Study On The Solid-liquid Phase Change Heat Transfer Characteristics In Thermal Energy Storage Heat Exchanger With Tree-shaped Fin And Its Structural Optimization

Owning to advantages of large thermal energy storage(TES)density and nearly constant working temperature,latent heat thermal energy storage(LHTES)has gradually become the most promising method of energy storage.However,the low thermal conductivity of phase change materials(PCMs)in LHTES units results in the problem of low efficiency in charge and discharge processes.Therefore,various technologies of thermal performance enhancement have been applied to improve the thermal performance of LHTES units.Among those techniques,the addition of fins has been widely utilized in many practical engineering applications due to the advantages of less operation and lower maintenance cost.However,the addition of fins decreases the PCM amount and weakens the potential of TES capacity of a LHTES unit and the structure of fin has important influence on the performance of LHTES units.Therefore,it is of great significance to optimize the fin geometry of LHTES units with a consideration of the trade-off between the higher TES capacity and the faster heat transfer rate.Inspired by the successful application of fractal geometry in the field of heat and mass transfer in micro-channels,an innovative fractal-tree-shaped structure is introduced to construct the metal fin of a shell-tube LHS unit.An unsteady model of solidification heat transfer in a LHS unit with tree-shaped fins is developed and numerically analyzed using commercial CFD software.The experimental study of the heat transfer characteristics of fractal-tree-shaped LHS unit is also carried out on the basis of numerical simulation.Moreover,according to the result of the research,a fractal tree-shaped-porous structure is used to optimize the fin geometry of LHTES unit.In conclusion,the research content and main results of this paper are as follows:(1)A numerical study of the discharging process in the fractal tree-shaped LHTES unit was carried out.The transient temperature distribution,solid-liquid interface evolution,and dynamic changes of the liquid fraction as well as sensible and latent heat in a tree-shaped LHS unit are presented and compared with a corresponding radial-fin LHS unit.Furthermore,the tree-shaped metal fin is optimized considering the length ratio and width index.The results indicate that the tree-shaped fin significantly improves the energy discharge performance of a shell-tube LHS unit.Compared with radial-fin LHS unit,the tree-shaped-fin LHS unit possesses a faster solidification rate,higher energy discharge rate,and stronger temperature uniformity due to the better heat flow access and more surface area.The energy discharge rate for the tree-fin LHS unit is almost the same as that for the radial-fin LHS unit in the early stage of discharging process.However,relative to the radial-fin LHS unit,the energy discharge rate is much faster for tree-fin LHS unit in the middle and later discharging process.To achieve the superior discharging performance,a fractal tree-shaped fin with length ratio of 1.3 and width index of 1 is recommended for the configuration of shell-tube latent heat storage unit in practical applications.(2)fractal tree-shaped unit and radial-fin LHS unit were designed and the experimental platform for charging and discharging was set up.The heat transfer characteristics in various directions during the phase change process and the effect of fin shape on the LHS units performance were studied.The results indicate that the tree-shaped-fin LHS unit possesses a faster melting and solidification rate and stronger temperature uniformity both in radial and axial directions;In the radial-fin LHS unit,convection plays the primary role in the discharging process while the heat conduction dominates in the solidification process.During the discharging process,the temperature uniformity along the axial direction is poor and the melting speed of the PCM in the upper part of the unit is higher than that in the lower part.In the fractal tree-shaped LHS unit,the temperature uniformity along the axial direction is stronger both in charging and discharging process as the fractal tree-shaped fin hinder the onset of convection;In radial-fin LHS unit,the temperature uniformity along the radial direction is strong during the charging process due to the influence of convection and conduction.However,in the discharging process,the temperature uniformity along the radial direction is poor due to the dominant role of conduction.In the fractal tree-shaped LHS unit,the temperature uniformity along the radial direction is strong both in the charging and discharging process due to the effect of multiple-level bifurcations.Furthermore,In the radial direction,the melting and solidification direction o f PCM is from both side to the middle in the fractal tree-shaped LHS unit,while that in radial-fin LHS unit is from inside to outside.(3)A numerical study of the discharging process in the fractal tree-shaped porous LHTES unit was carried out.The dynamic changes of the liquid fraction,transient temperature distribution and solid-liquid interface evolution in a tree-shaped-porous LHS unit are presented and compared with corresponding radial-fin LHS unit and fractal tree-shaped LHS unit.The results indicate that the main vein of the fractal tree-shaped porous fin can provide an optimized point-area heat flow access and the small venation can eliminate the areas with slow heat transfer rates.Compared to the fractal tree-shaped LHS unit and porous LHS unit,fractal tree-shaped porous LHS unit has faster discharging speed.In the fractal tree-shaped LHS unit,the PCM solidifies along the normal direction of the rib,forming a hysteresis zone between the fins.In the porous LHS unit,PCM solidifies uniformly outwards in radial direction,but its solidification speed is slow.In the fractal tree-shaped porous LHS unit,The solidification rate of phase change materials in different regions tends to be the same,and the solidification rate is faster.The fractal tree-shaped fin can collect thermal from area to point rapidly,but high temperature region will be formed between fins.The temperature difference between fins and PCM is not obvious in the porous LHS unit,but the thermal diffusion speed is slow in the radial direction,which lead to the poor temperature uniformity in the radial direction.The fractal tree-shaped fin can not only improve the temperature uniformity in the radial direction,but also eliminate the high temperature areas between the fins,thus improving the discharging efficiency of the LHS unit.