Influence Of Heat Conduction And Natural Convection On Melting Characteristics Of Low Melting Point Materials And Synergistic Enhancement For Melting Process

In recent years,with the rapid development of microelectronics technology,the heat flux of electronic devices has been rising rapidly.In order to ensure their safe,efficient and stable operation,it is urgent to explore effective cooling technology.There are many kinds of electronic devices,the heat flux and working mode are various.That means the corresponding cooling technology is quite different.For instance,the phase change material（PCM）heat sink is usually used to cool the electronic devices which work in the confined space for a short time.However,the commonly used low melting point material is unable to meet the heat dissipation requirements of high power electronic devices due to its poor thermal conductivity.To solve this problem,materials with high thermal conductivity such as porous media,fins and nanoparticles/tubes are added into the PCM.It is worth noting that these methods not only enhance the thermal conductivity of PCM,but also weaken the natural convection during the melting process of PCM,leading to a limited improvement of the PCM heat sink cooling performance.Therefore,this paper will explore the influence of heat conduction and natural convection on the melting characteristics of low melting point material,and explore a concurrent enhancement way of heat conduction and natural convection in the melting process.This article summarizes the research progress of PCM and the research status of the optimization of effective protection performance of PCM heat sinks.On this basis,the main research contents of this paper are proposed.Firstly,the directional melting characteristics of the low melting PCM were investigated experimentally.The influence of natural convection on the directional melting process of paraffin was investigated by different heating methods.Meanwhile,the influence of heat flux on the temperature field,target temperature and the impact ratio between heat conduction and natural convection were discussed.Moreover,the evolvement trend of paraffin melting temperature field,the effects of heat flux and physical parameters on the ratio between heat conduction and natural convection were found out.Then,based on the study of the directional melting characteristics of pure paraffin,an visual system was designed to explore the melting characteristics of paraffin after strengthening the thermal conductivity,and the coupling effects of thermal conductivity and natural convection on the melting process after strengthening the thermal conductivity of paraffin was clarified.Finally,based on strengthening the influence law of heat conduction on the paraffin melting characteristic,designed the synergy of strengthening heat conduction and natural convection new experiment scheme.The effects of co-enhanced heat conduction and natural convection on the melting characteristics of paraffin were further explored,and the effects of heat flux on wall temperature,cooling target,co-enhanced heat conduction and natural convection were discussed.The main conclusions can be given as follows:（1）When the PCM is heated from the bottom position,the melting process of paraffin is greatly accelerated by the appearance of natural convection,meanwhile,the temperature difference inside the PCM is reduced.The temperature evolution of paraffin at different locations goes through four stages:slow rise,rapid rise,slow fluctuation and continue to rise.In contrast,when the PCM is heated from the top position,the heat conduction dominates the melting process of paraffin.Moreover,the melting rate is relatively slow and the slow fluctuation stage has vanished.With the increase of heat flux on the heating wall,the impact proportion of natural convection on the directional melting of paraffin increased firstly and then decreased gradually.It reached the maximum value（58.7%）when the heat flux increased to3.11 W·cm^-2.（2）Fill copper foam can significantly enhance the thermal conductivity of paraffin,and make temperature distribution inside the PCM more uniform.However,it unavoidably reduces the theory of heat absorption of paraffin,extends the onset time of natural convection,shortens the duration of natural convection and weakens the intensity of natural convection.For instance,the impact ratio of natural convection decreased from 8.1 to 0.72 once the copper foam is added into the paraffin.In addition,the structure of copper foam has an important effect on the melting of paraffin.The copper foam with a larger pore diameter and internal skeleton can improve the thermal conductivity and reduce the inhibition for natural convection.Specifically,the effective protective performance of 10 PPI copper foam is 6.1%higher than that of 15 PPI copper foam.（3）Adding the low melting point alloy in paraffin is an effective way to improve the thermal conductivity of paraffin in the solid phase and early melting stage,and enhance the intensity of natural convection in the melting process.Paraffin/alloy can improve the effective protection time of PCM heat sink by 55.6%for electronic devices with a lower upper operating temperature（e.g.,80°C）.Moreover,as the mass fraction of the alloy increases from 39%to69%,the effective protection performance is further increased to 130%.However,with the increase of the upper operating temperature,the improvement of effective protection performance by filling low melting point alloy is gradually weakened.