Investigation On Performance Of Flat Loop Heat Pipe With Modified Carbon Fiber Felt Wick

With the progress of modern science and technology,microelectronic technology and information industry are rapidly developing.Volume of CPU,GPU and other chip devices are smaller.The heat generated during operation is also increasing.When the temperature is too high,the operation performance of electronic components will become worse,and operation failure may occur.At present,thermal control technology is mainly divided into active way and passive way.The main way of heat dissipation for integrated electronic devices is phase change which is a passive heat dissipation method.Heat pipe is an efficient passive heat exchanger using phase change.With the development of heat pipe technology,loop heat pipe has been developed.Loop heat pipe inherits the advantages of traditional heat pipe,such as high heat transfer efficiency,low heat transfer temperature difference,reliable operation and suitable for different shape spaces.It also has the advantages of small flow pressure drop,flexible pipeline layout,long-distance heat transfer,anti-gravity operation and so on.At present,loop heat pipe has been successfully used in aerospace equipment,large LED equipment and frequency emission system.The capillary wick in the loop heat pipe provides capillary force which is the driving force of working fluid cycle.Therefore,the capillary wick is the core component of the loop heat pipe.The research on capillary wick plays an important role in the development of loop heat pipe.At present,the material selection and preparation process of capillary wick are diversified.Most of them are made of wire mesh,powder sintered and porous foam.In the miniaturization process of loop heat pipe,small-size capillary wick not only needs to ensure good mechanical properties,but also needs to provide high capillary suction.Therefore,looking for capillary wick materials more suitable for small space has become the focus of research.Carbon fiber is a kind of flexible fiber material with excellent physical and chemical properties.Because of its convenient process and good comprehensive mechanical properties,Polyacrylonitrile based carbon fiber has become a commercial carbon fiber product with the fastest development,the most mature manufacturing process and the largest production scale.Carbon fiber felt is a kind of flexible material woven by carbon fiber bundles.The fiber bundles are connected by points.Carbon fiber felt has micro pores with different sizes and shapes.Unlike sintered wick and porous foam capillary wick,plugged holes will not happen in carbon fiber felt.In this thesis,carbon fiber felt was made into capillary wick of loop heat pipe.Its small pores were used to provide large suction force and large pores were used to exhaust vapor.Because of its softness,it can cling to the wall and reduce the thermal resistance.Besides,it overcomes the problem that hard materials are prone to brittle fracture in small size.In order to study the internal flow mechanism of fibrous porous media,Random Creation and Extension Method(RCEM)for constructing two-dimensional fibrous porous media model was proposed.Porous media models with the same porosity were generated by RCEM,Sierpinski Carpet and Quartet Structure Generation Set(QSGS)algorithm.The lattice Boltzmann method was used to calculate with models established by the three algorithms,and the results of flow field velocity,tortuosity and dimensionless permeability were obtained.The results shown that the tortuosity obtained by QSGS algorithm were consistent with that of granular porous media.While the RCEM model can reasonably predict the tortuosity and dimensionless permeability of fibrous porous media.Through the numerical simulation of models with different porosities and fiber diameters,it was found that the effect of porosity on the dimensionless permeability of tortuosity is greater than that of fiber diameter.Wettability is an important characteristic of capillary wick.When the capillary wick shows hydrophobicity to the working fluid,it cannot provide sufficient capillary driving force.Carbon fiber is a material that shows hydrophobicity to water and hydrophilicity to organic solvents.In order to broaden its application range,the surface of carbon fiber felt was modified by arc spraying.Before spraying,the carbon fiber felt shall be pretreated by degumming,degreasing,neutralization and flushing,so that the metal ions can be more evenly and closely combined with the matrix.In this thesis,copper,zinc and aluminum were used as spraying materials.By comparing the SEM images before and after spraying,it was found that the thinner metal coating attached to the surface of carbon fiber bundles,without destroying the morphology and continuity of carbon fiber.The porosity of carbon fiber samples before and after spraying was measured by Archimedes method.The porosity of unmodified carbon fiber felt was 90.47%,which decreased slightly after spraying.The capillary force and permeability of carbon fiber felt samples before and after spraying were obtained by experiment and fitting calculation.The capillary force of carbon fiber felt with aluminum coating is 4678.91 Pa,which is 2.04 times higher than that of unmodified carbon fiber felt.In order to study the effect of carbon fiber felt as capillary wick of loop heat pipe,a flat loop heat pipe was designed.For flat loop heat pipe,the common evaporator types are reverse liquid supply evaporator and longitudinal liquid supply evaporator,and there are three kinds of vapor channel positions.In this thesis,the flat loop heat pipe evaporator is designed as a longitudinal liquid supply type,and the capillary wick is designed as a comb shape.The capillary wick not only provides vapor channels,but also divides the evaporator into compensation chamber and evaporation chamber,which improves the utilization efficiency of narrow space.In order to predict the design rationality of loop heat pipe,five heat transfer limits were calculated.The calculation shows that the minimum heat transfer limit is greater than 500 W when the temperature is above 30? which meets the actual operation requirements.Through four comparative experiments of control variables,the effects of working medium,capillary wick coating,liquid filling rate and shell material on loop heat pipe start-up,variable power characteristics and heat transfer characteristics were investigated.Among methanol,acetone and water,methanol with moderate boiling point and latent heat of vaporization shown better working fluid characteristics.Among unmodified carbon fiber felt and three metal coated carbon fiber felts,aluminum coated carbon fiber felt shown the best comprehensive properties because of its highest capillary performance and small thermal conductivity.Results shown that when the filling rate was low,an unstable half start state occurred.Higher liquid filling rate shown higher thermal resistance.Filling rate at 70%shown the best operation performance.When the shell material changed from red copper to stainless steel,the thermal conductivity of the shell material greatly reduced.Besides,the thermal resistance was significantly reduced and the heat transfer coefficient was significantly improved.However,the temperature of the heating sheet increased significantly.The carbon fiber felt wick was applied to the loop heat pipe with small evaporator.The experimental results shown that carbon fiber felt still maintains excellent characteristics in small space.Through the experiments under different cooling water temperature and cooling system opening time,the influence law of cooling system on the start-up and operation characteristics of loop heat pipe was explored.The test results shown that in the range of0?30?,the higher the cooling water temperature was the more stable the start-up process of loop heat pipe was.When the cooling water temperature was 20?,the start-up speed of loop heat pipe was the fastest.When the cooling system was started earlier than the heating system,liquid working medium backflow with low-temperature occurred at the inlet of the condenser during startup,resulting in long start-up duration and poor stability.Therefore,the cooling system should be started later than the heating system.The capillary wicks with different coating positions were applied to the operation test of loop heat pipe.The results shown that after changing the double-sided coated capillary wick to single-sided coating and double-layer capillary wick,the heat transfer characteristics of loop heat pipe were obviously optimized.The double-layer capillary wick had better performance during startup and under low load,while the single-sided coated capillary wick had better performance under high heat load.In order to explore the feasibility of applying loop heat pipe to multi heat source heat dissipation,a parallel double evaporator loop heat pipe is designed and manufactured.Under three working conditions of single load,double equal load and double non-equal load,the loop heat pipe started smoothly.When operating under non-equal load conditions,the power of evaporator 1 decreased and the power of evaporator 2 increased,the stable temperature of two heating surfaces decreased at the same time.When the heat load is greater than 30 W,the heating surface temperatures of the double evaporator under different working conditions were less than that of the single evaporator loop heat pipe.When the heat load is greater than 50 W,the thermal resistance of the double evaporator loop heat pipe is less than that of the single evaporator loop heat pipe.In the case of multiple heat sources,the operation characteristics of double evaporator loop heat pipe were better than that of single evaporator loop heat pipe.In summary,this thesis put forward the physical model establishment algorithm of fibrous porous media.Arc spraying was used to modify the surface of carbon fiber felt to change the hydrophilicity.A flat loop heat pipe with carbon fiber felt as capillary wick was designed.Heat transfer limits were calculated.Five key effect factors were compared and analyzed through univariate comparison experiment.A parallel double evaporator loop heat pipe was designed.The experimental results provide reliable technical guidance for improving the performance and expanding the application scope of flat loop heat pipe.