Heat Transfer Properties Research And Numerical Simulation Of Efficient Composite Phase Change Materials

Energy is the material basis for human survival and development. Human began todevelop solar energy, wind energy, and other clean energy because primary energy wasexhausted and bringing all kinds of environmental problems. However, this kind of energywere supplied and demanded not match in time. However, the problem can be solved byenergy storage technology. Among these technology, phase change energy storage has becomethe preferred method for its high heat storage density, constant phase change temperature,little supercooling, little phase separation and easy control.The coefficient of thermal conductivity of traditional organic phase change materials isquite low. Two-dimensional graphite flake size in5~20μm was prepared by taking examplesfrom the preparation of expanded graphite and superfine powder. Then add it into the organicPCMs, so that a new kind of phase change material which named graphite/organic PCMs forthermal storage was developed. This thesis emphasizes on the preparation, thermalconductivity, heat storage density, phase change temperature and characteristics of thegraphite/paraffin composites. The main research contents in this paper are followed:（1）An orthogonal experimental was designed, comparing the shear time, shear powervolume ratio of graphite and water to finding the best reaction conditions of preparinggraphite sheets(GS). The best reaction conditions for graphite sheets were10min,16000rpm,volume ratio1:1, respectively.（2）Graphite/paraffin PCMs were synthesized by mixing GS with melted paraffin. Themicroscopic structure, cyclical stability and sedimentation of graphite/paraffin PCMs wereanalyzed, and the stable mechanism of the material was investigated.（3）The thermal conductivity of graphite/paraffin PCMs were measured by Hot-DiskThermal Constants Analyzer. Analyzing the influence of thermal conductivity ofgraphite/paraffin PCMs by changing the mass faction and temperature. The phase changetemperature and latent heat of graphite/paraffin were measured by Differential ScanningCalorimetry (DSC).The data show that the thermal conductivity of liquid and solidgraphite/paraffin were enhanced prominently with the mass fraction of GS and the causes oflatent heat changing of composite phase change materials were illustrated.（4）The temperature-time tests was used to verify the improvement of heat transfer ratein melting and solidification process. The experimental data indicated that the melting andsolidification rates of PCMs were significantly improved by the increasing of the massfraction of GS. （5）Compared the thermal conductivity of the graphite/paraffin PCMs which were madeby ultrasonic dispersion and high-speed shearing respectively. The data show that thegraphite/paraffin PCMs prepared by ultrasonic dispersion were more stable after multiplecycles, but the thermal conductivity of the graphite/paraffin PCMs prepared by high-speedshearing was still better.（6）Aheat pump water heater for thermal storage was built and16thermocouples weredecorated for testing the temperature changes in the energy storage system. The data showthat temperature distribution were relatively uniform in the energy storage system.Additionally, the data proved that the heat pump water heater with energy storage was steadierthan conventional heat pump water heater.（7）Based on the designed heat storage exchanger, physical model and mathematicalmodel were built. Fluent software was used to simulate the melting and solidification processof graphite/paraffin. Different parameters such as particle concentration, fin thickness, finsspacing were calculated to analyze the effect on heat transfer rate.