| Phase change heat storage technology is widely used in the fields of building energy saving,renewable energy utilization and industrial waste heat recovery.It is a key technology to improve energy utilization efficiency and reduce energy consumption per unit of GDP in China.Direct contact phase change heat storage technology has the characteristics of high heat storage density,high heat storage and discharge rate,and simple structure.In-depth study of phase change heat transfer and multiphase flow heat exchange process can help develop efficient phase change heat storage technology and help achieve the goal of"carbon peaking and carbon neutrality"in China.In this paper,the direct contact phase change heat storage process with heat transfer medium as heat transfer oil and phase change material as erythritol is studied,and the heat storage performance under different heat transfer oil flow rates is investigated experimentally.Through the experimental study of the direct contact phase change thermal storage performance,it was found that the whole thermal storage process can be divided into three stages:preheat thermal storage,mid-heat thermal storage and final heat thermal storage.At the first stage of heat storage,increasing the flow rate of heat transfer oil causes the temperature of phase change material to rise rapidly and the heat transfer rate to increase,thus accelerating the melting rate of solid phase change material,improving the heat transfer efficiency of direct contact heat storage device and shortening the heat storage time.When the flow rate of heat transfer oil was increased from 3.4 L/min to 10 L/min,the heat storage efficiency was increased by about 67%and the time for complete melting of the phase change material was shortened by 27%.In addition,the effect of heat-conducting oil flow rate on the heat storage rate was more obvious in the early and middle stages of heat storage,while the difference was smaller at the end.A one-dimensional theoretical model of direct contact phase change heat storage is established and the reliability of the model is verified by comparing with the experimental results.Through an in-depth analysis of the melting behavior and heat transfer characteristics of the phase change material under experimental conditions,it was found that the moving speed of the melting interface of the solid phase change material was accelerated when the flow rate of heat transfer oil was increased,and the moving speed of the interface increased by 57%when the flow rate was increased from 3.4 L/min to 10 L/min.In addition,the convective heat transfer coefficient of the heat transfer oil-erythritol phase change contact was also obtained,and it was found that the convective heat transfer coefficient was the largest at the early stage of heat storage,which was due to the small cross-sectional area of the heat transfer oil flow channel,the large flow rate,and the large temperature difference between the heat transfer oil and the erythritol.As the flow rate increased,the convective heat transfer coefficient also increased,and when the flow rate increased from 3.4 L/min to 10 L/min,the convective heat transfer coefficient increased from 517/~2to 1313/~2,an increase of154%.The convective heat transfer coefficients of direct contact and indirect were compared under the same conditions of heat-carrying fluid and phase change material,and although the values of both were greatest in the pre-storage period and decreased with increasing storage time,the convective heat transfer coefficients of direct contact were generally greater than the values of indirect. |
