,as well as good thermal property.Based on the developed manufacturing process of capillary wicks,The best ratio is:30%pore former,35%nickel powder with a particle size of 2.8μm,and 35%nickel powder with a particle size of 0.5μm for sintering..With the help of scanning electronic microscope(SEM),different specifications of pore-diameter distribution were found.Multi-pore-diameter capillary wick has a good thermal-hydraulic physical property based on the analysis of thermal conductivity coefficient,permeability and porosity.Otherwise,the pouring wick was proposed in order to simplify manufacture process of capillary wick and reduce the cost,utilizing cement and adhesive to manufacture the wick by the means of pouring method.The testing result of the pouring samples demonstrated that the pouring wick had a controllable dimension and high yield rate,and Pore size distribution in the range of 3-10μm...In addition,it had a low thermal conductivity coefficient,effectively reducing heat leak to the CC.The reasonable organization of flow and heat transfer in the evaporator is one of ways to improve the evaporator performance.In the improvement of the evaporator structure,an evaporator with a secondary wick,an evaporator with a shared CC and an evaporator with a strengthened rib plate were proposed,mainly aiming at the existing drawbacks of the evaporator and practical application.The evaporator with a secondary wick stabilizes the returning liquid into the CC,and the corresponding LHP system has a stable operation.The evaporator with a shared CC is able to dissipate waste heat from two adjacent heat sources,making the LHP system more simple and compact.The evaporator with a strengthened rib plate improve the flat evaporator strength,and has more choices of the working fluid,designed for addressing thermal issues with large footprint and high heat flux.The effective heat transfer area of the evaporator is increased to 64 cm~2,which is 4～5 times larger than the heat transfer area of the existing positive pressure evaporator.The maximum heat load that can be transferred when the heat source temperature is lower than 80℃is 160 W,and the heat source surface has a good surface.The temperature uniformity of each measuring point is less than 0.5℃from the average value.In the last,the design rule of the flat evaporator was summarized in the dissertation for choosing the suitable evaporator structure based on engineering applications.With respect to the LHP system,the positive pressure LHP system and negative pressure LHP system were investigated.The positive pressure LHP systems with ammonia or R245fa have better startup performance and low operating temperature,but high vapor pressure in the evaporator increases the difficulty in fabrication of the evaporator.The negative pressure LHP system with methanol possesses larger heat transfer capacity,low vapor pressure and easy fabrication of the evaporator,but operates unstablely.In the last,the flat type LHP with dual evaporators were designed for solving multi-heat-source dissipation with high heat flux,and tested their heat transfer performance with ammonia and methanol as the working fluid.By the means of reorganizing the circulation path of working fluid in the loop,the flat type LHP with dual evaporators and methanol as the working fluid operates successfully and has good heat transfer performance.In this dissertation,the capillary wick,evaporator structure and system of the flat type LHP were theoretically and experimentally investigated.The corresponding LHP systems were established to test the effect of charge ratio,heat sink temperature,working fluids and gravity tilt on their heat transfer performance and characteristics.This work has a profound influence on future design of the flat type LHP,and provides many guidlines for engineering applications.