Research On Heat Transfer Enhancement And Optimization Design Of Shell-and-tube Phase Change Heat Storage Unit

Phase change heat storage technology can well solve the problems of volatility and instability in the utilization of renewable energy.However,the existing alternative phase change materials all have the fatal defect of low thermal conductivity,resulting in a very slow heat storage/release rate,which seriously restricts the practical industrial application of phase change heat storage technology.To this end,based on the fractal structure of snowflake crystals,this paper proposes a novel fin structure to improve the heat storage/release rate of latent heat storage cells filled with phase change materials.First,a full three-dimensional multi-field coupled numerical simulation study is carried out for the heat storage/release process of this unit.The results show that,compared with the longitudinal fin unit with the same volume,the snowflake-type fin unit has a higher inlet velocity than 0.01 m/s,the inlet temperature of the heat storage process is 52 ℃,and the inlet temperature of the exothermic process is 22 ℃.The time for complete melting/solidification of the variable material is shortened by 26.87% and 32.01%respectively,and the average heat storage/release rate has been greatly improved,but the mechanism of the improvement is different,such as the liquid phase transition that characterizes the strength of natural convection heat transfer The integral average value of the material’s maximum velocity instead decreased by 26.83% during heat storage,and increased by 18.00% during heat release.Secondly,aiming at the minimum total melting/solidification time of the phase change material in the heat storage unit,the geometric parameters of the snowflake fin structure in the heat storage unit are optimized by the orthogonal experimental design method.The results show that the primary and secondary influence orders of the experimental factors are the branch angle,branch length and branch spacing of snowflake fins.Compared with the worst experimental scheme,the complete melting/solidification time of the phase change material in the optimal experimental scheme is shortened by13.48% and 18.18%,respectively.In terms of heat transfer performance,in the heat storage process,the average heat flux density increased by 6.32%,and the average Nusselt number increased by 29.81%;in the heat release process,the average heat flux density increased by 12.74%,and the average Nusselt number increased by 40.47% %.Finally,according to the optimal design model,the snowflake fins are processed by wire cutting process.A visual test bench was built to conduct experimental research on the heat storage/release performance of units without fins,with traditional longitudinal fins,and with snowflake fins.The results show that: when the inlet water temperature of the heat storage/release process is 80 ℃/10 ℃ and the flow rate is 2 L/min,the average heat transfer power of the unit with snowflake fins in the heat storage process is higher than that of the unit without fins Compared with the unit with traditional longitudinal fins,the average heat exchange power is increased by 85.71% compared with the unit without fins,which is higher than that of the unit with traditional longitudinal fins.The complete melting/solidification time was shortened by 79.58%,35.47% and 83.36%,51.81%,respectively.The research in this paper shows that snowflake fins can significantly improve the heat storage/release rate and the uniformity of temperature in the phase change heat storage unit.The research results can provide a reference for the design and operation optimization of the heat storage unit.