Research On High-Efficiency Flexible Heat Transfer Technologies Based On Loop Heat Pipes

Facing heat dissipation problems such as high power,complex long-distance layout and relative movements,this thesis carried out research on high-performance flexible loop heat pipe prototypes under the background of future aerospace project requirement.Combining experimental research and theoretical analysis,the working characteristics and heat transfer mechanism of loop heat pipes were analyzed in detail,and the influences of working fluid charging ratio,capillary wick pore parameters,evaporator structure,cooling conditions,gravity and movement modes on its heat transfer performance were explored.The main research contents are as follows:Firstly,two environmentally friendly refrigerants,R245 fa and R1234ze(E),were proposed as alternatives for traditional loop heat pipe working fluids according to the working temperature zone of the project.Then,the start-up and steady-state operation characteristics of loop heat pipes under different charging ratios(50% ～ 85%)using these two working fluids were experimentally studied.The results show that both working fluids could operate with the working temperature below 40 °C,providing good heat transfer ability.The thermal resistance of the system was smaller when the loop heat pipe was charged with R1234ze(E)as the working medium,and the system showed better steady-state heat transfer performance,while R245 fa could bear a higher heat load.At the same time,the optimal charging ratio of loop heat pipes was within the range of 65% ～ 70%.By summarizing the starting and operating characteristics of loop heat pipes at different charging ratios,reference for the failure analysis of loop heat pipes can be provided in combination with the vapor liquid distribution,which will provide guidance to subsequent experimental research.To better suppress the heat leakage and acquire a higher capillary driving force,PTFE and silicon nitride capillary wicks with low thermal conductivity were developed.Two evaporators,a flat plate type and cylindrical evaporator with an eccentric reservoir,were designed based on the original cylindrical evaporator.A two-dimensional flow model for the working fluid in the capillary wick was developed using the semi-implicit method(SIMPLE)algorithm based on the solution of the pressure coupling equation system.By discretizing the calculation domain and governing equations using finite volume method,and combining the capillary force calculation,the influence of pore structure parameters of the capillary wick on the vapor and liquid phase pressure drops and the effective capillary force inside the capillary wick was analyzed.The results show that the selection of appropriate capillary wicks needs to be accomplished by evaluating the effective capillary driving force in combination with the pressure drops of both the vapor and liquid phase.The calculated values of the proposed Si3N4 and PTFE wicks were better than those of copper wicks.To further evaluate the performance of the evaporators and the performance of loop heat pipes with different structures and materials,and to further study the heat and mass transfer phenomena of working fluids in the evaporator,a three-dimensional simulation model of the evaporator was constructed based on computational fluid dynamics methods using the Ansys Fluent software.Transient solutions were obtained.The influence of thermal conductivity of different materials and evaporator structures on heat leakage and re-wetting ability of the capillary wick were analyzed by observing the phase distribution and temperature distribution in the evaporator under given heat fluxes.The simulation results show that the capillary wick material and the evaporator structure design with low thermal conductivity can effectively suppress heat leakage and improve the liquid supply of working fluid to the capillary wick.Using R245 fa as working fluid,a flat-type evaporator loop heat pipe and an eccentric cylindrical evaporator loop heat pipe were fabricated and designed with Si3N4 and PTFE wicks.The results show that the capillary wick with low thermal conductivity and the new evaporator structure can effectively suppress heat leakage and can realize the fast and smooth start-up and steady-state operation under horizontal,gravity assisted and antigravity positions.The results of numerical analysis and experiment show good agreement.The transient temperature distribution of the condenser surface during the operation of the loop heat pipe was observed using an infrared thermal camera.The influence of heat sink temperature and condensing pipeline on the operation of the heat pipe under different heat loads was studied in combination with the visualization experiments of heat transfer and flow distribution.The results show that the lower the heat sink temperature,the greater the thermal resistance of the condenser under the same heat load,in other words,the worse the cooling efficiency in a small power range.Thus,the condensation efficiency utilization is gradually maximized as the thermal load increases.In contrast,a higher heat sink temperature easily releases the maximum efficiency of the condenser but also limits the maximum heat transfer of the system.In addition,the restart characteristics of the loop heat pipe under 400 W high-power thermal shock were experimentally studied under different cold environments(-60 °C～ 0 °C)and shutdown intervals.The results show that when the cooling cycle duration between the two working sessions of the heat pipe is long,start-up failure is more prone to happen,but normal operation can be restored by reducing the heat load.A flexible loop heat pipe prototype was fabricated by coupling PTFE metal hoses in both the vapor and liquid line.The low-power starting characteristics and heat transfer performance of the prototype under different heating powers were experimentally tested,and the internal pressure drop distribution during steady-state operation was analyzed.The results show that the transient response of the prototype is fast and stable,and the maximum heat transfer is higher than 700 W.The thermal resistance of the system is less than 0.01 K/W,and an operating temperature of 35 °C below could be achieved at an effective distance of more than 4 m.At the same time,the flow of working fluid through the vapor line is the main source of pressure loss inside the system,and the influence of the metal hose on the flow resistance of the loop heat pipe system can be ignored in comparison to the flexibility it provides.In this thesis,the working characteristics of flexible loop heat pipes in different swing modes of vertical and horizontal directions were studied.Results show that the symmetrical swing up and down of the evaporator can offset the influence of gravity,while the horizontal reciprocating swing has almost no effect on the operation of the heat pipe.