Heat Transfer Research And Structure Optimization Of Triple-tube Latent Heat Storage Unit

Phase change heat storage technology can realize the transfer and utilization of thermal energy across time and space,which is of great significance to solve the problem of mismatch between the supply and demand of renewable energy and promote the adjustment of energy structure.However,the low thermal conductivity of phase change materials(PCM)limits the development and application of PCM technology.Compared with the traditional double-tube latent heat storage unit,the triple-tube latent heat storage unit(TTLHSU)has the advantages of a large heat transfer area and good heat transfer uniformity.In this paper,the heat transfer characteristics of a three-tube phase change heat storage system with horizontal and vertical placement are studied numerically,and a novel fin structure is proposed and optimized.The coagulation and melting characteristics of PCM in a triple tube placed horizontally and vertically are numerically investigated for the same structural parameters using the coagulation/melting model and the Boussinesq assumption.The results show that the solidification and melting time of PCM in the horizontal placement triple tube are less than those in the vertical placement,and the time is reduced by 8%and 45.9%,respectively.The analysis shows that the heat storage process of the vertically placed phase change heat storage system would appear temperature stratification phenomenon under the action of buoyancy.The heat transfer state near the heat transfer wall and the top of the PCM region is dominated by natural convection,and the heat transfer state in the middle and bottom is dominated by heat conduction.It shows that the larger the vertical scale of the phase change heat storage system has a great adverse effect on the heat storage velocity,and the horizontal layout of the heat storage system with higher requirements for heat storage performance should be adopted.According to the shape characteristics of PCM cross section in the horizontally placed triple tube phase change heat storage unit,based on fractal theory and method,a novel type of fin structure with longitudinal rectangular fin tip extended cruciform branch is proposed.The calculation results show that the fin with longitudinal rectangular fin tip extended cruciform branch can make the fin more evenly distributed in PCM,and improve the heat transfer performance and heat storage and release rate significantly.By discussing the effects of fin height,fin length ratio,and fin angle on heat storage and release performance,it is found that increasing fin height and decreasing fin length ratio can increase the heat transfer area of the fin,and improve the heat transfer rate in the heat storage and release process.The heat storage and release time decreases first and then increases with the increase of fin angle.The Box-Behnken response surface method was used to optimize the structural parameters of the new branch fin with the goal to minimize the total time of completing a heat storage and release cycle.In the range of studied fin parameters,when the thickness of the branch fin is 0.8 mm,the fin length ratio is 1,and the fin Angle is 63.1°,the heat transfer enhancement effect of the fin is the best.Compared with the structure without fins,the total time to complete a heat storage and release cycle is reduced by 83.72%;compared with the structure with a single longitudinal rectangular fin,the time to complete a heat storage and release cycle is reduced by 51.04%.Based on the theoretical method,a new type of cross-branch fin can improve the performance of heat accumulator,but the fin shape is complex,the processing and manufacturing are difficult,and it is lack of practical application value.However,the optimized fin distribution has reference value for the design of fins with practical application value.Referring to the optimized distribution shape of the fin structure of the longitudinal rectangular fin with the cross branch extended at the top of the longitudinal rectangular fin in PCM,the ring-rectangular longitudinal fin with more practical value is designed by simplifying it.The effects of the length of rectangular fin,the number of fins,and the radius of annular fin on solidification and melting process are analyzed.Numerical results show that the heat storage and release time decreases with the increase of the fin length,and it is more beneficial to heat storage and release when the rectangular fin is connected with the heat transfer walls on both sides.The time required for the heat storage and release process decreases first and then increases with the increase of the radius of the annular fin.The structural parameters are optimized by the Central Composite Design method.In the given range of parameters,when the number of fins is 12 and the radius of annular fins is 35.6 mm,the total time to complete a heat storage and release process is the shortest,which is 21.9 minutes.Compared with the optimal structure of the new branch fin,it is reduced by 6.8%.It is proved that the ring-rectangle longitudinal fin structure can not only reduce the machining difficulty and enhance the practical value but also obtain better heat transfer performance.The regression equation of the total heat storage and release time and fin parameters of the ring-rectangular longitudinal fin structure TTLHSU is summarized,which can provide a calculation basis for the design and application of the heat storage structure.