Enhanced Heat Transfer And Structural Optimization Of Low Temperature Shell And Tube Phase Change Heat Storage Device

Among the phase change heat storage devices,the horizontal shell and tube phase change heat storage device is widely used and studied because of its simple structure and less heat loss,but it has the problem of low heat transfer efficiency.In view of this situation,firstly,taking the arcuate section phase change heat storage unit model as the research object at the two-dimensional level,the changes of solid-liquid phase fraction,temperature and flow field of phase change material（PCM）in the heat storage/release process are obtained through numerical simulation,and the heat transfer mechanism of melting and solidification process of phase change material in the phase change heat storage unit and the influence of arcuate section structure on the heat storage/release process are studied.After that,the structure is further optimized and adjusted by changing the arc center angle.After obtaining the optimal arc center angle,the optimal position and length of fins are determined by analyzing the influence of adding fins at different positions and adding fins with different heights on the heat storage/release process of heat storage unit.Finally,taking the cascade phase change heat storage device with different proportion of PCM as the research object,the heat storage/release process under different conditions is numerically simulated,and the simulation results are analyzed to explore the influence of different filling proportion of PCM on the heat storage/release rate of shell and tube phase change heat storage device.It is found that during heat storage,the arch section design increases the area of the upper area outside the inner tube,and more PCM melts under the action of convective heat transfer.At the end of melting,the plane design at the bottom increases the area of solid-liquid interface and heat transfer efficiency;During heat release,the heat transfer mode of PCM is mainly heat conduction,and there is slight convective heat transfer in the early stage of PCM solidification.This design slightly increases the intensity of convective heat transfer,thus slightly reducing the solidification time.After optimization,the complete melting time of PCM in the arched section heat storage unit with arc center angle of 70.6°is reduced by 31.26%and the complete solidification time is reduced by 5.51%compared with the traditional circular section.On this basis,simple fins are added in the arched section.Through the study of the simulation results,it is found that adding fins directly above and below the inner tube can significantly improve the melting and solidification rate of PCM in the heat storage unit,and the higher the fin height is,the more obvious the effect is.Compared with the traditional circular section,the complete melting time and complete solidification time of PCM are reduced by 56.95%and 40.99%respectively.At the three-dimensional level,compared with the single-stage RT60 phase change heat storage device,the step heat storage device shortens the complete melting time of PCM and increases the heat storage;Compared with the single-stage RT35phase change heat storage device,the complete solidification time of phase change materials in the cascade phase change heat storage device is shortened,but the heat release is reduced,and it is increased compared with the single-stage RT60 phase change heat storage device,but the heat release is increased.If there are higher requirements for heat storage rate,the proportion of materials with lower phase change temperature shall be appropriately increased.If there are higher requirements for heat release rate,the proportion of phase change materials with higher phase change temperature shall be appropriately increased.When the requirements are the same,under the condition of heating temperature of 80℃and cooling temperature of 10℃,the layout of l₁:l₂:l₃=1:2:3 shall be selected.The specific layout shall be selected according to the actual operating conditions.