Study On Optimization Of Phase Change Thermal Storage Device Based On Natural Convection

As an important part of peak cutting and valley filling technology,phase change thermal storage technology can overcome the negative effects of power supply and industrial waste heat,which is considered as a new excellent thermal storage technology.However,there are some problems in engineering application,such as complex structure,small heat storage density per unit volume,poor economy.Strengthening the heat exchange efficiency of phase change thermal storage device under the premise of ensuring economy becomes a major difficulty in the application.It has great significance to study the optimization of the device based on natural convection.In this paper,the influence of natural convection on the phase change heat transfer process in the device is studied by the method of experiment and numerical simulation,and the existing device and the new composite heat transfer pipe are optimized by the characteristic of natural convection.Firstly,the physical parameters of Ba(OH)2·8H2O such as latent heat,phase change temperature,dynamic viscosity and other important thermal physical parameters were measured.The influence of natural convection on the melting / solidification interface and liquid phase ratio in the phase change process was studied by simulation method,and the importance of natural convection for the phase change process was analyzed.Then,a visualization shell and tube thermal storage device was built and modeled by numerical method.Visual experiment and simulation were carried out to understand the characteristics of thermal storage/release processes,and to verify the effect of natural convection and the correctness of numerical method.Finally,the operation characteristics of existing thermal storage device were studied.According to the analysis of the natural convection and operation characteristics,the operation mode of existing thermal storage device and composite heat exchange pipe(convex type / concave type)of the new device were optimized,which can obtain the optimal pipe structure.The results showed that this material had excellent stability in multiple cycles,the phase change temperature of Ba(OH)2·8H2O is 78.7?,the latent heat is 252.2 J/kg,the dynamic viscosity of liquid-phase decreased with the increase of temperature.Natural convection plays an important role in the process of phase change heat transfer,which has a great influence on melting / solidification interface,heat transfer rate and internal temperature.The phase interface did not expand in a uniform circle but changed irregularly with time.Natural convection reduced the total melting time by 30min(19.23%).In the visualized thermal storage device,the melting process was affected by natural convection.The high temperature liquid-phase materials rise to the top of the solid-liquid interface which accelerated the melting in the upper part of the heat exchange tube,and the phase interface gradually move down from "inverted triangle" type to the lower part of the heat exchange tube.During the solidification process,the temperature of the phase-change materials decreased and the density increased.Under the effect of gravity and natural convection,the materials were easy to sink which accelerated the solidification of lower part of the heat exchange pipe.According to the operation characteristics of existing thermal storage device,a new operation strategy was proposed,which can shorten the thermal storage / release time by discarding 4% of the energy,improve the system economy and heat exchange efficiency,reduce the heat dissipation by 43% and increase the average heat release power by 132%.For new thermal storage device,composite heat exchange pipe with high / low conductive material were designed.The addition of low conductive material in convex pipe had little influence on the heat exchange process,which can solve the scaling and adhesion problem effectively.Under the optimized structure,the increase rate of heat exchange time was 5.16% and the decrease rate of average heat transfer rate was 1.13%.The low conductive material which embedded in the concave pipe strengthened the natural convection,improved the heat exchange efficiency and reduced the heat exchange time.Under the optimized structure,the heat exchange time decreased by 7.89% and the average heat transfer rate increased by 7.20%.