Study On The Heat Transfer Characteristics In Composite Phase-change Energy-storing Materials

As functional materials which are used to store heat,the composite phase change materials(PCMs)are of great importance in a wide range of areas,such as building energy conservation,thermal utilization of solar energy,heat insulation in aerospace engineering,peak load leveling of power system,cooling of microelectronic chips,etc.Due to the conjugate heat transfer and the nonlinear characteristic of phase change process of the composite PCMs,the heat transfer characteristics of composite PCMs has not yet fully understand on the macroscopic scale.Besides,the exploration of the heat transfer mechanism in composite PCMs is to be carried out from the microscopic scale.Based on the molecular dynamics method,the mechanism of phase change heat transfer in microscopic scale is studied.On this basis,the mathematical and physical models in macro,meso scale are established.The finite volume method and the lattice Boltzmann method are used to simulate the phase change heat transfer process of PCMs.The thermal conductivity of the composite PCMs were then studied by experiments.In addition,the phase change heat transfer process is observed in pore scale through visual experiment.At last,the heat transfer performance of phase change energy storage solar collector was analyzed.The main contents and conclusions of this thesis are as follows:1.Numerical study on composite PCMs in macro scale.The physical models of composite PCMs with different combination forms are established and the temperature fields of PCMs under constant temperature are simulated.Factors such as effective thermal conductivity,melting point of two element PCMs,porosity of porous medium and so on are considered.The phase transition processes are treated by means of appraent heat capacity method.The distribution characteristics of temperature field,flow field and phase in PCMs are simulated.The results show that the composite PCMs have a great advantage over the pure PCMs in heat transfer and heat storage.The two component cascaded composite PCMs have the characteristics of good heat storage and fast heat transfer process.Compared with other composite PCMs,the composite PCMs in porous medium have the fastest change in temperature during the heat transfer process.2.Numerical study on composite PCMs in porous medium under meso scale.The porous media models are constructed based on the fractal Brown motion method and random generation method.The pore shape and the connectivity of the skeleton and pore of the model is analyzed.The dual distribution function model is based on local non thermal equilibrium condition.The heat transfer process between porous medium skeleton and composite PCMs is considered.The phase transition processes are treated by enthalpy method.The melting and motion patterns of composite PCMs in the pore structure are simulated.The results show that:The porous metal skeleton with high thermal conductivity has priority over the composite PCMs in receiving the heat transfer from the fixed temperature wall.The heat transfer in the metal skeleton plays a leading role in the heat transfer mechanism of the composite PCMs.Due to the existence of the metal skeleton in the porous composite PCMs,the convection effect of the PCMs in the cavity has been greatly suppressed.3.Numerical study on phase change heat transfer in micro scale.A molecular dynamics model of solid n-octadecane is established.The microcosmic heat transfer mechanism of phase change materials is analyzed through reverse non-equilibrium molecular dynamics.Based on the Monte Carlo method,the change of molecular position in the phase transition process is studied.The kinetic behavior of the molecules in the microscale phase transition process is revealed.The results show that:The non-equilibrium molecular dynamics method can be used to obtain the temperature variation curve so as to predict the thermal conductivity of the material accurately.The Monte Carlo method can be used to predict the phase transition point of simple substance.4.Experimental study on phase change heat transfer of composite PCMs.The testing platform of thermal parameters of composite PCMs was designed and built.The macro theoretical thermal conductivity model of composite PCMs is established and verified by experiments.The process of phase change heat transfer about the composite PCMs in porous medium was studied by visual experiment.The difference of phase change process under macro scale and pore size about the composite PCMs in porous medium is revealed.The experimental results show that the finite volume method and the lattice Boltzmann method are reliable in simulating the phase change heat transfer process.5.Application study on the solar flat-plat collector integrated with composite PCMs.The two dimensional physical model of solar flat-plat collector is established.The binary mixture of paraffin and lauric acid is used as energy storage material.The temperature changes in heat exchanger during typical winter and summer weather conditions are studied.The results show that:Compared with a phase change energy storage plate collector without foam metal,the temperature distribution of the collector is even more uniform and the melting rate of the PCMs in the collector is greatly improved by the addition of foam metal.The collector with foam metal can be used in winter conditions and it can be used to provide hot water for the whole year in the environment without water tank.In this work,the heat transfer characteristics of composite PCMs,especially in porous media,are studied using the combined method of theory and experiment.The inflence factors of heat transfer enhancement of composite PCMs are analyzed.The application potential of composite PCMs in the field of solar heat utilization is explored.Research of the phase transition rule and the law of heat transfer happens in composite PCMs has important value.The research provides an effective supplement for the theory of flow and heat transfer in the foam metal composite PCMs.At the same time,it provides theoretical and technical support for the application of metal foam in the field of solar energy storage.