| As one of the most common and available energy modes,thermal energy is often accompanied by various energy conversion and utilization processes.Thermal energy storage can effectively solve the mismatch between energy supply and demand in time and space,thus becoming a key method to improve energy efficiency.Latent heat thermal energy storage(LHTES)is to use phase change materials(PCMs)to absorb and release a lot of heat in the process of state changes,having the advantages of high energy storage density and almost constant temperature during the process.Although common organic PCMs have large specific heat capacity and latent heat,low thermal conductivity and poor heat transfer performance restrict their practical application.In order to improve the heat transfer efficiency of the LHTES system,it is a common method to add high-conductive nano particles to PCM to enhance the heat transfer.In practical applications,spherical vessels are often used as heat storage units in packed bed energy storage systems.In order to study the energy charging and discharging processes of packed bed LHTES systems,the melting and solidification heat transfer in a single spherical vessel is especially important.Therefore,because the inward solidification process of PCM is difficult to achieve the visualization,a new experimental method that is based on the volume shrinkage to quantify the inward solidification heat transfer of PCM in spherical capsules is put forward.Moreover,by preparing nano-enhanced PCMs with different mass fraction,the heat transfer enhanced effect of high conductive nano particles on PCM is studied.1-tetradecanol(melting point 37?)is used as PCM in this paper,adopting the 'two step'method to prepare composite PCMs and the mass fraction of graphene nanoplatelets is 0%,0.5%,1%,3%.Thermo-physical properties of nano composite PCMs are tested,including thermal conductivity,latent heat,melting point,specific heat capacity,density and viscosity.A new indirect experimental method is proposed.By inserting the descending tube inside the spherical container,volume changes of voids produced by shrinkage during solidification is transformed to height changes of liquid PCM in the descending tube,so the instantaneous liquid fraction can be further calculate.The experimental results show that the maximum experimental relative deviation of the temperature curves of these three parallel tests is about 3.7%.The relative deviation of the descending height is not more than 6%,which indicates that the experimental repeatability is good.At the same time,the relative deviation between the measured final descending height and the theoretical maximum value based on the solid-liquid phase density difference is observed to lie between 10%and 15%.The liquid mass fraction is analyzed as a function of the product of Fo and Ste numbers,considering the differences in material,geometry and physical condition,which is consistent with the theoretical variations by predecessors.The experiment shows good reliability.Based on volume shrinkage method,the effect of nanoparticles on the solidification heat transfer process of PCM in spherical vessel is further study.The results show that the total solidification time of nano composite PCM in spherical container decreases as the increase of mass fraction of nano particles.Because adding nanoparticles can enhance the thermal conductivity of the composite PCMs.In addition,the liquid fraction curves of the three composite PCMs have crossover points.Because viscosity of liquid PCM grows dramatically as the increase of mass fraction,the descending height in the tube may have been delayed due to the low flowability.Therefore,for the NePCM samples of higher concentrations,there exists a time lag in the descending height,leading to an underestimated solidification rate during the initial phase.By fitting the experimental data,the liquid mass fraction and Nusselt number is analyzed as a function of the product of Fo and Ste numbers,which is of great significance for the engineering application. |
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