Dynamic Heat Transfer Characteristics Of Heat Transfer Devices With Latent Heat Storage Units

Latent heat storage(LHS)technology has a wide range of applications in energy and power,aerospace,information and communication.It has advantages of constant temperature and high heat storage density during phase change process.Heat pipes and the fluid loop,as typical efficient heat transfer devices,have been widely used in advanced manufacturing fields such as thermal management of electronic components.They have strong heat transfer capacity,convenient structure arrangement,safety and reliability.In view of this,the application of LHS technology in heat pipes,fluid loop and other efficient heat transfer devices has significant engineering value to meet the heat dissipation requirements of electronic components under the current intermittent high heat load.Existing researches on the comprehensive utilization of LHS technology mainly focus on the heat sink coupled with phase change materials(PCM).There are few researches on the combination of PCM and efficient heat transfer devices.In particular,the researches on the dynamic heat transfer behavior of heat transfer devices with LHS units under intermittent high heat load and the methods for improving heat transfer performance need to be strengthened.Therefore,in order to meet the demand of efficient thermal management of electronic components under intermittent working mode,gravity heat pipe,axially grooved heat pipe and micro-channel pump-driven fluid loop(MPFL)are combined with LHS technology there.Three forms of thermal management modes under intermittent heat load are proposed,and heat transfer devices such as gravity heat pipe with LHS units,axially grooved heat pipe with LHS units and MPFL with LHS units are established.The dynamic heat transfer characteristics and performance optimization of the three heat transfer devices with LHS units are studied by means of experimental and numerical simulation.It provides new materials for the development of novel and efficient thermal management under intermittent mode.Generally speaking,the main research contents and conclusions of this thesis are as follows:(1)Dynamic heat transfer characteristics of a heat storage assisted gravity heat pipeFor efficient transport of intermittent heat load on the ground,a three-dimensional dynamic heat transfer model of a heat storage assisted gravity heat pipe(HSGHP)is established.The difference of heat transfer performance between the HSGHP and the ordinary gravity heat pipe is compared.The effects of the location and configuration of the LHS units,the type and filling rates of PCMs on the HSGHP are studied.The results indicate that compared with ordinary gravity heat pipe,the average temperature of the heat source decreases by 48.4%,and the duration of the maximum heat source temperature decreases by 50%.As a result,it reduces the impact of transient heat load and improves the overall temperature uniformity of the gravity heat pipe.In addition,the closer the LHS units are to the heat source,the better the temperature uniformity of the HSGHP is.When the PCM is gallium,the maximum temperature of the heat source is 26% lower than that when the PCM is paraffin.The increase of PCM filling rates is conducive to delaying the occurrence time of the highest temperature and improving the temperature uniformity of the gravity heat pipe.The number of fins in LHS units has a significant influence on the competition mechanism between natural convection and heat conduction during the melting process.The maximum temperature of the heat source decreases by 29.7% when the number of fins increases from 38 to 150,and the overall temperature uniformity of the gravity heat pipe increases by 31.7%.However,the influence of the number of fins on the heat transfer enhancement of the HSGHP is limited.The research in this respect provides theoretical guidance for the design and optimization of the HSGHP.(2)Dynamic heat transfer characteristics of an axially grooved heat pipe-LHS systemFor the requirements of efficient temperature equalization and heat dissipation of electronic components under intermittent working mode,an experimental platform of trapezoidally grooved heat pipe(TGHP)is constructed,and the effects of heat load and inclination angles on the startup behaviors are analyzed.Based on Weber number and Bond number,the startup mechanism diagram in evaporator of the TGHP is given.The variation of heat transfer performance of the TGHP with inclination angles under different heat load conditions are explored,in which the evaporator is set at the bottom and the condenser is set on the top.The temperature distributions of the TGHP along the axial direction under different working conditions are obtained.The influence of gravity on the thermal resistance and equivalent thermal conductivity of the TGHP is studied.The results indicate that when the TGHP is placed horizontally,only gradual start-up process occurs inside grooves.Under the inclined condition,with the increase of heat load,gradual startup and overshoot startup appear in turn in the evaporator of the TGHP.The transition from fin-film evaporation to corner-film evaporation contributes to the evaporative heat transfer changes from gradual startup to overshoot startup.Gradual startup occurs at small Weber number,while overshoot startup occurs at large Weber number.As the Bond number increases,so does the Weber number of the start-up process in the trapezoidal grooves.In addition,under different heat loads,gravity has an important influence on the axial temperature distributions of the TGHP.The appropriate inclination angle is beneficial for improving the axial temperature distributions.Moreover,with the increase of heat load,gravity promotes the evaporative heat transfer characteristics of the TGHP.On the basis of mastering the heat transfer characteristics of the TGHP,the experimental platform of an axially grooved heat pipe-LHS(AGHP&LHS)system is built,and the three-dimensional dynamic heat transfer model is established as well.Through the combination of experiment and simulation,the effects of the presence and absence of LHS units and the placement of LHS units on dynamic temperature of the AGHP&LHS are compared.The effects of filling rates of PCM and cold source temperature on the heat transfer performance of the AGHP&LHS are compared.The results indicate that the temperature control time of the heat source under thermal shock can be well prolonged by LHS units.The temperature control effect of the heat source varies significantly with the placement of LHS units.When the LHS units are placed in the evaporator of the AGHP&LHS,the temperature control level of the heat source against thermal shock and the overall temperature uniformity of the AGHP&LHS can be maximized.With the increase of heat load,the temperature at the heat source rises rapidly,and the overall temperature uniformity of the AGHP&LHS becomes worse.Properly increasing the PCM filling rate can prolong the temperature control time of the heat source,thus improving the overall temperature uniformity of the AGHP&LHS.The temperature of the cold source has a great influence on the temperature homogeneity of the AGHP&LHS in heating process,which increases with the cold source temperature.In order to better improve the temperature control ability of the AGHP&LHS under intermittent thermal shock,the cold source temperature can be appropriately reduced.The research provides technical support for heat dissipation of electronic components in aerospace with uniform temperature under intermittent heat load.(3)Dynamic heat transfer characteristics of a microchannel pump-driven fluid loop with LHS unitsFor the thermal management requirements of long distance under intermittent high heat load,the heat transfer model of the microchannel pump-driven fluid loop(MPFL)with LHS units is established.The influence of the presence and absence of LHS units and the location of LHS units on the dynamic temperature characteristics of the MPFL are systematically studied through numerical simulation.The dynamic temperature response of the MPFL under the optimal configuration of LHS units is analyzed under different heat loads,PCM filling rates,heating duration and flow rates.The results indicate that the LHS units can effectively reduce the temperature of the MPFL under transient thermal load,and improve the operation stability of the fluid loop.In addition,when the LHS unit is located upstream of the cooler,the heat storage capacity of it can be maximized.With the increase of heat load,the melting mode of PCM inside LHS unit has experienced semi-melting state,exact-melting state and over-melting state.When the PCM is in exact-melting state,the LHS unit has the best heat storage capacity.In addition,when the heat storage capacity of the LHS units just matches the heat production of the microchannel heat sink,further increasing the PCM filling rates will weaken the heat storage effect of the MPFL.When the heat storage capacity of the LHS units is certain,prolonging the heating time is not conducive to the heat dissipation effect of the MPFL.Increasing the flow rate of heat transfer fluid is also limited to improving the heat dissipation effect of the MPFL.This research will provide data support for long-distance thermal management under intermittent high heat load.