Study On Solid-liquid Phase Change Heat Transfer Characteristics In Fractal Tree-shaped Heat Exchanger

Solid-liquid phase change is an important way to achieve latent heat storage and release.Latent heat storage has the advantages of high heat storage density,wide range of phase transition temperature,abundant phase change materials,and low price.Therefore it is widely used in the application of solar energy,the power industry in terms of "peak-cutting and valley-filling" and waste-heat recovery field.As the phase change materials(PCM)usually have the defect of low thermal conductivity which affects the heat transfer performance greatly,it is of great significance to carry out solid-liquid phase change heat transfer enhancement studies for the development of phase change energy storage systems and efficient energy use.At present,enhancing solid-liquid phase change heat transfer mainly implemented via structural design of the device or selection of PCM.Among them,the optimization method based on selecting PCM relies on the development of the material industry,and the available core materials are also scarce.The optimal structural design of the device is mainly the utilization of fins,whose shape is simple.Besides,the spatial structure configuration is scarce and the improvement of the temperature uniformity inside the system is not obvious.Inspired by the successful application of fractal tree structure in electronic chip heat dissipation,reactor structure optimization and other fields,the fractal tree structure is introduced into the field of solid-liquid phase change energy storage,and a new solid-liquid phase change heat transfer device is proposed to improve the heat transfer performance of PCM energy storage process.At present,the mechanism of heat transfer enhancement for this kind of phase change heat exchanger is not clear yet,and the layout of the tree structure needs to be optimized.For this reason,a theoretical model of solid-liquid phase change heat transfer process of heat exchanger with fractal tree structure is established,and the heat transfer behavior of phase change process and the heat transfer enhancement characteristics of fractal structure is numerically studied.Moreover,the experimental study of the heat transfer characteristics of fractal tree heat exchanger is also carried out in this paper.In conclusion,the research content and main results of this paper are as follows:(1)A numerical study of the heat transfer process in the fractal tree heat exchanger was carried out.A two-dimensional theoretical model for the solid-liquid phase change heat transfer process in a fractal tree heat exchanger was established and numerically simulated.The solid-liquid phase change dynamic heat transfer performance of the fractal tree heat exchanger was studied.Then the temperature characteristics,phase transition interface movement and heat release characteristics of the solidification process are given.And the geometric parameters of the fractal tree structure and the influence of the metal material on heat transfer performance are also analyzed.The results indicate that the temperature of the fractal tree skeleton changes faster than the phase change region.The isotherms in the fractal tree skeleton are parallel to each other,and the isotherms in the phase transition region are distributed along the surface of the fractal tree skeleton.The closer the isotherm is to the fractal tree skeleton,the denser the isotherm is,then the greater the temperature difference is,and the isotherm in the middle of the skeleton branch is ellipse.In the early stage of phase change process,the solidification range is roughly distributed along the fractal tree skeleton and extends outward along the skeleton.In the middle stage of the phase change process,the phase change area presents a uniformly distributed "water drop" shape and a "triangular shape",a large "water drop" appears at a 0-level bifurcation,a small "water drop" appears at a 1-level bifurcation,and a "triangle shape" appears at the edge of the disc.At the end of the phase change process,small “water droplets” disappear first,then “big water droplets” disappear,and “triangular shape” finally disappears.The amount of latent heat released is more than the sensible heat during the solidification process,and the latent heat release rate is much greater than the sensible heat release rate.Fractal tree series have the greatest influence on the heat transfer performance of solid-liquid phase change process,followed by the total number of branches and the length fractal dimension.Meanwhile the width fractal dimension has little effect on the heat transfer performance.When the fractal tree series k = 2,the total number of branches F = 6,the fractal dimension of length D =-1.8,the heat transfer performance of the system is relatively better than those with other structural parameters.(2)A heat exchanger based on fractal tree fins was designed and the experimental platform for solid-liquid phase change energy storage was set up.The heat transfer characteristics in various directions during the phase change process and the effect of operating conditions on the heat transfer performance were studied.The results show that convection plays the primary role in the melting process while the heat conduction dominates in the solidification process.After experiencing the same working time,the heat accumulated during the melting process is greater than the heat released during the solidification process,indicating that the convection accelerates the heat transfer greatly.The variation of the temperature during the entire phase change process experienced rapid rise(fall)period,gradual change period during phase change,second rapid rise(fall)period after phase change,and second gradual change period after phase change.In the axial direction,the temperature curve of each layer basically coincides with each other during solidification.In the radial direction,variation of the temperature follows almost the same tendency,and the first and second rapid rising period are basically the same in each direction.In the circumferential direction,the temperature change process of the PCM region lags behind that of the fractal tree skeleton,and the temperature of the solidification process changes faster than the melting process."Secondary melting" phenomena is observed frequently in the melting process.As the initial temperature difference and flow rate increase respectively,the time required for completing phase change of the PCM in the system is nonlinearly reduced.In this paper,the theoretical and experimental studies on the solid-liquid phase change heat transfer of fractal tree heat exchangers are systematically carried out.The research work provides theoretical support for the development of heat transfer enhancement technology in phase change energy storage.