A helical phase change heat storage structure based on gradient reinforcement

Abstract

The application discloses a spiral phase change heat storage structure based on gradient reinforcement, and belongs to the technical field of phase change heat storage, which comprises an outer shell, a heat exchange pipe, spiral main fins, gradient secondary fins, phase change heat storage materials and the like. The heat exchange pipe is arranged in the outer shell along the axial direction of the outer shell, and the spiral main fins are fixed to the outer wall of the heat exchange pipe and continuously extend along the axial direction of the heat exchange pipe. A plurality of gradient secondary fins are arranged on the upper surface of the spiral main fins and are arranged in sequence and at intervals along the radial direction from the outer wall of the heat exchange pipe to the inner wall of the outer shell, a flow channel for the flow of liquid phase change heat storage materials is formed between adjacent gradient secondary fins, and the height of the plurality of gradient secondary fins gradually increases in the direction away from the heat exchange pipe. The application can shorten the radial heat conduction path of the phase change heat storage materials, reserve suitable natural convection space for the liquid phase change heat storage materials, improve the heat conduction capacity of the peripheral region of the heat storage device, make the phase change process more uniform, and improve the heat storage rate and heat transfer efficiency of the phase change heat storage device.

Description

Technical Field

[0001] This invention relates to the field of phase change thermal energy storage technology, and specifically to a gradient-enhanced spiral phase change thermal energy storage structure. Background Technology

[0002] Phase change thermal energy storage (PCE) technology utilizes the absorption or release of latent heat during the phase change process between a solid and liquid phase using PCE materials to achieve thermal energy storage and release. Compared to sensible heat storage, PCE technology features higher thermal density, relatively stable temperature during the phase change process, and higher energy utilization efficiency. It can be applied to fields such as solar thermal utilization, industrial waste heat recovery, building energy conservation, district heating, and medium- and high-temperature thermal energy storage.

[0003] However, existing phase change thermal storage materials generally suffer from low thermal conductivity. During the thermal storage process, after heat is transferred from the heat exchange medium to the phase change thermal storage material through the heat exchange tube wall, it continues to be transferred to areas away from the heat exchange tube mainly through the internal thermal conductivity of the phase change thermal storage material. This easily causes the phase change thermal storage material near the heat exchange tube to melt first, while a large unmelted area remains in the outer area and bottom of the thermal storage device. This results in a long melting time for the phase change material, uneven temperature distribution, and low thermal storage efficiency.

[0004] To improve the heat transfer performance of phase change thermal storage devices, existing technologies typically incorporate straight fins, annular fins, or spiral fins on the outside of the heat exchange tubes to expand the heat exchange area and shorten the internal heat conduction path of the phase change thermal storage material. Among these, spiral fins can form continuous heat conduction paths along the axial and radial directions of the heat exchange tubes, resulting in a superior heat transfer enhancement effect.

[0005] However, existing helical fin reinforcement structures typically improve thermal conductivity by adjusting the pitch, thickness, width, or number of helical fins. While increasing the number of fins, decreasing the fin spacing, or increasing the fin size can further shorten the heat conduction path, it also occupies the flow space of the liquid phase change thermal storage material, inhibiting its natural convection. On the other hand, simply adjusting the overall size of the main helical fins makes it difficult to specifically configure the thermal conductivity according to the heat transfer requirements of different radial regions, resulting in low-temperature zones or unmelted zones easily forming in the peripheral areas far from the heat exchange tubes.

[0006] Therefore, there is a need to provide a phase change thermal storage device that can take into account both radial thermal conductivity enhancement and natural convection of liquid phase change thermal storage materials. Summary of the Invention

[0007] The purpose of this invention is to provide a gradient-enhanced spiral phase change thermal storage structure to solve the problem that existing phase change thermal storage devices cannot simultaneously achieve radial thermal conductivity enhancement of phase change thermal storage materials and natural convection of liquid phase change thermal storage materials.

[0008] To solve the above-mentioned technical problems, the present invention provides the following technical solution: This invention provides a gradient-enhanced spiral phase change thermal storage structure, including an outer shell, heat exchange tubes, spiral main fins, multiple gradient secondary fins, phase change thermal storage material, a thermal fluid inlet, and a thermal fluid outlet.

[0009] The heat exchange tubes are arranged axially within the outer shell, forming a heat storage cavity between the outer shell and the heat exchange tubes. The phase change heat storage material is filled into the heat storage cavity. The heat fluid inlet and heat fluid outlet are respectively located at both ends of the heat exchange tubes to allow the heat exchange medium to flow into and out of the heat exchange tubes.

[0010] The spiral main fins are fixedly connected to the outer wall of the heat exchange tube and extend continuously spirally along the axial direction of the heat exchange tube. The spiral main fins can transfer the heat from the heat exchange tube wall to the interior of the phase change thermal storage material along the axial and radial directions of the heat exchange tube.

[0011] Multiple gradient secondary fins are disposed on the upper surface of the helical main fins and are arranged in a radial pattern from the outer wall of the heat exchange tube to the inner wall of the outer shell at intervals. A flow channel communicating with the heat storage cavity is formed between two adjacent gradient secondary fins to allow the molten liquid phase change heat storage material to flow.

[0012] Along the direction away from the heat exchange tube, the height of multiple gradient secondary fins gradually increases. The gradient secondary fins closer to the heat exchange tube have a smaller height to reduce the obstruction of the flow of the liquid phase change thermal storage material; the gradient secondary fins further away from the heat exchange tube have a larger height to increase the heat conduction area of ​​the outer region of the thermal storage device and shorten the radial heat conduction path.

[0013] Furthermore, the gradient secondary fins are strip-shaped protrusions and are arranged perpendicular to the upper surface of the helical main fins.

[0014] Furthermore, each spiral cycle of the main spiral fin is provided with at least one set of gradient secondary fins, and each set of gradient secondary fins includes a first gradient secondary fin, a second gradient secondary fin, and a third gradient secondary fin arranged in sequence along the radial direction.

[0015] Furthermore, the heights of the first gradient secondary fin, the second gradient secondary fin, and the third gradient secondary fin are H1, H2, and H3, respectively, and satisfy H1 < H2 < H3.

[0016] Furthermore, the heights of the first-gradient secondary fins, the second-gradient secondary fins, and the third-gradient secondary fins are set in an arithmetic progression or a proportional progression.

[0017] Furthermore, the height ratio of the first gradient secondary fin, the second gradient secondary fin, and the third gradient secondary fin is H1∶H2∶H3=1∶2∶3.

[0018] Furthermore, the thickness of the gradient secondary fins is 0.5~2 mm, and the width of the flow channel formed between two adjacent gradient secondary fins is 5~50 mm.

[0019] Furthermore, the thickness of the main spiral fin is 1~3 mm, the pitch is 15~50 mm, and the radial width is 20~80 mm.

[0020] Furthermore, the phase change thermal storage material can be an organic phase change material, an inorganic phase change material, or a eutectic phase change material.

[0021] Furthermore, the phase change thermal storage material is a eutectic phase change material formed by adipic acid and succinic acid, with a mass ratio of adipic acid to succinic acid of 2.9:1.

[0022] Furthermore, the heat exchange tube, the spiral main fin, and the gradient secondary fin are integrally formed using selective laser melting additive manufacturing; or, the gradient secondary fin is fixedly connected to the spiral main fin by welding or brazing.

[0023] Compared with the prior art, the present invention can achieve the following beneficial effects: 1. This invention constructs a helical phase change thermal storage structure with gradient-enhanced characteristics by incorporating secondary fins with radially varying heights on the surface of the helical primary fins. Through rational configuration of the secondary fin height distribution, the thermal conductivity of areas far from the heat source is enhanced while retaining suitable flow space for the liquid phase change material. This ensures that the enhanced structure matches the temperature and flow field evolution during the phase change process, achieving a synergistic effect of enhanced thermal conductivity and enhanced natural convection, thereby improving the overall heat transfer performance of the thermal storage device.

[0024] 2. The gradient secondary fins increase the effective heat exchange area, shorten the radial heat conduction path of the phase change material, enhance the heat transfer capability to the outer region of the heat storage device, and promote the uniform outward propagation of the phase change interface. Numerical simulation results show that, under the same operating conditions, the medium gradient structure (H1:H2:H3=1:2:3) proposed in this invention has superior heat storage performance, with an average liquid phase fraction of 0.75 at 5000 s, which is approximately 53.0%, 14.0%, and 7.3% higher than the bare tube structure, the spiral fin structure, and the structure without gradient secondary fins, respectively, significantly improving the phase change heat storage efficiency.

[0025] 3. The continuous flow channels formed between the gradient secondary fins can guide the liquid phase change material to form a more stable and uniform natural convection circulation, reduce the low-speed flow zone and unmelted zone at the bottom of the heat storage tank and in the area far from the heat exchange tube, make the internal temperature distribution of the heat storage tank more uniform, and improve the effective utilization rate of the phase change material and the heat storage stability.

[0026] 4. The gradient reinforcement structure described in this invention can be optimized by adjusting parameters such as the height, number, spacing, thickness and gradient amplitude of the secondary fins to meet the needs of different sized thermal storage devices, different phase change materials and different operating conditions, and has good structural adaptability and engineering design flexibility.

[0027] 5. This invention adds gradient secondary fins to the traditional spiral fins without changing the overall structure of the shell-and-tube phase change thermal energy storage device. It can be manufactured using integral forming processes such as additive manufacturing, or by machining, welding, or brazing. It has the advantages of compact structure, good thermal conductivity continuity, high manufacturing consistency, and convenient engineering implementation. It can be widely used in industrial waste heat recovery, solar thermal utilization, medium and high temperature phase change thermal energy storage, geothermal energy storage, and other thermal energy storage fields. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the structure and an axial cross-sectional view of the spiral phase change thermal energy storage device of the present invention; Figure 2 This is a partially enlarged schematic diagram of the gradient secondary fin configuration structure of the present invention; Figure 3 A comparative schematic diagram of the light tube structure, spiral fin structure, non-gradient secondary fin structure, medium-gradient secondary fin structure, and high-gradient secondary fin structure; Figure 4 Liquid fraction cloud diagrams for different phase change thermal storage structures at different thermal storage times; Figure 5 Velocity contour plots for different phase change thermal storage structures at 5000 s.

[0029] In the diagram: 1. Shell; 2. Heat exchange tube; 3. Spiral main fins; 4. Gradient secondary fins; 5. Phase change thermal storage material; 6. Heat fluid inlet; 7. Heat fluid outlet. Detailed Implementation

[0030] The present invention will now be described in further detail with reference to the accompanying drawings. It should be noted that the following specific embodiments are only used to further illustrate the present application and should not be construed as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above application content.

[0031] In the description of this invention, "axial" refers to the direction extending along the central axis of the heat exchange tube 2; "radial" refers to the direction from the outer wall of the heat exchange tube 2 along the width direction of the spiral main fin 3 toward the inner wall of the outer shell 1; and "upper surface" refers to the side surface of the spiral main fin 3 facing its previous spiral cycle.

[0032] Example 1 like Figure 1 and Figure 2 As shown, this embodiment provides a gradient-enhanced spiral phase change thermal storage structure, including a shell 1, a heat exchange tube 2, spiral main fins 3, gradient secondary fins 4, phase change thermal storage material 5, a thermal fluid inlet 6, and a thermal fluid outlet 7.

[0033] The outer shell 1 is a cylindrical structure extending in the vertical direction. The heat exchange tube 2 is arranged inside the outer shell 1 along the axial direction of the outer shell 1 and is located in the central region of the outer shell 1. The heat exchange tube 2 is spaced apart from the outer shell 1, thereby forming a heat storage cavity between the outer wall of the heat exchange tube 2 and the inner wall of the outer shell 1.

[0034] Phase change thermal storage material 5 is filled into the thermal storage cavity; the phase change thermal storage material 5 can be an organic phase change material, an inorganic phase change material, or a eutectic phase change material; in this embodiment, the phase change thermal storage material 5 is preferably a eutectic phase change material formed by adipic acid and succinic acid, with a mass ratio of adipic acid to succinic acid of 2.9:1, a phase change temperature of 130~140℃, a latent heat of phase change of approximately 220kJ / kg, a thermal conductivity of approximately 0.40 W / (m·K), a solid phase heat capacity of 1491 J / (kg·K), a liquid phase heat capacity of 2433 J / (kg·K), and a solid density of approximately 1500 kg / m³. 3 The liquid phase density is approximately 1350 kg / m³. 3 The solid-phase thermal expansion coefficient is approximately 1.0 × 10⁻⁶. -4 K -1 The coefficient of thermal expansion of the liquid phase is approximately 8.0 × 10⁻⁶. -4 K -1 The dynamic viscosity of the liquid is 4.5 × 10⁻⁶. -3 Pa·s.

[0035] The hot fluid inlet 6 and the hot fluid outlet 7 are respectively located at both ends of the heat exchange tube 2 and are connected to the internal channels of the heat exchange tube 2 for the entry and exit of the high-temperature heat exchange medium. The heat exchange medium can be heat transfer oil, water, molten salt or other heat exchange media suitable for medium and high temperature heat storage systems. During the heat storage process, the high-temperature heat exchange medium enters the heat exchange tube 2 through the hot fluid inlet 6 and is discharged through the hot fluid outlet 7 after passing through the heat exchange tube 2.

[0036] The spiral main fin 3 is fixedly connected to the outer wall of the heat exchange tube 2 and extends continuously spirally along the axial direction of the heat exchange tube 2; the inner edge of the spiral main fin 3 is connected to the outer wall of the heat exchange tube 2, and the outer edge of the spiral main fin 3 extends toward the inner wall of the outer shell 1, thereby forming a heat-conducting skeleton that is continuously distributed along the axial and radial directions.

[0037] In this embodiment, the thickness of the spiral main fin 3 is 1~3 mm, the pitch is 15~50 mm, and the width of the spiral main fin 3 extending radially along the heat exchange tube 2 is 20~80 mm. The specific dimensions of the spiral main fin 3 can be determined according to the dimensions of the shell 1 and the heat exchange tube 2, the thermophysical parameters of the phase change heat storage material 5, and the predetermined heat storage power.

[0038] Each spiral cycle of the main spiral fin 3 is provided with a set of gradient secondary fins 4. The gradient secondary fins 4 and the main spiral fin 3 are integrally formed, or they can be fixedly connected by welding or brazing.

[0039] The height gradient direction of the gradient secondary fins 4 is the radial direction of the spiral main fins 3, that is, from the outer wall of the heat exchange tube 2 to the inner wall of the heat storage shell 1, rather than changing along the axial direction or spiral circumference of the heat exchange tube 2; preferably, three gradient secondary fins 4 are provided in each spiral cycle; the gradient secondary fins 4 are strip-shaped raised reinforcement structures, and multiple gradient secondary fins 4 are arranged in sequence along the radial direction of the spiral main fins 3, forming a continuous millimeter-level flow channel between adjacent gradient secondary fins 4 for the flow of liquid phase change material.

[0040] In one embodiment, each set of gradient secondary fins 4 includes a first gradient secondary fin, a second gradient secondary fin, and a third gradient secondary fin; the first gradient secondary fin, the second gradient secondary fin, and the third gradient secondary fin are all strip-shaped protrusions and are arranged perpendicular to the upper surface of the spiral main fin 3.

[0041] The first gradient secondary fin, the second gradient secondary fin, and the third gradient secondary fin are arranged in sequence at intervals along the radial direction of the spiral main fin 3; wherein, the first gradient secondary fin is located close to the heat exchange tube 2, the third gradient secondary fin is located close to the inner wall of the outer shell 1, and the second gradient secondary fin is located between the first gradient secondary fin and the third gradient secondary fin.

[0042] The heights of the first gradient secondary fin, the second gradient secondary fin, and the third gradient secondary fin are H1, H2, and H3, respectively, and satisfy: H1 < H2 < H3; the heights of the three gradient secondary fins 4 can be increased by arithmetic progression, proportional progression, or other increasing rules, and can be determined according to the structural dimensions of the heat storage device and the thermophysical parameters of the phase change heat storage material 5.

[0043] In this embodiment, the heights of the three gradient secondary fins 4 are preferably arranged in an arithmetic progression, and the height ratio of the first gradient secondary fin, the second gradient secondary fin, and the third gradient secondary fin is: H1∶H2∶H3=1∶2∶3.

[0044] Specifically, H1, H2 and H3 can be set to 3 mm, 6 mm and 9 mm respectively; the average height of the three gradient secondary fins 4 is 6 mm; the thickness of each gradient secondary fin 4 is 0.5~2 mm, and a continuous flow channel with a width of 5~50 mm is formed between adjacent gradient secondary fins.

[0045] It should be noted that the above height ratio and specific height are only a preferred setting in this embodiment and do not constitute a limitation on the height parameters of the gradient secondary fins 4. Under the condition that the height of the gradient secondary fins 4 increases stepwise in the direction away from the heat exchange tube 2, the height of each gradient secondary fin 4 and its increment can be adaptively adjusted according to parameters such as the inner diameter of the shell 1, the outer diameter of the heat exchange tube 2, the radial width of the spiral main fins 3, and the viscosity and thermal conductivity of the phase change heat storage material 5.

[0046] During the heat storage process, the high-temperature heat exchange medium enters the heat exchange tube 2 through the hot fluid inlet 6; the heat of the high-temperature heat exchange medium is transferred through the tube wall of the heat exchange tube 2 to the spiral main fin 3, and further transferred to the gradient secondary fin 4, and finally diffused into the interior of the phase change heat storage material 5.

[0047] In the initial stage of thermal storage, the phase change thermal storage material 5 is mainly in a solid state. The heat is mainly transferred outward through the heat exchange tube 2, the spiral main fin 3, the gradient secondary fin 4, and the thermal conduction between the solid phase change thermal storage material 5. The spiral main fin 3 forms a primary thermal conduction path that is continuously distributed along the axial and radial directions. The gradient secondary fin 4 forms a secondary thermal conduction path that extends into the thermal storage cavity based on the spiral main fin 3. This can shorten the average thermal conduction distance inside the phase change thermal storage material 5 and improve the heat transfer rate in the initial stage of thermal storage.

[0048] Because the first gradient secondary fins are positioned close to the heat exchange tube 2, the phase change thermal storage material 5 near the heat exchange tube 2 is heated first and gradually melts. The height of the first gradient secondary fins is relatively low, which can both enhance the thermal conductivity of the near heat source area and reduce the occupation of the flow space near the heat exchange tube 2. As the phase change thermal storage material 5 gradually melts, the amount of liquid phase change thermal storage material increases. The heated liquid phase change thermal storage material moves upward under the action of buoyancy and flows along the flow channel formed between adjacent gradient secondary fins 4 and the space between adjacent spiral cycles. The relatively low-temperature liquid phase change thermal storage material is replenished to the vicinity of the heat exchange tube 2, thereby gradually forming a natural convection circulation in the thermal storage cavity.

[0049] The heights of the second and third gradient secondary fins increase sequentially in the direction away from the heat exchange tube 2, giving the central and peripheral regions of the heat storage cavity a gradually increasing metal heat conduction area. Since the peripheral region of the heat storage cavity is far from the heat exchange tube 2, the third gradient secondary fins can further transfer the heat on the spiral main fins 3 to the inner wall of the outer shell 1, thereby shortening the heat conduction path of the peripheral region.

[0050] The continuous flow channels maintained between adjacent gradient secondary fins 4 allow the liquid phase change thermal storage material to circulate around the top or end of the gradient secondary fins 4; therefore, the gradient secondary fins 4 can not only increase the heat exchange area, but also distribute the thermal conductivity and flow space of the liquid phase change thermal storage material in different radial regions through the combination of height and spacing.

[0051] In the later stages of thermal storage, the spiral main fins 3 and the gradient secondary fins 4 continuously transfer heat to the outer area of ​​the thermal storage cavity. The natural convection formed by the liquid phase change thermal storage material further drives the heat to be transferred to the area away from the heat exchange tube 2, so that the phase change interface gradually moves from the vicinity of the heat exchange tube 2 toward the inner wall of the outer shell 1. As a result, the amount of unmelted phase change thermal storage material remaining in the outer and bottom areas of the thermal storage cavity can be reduced, making the melting process of the phase change thermal storage material 5 more uniform and improving the effective utilization rate of the phase change thermal storage material 5.

[0052] In summary, the lower first-gradient secondary fins can balance the enhancement of thermal conductivity near the heat exchange tube 2 with the retention of flow space, while the higher third-gradient secondary fins can enhance thermal conductivity in areas far from the heat exchange tube 2. The flow channels between adjacent gradient secondary fins 4 can provide flow space for the natural convection of the liquid phase change thermal storage material, thereby achieving synergistic enhancement of thermal conductivity and natural convection.

[0053] Example 2 This embodiment is used to illustrate the manufacturing method of heat exchange tube 2, spiral main fin 3 and gradient secondary fin 4. The structures and connection relationships not specifically described are the same as in Embodiment 1.

[0054] In this embodiment, preferably, the heat exchange tube 2, the spiral main fin 3, and the gradient secondary fin 4 are integrally formed using selective laser melting additive manufacturing process. Through integral forming, a continuous metal structure can be formed between the heat exchange tube 2, the spiral main fin 3, and the gradient secondary fin 4, reducing the contact thermal resistance at the interface between different components and improving the connection strength and dimensional consistency of the spiral main fin 3 and the gradient secondary fin 4.

[0055] In the specific sample preparation process, an SLM350 metal additive manufacturing equipment can be used, with AlSi10Mg aluminum alloy spherical metal powder as the printing material, and the powder particle size is 15~53 μm. During the printing process, the powder layer thickness is set to 0.04 mm, and the scanning strategy adopts the Line linear scanning method. The scanning direction between adjacent forming layers is rotated by 67° to reduce the accumulation of thermal stress in the same direction during continuous forming, reduce residual stress, and improve the density and dimensional accuracy of the formed parts. After printing, the formed parts are subjected to support removal and sandblasting to remove the printing supports and residual powder on the surface. Subsequently, the ends, connection positions, and surfaces that need to be fitted or sealed of the heat exchange tube 2 are machined to ensure the channel size, assembly accuracy, and sealing performance of the heat exchange tube 2.

[0056] In other embodiments, the heat exchange tube 2, the spiral main fin 3, and the gradient secondary fin 4 can also be processed separately; the spiral main fin 3 is fixed to the outer wall of the heat exchange tube 2 by welding or brazing, and the gradient secondary fin 4 is fixed to the upper surface of the spiral main fin 3 by welding or brazing.

[0057] When using separate processing and connection methods, it should be ensured that there is a continuous and effective connection area between the spiral main fin 3 and the heat exchange tube 2, and between the gradient secondary fin 4 and the spiral main fin 3, so as to reduce the contact thermal resistance at the connection interface and ensure that heat can be transferred from the heat exchange tube 2 to the spiral main fin 3 and the gradient secondary fin 4 in sequence.

[0058] Example 3 This embodiment is used to numerically simulate and verify the enhanced heat transfer effect of the radial height gradient secondary fin 4 structure described in Embodiment 1.

[0059] (1) Numerical simulation method To verify the enhanced heat transfer performance of the helical phase change thermal storage structure with radially gradient secondary fins proposed in this invention during the phase change thermal storage process, computational fluid dynamics (CFD) was used to perform three-dimensional transient numerical simulations of the phase change thermal storage process of different thermal storage structures, and the heat transfer performance of each structure was compared and analyzed. A three-dimensional shell-and-tube phase change thermal storage device model was established using ANSYS Fluent, and the phase change process was solved using the enthalpy-porosity method. The flow of the liquid phase change material was modeled using a laminar flow model, and natural convection was considered. To improve the accuracy of the numerical simulation, this embodiment uses user-defined functions to define the density and viscosity of the liquid phase change material in a temperature-dependent manner.

[0060] The initial temperature of all models was set to 120°C. oC, The phase change material is initially in a completely solid state; an isothermal boundary condition is applied to the inner wall of heat exchanger tube 2, with the temperature set at 150°C. o C. The outer shell 1 wall adopts an adiabatic boundary condition, and the gravity direction is vertically downward. Numerical calculations use a pressure-based transient solver, the pressure-velocity coupling uses the Coupled algorithm, and the momentum and energy equations are discretized using a second-order upwind scheme. The time step is set to 1 s, the maximum number of iterations per time step is 20, and the residual convergence criteria are set to continuity and a momentum equation value less than 10. -5 The energy equation is less than 10. -6 .

[0061] In this embodiment, adipic acid / succinic acid eutectic phase change material is used as the heat storage medium. Its mass ratio and thermophysical parameters are set according to the parameters listed in Example 1, and will not be repeated.

[0062] (2) Comparison of structural design To evaluate the enhanced heat transfer performance of the gradient secondary fin structure 4 of this invention, five different heat storage structures were established for comparative analysis, and their structural schematic diagrams are shown below. Figure 3 As shown, the abbreviations and descriptions of each structure are shown in Table 1. To ensure comparability between different structures, each calculation model maintains the same shell 1 size, heat exchange tube 2 size, phase change material mass, total metal material volume, boundary conditions, initial conditions and calculation parameters. Only the height distribution of the gradient secondary fins 4 is changed, and the other structural parameters remain the same to eliminate the influence caused by heat exchange area, metal usage and other factors.

[0063] Table 1. Abbreviations and related explanations for different phase change thermal storage structures.

[0064] Besides the light tube (BT) and helical fin structure (SF), three different height distribution schemes were set up for analysis, only changing the height gradient of the secondary fins along the radial direction of the helical main fin 3. The specific parameters are as follows: Option I (no gradient): H1=6 mm, H2=6 mm, H3=6 mm (height ratio is 1:1:1); Option II (Medium gradient): H1=3 mm, H2=6 mm, H3=9 mm (height ratio is 1:2:3); Scheme III (High Gradient): H1=1 mm, H2=6 mm, H3=11 mm (height ratio is 1:6:11). All three schemes employ three strip-shaped secondary fins arranged radially along the main helical fin 3, and satisfy the following conditions: H1 < H2 < H3; That is, the height of the gradient secondary fins 4 increases gradually from the side closer to the heat exchange tube 2 to the side farther away from the heat exchange tube 2; the average secondary fin height of the three structures is 6 mm, so the total metal volume and heat transfer area are basically the same, only the height gradient amplitude is different, which can effectively eliminate the influence of increased metal content on the strengthening effect; by comparing the average liquid fraction, average temperature and other indicators of the phase change material under different height gradient schemes, the influence of the height gradient amplitude on the synergistic effect of thermal conductivity strengthening and natural convection strengthening is analyzed, and the optimal height distribution of the gradient secondary fins is determined.

[0065] (3) Analysis of simulation results To evaluate the enhanced heat transfer performance of the radially height-gradient secondary finned spiral phase change thermal storage structure proposed in this invention, numerical simulations were performed to compare and contrast bare tubes (BT), spiral fins (SF), non-gradient secondary finned spiral structures (SF-NGSF), medium-gradient secondary finned spiral structures (SF-MGSF), and high-gradient secondary finned spiral structures (SF-HGSF). The average temperature and average liquid fraction of the phase change material were used, combined with a liquid fraction cloud map (…). Figure 4 ) and velocity cloud map ( Figure 5 The thermal storage performance of each structure was comprehensively evaluated, and the relevant calculation results are shown in Tables 2 and 3.

[0066] Table 2. Average temperature (K) of phase change materials in different phase change thermal storage structures

[0067] Table 3. Average liquid fraction (-) of phase change materials in different phase change thermal storage structures

[0068] Simulation results show that the phase change process of each thermal storage structure starts near the heat exchange tube and gradually expands to the periphery of the thermal storage device. In the early stage of thermal storage (0~1000 s), the liquid phase change material has not yet formed in large quantities, and the heat is mainly transferred by conduction. The melting area is concentrated near heat exchange tube 2. As the thermal storage process proceeds, the liquid PCM increases continuously and gradually forms a natural convection circulation under the action of buoyancy. The combined effect of conduction and natural convection causes the melting interface to continue to advance to the periphery.

[0069] As shown in Table 2, all the reinforcement structures can increase the average temperature of the phase change material. Among them, the average temperature of the BT structure is only 407.90 K at 5000 s, while that of the SF structure is increased to 409.57 K. After further adding secondary fins, the average temperatures of the SF-NGSF, SF-MGSF and SF-HGSF structures reach 409.99 K, 410.51 K and 410.13 K respectively. Among them, SF-MGSF always maintains the highest average temperature, indicating that appropriate gradient design can improve the heat transfer capacity to the outer area of ​​the heat storage device, so that the heat storage device as a whole is heated more fully.

[0070] Table 3 shows that each reinforcement structure improved the melting rate of the phase change material to varying degrees. At 5000 s, the average liquid phase fraction of the BT structure was only 0.49. After adding helical fins, the SF structure increased to 0.65, which is about 34.2% higher than that of BT, indicating that continuous helical fins can effectively increase the heat exchange area and establish a continuous heat conduction path. After further adding non-gradient secondary fins, the average liquid phase fraction of SF-NGSF reached 0.69, which is about 6.2% higher than that of SF, indicating that adding secondary fins can further enhance the thermal conductivity. After adopting the medium gradient secondary fin structure (SF-MGSF) proposed in this invention, the average liquid phase fraction reached 0.75, which is about 53.0%, 14.0%, and 7.3% higher than that of BT, SF, and SF-NGSF, respectively, showing a better heat storage enhancement effect. However, after further increasing the gradient to SF-HGSF, the average liquid phase fraction decreased to 0.71, indicating that a larger gradient is not always better, but there is an optimal gradient range.

[0071] Combination Figure 4 Liquid phase density contour plots can further analyze its enhancement mechanism; due to the lack of enhanced heat conduction units, the melting of the BT structure is mainly concentrated near heat exchanger tube 2, and a large unmelted area always exists around the periphery and bottom of the heat storage tank; the SF structure relies on continuous spiral fins to establish a heat conduction network, which significantly expands the melting range, but a certain unmelted area still exists in the peripheral region; the SF-NGSF structure further shortens the radial heat conduction path by adding secondary fins, allowing the liquid phase region to continue to expand; in contrast, the SF-MGSF structure shows poor performance at 1000 s, 3000 s, and 5000 s. The SF-HGSF structure exhibits a more uniform melting interface at all time points, and the liquid phase region can expand to the periphery of the heat storage device more quickly. The unmelted areas at the bottom and corners are significantly reduced, indicating that it can balance thermal conductivity enhancement and natural convection enhancement, making the phase change process more uniform. Although the SF-HGSF structure further increases the thermal conductivity of the peripheral region, the gradient amplitude is too large, which weakens the thermal conductivity enhancement in the near-heat source region. At the same time, the higher secondary fins on the periphery compress the liquid PCM flow space, reducing the matching degree between thermal conductivity enhancement and natural convection enhancement. Therefore, its overall thermal storage performance is slightly lower than that of the SF-MGSF structure.

[0072] Figure 5 Velocity cloud maps further illustrate the development patterns of natural convection in the liquid PCM within different structures. The BT structure exhibits a low liquid PCM velocity, resulting in only limited natural convection circulation and a weak overall flow field. The SF structure benefits from enhanced natural convection due to the guiding effect of the spiral main fins. The SF-NGSF structure creates multiple localized circulating flow fields between adjacent secondary fins, improving the flow capacity of the liquid PCM; however, the uniform height of each secondary fin lacks specificity in enhancing flow across different radial regions. In the SF-MGSF structure, lower secondary fins are used near heat exchanger tube 2, ensuring enhanced thermal conductivity in the near-heat source region while preserving sufficient flow space for the liquid PCM. The gradually increasing radial height of the secondary fins further enhances the flow. The SF-HGSF structure enhances the thermal conductivity of the outer region and forms a continuous and stable flow channel between adjacent secondary fins, enabling the liquid PCM to form a more uniform and complete natural convection circulation. This achieves a synergistic effect of enhanced thermal conductivity and enhanced natural convection, resulting in superior heat transfer performance. In contrast, while the SF-HGSF structure further enhances the thermal conductivity of the outer region, the excessively large gradient amplitude and the low height of the secondary fins near the heat exchange tube weaken the thermal conductivity enhancement effect in the near-heat source region. At the same time, the higher secondary fins on the outer periphery occupy more of the liquid PCM flow space, which inhibits the development of natural convection to some extent, thus weakening the synergistic effect between enhanced thermal conductivity and enhanced natural convection. Therefore, the overall enhancement effect is slightly lower than that of the SF-MGSF structure.

[0073] In summary, the spiral phase change thermal storage structure with radially height-gradient secondary fins proposed in this invention can enhance the thermal conductivity of the outer periphery while retaining the flow space of the liquid phase change material through reasonable configuration of the secondary fin height distribution, achieving a synergistic effect of enhanced thermal conductivity and enhanced natural convection. Among them, the medium gradient structure (SF-MGSF, H1:H2:H3=1:2:3) exhibits superior comprehensive performance, with the highest average temperature, the highest average liquid phase ratio, and the most uniform melting interface, which can significantly improve the thermal storage rate and heat transfer efficiency of the phase change thermal storage device, providing a basis for optimizing the structural parameters of the gradient secondary fins.

[0074] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A gradient-enhanced helical phase change thermal storage structure, comprising a shell (1), heat exchange tubes (2), helical main fins (3), multiple gradient secondary fins (4), phase change thermal storage material (5), a thermal fluid inlet (6), and a thermal fluid outlet (7), characterized in that: The heat exchange tube (2) is arranged in the shell (1) along the axial direction of the shell (1), and a heat storage cavity is formed between the shell (1) and the heat exchange tube (2), and the phase change heat storage material (5) is filled in the heat storage cavity; The spiral main fin (3) is fixedly connected to the outer wall of the heat exchange tube (2) and extends continuously spirally along the axial direction of the heat exchange tube (2); Multiple gradient secondary fins (4) are disposed on the upper surface of the spiral main fin (3) and are arranged in sequence at intervals along the radial direction from the outer wall of the heat exchange tube (2) to the inner wall of the outer shell (1). A flow channel communicating with the heat storage cavity is formed between two adjacent gradient secondary fins (4). Along the direction away from the heat exchange tube (2), the height of the plurality of gradient secondary fins (4) increases step by step.

2. The gradient-enhanced spiral phase change thermal storage structure according to claim 1, characterized in that, The gradient secondary fin (4) is a strip-shaped protrusion structure, and the gradient secondary fin (4) is arranged perpendicular to the upper surface of the spiral main fin (3).

3. The gradient-enhanced spiral phase change thermal storage structure according to claim 1, characterized in that, Each spiral cycle of the spiral main fin (3) is provided with at least one set of gradient secondary fins (4), and each set of gradient secondary fins (4) includes a first gradient secondary fin, a second gradient secondary fin and a third gradient secondary fin arranged sequentially along the radial direction.

4. The gradient-enhanced spiral phase change thermal storage structure according to claim 3, characterized in that, The heights of the first gradient secondary fin, the second gradient secondary fin, and the third gradient secondary fin are H1, H2, and H3, respectively, and satisfy: H1 < H2 < H3.

5. The gradient-enhanced spiral phase change thermal storage structure according to claim 4, characterized in that, The heights of the first gradient secondary fin, the second gradient secondary fin, and the third gradient secondary fin are set in an arithmetic progression, and H1∶H2∶H3=1∶2∶3.

6. The gradient-enhanced spiral phase change thermal storage structure according to claim 1, characterized in that, The thickness of the gradient secondary fin (4) is 0.5~2 mm, and the width of the flow channel formed between two adjacent gradient secondary fins (4) is 5~50 mm.

7. The gradient-enhanced spiral phase change thermal storage structure according to claim 1, characterized in that, The thickness of the spiral main fin (3) is 1~3 mm, the pitch is 15~50 mm, and the width of the spiral main fin (3) extending radially along the heat exchange tube (2) is 20~80 mm.

8. The gradient-enhanced spiral phase change thermal storage structure according to claim 1, characterized in that, The phase change thermal storage material (5) is an organic phase change material, an inorganic phase change material, or a eutectic phase change material; wherein, when the phase change thermal storage material (5) is a eutectic phase change material, the eutectic phase change material includes adipic acid and succinic acid, and the mass ratio of adipic acid to succinic acid is 2.9:

1.

9. The gradient-enhanced spiral phase change thermal storage structure according to claim 1, characterized in that, The heat exchange tube (2), the spiral main fin (3) and the gradient secondary fin (4) are integrally formed by additive manufacturing process.

10. The gradient-enhanced spiral phase change thermal storage structure according to claim 9, characterized in that, The additive manufacturing process is selective laser melting forming, the forming material is AlSi10Mg aluminum alloy powder, the particle size of the aluminum alloy powder is 15~53 μm, the powder layer thickness is 0.04 mm, and the scanning direction rotation angle between adjacent forming layers is 67°.

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