Three-dimensional incongruous fractal fin latent heat storage device and fin optimization method

Abstract

The invention discloses a three-dimensional incongruous fractal fin latent heat storage device which comprises an outer pipe shell and a fluid inner pipe, the fluid inner pipe is arranged in the outer pipe shell in a penetrating mode, the space between the outer pipe shell and the fluid inner pipe is filled with phase change heat storage materials, and a plurality of three-dimensional incongruous fractal fins are arranged on the outer side wall of the fluid inner pipe. Each three-dimensional incongruous fractal fin comprises a linear first-stage main fin, two second-stage fractal fins and four third-stage fractal fins, the first end of each first-stage main fin is connected to the outer side wall of the fluid inner pipe, and the second-stage fractal fins are distributed in a V shape on the radial section of the fluid inner pipe and jointly connected to the second end of the first-stage main fin; every two third-stage fractal fins are distributed in a V shape on the axial section of the fluid inner pipe and jointly connected to the ends, away from the first-stage main fins, of the second-stage fractal fins. The invention further discloses a fin optimization method of the heat exchanger. The melting time can be shortened while the heat storage amount can be guaranteed, and efficient heat exchange of the phase change latent heat storage device is achieved.

Description

Technical Field

[0001] The present invention relates to a fin latent heat storage device and a method for optimizing its structure, belonging to the technical field of latent heat storage devices. Background Art

[0002] The latent heat storage system based on phase change materials has a high energy storage density and is expected to be applied in fields such as solar energy storage, waste heat recovery and utilization, and building area heating. However, the phase change materials themselves have a low thermal conductivity, and the heat transfer rate in the phase change materials is slow, making the heat storage and heat release processes of the horizontal shell-and-tube phase change heat storage device both time-consuming. This defect reduces the dynamic response performance of the device and greatly limits its application in some scenarios that require rapid heat storage or release.

[0003] Adding high thermal conductivity fins to the heat transfer wall surface is an important way to strengthen the heat transfer between the solid-liquid phase change materials in the heat storage device. The prior art has used non-uniformly distributed fins to strengthen the heat transfer process between the cold and hot fluids and the phase change materials. However, if we want to further improve the heat transfer efficiency to shorten the melting time of the phase change heat storage materials, we often need to increase the volume ratio of the fins in the heat storage device, which sacrifices the heat storage capacity of the heat storage device. Summary of the Invention

[0004] Aiming at the defects of the above-mentioned prior art, the present invention provides a three-dimensional anisotropic fractal fin latent heat storage device to solve the contradiction between shortening the melting time of the phase change materials and ensuring the heat storage capacity of the latent heat storage device. The present invention also provides a method for optimizing the fins of the three-dimensional anisotropic fractal fin latent heat storage device.

[0005] The technical solution of the present invention is as follows: A three-dimensional anisotropic fractal fin latent heat storage device includes an outer tube housing and an inner fluid tube. The inner fluid tube is arranged inside the outer tube housing. Phase change heat storage materials are filled between the outer tube housing and the inner fluid tube. A plurality of three-dimensional anisotropic fractal fins are provided on the outer side wall of the inner fluid tube. Each three-dimensional anisotropic fractal fin includes a primary main fin, two secondary fractal fins, and four tertiary fractal fins. The primary main fin, the secondary fractal fins, and the tertiary fractal fins are all straight. The first end of the primary main fin is connected to the outer side wall of the inner fluid tube. The secondary fractal fins are distributed in a V shape in the radial cross-section of the inner fluid tube and are commonly connected to the second end of the primary main fin. Every two tertiary fractal fins are distributed in a V shape in the axial cross-section of the inner fluid tube and are commonly connected to the end of the secondary fractal fin far from the primary main fin.

[0006] Furthermore, the three-dimensional anisotropic fractal fins are equally distributed circumferentially on the outer side wall of the inner fluid tube and are equally spaced axially on the outer side wall of the inner fluid tube.

[0007] Furthermore, the included angle of the V shape formed by the secondary fractal fins is 15° to 35°, and the included angle of the V shape formed by the tertiary fractal fins is 15° to 35°.

[0008] Furthermore, the length ratio of the secondary fractal fins to the primary main fins is 0.7 to 1.3, and the length ratio of the tertiary fractal fins to the primary main fins is 0.7 to 1.3.

[0009] Furthermore, six three-dimensional anisotropic fractal fins are equally distributed circumferentially along the outer sidewall in the radial cross-section of the fluid inner tube.

[0010] Furthermore, the phase change heat storage material can be inorganic hydrated salts, paraffin, fatty acids, esters, or composite phase change materials.

[0011] Furthermore, the cross-section of the outer tube housing is circular, and the fluid inner tube is arranged on the central axis of the outer tube housing.

[0012] Another technical solution of the present invention is: a fin optimization method for a three-dimensional anisotropic fractal fin latent heat storage device, including the steps of: determining the quadratic regression equation between the melting time of the phase change heat storage material, the average heat storage rate of the three-dimensional anisotropic fractal fin latent heat storage device, and the temperature distribution non-uniformity index and the three-dimensional anisotropic fractal fin parameters based on response surface optimization, and using the non-dominated genetic algorithm to optimize and calculate the three-dimensional anisotropic fractal fin parameters with the melting time, average heat storage rate, and temperature distribution non-uniformity index as optimization objectives to obtain the optimal three-dimensional anisotropic fractal fin parameters. The three-dimensional anisotropic fractal fin parameters include the length ratio of the secondary fractal fins to the primary main fins, the length ratio of the tertiary fractal fins to the secondary fractal fins, the included angle of the V shape formed by the secondary fractal fins, and the included angle of the V shape formed by the tertiary fractal fins.

[0013] Furthermore, the quadratic regression equation of the melting time is:

[0014] Melting time = a 1 + a 2 × L 1 + a 3 × L 2 + a 4 × θ 1 + a 5 × θ 2 + a 6 × L 1 × L 2 + a 7 × L 1 × θ 1 + a 8 × L 1 × θ 2 + a 9 × L2 ×θ 1 +a 10 ×L 2 ×θ 2 +a 11 ×θ 1 ×θ 2 +a 12 ×L 1 2 +a 13 ×L 2 2 +a 14 ×θ 1 2 +a 14 ×θ 2 2 ;

[0015] Average heat storage rate = b 1 +b 2 ×L 1 +b 3 ×L 2 +b 4 ×θ 1 +b 5 ×θ 2 +b 6 ×L 1 ×L 2 +b 7 ×L 1 ×θ 1 +b 8 ×L 1 ×θ 2 +b 9 ×L 2 ×θ 1 +b 10 ×L 2 ×θ 2 +b 11 ×θ 1 ×θ 2 +b 12 ×L 1 2 +b 13 ×L 2 2 +b 14 ×θ 1 2 +b 14 ×θ 2 2 ;

[0016] Temperature distribution non-uniformity index = c 1 +c 2 ×L 1 +c 3 ×L2 +c 4 ×θ 1 +c 5 ×θ 2 +c 6 ×L 1 ×L 2 +c 7 ×L 1 ×θ 1 +c 8 ×L 1 ×θ 2 +c 9 ×L 2 ×θ 1 +c 10 ×L 2 ×θ 2 +c 11 ×θ 1 ×θ 2 +c 12 ×L 1 2 +c 13 ×L 2 2 +c 14 ×θ 1 2 +c 14 ×θ 2 2 ;

[0017] Wherein, a 1 ~a 14 、b 1 ~b 14 、c 1 ~c 14 are the coefficients of each term, L 1 is the length ratio of the secondary fractal fin to the primary main fin, L 2 is the length ratio of the tertiary fractal fin to the secondary fractal fin, θ 1 is the included angle of the V shape formed by the secondary fractal fins, θ 2 is the included angle of the V shape formed by the tertiary fractal fins.

[0018] Furthermore, the melting time = 4758.65 - 202.55×L 1 - 1802.82×L 2 - 115.28×θ 1 - 32.3×θ 2

[0019] + 252.78×L 1 ×L 2 + 7.92×L 1 ×θ 1 - 8.67×L1 ×θ 2 + 13.17×L 2 ×θ 1 + 9.92×L 2 ×θ 2 + 0.33×θ 1 ×θ 2

[0020] - 26.85×L 1 2 + 306.48×L 2 2 + 1.49×θ 1 2 + 0.29×θ 2 2 ;

[0021] Average heat storage rate = - 104.94 + 0.96×L 1 + 124.87×L 2 + 10.05×θ 1 + 2.32×θ 2 - 11.28×L 1 ×L 2

[0022] - 1.28×L 1 ×θ 1 + 1.04×L 1 ×θ 2 - 0.65×L 2 ×θ 1 - 0.55×L 2 ×θ 2 - 0.02×θ 1 ×θ 2 + 9.04×L 1 2 - 24.61×L 2 2

[0023] - 0.13×θ 1 2 - 0.03×θ 2 2 ;

[0024] Temperature distribution non - uniformity index = 10.35 - 0.156×L 1 - 0.04×L 2 - 0.124×θ 1 + 0.034×θ 2 - 0.998×L 1 ×L 2

[0025] +0.057×L 1 ×θ 1 -0.022×L 1 ×θ 2 +0.011×L 2 ×θ 1 +0.039×L 2 ×θ 2 -0.002×θ 1 ×θ 2 -0.014×L 1 2

[0026] -0.114×L 2 2 +0.001×θ 1 2 -0.0005×θ 2 2 。

[0027] Compared with the prior art, the advantages of the technical solution provided by the present invention are as follows:

[0028] In the three-dimensional anisotropic fractal fin structure of the present invention, the anisotropic fractal fins in this structure can be more evenly distributed in the phase change heat storage region. Its multi-level anisotropic fractal characteristics can take into account the heat transfer in the region far from the heat source in the phase change heat storage region, and it is not necessary to change the volume fraction of the fins in the phase change heat storage material filling space, without sacrificing the overall heat storage capacity of the latent heat storage device. The melting time of the phase change heat storage material in this novel three-dimensional anisotropic fractal fin latent heat storage device is greatly shortened, the average heat storage rate is effectively improved, the temperature distribution non-uniformity index is reduced, and the phenomenon of supercooling or overheating regions in the heat transfer process of the heat storage material region is avoided. At the same time, the complexity of this structure is low, it is easy to manufacture, and it is expected to be practically applied in engineering.

[0029] In addition, the present invention combines the response surface method and the non-dominated genetic algorithm for multi-objective optimization, which can quickly optimize the size parameters of the three-dimensional anisotropic fractal fins, thereby achieving the optimal heat storage effect. Compared with the traditional six-rectangular fin device with radial stretching, the melting time of its phase change heat storage material is shortened by 65.01%, the average heat storage rate is increased by 182.46%, and the temperature distribution non-uniformity index is reduced by 26.42%. Brief Description of the Drawings

[0030] Figure 1 It is a schematic structural diagram of the three-dimensional anisotropic fractal fin latent heat storage device of the embodiment.

[0031] Figure 2 It is an exploded structural diagram of the three-dimensional anisotropic fractal fin latent heat storage device of the embodiment.

[0032] Figure 3 It is a three-dimensional schematic diagram of an internal fluid tube with three-dimensional anisotropic fractal fins.

[0033] Figure 4 It is a front view schematic diagram of an internal fluid tube with three-dimensional anisotropic fractal fins.

[0034] Figure 5 It is a schematic diagram of the radial cross-sectional structure of a three-dimensional anisotropic fractal fin latent heat storage device of an embodiment.

[0035] Figure 6 It is a schematic diagram of a group of three-dimensional anisotropic fractal fin structures of a three-dimensional anisotropic fractal fin latent heat storage device of an embodiment on the same radial cross-section. Detailed implementation manners

[0036] The present invention will be further described below in conjunction with embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. After reading this description, those skilled in the art's various equivalent modifications of this description all fall within the scope defined by the appended claims of this application.

[0037] Please refer to Figures 1 to 4 As shown, the three-dimensional anisotropic fractal fin latent heat storage device of this embodiment is a horizontal latent heat storage device, mainly including an outer tube housing 1, an internal fluid tube 2, and a phase change heat storage material 5. The outer tube housing 1 is a circular tube, and its radial cross-section is circular. The internal fluid tube 2 is arranged on the central axis of the outer tube housing 1, that is, the outer tube housing 1 and the internal fluid tube 2 are coaxially arranged. The phase change heat storage material 5 is filled between the internal fluid tube 2 and the outer tube housing 1. Sealing end caps 3 are respectively connected to both ends of the outer tube housing 1 to prevent the phase change heat storage material from leaking. The phase change heat storage material can be inorganic hydrated salts, paraffin, fatty acids, esters, or composite phase change materials. The internal fluid tube 2 penetrates through the outer tube housing 1, and a hot and cold fluid channel 4 for hot and cold fluids to pass through is formed inside the internal fluid tube 2.

[0038] A number of three-dimensional anisotropic fractal fins are provided on the outer side wall of the internal fluid tube 2. The three-dimensional anisotropic fractal fins are evenly distributed. Specifically, the three-dimensional anisotropic fractal fins are equally distributed circumferentially along the outer side wall on the radial cross-section of the internal fluid tube 2 to form an annular array, and the annular array of the three-dimensional anisotropic fractal fins is equally spaced in the axial direction of the internal fluid tube 2 to form a linear array.

[0039] The structure of each three-dimensional anisotropic fractal fin is such that it includes a primary fin 6, two secondary fractal fins 7, and four tertiary fractal fins 8. The primary fin 6, secondary fractal fins 7, and tertiary fractal fins 8 are all linear. The first end of the primary fin 6 is connected to the outer wall of the internal fluid pipe and is arranged radially with respect to the internal fluid pipe. The secondary fractal fins 7 are formed by fractal of the primary fin 6 in the radial cross-section of the internal fluid pipe. The two secondary fractal fins 7 are distributed in a V shape and are jointly connected to the second end of the primary fin 6. The tertiary fractal fins 8 are formed by fractal of the secondary fractal fins 7 in the axial cross-section of the internal fluid pipe. The two tertiary fractal fins 8 are distributed in a V shape and are jointly connected to the end of the secondary fractal fins 7 far from the primary fin 6. Since the primary fin 6 fractally forms two secondary fractal fins 7, and each secondary fractal fin 7 further fractally forms two tertiary fractal fins 8, there are four tertiary fractal fins 8 in a three-dimensional anisotropic fractal fin.

[0040] The included angle of the V shape formed by the secondary fractal fins 7 is 15° - 35°, and the included angle of the V shape formed by the tertiary fractal fins 8 is 15° - 35°. The length ratio of the secondary fractal fins 7 to the primary fin 6 is 0.7 - 1.3, and the length ratio of the tertiary fractal fins 8 to the primary fin 6 is 0.7 - 1.3.

[0041] Regarding the specific structural dimensions of the primary fin 6, secondary fractal fins 7, and tertiary fractal fins 8, the present invention uses the response surface method and non-dominated genetic algorithm for optimization. In this example, the diameter Φ of the outer tube housing 1 of the latent heat storage device 2 is 80 mm, and the diameter Φ of the internal fluid pipe 2 of the latent heat storage device 1 is 20 mm. The wall thickness of both is 1 mm. The angle θ between the primary fins 6 is 60°. The initial temperature of the phase change heat storage material 5 is 20 °C, the inflow temperature of the central hot fluid is 90 °C, and the velocity is 0.08 m / s. The length l of the secondary fractal fins 7 2 and the length l of the primary fin 6 1 The ratio is L 1 , and the length l of the tertiary fractal fins 8 3 and the length l of the primary fin 6 1 The ratio is L 2 . The fractal angle of the secondary fractal fins 7 is θ 1 , and the fractal angle of the tertiary fractal fins 8 is θ 2 .

[0042] The volume ratio of the three-dimensional anisotropic fractal fins in the entire phase change heat storage chamber (the space between the outer tube housing 1 and the internal fluid pipe 2) is 5%. The number of fins in the circular array of the three-dimensional anisotropic fractal fins is 6, and the number of circular arrays in the linear array is 10.

[0043] For l by the response surface method and non-dominated genetic algorithm1 , l 2 , l 3 , θ 1 and θ 2 to screen the numerical values. The specific steps include: to ensure that the volume fraction of the fins in the entire phase change material heat storage chamber remains unchanged, set the length ratio L 1 and the length ratio L 2 to determine l 1 , l 2 , l 3 size. Thus, four independent variable parameters L 1 , L 2 , θ 1 and θ 2 are determined. Then, taking the minimization of the phase change material melting time t full (s), the maximization of the average heat storage rate P s (W) and the minimization of the temperature distribution non-uniformity index σ ave (K) as the three main optimization objectives, the corresponding quadratic regression equations under the response surface optimization are obtained respectively.

[0044] The regression equations for the three optimization objectives are expressed as:

[0045] Melting time = a 1 + a 2 × L 1 + a 3 × L 2 + a 4 × θ 1 + a 5 × θ 2 + a 6 × L 1 × L 2 + a 7 × L 1 × θ 1 + a 8 × L 1 × θ 2 + a 9 × L 2 × θ 1 + a 10 × L 2 × θ 2 + a 11 × θ 1 × θ 2 + a 12 × L 1 2 + a 13 × L 2 2 + a 14 × θ 1 2 + a14 ×θ 2 2 ;

[0046] Average heat storage rate = b 1 +b 2 ×L 1 +b 3 ×L 2 +b 4 ×θ 1 +b 5 ×θ 2 +b 6 ×L 1 ×L 2 +b 7 ×L 1 ×θ 1 +b 8 ×L 1 ×θ 2 +b 9 ×L 2 ×θ 1 +b 10 ×L 2 ×θ 2 +b 11 ×θ 1 ×θ 2 +b 12 ×L 1 2 +b 13 ×L 2 2 +b 14 ×θ 1 2 +b 14 ×θ 2 2 ;

[0047] Temperature distribution non-uniformity index = c 1 +c 2 ×L 1 +c 3 ×L 2 +c 4 ×θ 1 +c 5 ×θ 2 +c 6 ×L 1 ×L 2 +c 7 ×L 1 ×θ 1 +c 8 ×L 1 ×θ 2 +c 9 ×L 2 ×θ 1+c 10 ×L 2 ×θ 2 +c 11 ×θ 1 ×θ 2 +c 12 ×L 1 2 +c 13 ×L 2 2 +c 14 ×θ 1 2 +c 14 ×θ 2 2 ;

[0048] wherein, a 1 ~a 14 , b 1 ~b 14 , c 1 ~c 14 are coefficients

[0049] The optimized quadratic regression equation obtained in this embodiment is:

[0050] Melting time = 4758.65 - 202.55×L 1 -1802.82×L 2 -115.28×θ 1 -32.3×θ 2 +252.78×L 1 ×L 2

[0051] +7.92×L 1 ×θ 1 -8.67×L 1 ×θ 2 +13.17×L 2 ×θ 1 +9.92×L 2 ×θ 2 +0.33×θ 1 ×θ 2

[0052] -26.85×L 1 2 +306.48×L 2 2 +1.49×θ 1 2 +0.29×θ 2 2 ;

[0053] Average heat storage rate = -104.94 + 0.96×L 1 + 124.87×L 2 + 10.05×θ 1 + 2.32×θ 2 - 11.28×L 1 ×L 2

[0054] - 1.28×L 1 ×θ 1 + 1.04×L 1 ×θ 2 - 0.65×L 2 ×θ 1 - 0.55×L 2 ×θ 2 - 0.02×θ 1 ×θ 2 + 9.04×L 1 2 - 24.61×L 2 2

[0055] - 0.13×θ 1 2 - 0.03×θ 2 2 ;

[0056] Temperature distribution non-uniformity index = 10.35 - 0.156×L 1 - 0.04×L 2 - 0.124×θ 1 + 0.034×θ 2 - 0.998×L 1 ×L 2

[0057] + 0.057×L 1 ×θ 1 - 0.022×L 1 ×θ 2 + 0.011×L 2 ×θ 1 + 0.039×L 2 ×θ 2 - 0.002×θ 1 ×θ 2 - 0.014×L 1 2

[0058] - 0.114×L 2 2 + 0.001×θ 1 2 - 0.0005×θ2 2 .

[0059] Further, the non - dominated genetic algorithm is used to optimize with the above - mentioned melting time, average heat storage rate, and temperature distribution non - uniformity index as the optimization objectives, and the corresponding Pareto - front solutions are obtained. The best parameter solution is selected as L 1 = 1.29, L 2 = 1.29, θ 1 = 28.96°, θ 2 = 34.97°.

[0060] The three - dimensional anisotropic fractal fin latent heat storage device with the obtained best parameters is evaluated, and the evaluation index scheme is as follows:

[0061] The melting rate is used to represent the proportion of the volume of the liquid - phase phase - change material in the total volume of the phase - change material during the melting process:

[0062]

[0063] where V PCM is the total volume of the phase - change heat - storage material, is the liquid - phase rate of the phase - change material at different times.

[0064] The heat storage amount of the phase - change material in the latent heat storage device can be divided into three parts according to its melting characteristics: solid - state sensible heat, latent heat of solid - liquid phase transition, and liquid - state sensible heat. The average heat storage rate is used to represent the heat transfer rate of the phase - change material in the three - dimensional anisotropic fractal fin latent heat storage device during the entire heat storage process:

[0065]

[0066] where T 0 , T m and T ave are the average temperature at the initial moment of the phase - change material, the temperature at which phase transition occurs, and the average temperature of the liquid - phase phase - change material respectively. M s and M l represent the mass of the solid state and the liquid state of the phase - change material respectively. C p,s and C p,l are the specific heat capacities of the solid state and the liquid state of the phase - change material respectively, and L is the latent heat of the phase - change material.

[0067] The temperature distribution non - uniformity index is used to represent the magnitude of the temperature gradient in the phase - change material filling area in the three - dimensional anisotropic fractal fin latent heat storage device during the heat storage process:

[0068]

[0069] In the formula, T is the temperature matrix of the phase change material region, σ(t) represents the magnitude of the instantaneous temperature distribution non-uniformity index of the phase change material, and σ ave represents the magnitude of the average temperature distribution non-uniformity index of the phase change material during the entire melting process.

[0070] According to the determined structural shape and structural parameters, the following conclusions can be obtained through numerical simulation calculations:

[0071] For the three-dimensional anisotropic fractal fin latent heat storage device proposed in this embodiment, when its design parameters L 1 = 0.7, L 2 = 0.7, θ 1 = 15°, θ 2 = 15°, compared with the traditional radially stretched six-rectangular fin device, the melting time of its phase change material is shortened by 54.11%, the average heat storage rate is increased by 117.2%, and the temperature distribution non-uniformity index is reduced by 8.3%. When its design parameters L 1 = 1.3, L 2 = 1.3, θ 1 = 35°, θ 2 = 35°, compared with the traditional structure device, the melting time of its phase change material is shortened by 64.64%, the average heat storage rate is increased by 181.85%, and the temperature distribution non-uniformity index is reduced by 21.39%. When its parameters are the selected Pareto optimal solution, at this time L 1 = 1.29, L 2 = 1.29, θ 1 = 28.96°, θ 2 = 34.97°, compared with the traditional structure device, the melting time of its phase change material is shortened by 65.01%, the average heat storage rate is increased by 182.46%, and the temperature distribution non-uniformity index is reduced by 26.42%.

Claims

1. A three-dimensional anisotropic fractal fin latent heat storage device, comprising an outer tube shell and a fluid inner tube, wherein the fluid inner tube is inserted into the outer shell, and a phase change heat storage material is filled between the outer tube shell and the fluid inner tube, characterized in that: The outer side wall of the fluid inner tube is provided with a plurality of three-dimensional anisotropic fractal fins, each of the three-dimensional anisotropic fractal fins comprises a primary main fin, two secondary fractal fins and four tertiary fractal fins, the primary main fin, the secondary fractal fins and the tertiary fractal fins are all straight, the first end of the primary main fin is connected to the outer side wall of the fluid inner tube, the secondary fractal fins are distributed in a V shape on the radial cross section of the fluid inner tube and are commonly connected to the second end of the primary main fin, and every two of the tertiary fractal fins are distributed in a V shape on the axial cross section of the fluid inner tube and are commonly connected to one end of the secondary fractal fin away from the primary main fin.

2. The three-dimensional anisotropic fractal fin latent heat storage device according to claim 1, characterized in that: The three-dimensional anisotropic fractal fins are equally distributed in the circumferential direction of the outer wall of the inner fluid tube, and are equally spaced in the axial direction of the outer wall of the inner fluid tube.

3. The three-dimensional anisotropic fractal fin latent heat storage device according to claim 1, characterized in that: The included angle of the V-shape formed by the secondary fractal fins is 15° to 35°, and the included angle of the V-shape formed by the tertiary fractal fins is 15° to 35°.

4. The three-dimensional anisotropic fractal fin latent heat storage device according to claim 1, characterized in that: The length ratio of the secondary fractal fin to the primary main fin is 0.7 to 1.3, and the length ratio of the tertiary fractal fin to the primary main fin is 0.7 to 1.

3.

5. The three-dimensional anisotropic fractal fin latent heat storage device according to claim 1, characterized in that: There are six three-dimensional anisotropic fractal fins equally distributed along the circumference of the outer side wall on the radial cross section of the fluid inner tube.

6. The three-dimensional anisotropic fractal fin latent heat storage device according to claim 1, characterized in that: The phase change thermal storage material may be inorganic hydrated salts, paraffin, fatty acids, esters or composite phase change materials.

7. The three-dimensional anisotropic fractal fin latent heat storage device according to claim 1, characterized in that: The cross section of the outer tube shell is circular, and the fluid inner tube is arranged on the central axis of the outer tube shell.

8. A fin optimization method for a three-dimensional anisotropic fractal fin latent heat storage device according to any one of claims 1 to 7, characterized in that: The invention comprises the following steps: determining the melting time of a phase change thermal storage material, the average heat storage rate of a three-dimensional anisotropic fractal fin latent heat storage device, the quadratic regression equation between the temperature distribution non-uniformity index and the three-dimensional anisotropic fractal fin parameters based on response surface optimization; using a non-dominated genetic algorithm to optimize the three-dimensional anisotropic fractal fin parameters with the melting time, the average heat storage rate and the temperature distribution non-uniformity index as optimization targets to obtain the best three-dimensional anisotropic fractal fin parameters; the three-dimensional anisotropic fractal fin parameters include the length ratio of a secondary fractal fin to a primary main fin, the length ratio of a tertiary fractal fin to a secondary fractal fin, the V-shaped angle formed by the secondary fractal fins and the V-shaped angle formed by the tertiary fractal fins.

9. The fin optimization method of the three-dimensional anisotropic fractal fin latent heat storage device according to claim 8, characterized in that: The quadratic regression equation for the melting time is: Melting time = a1+a2×L1+a3×L2+a4×θ1+a5×θ2+a6×L1×L2+a7×L1×θ1+a8×L1×θ2+a9×L2×θ1+a 10 ×L2×θ2+a 11 ×θ1×θ2+a 12 ×L1 2 +a 13 ×L2 2 +a 14 ×θ1 2 +a 14 ×θ2 2 ; Average heat storage rate = b1+b2×L1+b3×L2+b4×θ1+b5×θ2+b6×L1×L2+b7×L1×θ1+b8×L1×θ2+b9×L2×θ1+b 10 ×L2×θ2+b 11 ×θ1×θ2+b 12 ×L1 2 +b 13 ×L2 2 +b 14 ×θ1 2 +b 14 ×θ2 2 ; Temperature distribution non-uniformity index = c1+c2×L1+c3×L2+c4×θ1+c5×θ2+c6×L1×L2+c7×L1×θ1+c8×L1×θ2+c9×L2×θ1+c 10 ×L2×θ2+c 11 ×θ1×θ2+c 12 ×L1 2 +c 13 ×L2 2 +c 14 ×θ1 2 +c 14 ×θ2 2 ; Among them, a1~a 14 、b1~b 14 、c1~c 14 are the coefficients, L1 is the length ratio of the secondary fractal fin to the primary main fin, L2 is the length ratio of the tertiary fractal fin to the secondary fractal fin, θ1 is the V-shaped angle formed by the secondary fractal fins, and θ2 is the V-shaped angle formed by the tertiary fractal fins.

10. The fin optimization method of the three-dimensional anisotropic fractal fin latent heat storage device according to claim 9, characterized in that: Melting time = 4758.65-202.55×L1-1802.82×L2-115.28×θ1-32.3×θ2+252.78×L1×L2+7.92×L1×θ1-8.67×L1×θ2+13.17×L2×θ1+9.92×L2×θ2+0.33×θ1×θ2-26.85×L1 2 +306.48×L2 2 +1.49×θ1 2 +0.29×θ2 2 ; Average heat storage rate = -104.94 + 0.96 × L1 + 124.87 × L2 + 10.05 × θ1 + 2.32 × θ2 - 11.28 × L1 × L2 - 1.28 × L1 × θ1 + 1.04 × L1 × θ2 - 0.65 × L2 × θ1 - 0.55 × L2 × θ2 - 0.02 × θ1 × θ2 + 9.04 × L1 2 -24.61×L2 2 -0.13×θ1 2 -0.03×θ2 2 ; Temperature distribution non-uniformity index = 10.35-0.156×L1-0.04×L2-0.124×θ1+0.034×θ2-0.998×L1×L2+0.057×L1×θ1-0.022×L1×θ2+0.011×L2×θ1+0.039×L2×θ2-0.002×θ1×θ2-0.014×L1 2 -0.114×L2 2 +0.001×θ1 2 -0.0005×θ2 2 .

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