Study On Heat Transfer Characteristics Of Low Temperature Loop Heat Pipes Of Ammonia Medium Based On Capillary Wicks Modification

Since the beginning of the 21st century,aviation technology has already become an important index to judge a country's comprehensive strength.When the spacecraft is in orbit,some factors,like high vacuuum,low temperature,radiation from sun and planets and so on,have an important effect on heat transfer of the spacecraft.In recent years with the rapid development of high-power devices,the high heat flux was a big problem to further improving the performance of the processor and chip.In order to meet the poor working environment and electronic cooling requirement,LHP(loop heat pipe)was born in this condition as a new requirement with two-phase flow and heat transfer.The LHP is a kind of diode like one-way heat conduction and it is a shunt heat pipe with two-phase flow.In its interior,the working substance circulates in the form of gaseous and liquid forms and completes the transfer of heat.Compared with a normal heat pipe,the LHP has the advantages of long heat transfer,strong heat transfer ability and can work under different gravity acceleration environment.At present,the temperature of electronic products is maintained between 50 and 80 Centigrade,and the lower the temperature is,the longer the service life is.Based on this,the low temperature loop heat pipe was prepared in ammonia as the circulating medium,and the threshold value of ammonia working in low temperature LHP was lower and the response rate was faster.The LHP system can run smoothly at 50 Centigrade.The capillary wicks wick is the core component of the LHP.Its main function is to provide capillary power for the LHP operation.But at the same time it is the main source of resistance for LHP.Therefore,the key parameters such as pore size structure,capillary force,permeability and porosity of the capillary wicks was optimized to obtain a high-performance capillary wicks,so as further improve the heat transfer limit of the LHP.Based on this,this paper starts with the Ccapillary wicks modification,and proposes a method of adding a salt-enriching pore-forming agent to the nickel powder to optimize the capillary force and the permeation resistance of the capillary wicks,and develop a high-stability LHP by regulating the proportion of the pore-forming agent.In order to characterize the key parameters of the capillary wicks,a capillary suction test platform and a permeation resistance test platform were developed.The pore diameter characteristics,porosity,capillary force and permeability of the prepared capillary wicks were experimentally tested and numerically simulated,verification.The experimental results show that the porosity and permeability of the capillary wicks increase with the proportion of pore-forming agent;the suction performance of the capillary wicks increases first and then decreases with the proportion of the pore-forming agent.The best ratio is 30%.Compared to the cold-pressed sintered capillary wick,hot pressing sintered capillary core has better molding characteristics and processing characteristics.However,porosity,permeability and capillary suction are all lower than those of cold pressed sintered core.Not only does the internal structure of the wick have an impact on its performance,but the shape structure also affects its capillary force and resistance to penetration.Based on this,the capillary forces and permeability tests were carried out for four different capillary wicks with different inner diameters.The test results showed that the capillary wicks with the inner diameter of 12mm has the worst performance,and the capillary wicks with the inner diameter of 6mm,8mm and 10mm has the suction performance in the early stage about the same,the late suction diameter of 10mm slightly worse.Compared to 6mm and 8mm diameter of the capillary wicks,the suction performance is basically the same.On the basis the aspiration test,the capillary force of the capillary wick was characterized and verified by numerical simulation.Finally,based on the preparation of high-performance capillary wicks,LHP is designed and manufactured.The preparation process and optimization method of the LHP are introduced in detail.A LHP heat transfer test platform was designed to verify the heat transfer performance of the LHP using the prepared LHP.The starting characteristics of LHP under different heating power and different condensing temperatures were experimentally studied,and the heat transfer performance of the loop heat pipe was analyzed.The starting characteristics of the LHP under different heating power and different condensing temperatures were experimentally studied,and the heat transfer performance of the LHP was analyzed.The results show that the start-up threshold of the heat pipe in the ammonia low temperature loop is only 5W.With the increase of the heating power of the evaporator,the thermal resistance of the LHP is getting lower and lower and the heat transfer efficiency is getting higher and higher.As the heating power increases,the temperature of the evaporator and the accumulator and the steam line increases as the heating power increases.When the heating power of the loop heat pipe is too high,the capillary wick may not be able to continue sucking the working substance inside the accumulator back to the evaporator.Therefore,the preparation process of the high-performance capillary wicks needs further research.