Numerical Analysis Of Heat Transfer Enhancement With Fractal Fins In The Phase Change Energy Storage Heat Exchanger

Phase change energy storage heat exchangers have the advantages of high energy storage density and near constant temperature heat storage and heat release and are widely used.The volume change of the solid-liquid phase change material before and after the phase change is small,which makes the phase change material easier to store and package and has a larger latent heat of phase change.However,its generally low thermal conductivity results in a very slow heat storage and heat release process,which has become a key factor restricting the wide application of phase change materials.As a simple,economical and effective heat transfer enhancement method,fins have been widely used to enhance the solid-liquid phase change heat transfer process.On the basis of previous research,the fractal structure fin has been proved to be a fin structure with the best heat conduction path.Compared with the traditional fin,it has a smaller heat conduction resistance and can more uniformly transfer heat from points to surfaces.The advantage of diffusion into phase change materials with low thermal conductivity.However,the spatial distribution of the fractal fins in the phase change energy storage heat exchanger,fin parameters,heat exchanger geometric parameters and thermodynamic parameters have a significant impact on the phase change heat transfer process.Therefore,it is necessary to optimize the structural parameters of the fractal phase change heat exchangerInspired by the successful application of fractal theory in the field of microchannel heat dissipation,this paper introduces the structure of fractal fins into the shell-and-tube solid-liquidphase heat exchanger,and optimizes the structural parameters of the fractal phase-change heat exchanger with fractal fins.Firstly,considering the combined effect of the structural parameters of the fractal fins and the structural parameters of the heat exchanger on the solid-liquid phase change heat transfer process,the ratio of the radial length of the fractal fins to the radius of the heat exchange tube is studied;secondly,a combined fractal fin is proposed.The fin structure solves the problem that the inherent fractal law causes some phase change materials to be difficult to melt;finally,the fractal law of the fractal fin structure is comparatively studied,and a more reasonable length ratio and width ratio are found.In this paper,a fractal phase-change heat exchanger model with different aspect ratios,a solid-liquid-liquid phase-change heat exchanger model with combined fractal fins,and a phase-change heat exchanger model with different fractal laws are established.The influence of the ratio of the radial length of the fractal fins to the radius of the heat exchange tube,the combined types of the combined fractal fins,and the fractal law of the fractal fins on the solid-liquid phase change heat transfer process is analyzed.The research content and main conclusions of this article are as follows:(1)A numerical simulation study of the process of enhancing the heat transfer between solid and liquid phases with fractal fins was carried out.Based on enthalpy theory and fractal theory,a two-dimensional mathematical model of heat transfer in a fractal fin phase change heat exchanger is established and simulated.The solid-liquid dynamic heat transfer performance of the fractal fin heat exchanger is studied.The phase interface movement characteristics,temperature change characteristics and PCM latent heat storage heat transfer characteristics of the PCM melting process are given.The geometry of the fractal fin structure is analyzed.The influence of parameters,material thermal conductivity,and initial heat transfer temperature difference on phase change heat transfer performance.The research results show that:(a)When the aspect ratio is 12,the enhanced heat transfer performance of the fins reaches the best.(b)There are different heat transfer laws in the early melting stage and the late melting stage.The dimensionless melting time increases with the increase of the aspect ratio and tends to be stable.(c)The more even the fractal fins divide the PCM area,the faster the melting rate and the more uniform the temperature distribution.(2)Optimize the existing fractal fin structure,adjust the refractory PCM area that is not conducive to heat transfer,and achieve uniform cutting by combining with fins of different lengths and shapes.The purpose of the PCM area.The dynamic heat transfer characteristics of a phase-change heat exchanger with branches of vice fins added in the refractory zone are studied.The evolution characteristics of the solid-liquid interface of the combined fractal fins with different shapes of vice fins and the temperature distribution of the heat exchanger are given.The research results show that:(a)The combination of straight fins and fractal fins has the shortest PCM complete melting time;the combination of V-shaped ribs and fractal fins has the longest PCM complete melting time.(b)The larger the heat transfer area of the vice fins,the greater the heat flux through the wall of the heat exchange tube,and the faster the PCM melting rate;the longer the radial length of the vice fins and the thicker the fin thickness,the more conducive to shortening the PCM Melting time.(c)The shape and size of the PCM segmentation area,especially the thickness of the PCM between the fins,have a significant impact on the melting rate and complete melting time of the PCM.(3)Fractal law is the key factor that determines the performance of fractal fins.The length ratio and width ratio of the fractal fins play a decisive role in the heat transfer performance of the fractal fins.Therefore,in this paper,a fractal phase-change heat exchanger model with different length ratio and width ratio is established and numerically simulated.In the case of constant fin material volume,the influence of length ratio and width ratio on the heat transfer process of solid-liquid phase change is analyzed.The research results show that:(a)In the melting heat transfer process,increasing the length ratio of the fractal fins will speed up the average temperature of the fractal fins and the average temperature of the PCM,which will shorten the time for the PCM to completely melt.(b)In the melting heat transfer process,increasing the width ratio of the fractal fins will reduce the melting rate of the PCM and increase the complete melting time.(c)The length ratio and width ratio of the fractal fins interact with each other to affect the melting heat transfer process of the PCM.In this paper,a numerical simulation study of the process of enhancing the heat transfer between solid and liquid phase with fractal fins is carried out,revealing the mechanism of the enhancement of heat transfer between solid and liquid phase by fractal fins,and optimizing the geometric structure parameters of fractal fins and heat exchangers.The concept of combined fractal fins re-optimizes the length ratio and width ratio in the fractal law of fractal fins.The research work provides a theoretical basis for the development of fractal fin enhanced heat transfer technology,and also provides data support for the structural design of fractal phase change heat exchangers.