Study On Heat Transfer Characteristics Of PCM Melting In A Fractal Heat Exchanger

In recent years,phase change material(PCM)based heat storage technology is widely used in building energy conservation,fuel cell thermal control,electronic equipment cooling and heating,ventilation and air conditioning(HVAC).However,because of low thermal conductivity of most PCMs,the application of phase change thermal storage technology in practical engineering is restricted.Due to the advantages of easy operation and low cost,the method of adding fins is most commonly adopted for the enhancement of solid-liquid phase heat transfer.However,as the heat storage capacity is reduced when the fins are added,it is of great significance for the improvement of performances of the phase change heat storage devices by optimizing the geometry of the fins.At present,few researches have been carried out on the geometric structure optimization of phase change heat storage devices,and the geometry of the traditional heat exchange device is usually quite simple,so it is necessary to optimize its spatial configuration to improve the efficiency of phase change heat storage.In this context,the fractal geometry is introduced for the enhancement of solid-liquid phase change heat transfer,and the exchangers with cross-shaped fractal metal structure and snowflake fractal fins have been designed in this thesis respectively,which provides a new design concept for the efficiency improvement of the solid-liquid phase change heat exchanger.In this paper,based on the geometric construction of the exchanges with the cross-shaped fractal and snowflake fractal,a two-dimensional model of the PCM melting process in the heat exchanger has been developed and simulated.The influence mechanisms of solid skeleton structural characters and working parameters on the heat transfer characteristics of solid-liquid phase change have been quantitatively analyzed.The internal relationship between the phase change heat transfer performance and the porous metal structural parameters(porosity,fractal dimension),the index ? of the snowflake fractal fins and working conditions have been established,which provides the supportive data for the optimization design of the solid-liquid phase change heat transfer devices.Furthermore,a visualization experimental system is designed and built to study of the solid-liquid phase change heat transfer characteristics of the snowflake fractal heat exchanger.The morphology evolution of the solid-liquid interface during the melting process of the PCM in the snowflake fractal heat exchanger has been observed.The dynamic characteristics of axial and radial temperature variations in the snowflake fractal heat exchanger are analyzed and compared with the traditional heat exchanger with straight fins.The effect of natural convection and working parameters on the solid-liquid phase change heat transfer characteristics is quantitatively analyzed,and the strengthening mechanisms of the snowflake fractal structure on the solid-liquid phase heat transfer process is further revealed.