Performance Study And Fractal Optimization Of Latent Heat Storage Units

Latent heat storage（LHS）technology has the advantages of high heat storage density,stable working temperature and high safety.It is used to solve the problems of uneven distribution of energy supply and demand in space,fluctuation and intermittency in time.However,the thermal conductivity of most phase change materials（PCMs）is small.This leads to low heat transfer efficiency,long heat storage time and high thermal stress inside existing LHS units.Therefore,there is an urgent need to develop the thermal performance enhancement technology for LHS units.In order to promote the practical engineering application of LHS technology.In this paper,numerical simulations coupled with experimental studies are used to study a shell-and-tube LHS unit.Based on the bionic optimization principle,the fin structure and heat exchanger tube layout are optimized to enhance the heat storage performance.The melting process of different configurations of LHS units is studied and discussed more comprehensively and thoroughly.In summary,the main research components and conclusions are as follows.（1）A numerical study of the thermal storage performance of stepped fractal finned LHS units is carried out.No fins,rectangular fins,tree-shaped fins,and stepped fractal finned LHS units are designed.The melting behavior,temperature profiles,natural convection evolution,and heat storage characteristics of the four LHS units are comprehensively analyzed.In addition,the mechanisms of heat transfer fluid temperature and fin material on the thermal storage characteristics are analyzed.In addition,the structural parameters of stepped fractal fins were optimized by the response surface method（RSM）.The results show that: The graded stepped fractal fins synergistically enhance the heat convection and heat conduction of the PCMs above and below the horizontal LHS unit,effectively improving the heat storage rate and temperature uniformity.Compared with the tree-shaped fins,the graded stepped fractal fins greatly improve the thermal performance in the later melting stages and reduce the melting time by 35.9%.Natural convection develops as the dominant mode in the middle melting stage,but decreases in the later stages.In contrast,heat conduction dominates mainly in the early and late melting stages.It should be noted that an increase in heat transfer fluid temperature or fin thermal conductivity improves the heat storage performance of finned LHS units.In particular,the effect is most pronounced for tree-shaped fined LHS units,but the extent of this effect is gradually diminishing.The RSM results showed that the thickness gradient of the main bifurcation,the thickness fractal dimension,and the length ratio of the stepped fractal fins are 3.98,1.34,and 1.11,respectively.The optimized stepped fractal fins reduced the complete melting time by 41.1% compared to the tree-shaped fins.（2）An experimental platform for visualization of the storage and exothermic performance testing in the LHS unit is constructed.The heat storage and release processes are carried out in a stepped fractal finned LHS unit and a conventional rectangular finned LHS unit.The solid-liquid phase distribution at typical moments and the temperature response characteristics at typical locations are studied for comparison.The results show that the stepped fractal fins have the advantage of high specific surface area and optimize the heat transfer path inside the LHS unit.The higher specific surface area and fast and efficient heat transfer path of the graded layered fins is used in the heat storage process.The structure accelerates the melting rate while improving the temperature inhomogeneity and realizing the synergy between natural convection and heat conduction.During the heat release process,the uneven fin layout leads to a deterioration of the temperature inhomogeneity in the LHS unit during the late solidification stage.However,the heat transfer rate is accelerated due to the high specific surface area of the fin structure.Therefore,the total solidification rate of the stepped fractal finned LHS unit is still faster than that of the rectangular finned LHS unit.（3）A numerical model of the heat storage process of a multi-tube tree-shaped finned LHS unit is developed.The influence mechanism of natural convection is targeted from three aspects,i.e.,liquid phase fraction,temperature profile,and heat storage rate.The melting characteristics of three different multi-tube LHS units with no fins,rectangular fins,and tree-shaped fins are comprehensively evaluated.Moreover,the mechanisms of the influence of heat transfer fluid（HTF）temperature and PCM type on the melting characteristics are revealed.The results show that the tree-shaped fin has significantly enhanced heat transfer benefits.The complete melting time of the multi-tube tree-shaped finned LHS unit is reduced by 80.2% and 34.4% compared to the multi-tube LHS unit without fins and rectangular fins,respectively.The corresponding temperature uniformity is also improved.The evolution mechanism of natural convection undergoes three stages: pre-induction,mid-term development,and final-decay.The natural convection induction is slow and its development is limited in the multi-tube LHS unit with fins compared to those without fins.However,the influence of natural convection is significantly expanded,which is especially true in the multi-tube tree-shaped finned LHS unit.It is important to note that increasing the HTF temperature can effectively improve the heat storage efficiency of multi-tube LHS units.However,the magnitude of the increase gradually decreases.Moreover,the selection of PCMs in practical engineering should consider their heat storage rate and thermal efficiency to maximize the thermal storage performance.Compared with RT58 and lauric acid,the complete melting time of the multi-tube LHS unit using NaNO₃ is reduced by 25.1% at maximum.The corresponding total heat storage capacity is increased by 120.4%.