Research On Enhanced Heat Transfer Performance Based On Tube Phase Change Heat Storage Device

Solar radiant energy stores heat in the thermal storage device through phase change thermal storage technology,that is,phase change material（Phase Change Material PCM）,which solves the problem of intermittent and unstable solar energy.The advantages and disadvantages of the structure of the heat storage device can often determine the heat storage/release performance of the device,thermal storage devices are mainly divided into distributed and integral,the integrated phase change heat storage device has become the focus of current research at home and abroad due to its advantages of large heat storage per unit volume,less leakage of PCM,and simple processing technology.In order to improve the heat storage/release performance of the integral heat storage device,the structure of the constant pitch spiral coil heat storage device and the straight tube heat storage device are optimized in this paper.Based on this,the following research work is carried out:1)Structural design of the heat storage device,tetermine the box height 800mm,diameter 250mm,coil inner diameter 10mm,wall thickness 1mm,curvature radius 150mm,and the number of turns 15.Aiming at the problems of long pipe path and large temperature difference between the outlet temperature and the inlet temperature of the constant pitch spiral coil,the pitch is optimized,and the gradient spiral coil is designed,and the optimal gradient pitch is determined:d₁=17mm、d_n+1=dn+C、C=4.238mm、d₁₅=76.332mm.Under the condition that the quality of the composite PCM paraffin,the number of coils,the radius of curvature,and the height of the coil are the same for the two heat storage devices,the heat storage/release characteristics of the two heat storage devices were explored by changing the inlet flow rate,inlet temperature,and inlet method.The heat transfer process between heat transfer fluid（Heat Transfer Fluid,HTF）and PCM was analyzed by combining the temperature nephogram and the liquid phase rate curve.The research shows that the heat storage/release efficiency increases with the increase of the inlet flow rate and the temperature difference between the PCM and the HTF;The heat release efficiency of the bottom-in,top-exit mode is improved to a certain extent compared with the top-in,bottom-out mode;Under different working conditions,the heat storage/release efficiency of the gradient spiral coil heat storage device is greatly improved compared with the constant spiral coil heat storage device.2)The structure of the single-phase straight-tube heat storage device was improved,and a three-phase straight-tube heat storage device filled with three PCMs was designed.In the design,along the axial direction of the HTF,composite PCM paraffins with phase transition temperatures of 61°C,57.5°C,and 49°C are arranged from bottom to top.The temperature distribution cloud map of the tube wall determines the volume of different PCMs from bottom to top 1/6,1/3,1/2.The experimental results under different working conditions show that the three-phase straight-tube heat storage device can gradually reduce the temperature of HTF to close to the phase transition temperature of the final stage PCM during the heat storage process,which solves the problem of PCM settlement in the single-phase straight-tube heat storage device.problems,and the heat storage efficiency is greatly improved compared with the single-phase straight tube heat storage device.In this paper,the combination of experiments and simulations is used to explore the heat storage/release performance of the tubular heat storage device,and to analyze the heat transfer process between HTF and PCM.The application and optimization of the thermal storage device in the solar heating system has laid a theoretical and experimental foundation.