Study On Heat Transfer And Flow Characteristics Of Microencapsulated Phase Change Material During Thermal Energy Storage Process

With the development of manufacture and economy,energy demand is increasingly important for countries all around world.The consumption of fossil energy such as coal and petroleum increases rapidly,which also cause problems of energy shortage and environmental pollution.Therefore energy conservation,emission reduction and improvement of energy utilization rate are extremely urgent.Phase change material(PCM)can solve the mismatching problem of heat supply and demand in time and space and improve the energy utilization efficiency.It has broad application prospect in industrial waste heat recovery,solar heat utilization,building energy conservation,thermal management of electronic device,battery thermal management and so on.Meanwhile,the microencapsulated phase change material(Micro EPCM)prepared via microcapsule techniques can efficiently solve the pollution of environment and facility which is caused by the leakage of PCM during solid-liquid phase change process.However,because of the low thermal conductivity of paraffin wax employed as core materials in Micro EPCM,the practical applications are limited by the inferior efficiency of thermal storage and carriage.For the problems mentioned above,the heat transfer enhancement of Micro EPCM,heat transfer and flow characterization of latent functionally thermal fluid,the preparation and characterization of salt hydrate Micro EPCM are developed in this paper.The major investigations and conclusions are as follows:(1)In order to enhance the heat storage efficiency of Micro EPCM,expanded graphite(EG),graphene nanoplatelet(GN)and nanocopper with different mass fractions(0.1 wt.%,0.5 wt.%,1.0 wt.%,1.5 wt.% and 2.5 wt.%)were added to the Microe PCM of paraffin/melamine resin as the effective heat transfer promoters.The thermal property and heat storage profermance of Micro EPCM and composites were investigated.The results indicated that thermal conductivity of the Micro EPCM was increased by 8.72%,28.27% and 39.62% respectively which dues to the addition of nanocopper,GN and EG when each mass fraction was 2.5%.In addition,the thermal conductivities of Micro EPCM and composites decreased with the increasing of test temperature.The heat storing/releasing efficiency of the Micro EPCM/EG(2.5wt.%)compared to that of Micro EPCM was increased by 14.98% and 26.63%,respectively,when the flow rate of heat transfer fluid was 30 m L/min.Furthermore,the heat storage efficiency of the composite was increased with the increasing of the mass fraction of the EG.On the basis of all results,it was concluded that the addition of EG can be considered as an effective method to increase thermal conductivity and heat storage efficiency of Micro EPCM without greatly reducing its latent heat storage capacity.(2)Microencapsulated phase change material(Micro EPCM)slurry as LFTF,which was composed of mixture of water/ethanol as base fluid and parafin/melaine resin Micro EPCM additives as dispersion,is prepared.The density,viscosity,latent heat and thermal conductivity of the slurry with different Micro EPCM mass fractions have been studied.In additon,the convective heat transfer behaviors of base fluid and Micro EPCM slurry in a horizontal circular tube have been experimentally investigated.The relationship between the concentration of Micro EPCM and heat transfer coefficient has been discussed in detail.The results showed that the heat storage capacity of themal fluid can be enhanced by adding the concentration of Micro EPCM.Moreover,the convective heat transfer coefficients of Micro EPCM slurry are about 2 times and 3 times higher than that of base fluid at the same Reynolds number when the mass fractions of Micro EPCM are 5% and 10%,respectively.This indicated that the Micro EPCM have great potential applications in heat transport and thermal energy storage.(3)Microencapsulated phase change material(Micro EPCM)was successfully synthesized by emulsion polymerization method,using magnesium sulfate heptahydrate(MSH)as core material and urea resin(UR)as shell material.The impact of process condition on the surface morphology and thermal property of Micro EPCM was investigated.The results indicated that the additive amount of emulsifier and reaction time display great effects on the surface morphology and particle size distribution of Micro EPCM.The additive amount of emulsifier with 0.5 g and the reaction time at about 240 min were the most admirable parameters for the microencapsulation process.The product S3 Micro EPCM appeared to have a smooth and compact surface,spherical shape and mean diameter at around 34.99 ?m.Although the MSH/UR Micro EPCM for the first time was successfully synthesized,the encapsulation rate and thermal conductivity of Micro EPCM is very low.Therefore,improve the encapsulation rate and thermal conducticity of Micro EPCM still need further research.(4)The Micro EPCM using sodium thiosulfate pentaydrate(So TP)as core material and polystyrene(PS)as shell material were successfully synthesized by solvent evaporation method.The influences of additive amount of emulsifier and stirring rate on the morphology and particle size distribution of the Micro EPCM were investigated.The results showed that the additive amount of emulsifier and stirring rate had little effect on surface morphology but a significant effect on particle size distribution of the Micro EPCM.In addition,a lot of studies including chemical,phase analysis,thermal property and thermal stability of Micro EPCM were investigated.The melting peak temperature and enthalpy of the Micro EPCM measured were 57.5 °C and 57.82 k J/kg,respectively.In addition,the thermogravimetric analysis showed that the Mciro PCM had good thermal stability.(5)The So TP/silica Micro EPCM were successfully fabricated using sol-gel method.The effects of core/shell mass ratio,concentration of emulsifier and stirring rate on the morphology of Micro EPCM were studied.In addition,the core/shell mass ratio impact on the latent heat,thermal stability and supercooling of Micro EPCM were also tested.The results indicates that the Micro EPCM have relatively spherical shape.The encapsulation ratio of Micro EPCM decrease with the decrease of So TP mass percentage.The highest encapsulation ratio of the Mciro EPCM is 94.65%.The thermal properties,supercooling degree and life cycle have been greatly improved.Therefore,the prepared So TP/silica Micro EPCM have great potential for being used in thermal energy storage applications.In summary,the heat transfer enhancement of Micro EPCM,heat transfer and flow characterization of latent functionally thermal fluid,the preparation and characterization of inorganic hydrous salt Micro EPCM were investigated in this paper.Results and methods of this research can be references for the heat transfer enhancement of thermal energy storage and the preparation of salt hydrate Micro EPCM.