Theoretical And Experimental Study On The Heat Transfer And Storage Characteristics Of Phase Change Materials And Cascaded Systems

Renewable energy?e.g.,solar energy and wind energy?and industrial waste heat are usually intermittent and fluctuant,and then fail to produce steady energy output,which has a negative effect on the direct heat utilization and grid-connected power generation on the user side.Besides,the mismatch between energy supply and energy demand in time and space would result in the low efficiency of energy untilization.Thermal energy storage is a promising way to solve the above problems.In the available methods,latent heat storage is a kind of method with balanced performance due to its higher energy density,compact structure,constant operation temperature,easy control,safty and so on,which would promote its application.Though the application of latent heat storage has been carried out,there still exist several problems,such as heat transfer enhancement,material corrosion,capsulation,system optimization and so on.Based on the application background and current research situation,the theoretical research on solid-liquid phase change using phase field method,the numerical research on the solid-liquid phase change in metal foams,the experimental research on cascaded latent heat storage systems and the experimental research on the heat storage performance of Al-Si alloys are selected as key points to study the efficient latent heat storage technology through numerical and experimental methods in this thesis.A mathematical model of solid-liquid phase change is established based on phase field method in which undercooling?or superheating?effect could be considered.A feasibility study of the mathematical model on the macro scale is done through the one-dimensional heat conduction problem with solid-liquid phase change and the convection melting problem in a two-dimensional square cavity,respectively.In addition,the effect of kinetic coefficient on the undercooling effect is also discussed.It is found that the phase field model is a reliable method to simulate more realistic solid-liquid phase change problems no matter whether natural convection is considered or not on the macro scale;when kinetic coefficient is large enough to make the effect of surface tension obvious,the undercooling?or superheating?effect can be observed.Based on the phase field model and the flow&heat transfer theory in porous media,a mathematical model of the solid–liquid phase change in open-cell metal foams is established on the REV scale.The validation of the mathematical model is performed through comparison with the numerical and experimental results.The effects of key parameters,such as Rayleigh number,porosity and pore density,on the melting and solidification processes are investigated.The phase field,flow field and temperature field in the melting and solidification processes are obtained.It is found that Rayleigh number,porosity and pore density all have great influence on the solid-liquid phase change,but the effects on the melting and solidification processes differ with each other a little;heat conduction through the ligament of metal foams dominates the whole heat transfer;kinetic undercooling effect is weakened as kinetic coefficient decreases.The heat storage process of a three-stage latent heat storage system is experimentally studied.Its temperature evolution in each stage during the heat charging process is measured and the corresponding thermodynamic performance is analyzed.The effects of stage number,HTF inlet temperature and HTF flow rates on the thermodynamic performance are discussed,respectively.The results show that the solid-liquid phase change in the three stages does not take place simultaneously due to the poor heat transfer and the large melting temperature difference;more stages could improve energy storage efficiency,exergy storage efficiency and entransy storage efficiency;higher HTF inlet temperatures and larger HTF flow rates could increase transfer and storage rates of energy,exergy and entransy,but the storage efficiency of energy,exergy and entransy could only be obviously improved by higher HTF inlet temperatures.The heat storage performance of Al-Si alloys is experimentally investigated,including cycling stability and corrosive property.The cycling stability study is carried out through microstructure observation,measurement of thermophysical properties?melting temperature,latent heat and thermal diffusivity?and calculation of thermal conductivity under different cycling numbers.The compatibility between Al-Si alloys and common ceramic materials?Al₂O₃,AlN,SiC,Si₃N₄,BN and ZrO₂?at high temperature is examined and the mechanism of corrosion is revealed.It is concluded that after several cycles,the microstructures of Al-Si alloys all changes;the melting temperatures of the four kinds of Al-Si alloys all changes a little,but only AlSi12 and AlSi20 show good stability in latent heat;for different Al-Si alloys,the variations of thermal conductivity with cycle number differ much and the thermal conductivity of AlSi12 almost remains stable;Al₂O₃ ceramics and AlN ceramics could resist corrosion against molten Al-Si alloys;the corrosion resistance of SiC ceramics against molten Al-Si alloys depends on the Si content in Al-Si alloys;Si₃N₄ ceramics,BN ceramics and ZrO₂ ceramics would react with molten Al-Si alloy,but the reaction products consist of AlN or Al₂O₃,which have been proven corrosion resistant in molten Al-Si alloys,so it still needs further study.