PIV Experimental And Numerical Studies On Phase Change Heat Transfer Of Composite Phase Change Materials Under The Background Of Energy Storage Safety

Usage of renewable energy has intermittent characteristic,which requires the support of energy storage technology.Latent heat thermal energy storage technology(LHTES)is one of the key points to pursuit green development.In LHTES,phase change materials(PCM)are widely used in the apply of energy storage and thermal safety management due to the advantages of high heat storage and constant temperature during phase change.But the low thermal conductivity of most PCM also limits the further development,thus studies on heat transfer enhancement of high thermal conductivity composite PCM(CPCM)attracted the attention of researchers.Aiming at the heat transfer enhancement of CPCM,this subject conducted research on phase change heat transfer characteristics by methods combining particle image velocimetry(PIV)experimental measurement and three-dimensional numerical simulation.The coupling phenomenon of multi-physics(temperature field,flow field and phase change process,etc.)in heat transfer process was analyzed.And the multi-factor heat transfer mechanism of composite phase change system was explored,which can promote a theoretical basis for PCM application in energy storage and thermal safety management as well as performance evaluation.Firstly,this study took the CPCM wtih partially porous filling as experimental research objects.PIV visualization experimental measurement of melting heat transfer process was carried out.The optimal partial filling mode of porous media was studied.N-octadecane paraffin was the PCM in experiment.Copper foam was the porous medium for partially filling.The melting experiments of this CPCM were carried out in a three-dimensional square cavity unit heated vertically in one direction on the left.In the visualization system,PIV was used to measure the quasi-two-dimensional flow field in liquid PCM,a camera was used to record the change of phase change interface.In addition,hermocouples and an infrared camera were used to measure the temperature.The visualization data of flow field,temperature field and phase change interface evolution as well as the calculation of heat transfer and energy storage performance were analyzed.Results showed that the melting of pure PCM without porous media filling can be divided into four heat transfer stages and mainly dominated by thermal convection.For the CPCM with partially porous filling,the mixed regime of thermal convection and thermal conduction dominated the phase change heat transfer process.The partially porous filling mode can reduce the suppression of thermal convection about porous media and effectively enhanced the thermal conduction.The influence on the heat storage was reduced.And the heat transfer of the phase change was more uniform.The phase change process can be shortened.Then,the numerical simulation in this thesis added the heat transfer effect of nanoparticles based on the experimental study,taking the CPCM that coupled with porous media,nanoparticles and PCM as research objects.This research on CPCM melting heat transfer was carried out in a three-dimensional square cavity with a central internal heater.Effects of Rayleigh number,nanoparticle volume fraction,porous media porosity,and heater size on melting heat transfer process were analyzed.Enthalpy method,local thermal non-equilibrium effect and non-Newtonian rheological properties were considered in mathematical model.Results were mainly based on the evolution of phase change interface,temperature distribution,flow field change and final melting time.Results showed that the heat transfer mechanism of CPCM was dominated by initial conduction and later convection.Changes of different factors mainly affected the thermal convection stage in mid-term.Increasing Rayleigh number can enhance convective heat transfer and accelerate melting.Positive contribution of thermal conductivity enhancement by increasing nanoparticle volume fraction outweighed the negative effect of its increasing viscosity.Reducing porosity can enhance the thermal conduction of metal foam and thus increased melting rate of PCM.Increasing the size of internal heater can increase the heat transfer area and thus significantly improved the melting rate of CPCM.By the way,the contribution of changing porosity to heat transfer enhancement was greater than that of changing nanoparticles volume fraction.