| Loop heat pipe(LHP)is a kind of flexible and efficient two-phase flow heat transfer device,which can achieve high heat transfer efficiency through phase change of working fluid and adsorption capability of porous wick.With more and more attention and recognition on the research of loop heat pipe at home and abroad,loop heat pipe has experienced the continuous expansion of the application scope and the continuous improvement of the application demand.Therefore,the loop heat pipe based on the different application conditions has emerged in recent years.The multievaporator loop heat pipe(Me LHP)is a new structure developed on the basis of traditional loop heat pipe.With parallel connection of multiple evaporators,the Me LHP is able to achieve efficient heat collection and dissipation of multiple heat sources,which changes the point-to-point heat transfer mode of traditional loop heat pipe and is suitable for the cooling of multi-array infrared detectors in space exploration technology.In this paper,aiming at the application background of multi-detector refrigeration in wide field X-ray telescope(WXT),several multi-evaporator loop heat pipe prototypes were developed.The start-up and operating rules of the prototype were studied from three aspects: design theory,numerical simulation and experimental exploration.Firstly,the basic principle of loop heat pipe was discussed.From the analysis of capillary principle,heat transfer limit and P-T diagram of working fluid in the loop,the mechanism analysis and thorough cognition of the driving force,phase transformation process,heat and mass transfer process of loop heat pipe were carried out,from which the factors affecting the operation of a multiple evaporators loop heat pipe and the key problems in the design and manufacturing process were excavated.After clarifying the operation mechanism,the research status of multi-evaporator loop heat pipe and other two-phase circuits with multi-evaporators was sorted out and summarized,the shortcomings of the existing development were clarified,and the research methods and directions of this paper were determined.Based on some design theories,this paper has developed three loop heat pipe prototypes.According to their structures,they were divided into two types,namely double evaporators loop heat pipe and triple evaporators loop heat pipe with reticular parallel pipelines structure and quadruple evaporators loop heat pipe with dual-layer condenser structure.The design theory not only focused on the material selection and working fluid selection of evaporator,condenser,compensator,capillary core and pipeline structure encountered in the design of general loop heat pipe,but also took a complete consideration in view of the problems specific to the multiple evaporators loop heat pipe,such as the size design of compensation,the number constraint of evaporators and the design of liquid charging amount.The research focuses of each prototype above were different.Among them,the research purpose of double evaporators loop heat pipe was to explore the influence of parallel evaporator structure on start-up and operating characteristics in different temperature zones.The research purpose of triple evaporators loop heat pipe was to study the influence of different heating distribution modes and different charging ratio conditions on the operation of multi-evaporator loop heat pipe under the pipeline asymmetry condition.Quadruple evaporators loop heat pipe was to improve the heat transfer limit of multi-evaporator loop heat pipe through structural design after summarizing the above conclusions,and then its operating reliability and stability were tested.The one-dimensional steady-state model of thermal resistance network and flow resistance network were established by taking the structure of the triple evaporators loop heat pipe as the physical model,and the simulation calculation was carried out by numerical iteration.Through the analysis of the heat transfer process in the parallel pipeline,the pressure balance relationship and the energy balance relationship were established in the loop.The main loop which meant a loop with the largest pressure drop in the parallel loop was found through the pressure drop relationship,and the distribution relationship of heat and flow among the three evaporators was found through simulation calculation.Based on the energy conservation of the compensation in the main loop,the iterative calculation of the loop’s temperature field and pressure field were carried out.The model has been verified by experimental data,which provides a theoretical basis for the design and operation of multi-evaporator loop heat pipe,and further parametric research and design guidance optimization can be carried out.Experimental study on multi-evaporator loop heat pipe with reticular parallel pipes was carried out.Two evaporators and three evaporators were used for the multievaporator loop heat pipe prototype with parallel mesh pipes.The working temperature was 170 K and the working fluid was ethane.The dual evaporators loop heat pipe explored the similarities and differences between the parallel structure and the single evaporator loop heat pipe,and studied the start-up and operating characteristics in the working temperature range of 170～250 K.It was found that the prototype could directly start under two heating modes,and there was a heat sharing characteristic between the evaporators in the operating process.The temperature oscillation occurred at 170 K and the reason for the oscillation was then analyzed as insufficient charging ratio.Subsequently,a triple evaporators loop heat pipe was developed for the influence of different heating power distribution and asymmetric structure on the loop fluid distribution and heat transfer performance.The heat sharing characteristics of the evaporator of the prototype was verified,and it was found that the heat sharing anisotropy existed due to the specific structure.Which is the working mode of multievaporator loop heat pipe compensation chamber is determined by comparing the performance under the two different charging ratios.Under the limitation of heat sharing characteristics and capillary limit of the evaporator,the heat transfer limit of the prototype reached 50 W.In order to improve the heat transfer limit of multi-evaporator loop heat pipe,a quadruple evaporators loop heat pipe with a dual-layer condenser structure was developed.The prototype used propylene as working fluid,and the optimization of the condenser was reflected as following aspects.Firstly,the layered structure was easy to realize the symmetrical distribution of multiple parallel pipelines,which avoided the difference of pressure drop caused by the three-way structure and effectively reduced the uneven flow distribution of each loop.Secondly,the condenser design increased the total volume of the closed loop,thus increased the liquid storage capacity of the loop and to some extent guaranteed the insufficient supply of liquid in the main loop.Through the performance test of the prototype,the heat sharing characteristics of the prototype with different heating power distribution modes were proved.Under the premise of reducing the flow resistance difference in the loop,the maximum heat transfer capacity was improved to 230 W,which was close to the capillary limit of one evaporator on average.At the same time,for further considering on the application,the operating reliability analysis of the prototype under the inverse gravity condition and the reliability analysis under the variable power heating condition were carried out.The results showed that the prototype could maintain the operating state within 30 mm adverse elevation of all evaporators and adapt to various heating power,which demonstrated good stability.The paper focuses on the experimental exploration of multi-evaporator loop heat pipes,improves the design of multi-evaporator loop heat pipes under the support of theoretical model,summarizes the operating characteristics and existing problems of different prototypes,and puts forward solutions.The work in this paper provides references and supports for the further development and application of the multievaporator loop heat pipe technology in the future. |
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