| As a new type of heat storage medium,phase change capsule can not only store and release a large amount of energy to improve the utilization rate of energy.Bedides,it can adjust the temperature to realize the functions of information transmission,dynamic anti-counterfeiting and infrared stealth.It has a wide application prospect in the fields of solar heat utilization,thermal management of electronic equipment and thermal regulation of functional materials.Nowadays,the research on the thermal control performance of phase change capsules has made some progress at home and abroad.However,the discussion and analysis of information coding performance and influencing factors of phase change capsules are not comprehensive enough.The efficient and reliable preparation methods of phase change capsules need to be developed urgently.In addition,the influence mechanism of flow and heat transfer in packed bed heat storage system based on phase change capsules has not been fully revealed.In order to solve the above problems,numerical simulation and experiment were used to reveal the transmission characteristics of coded information and the internal mechanism of thermal control membrane with phase change capsule.Based on gravity-assisted coflow microfluidic packaging method,phase change capsules with stable core-shell structure and good monodispersity were prepared,and their morphology,thermal properties and stability were characterized.On the other hand,a phase change capsule packed bed heat storage system which can quickly realize orderly packing is proposed.Numerical study is made on the flow and heat transfer characteristics of the packed bed heat storage system.The influence of working conditions and packed bed structure on the flow and heat transfer performance is systematically revealed.To sum up,the main research contents and conclusions of this paper are as follows:(1)The numerical model of thermal control membrane of phase change capsule is established,and the information coding performance of phase change membrane under different phase change capsule layouts and external conditions is studied.The influence of heating temperature,thermal physical parameters of phase change capsule and particle size of phase change capsule on information coding of phase change membrane is analyzed.The results show that the phase change capsule thermal control membrane has good information coding ability during heating,but the coded information transmission only occurs in a certain time range and the coded information will gradually become unclear or even distorted with time.Moreover,although different coded information has similar appearance time,the stable transmission time and information distortion are different.Moreover,with the increase of heating temperature,the decrease of PCM thermal conductivity,the increase of PCM phase change enthalpy and the increase of the diameter of the phase change capsule will lead to the earlier information transmission of the membrane and the higher information clarity.However,the increase of heating temperature will shorten the stable transmission time of coded information.The influence of PCM thermal conductivity on the transmission time of coded information is almost negligible,while the increase of PCM phase change enthalpy and the diameter of phase change capsule will prolong the transmission time of coded information.(2)Phase change capsules were prepared by multi-phase flow microfluidic packaging method,and the surface morphology,thermal properties and thermal stability of phase change capsules were characterized.At the same time,phase change capsule thermal control film was prepared based on phase change capsules.The thermal control performance of phase change capsule thermal control film was tested.The results show that the gravity-assisted synergetic microfluidic device can controllably package PCM into the core-shell structural rubber bag with high monodispersity and reliability.It can also accurately control the size of the phase change capsule by adjusting the flow rate of the internal phase and the external phase.Moreover,the phase change capsule prepared has the advantages of complete core-shell structure,compact shell,good monodispersity and the encapsulation success rate close to 100%.Compared with pure phase change materials,the melting/solidification enthalpy of phase change capsules has decreased,but its thermal stability and heat resistance have been significantly improved.In addition,it is further demonstrated that the thermal control membrane based on phase change capsule can only realize the stable transmission of coded information in a certain period of time.(3)The theoretical model of phase change thermal storage system in packed bed based on phase change capsules is established.Under different external conditions,the flow and heat transfer characteristics of the system are numerically studied to analyze the influence of temperature and velocity of heat exchange fluid,initial temperature,stacking depth and particle size of phase change capsules on the thermal storage process of packed bed.The results show that the fluid in the packed bed heat storage system shows an approximate periodic flow law along the flow direction.Between the upstream and downstream of the phase change capsule,there are some recirculation zones which leads to uneven local velocity distribution.Moreover,due to the heat transfer lag effect,the temperature of the surface of the phase change capsule is higher when it is closer to the inlet and the temperature of the inlet surface of the phase change capsule is higher than that of the back surface.Similarly,the temperature rise rate of each point inside the phase change capsule is different and the temperature rise rate of the measuring point closer to the center is slower.However,the melting trend of each phase change capsule in the packe bed heat storage system is very similar,with the heat storage time of about 720 s and the stored heat of 31 J.It should be noted that the increase of heat exchange fluid temperature can not only speed up the heat storage process of the packed bed,but also increase the heat storage capacity to a certain extent.The increase of heat exchange fluid velocity can shorten the heat storage time of packed bed,but does not change the total heat storage.The increase of initial temperature has little effect on the heat storage time of packed bed,but the total heat storage capacity decreases.The increase of stacking depth can effectively improve the heat storage capacity of the packed bed,but the heat storage time is prolonged.The particle size of the phase change capsule decreases,which obviously shortens the heat storage time of the packed bed under the same heat storage capacity.The research results of this paper can provide new ideas for the application of phase change capsules in the field of thermal regulation and packed bed phase change heat storage.Besides,it can provide strong data support for the information coding function of phase change capsule thermal regulation membrane. |
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