| Based on the inherent need to achieve the vision of "peak carbon" and "carbon neutral",latent heat storage(LHS)technology has become the preferred solution for the supply and demand adaptation of renewable energy sources such as solar and geothermal energy,producing significant social and economic value in the lowtemperature thermal storage field for daily life.Given this,the study of advanced LHS technology and solutions for existing key scientific problems have significant engineering application value and hold important scientific significance for further revealing the multi-scale solid-liquid phase change heat transfer mechanism.As the core component,the existing shell-and-tube latent heat exchangers(LHEs)lack optimization in spatial structure and heat transfer path,resulting in fatal defects such as low thermal efficiency,large thermal anisotropy,significant temperature gradients and concentrated thermal stress,seriously affecting the thermal performance and service life of LHS systems.Moreover,the multi-scale mechanisms of solid-liquid phase change behavior have not been fully revealed,especially the melting and solidification enhancement mechanisms.Inspired by nature,this paper extends the application of fractal tree-shaped structures to the bionic construction and optimization of LHEs,and proposes innovative LHEs with tree-shaped fins and sandwiched treechannels.To carry out systematic research on the heat storage/release performance enhancement of LHEs,the transient theoretical models of solid-liquid phase change processes are established,and the visualization experiments and structural optimization methods are utilized as well.The following conclusions are ultimately drawn:(1)A non-stationary three-dimensional(3D)numerical model of solid-liquid phase change heat transfer considering natural convection is established.The complex phase change heat transfer mechanism and evolution behavior of solid-liquid phase fronts in the heat storage and release processes of tree-shaped finned LHEs is fully revealed and compared to conventional rectangular finned LHEs.Moreover,the influence of the inlet temperature,volume flow rate,and flow direction of the heat transfer fluid(HTF)on the heat storage and release performance is revealed.The results show that for vertical LHEs,tree-shaped fins effectively improve the temperature uniformity of phase change materials(PCM)and increase the melting and solidification rates of the heat storage and release processes.Compared to the conventional rectangular fins,the duration of heat storage and release for tree-shaped fined LHEs is reduced by 34.5% and 49.2%,respectively,and the average heat storage and release rate is increased by 49.4% and96.4%,respectively.It should be noted that while natural convection significantly improves the heat storage performance of LHEs,its effect on the heat release process is negligible.Beyond this,the HTF inlet temperature severely affects the thermal performance of LHEs,but the volume flow rate plays a less prominent role.Interestingly,the HTF upward flow favors improved heat storage performance,while the downward flow mode favors higher heat release rates,a finding that provides useful suggestions for practical engineering applications of vertical LHEs.(2)A visual experimental platform for studies of the heat storage and release performance of LHEs with uniform tree-shaped fins is built.The evolutionary behavior of solid-liquid phase fronts and dynamic thermal characteristics of tree-shaped finned LHEs during the heat storage and release processes are studied and compared to the conventional rectangular finned LHEs.The focus is on the influence mechanism of the inclination angle on the heat storage/release performance.The results indicate that the differences in the melting and solidification mechanism of LHEs are mainly caused by the different excitation moments and durations of natural convection,and the influence of inclination angles on the heat storage process is greater than that of the heat release process,especially the axial thermal characteristics of LHEs in the heat storage process are most sensitive to the inclination angle.Compared to conventional LHEs,the heat storage and release characteristics of tree-shaped finned LHEs are less affected by inclination angles,and their charge and discharge rates are higher at any inclination angle,with a maximum reduction in complete melting time of 52.6% for vertical treeshaped finned LHEs and an increase in charge and discharge rates of 94.7% and 101.5%respectively for horizontal tree-shaped finned LHEs.(3)A two-dimensional numerical model of melting heat transfer in horizontal treeshaped finned LHEs considering natural convection is constructed,and the design optimization of the geometry and number of fins for uniform tree-shaped fins is carried out.In addition,two non-uniform fin layouts,heterogeneous tree-shaped fins and gradient tree-shaped fins,are proposed to improve the melting performance of LHEs.The influence mechanism of non-uniform tree-shaped fins on the thermal performance of horizontal LHEs is investigated,and the spatial layout optimization of non-uniform tree-shaped fins is achieved.The results show that equal branch lengths at each branch level,a width fractal dimension of 1,and a number of fins of 16 are recommended in actual engineering projects to maximize the thermal storage performance for a horizontal LHE with uniform tree-shaped fins.It is worth noting that the non-uniform tree-shaped fins allow for a faster melting rate and a more uniform temperature distribution of PCMs by synergistically improving the natural convection and heat conduction in the upper and lower parts of horizontal LHEs.For maximizing the melting rate of horizontal LHEs with non-uniform fin layouts,a filling angle of 300°and a bifurcation angle gradient of 8° are recommended in practical engineering for LHEs with heterogeneous tree-shaped fins and gradient tree-shaped fins.Compared to uniform tree-shaped fins,the optimized LHEs with heterogeneous tree-shaped fins and gradient tree-shaped fins have a maximum reduction in the complete melting time of49% and 46%,respectively.(4)A visual experimental platform for studies on the heat storage and release processes in gradient finned LHEs is established.The evolution behaviors of solidliquid phase fronts and dynamic melting and solidification characteristics of LHEs with gradient tree-shaped fins are investigated and compared to LHEs with uniform treeshaped fins.Furthermore,a comparative study of numerical predictions and experimental measurements optimized the non-stationary 3D numerical model of melting heat transfer.The results imply that for horizontal LHEs,the gradient treeshaped fins enhance the heat conduction in lower PCM zones of LHEs and prolong the natural convection duration in the upper part,thus promoting a synergistic enhancement of natural convection and heat conduction in the later melting stages.Compared to uniform tree-shaped fins,gradient tree-shaped fins effectively increase the charge rate of horizontal LHEs,reducing the complete melting time by 9% and further improving overall temperature uniformity.However,the non-uniform heat transfer path constructed by gradient tree-shaped fins is detrimental to the heat release process where heat conduction is the primary thermal mechanism,which increases the overall temperature gradient of horizontal LHEs,and extends the complete solidification time by 57.4% compared to the uniform tree-shaped fins.(5)An innovative LHE with sandwiched tree-channels is designed,and a 3D numerical model of its melting process is developed.The melting behavior,temperature response characteristics,and heat storage performance in the innovative LHE with sandwiched tree-channels are investigated and compared to the traditional LHE with serpentine channels.Furthermore,the structural optimization of this innovative LHE is carried out by the response surface method(RSM).The results show that the efficient heat transfer path inherent in tree-shaped channels increases the melting rate,and the fully extended spatial layout of the sandwiched tree-channels improves the temperature uniformity level of PCMs.Compared to conventional LHEs,the innovative LHE reduces the complete melting time by 51.5% while increasing the overall temperature uniformity level by 21.1%.It is worth noting that the natural convection intensity in the vertical direction is more intense than in the horizontal direction during the middle melting stages,and the heat storage performance of any LHE depends to a large extent on its thermal transfer capacity in the later stages.The RSM results show that the structural parameters of tree-shaped channels contribute to the thermal storage performance of innovative LHEs in the order of branch level,width fractal dimension,and length fractal dimension.To maximize the heat storage performance,it is recommended that the branch level,width fractal dimension,and length fractal dimension of the innovative LHE in practical engineering applications should be set to5,2.89,and 1.99,respectively. |
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