Multi-tube latent heat system heat storage device and method based on fractal optimization

Abstract

The invention relates to the technical field of phase change heat storage, in particular to a multi-tube latent heat system heat storage device and method based on fractal optimization. The multi-tube latent heat system heat storage device comprises an outer shell and a multi-stage tube unit, a multi-stage tube structure is arranged in the outer shell, the multi-stage tube unit is integrally and eccentrically arranged in the outer shell, the eccentric angle of the multi-stage tube unit is 28.5-31.5 degrees, and the multi-stage tube unit is arranged in the outer shell. And the eccentric distance of the multi-stage pipe unit is 37.5-42.5 mm. The technical scheme focuses on a multi-stage pipe heat storage technology, and aims to solve the problem of low heat storage efficiency of a current multi-pipe latent heat system. Through structural design and performance analysis of the multistage pipe heat storage system, the internal heat and mass transfer process of the multistage pipe heat storage system is deeply explored by applying computational fluid mechanics software. Research results show that the multi-stage pipe structure can remarkably improve the heat storage density, and research finds that under the same volume coefficient, the optimal fractal simplified structure is located in about 12 pipelines. Compared with a traditional non-fractal heat storage structure, the heat storage capacity and the heat storage rate are increased by 127.32%, and the melting time is shortened by 85.47%.

Description

Technical Field

[0001] The present invention relates to the technical field of phase change heat storage, and in particular to a heat storage device and method for a multi-tube latent heat system based on fractal optimization. Background Art

[0002] Phase change thermal storage is a high-tech energy storage technology developed based on phase change energy storage materials. Among the many heat storage methods, although thermochemical heat storage has considerable heat storage density, it has problems such as poor safety and difficult process control, which greatly hinders its promotion and application; although sensible heat storage is widely used, it is limited by the shortcoming of low heat storage density. In comparison, phase change thermal storage has obvious advantages. Its heat storage density can reach 5-10 times or even higher than that of sensible heat storage, and it can maintain a constant temperature during the phase change process. This makes phase change thermal storage technology show unique value in complex working conditions where heat supply is discontinuous or supply and demand are not coordinated. It becomes an effective means to solve the contradiction between energy supply in time and space, and is an important way to improve energy utilization.

[0003] In the process of continuous development of phase change thermal storage technology, multi-tube phase change thermal storage, as an important branch, has gradually emerged. The multi-tube structure can effectively improve the heat storage density and heat transfer efficiency. Through clever design, the phase change material undergoes phase change in an orderly manner within the multiple tubes, realizing efficient storage and release of heat. This structure is also convenient for modular design and expansion, providing convenience for large-scale applications. However, the current multi-tube phase change thermal storage system also faces some challenges. For example, the arrangement of internal pipes leads to slow heat storage, which to some extent affects the overall performance and urgently needs further research and optimization. Summary of the Invention

[0004] The purpose of the present invention is to provide a multi-tube latent heat storage system heat storage device and method based on fractal optimization, so as to solve the problem that the arrangement of internal pipes in the multi-tube phase change heat storage system in the prior art leads to slow heat storage, which to a certain extent affects the overall performance and urgently needs further research and optimization.

[0005] To achieve the above objectives, the present invention provides a multi-tube latent heat system heat storage device based on fractal optimization, the multi-tube latent heat system heat storage device based on fractal optimization comprising an outer shell and a multistage tube unit, the multistage tube structure being arranged inside the outer shell, the multistage tube unit being eccentrically arranged inside the outer shell as a whole, the eccentric angle of the multistage tube unit being 28.5°-31.5°, the eccentric spacing of the multistage tube unit being 37.5-42.5 mm, the multistage tube unit comprising a central tube and a plurality of peripheral tubes, the plurality of peripheral tubes being evenly arranged around the periphery of the central tube, the center spacing between the plurality of peripheral tubes and the central tube being 35 mm, the pipe diameters of the central tube and the plurality of peripheral tubes being both 13.86 mm, the interiors of the central tube and the plurality of peripheral tubes being filled with phase change material, and the phase change material being preferably paraffin RT55.

[0006] Among them, one of the specific structures of the multiple peripheral tubes in the multi-stage tube unit is: the number of the multiple peripheral tubes is specifically 12, the 12 peripheral tubes are evenly arranged around the periphery of the central tube, and the bifurcation angle between every two adjacent peripheral tubes and the central tube is 30°.

[0007] Among them, one of the specific structures of the multiple peripheral tubes in the multi-stage tube unit is: the number of the multiple peripheral tubes is specifically 11, the bifurcation angle between the two adjacent peripheral tubes located at the inner top of the outer shell and the central tube is 60°, and the bifurcation angle between each of the remaining two adjacent peripheral tubes and the central tube is 30°.

[0008] The present invention further provides a multi-tube latent heat system heat storage method based on fractal optimization, which is applied to the multi-tube latent heat system heat storage device based on fractal optimization as described above, and comprises the following steps:

[0009] Establish a physical model of a multi-tube latent heat system, and perform a three-dimensional simulation on the physical model to obtain a three-dimensional model of the multi-tube latent heat system;

[0010] Conduct simulation analysis on a three-dimensional model of a multi-tube latent heat system, simulate the melting and solidification process of phase change materials, and analyze the system's heat transfer performance and energy storage efficiency;

[0011] Designing the specific structure of the multi-stage tube unit in the multi-tube latent heat system to keep the volume of the phase change material constant;

[0012] performing fractal optimization on the structure of the multi-stage tube unit to determine the fractal structure of the multi-stage tube unit;

[0013] Simplifying the fractal structure of the multi-stage tube unit to determine the optimized structure of the multi-stage tube unit;

[0014] Determining the eccentric angle and eccentric spacing of the multi-stage tube unit;

[0015] Response surface methodology and NSGA-II algorithm were used for multi-objective optimization to provide guidance for the optimal design of multi-tube latent heat system.

[0016] Among them, in the step of "simulating and analyzing the three-dimensional model of the multi-tube latent heat system to simulate the melting and solidification process of the phase change material", the enthalpy porosity method is used to simulate the melting process of the phase change material in the multi-tube latent heat system, and the control equation is as follows:

[0017] Energy equation:

[0018]

[0019] Where ρ is the density of the phase change material, H is the enthalpy of the phase change material, ▽ is the vector differential operator, and k is the thermal conductivity;

[0020] Continuity equation:

[0021]

[0022] Where, is the velocity vector;

[0023] Momentum equation:

[0024]

[0025] Where t is time; ρ is density; p is pressure; μ is dynamic viscosity; ξ is thermal expansion coefficient; T ref is the reference temperature, the source term The damping term of Darcy's law can be expressed as:

[0026]

[0027] The liquid phase rate at different times is expressed as follows:

[0028]

[0029] Among them, is T solid Solidus temperature, T liquid is the liquidus temperature of the phase change material;

[0030] The total enthalpy consists of sensible enthalpy and latent enthalpy:

[0031]

[0032] ΔH=λL

[0033] H=λL+h

[0034] Where ΔH is the latent heat enthalpy and h is the sensible heat enthalpy.

[0035] In the step "Simulating and analyzing the three-dimensional model of the multi-tube latent heat system", the temperature of the heat transfer fluid is simplified to the pipe wall temperature. At the initialization moment t = 0, the temperature of the phase change material is 293K and is in a solidified state. When t>0, the temperature of the heat transfer fluid is set to 363K to melt the phase change material. The formula is as follows:

[0036]

[0037] Among them, the specific steps of the step "Using response surface methodology and NSGA-II algorithm for multi-objective optimization" are:

[0038] First, the target program is set, and the sample points are set using the CCD method. Then the sample amount is input into the program, and then the response surface is obtained. The objective function of the response surface is analyzed, and the objective function is brought into the genetic algorithm for optimization to obtain the parameter combination.

[0039] The present invention discloses a multi-tube latent heat storage system heat storage device and method based on fractal optimization, comprising an outer shell and a multi-stage tube unit. The multi-stage tube structure is disposed within the outer shell, and the multi-stage tube unit is eccentrically disposed within the outer shell. The eccentric angle of the multi-stage tube unit is 28.5°-31.5°, and the eccentric spacing of the multi-stage tube unit is 37.5-42.5 mm. This technical solution focuses on multi-stage tube heat storage technology, aiming to address the low heat storage efficiency of current multi-tube latent heat storage systems. Through structural design and performance analysis of the multi-stage tube heat storage system, computational fluid dynamics software was used to deeply explore its internal heat and mass transfer processes. The research results show that the multi-stage tube structure can significantly improve the heat storage density. The study found that under the same volume coefficient, the optimal fractal simplified structure is approximately 12 tubes. Compared with traditional non-fractal heat storage structures, the heat storage capacity and heat storage rate increased by 127.32%, and the melting time decreased by 85.47%. Through single-factor comprehensive heat storage analysis, it was found that the heat storage capacity and heat storage rate were higher when the eccentricity angle was 28.5°. In addition, the larger the eccentricity, the optimal comprehensive heat transfer situation. Finally, multi-objective optimization of the multi-tube latent heat system was performed to obtain the best quality. The heat storage and melting time values were 1439.39 J and 364.44 s, respectively. The corresponding spacing and angle values were 41.59 mm and 29.95°, respectively. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0041] Figure 1 It is a structural schematic diagram of a multi-tube latent heat system heat storage device based on fractal optimization provided by the present invention.

[0042] Figure 2 It is a comparison diagram of liquid phase cloud maps and temperature cloud maps of different fractal structures provided by the present invention.

[0043] Figure 3 This is a data comparison diagram of different fractal structures provided by the present invention.

[0044] Figure 4 It is a comparison diagram of the liquid phase cloud diagram and the temperature cloud diagram of the simplified structure provided by the present invention.

[0045] Figure 5 This is a comparison diagram of liquid phase data of the simplified structure provided by the present invention.

[0046] Figure 6 It is a comparison diagram of the liquid phase cloud map and the temperature cloud map at different eccentric angles provided by the present invention.

[0047] Figure 7 This is a data comparison diagram of different eccentricity angles provided by the present invention.

[0048] Figure 8 This is a data comparison chart of different eccentric spacings provided by the present invention.

[0049] Figure 9 This is a flowchart of multi-objective optimization based on RSM and NSGA-II provided by the present invention.

[0050] Figure 10 It is a response surface optimization analysis diagram provided by the present invention.

[0051] Figure 11 It is a Pareto front diagram of multi-objective optimization provided by the present invention. DETAILED DESCRIPTION

[0052] The following describes embodiments of the present invention in detail, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and are not to be construed as limiting the present invention.

[0053] See also Figures 1 to 11The present invention provides a multi-tube latent heat system heat storage device based on fractal optimization, the multi-tube latent heat system heat storage device based on fractal optimization includes an outer shell and a multistage tube unit, the multistage tube structure is arranged inside the outer shell, the multistage tube unit is eccentrically arranged inside the outer shell as a whole, the eccentric angle of the multistage tube unit is 28.5°-31.5°, the eccentric spacing of the multistage tube unit is 37.5-42.5mm, the multistage tube unit includes a central tube and multiple peripheral tubes, the multiple peripheral tubes are evenly arranged around the periphery of the central tube, the center spacing between the multiple peripheral tubes and the central tube is 35mm, the pipe diameters of the central tube and the multiple peripheral tubes are both 13.86mm, the interiors of the central tube and the multiple peripheral tubes are filled with phase change material, and the phase change material is preferably paraffin RT55.

[0054] Furthermore, Table 1 lists the physical properties of the phase change material paraffin:

[0055] Table 1. Physical properties of the phase change material preferably paraffin RT55

[0056] Physical properties Phase change materials Main ingredients RT55 <![CDATA[Density / kg / m 3 > 770 Specific heat / J / (kg·K) 2000 Thermal conductivity / W / (m·K) 0.2 <![CDATA[Thermal conductivity (K -1 )]]> <![CDATA[4.5×10 -4 ]]> Latent heat / kJ / (kg) 170 Dynamic viscosity / kg / (m·s) <![CDATA[3.408×10 -4 ]]> Solid phase temperature / K 324 Liquidus temperature / K 330

[0057] Furthermore, one of the specific structures of the multiple peripheral tubes in the multi-stage tube unit is: the number of the multiple peripheral tubes is specifically 12, the 12 peripheral tubes are evenly arranged around the periphery of the central tube, and the bifurcation angle between each adjacent two peripheral tubes and the central tube is 30°.

[0058] Furthermore, one of the specific structures of the multiple peripheral tubes in the multi-stage tube unit is: the number of the multiple peripheral tubes is specifically 11, the bifurcation angle between the two adjacent peripheral tubes located at the inner top of the outer shell and the central tube is 60°, and the bifurcation angle between each of the remaining two adjacent peripheral tubes and the central tube is 30°.

[0059] The present invention further provides a multi-tube latent heat system heat storage method based on fractal optimization, which is applied to the multi-tube latent heat system heat storage device based on fractal optimization as described above, and comprises the following steps:

[0060] Establish a physical model of a multi-tube latent heat system, and perform a three-dimensional simulation on the physical model to obtain a three-dimensional model of the multi-tube latent heat system;

[0061] Conduct simulation analysis on a three-dimensional model of a multi-tube latent heat system, simulate the melting and solidification process of phase change materials, and analyze the system's heat transfer performance and energy storage efficiency;

[0062] Designing the specific structure of the multi-stage tube unit in the multi-tube latent heat system to keep the volume of the phase change material constant;

[0063] performing fractal optimization on the structure of the multi-stage tube unit to determine the fractal structure of the multi-stage tube unit;

[0064] Simplifying the fractal structure of the multi-stage tube unit to determine the optimized structure of the multi-stage tube unit;

[0065] Determining the eccentric angle and eccentric spacing of the multi-stage tube unit;

[0066] Response surface methodology and NSGA-II algorithm were used for multi-objective optimization to provide guidance for the optimal design of multi-tube latent heat system.

[0067] Among them, in the step of "simulating and analyzing the three-dimensional model of the multi-tube latent heat system to simulate the melting and solidification process of the phase change material", the enthalpy porosity method is used to simulate the melting process of the phase change material in the multi-tube latent heat system, and the control equation is as follows:

[0068] Energy equation:

[0069]

[0070] Where ρ is the density of the phase change material, H is the enthalpy of the phase change material, ▽ is the vector differential operator, and k is the thermal conductivity;

[0071] Continuity equation:

[0072]

[0073] Where, is the velocity vector;

[0074] Momentum equation:

[0075]

[0076] Where t is time; ρ is density; p is pressure; μ is dynamic viscosity; ξ is thermal expansion coefficient; T ref is the reference temperature, the source term The damping term of Darcy's law can be expressed as:

[0077]

[0078] The liquid phase rate at different times is expressed as follows:

[0079]

[0080] Among them, is T solid Solidus temperature, T liquid is the liquidus temperature of the phase change material;

[0081] The total enthalpy consists of sensible enthalpy and latent enthalpy:

[0082]

[0083] ΔH=λL

[0084] H=λL+h

[0085] Where ΔH is the latent heat enthalpy and h is the sensible heat enthalpy.

[0086] In the step "Simulating and analyzing the three-dimensional model of the multi-tube latent heat system", the temperature of the heat transfer fluid is simplified to the pipe wall temperature. At the initialization moment t = 0, the temperature of the phase change material is 293K and is in a solidified state. When t>0, the temperature of the heat transfer fluid is set to 363K to melt the phase change material. The formula is as follows:

[0087]

[0088] Furthermore, in the step of "designing the specific structure of the multi-stage tube unit in the multi-tube latent heat system to maintain a constant volume of the phase change material", since the sum of the multi-tube areas of all fractals is a constant value, the volume of the phase change material is maintained constant; all fractals are fracted from a single circle at the beginning to 13 circles, and the corresponding diameters are obtained based on equal areas for modeling. The structure of the outermost large circle is constant, and the small circles inside are fracted. The total area remains unchanged, so the mass of the phase change material remains unchanged.

[0089] In this embodiment, Table 2 lists the specific structural dimensions of different models:

[0090] Table 2. Specific structural parameters of different models

[0091]

[0092]

[0093] Furthermore, in the step of "performing fractal optimization on the structure of the multi-stage tube unit to determine the fractal structure of the multi-stage tube unit", the multi-stage tube structure is fractal optimized, as shown in the attached figure. Figure 2 As shown, keep the total area of the pipe constant by attaching Figure 3 The data comparison determined that 13 pipes (1 central pipe and 12 peripheral pipes) were the optimal fractal structure.

[0094] In this embodiment, if Figure 2 This part shows the fractal processing of a shell-and-tube heat accumulator. From Case-A to Case-E, the phase change material area of each heat accumulator is constant, and the pipe area is also constant. Figure 2A multi-tube phase change thermal storage cloud diagram is shown. The cloud on the left represents the liquid phase fraction, and the cloud on the right represents the temperature. Over time, the phase change material begins to transform from solid to liquid. Due to thermal lift, natural convection occurs during the melting process, causing the upper half of the tube-in-tube phase change thermal storage system to melt faster than the lower half. Because the phase change material area does not decrease in the case of an equal-area fractal, the heat transfer area of the heat exchange pipes increases. Therefore, Cases C to E clearly outperform Cases A and B in heat transfer performance. According to Newton's cooling formula, increasing the heat transfer area increases the heat transfer rate.

[0095] from Figure 3 (a) It can be seen that within 1000s, as time increases, the average temperature of the phase change materials in Case-C, Case-D and Case-E increases the fastest, and all begin to slow down around 400s. This is because at 400s, most of the phase change materials have melted completely, leaving only the phase change material in the bottom area that is difficult to melt. This is because due to natural convection, heat transfer is difficult to reach. Figure 3 (b) The change of liquid phase rate over time shows that Case-D has the fastest melting effect. This is mainly because not only the fractal increases the heat transfer area, but also the pipe area is larger than that of Case-E, which can better contact the dead zone of the phase change material at the bottom that is difficult to melt. Figure 3 (c) It can be found that the Nusselt number of Case-D is the largest from the beginning, which means that the heat transfer effect of Case-D is the best. Figure 3 Analysis of the data in (d) reveals that Case-D is optimal in all aspects. Compared to the unfracted Case-A, the average temperature of the phase change material increases by 11.81%, the heat storage capacity and heat storage rate increase by 127.32%, and the melting time decreases by 85.47%. Therefore, Case-D is a suitable fractal result for further study.

[0096] Furthermore, in the step of "simplifying the fractal structure of the multi-stage tube unit and determining the optimized structure of the multi-stage tube unit", as shown in the attached figure, Figure 4 The fractal structure is simplified as shown in the figure. Figure 5 Comparison of the data revealed that removing one pipe from the upper half formed an optimized structure (one central pipe and 11 peripheral pipes).

[0097] In this embodiment, the natural convection generated by the phase change material during melting can lead to uneven temperature distribution in the sleeve phase change thermal storage system. Therefore, reasonable fractal simplification based on the phase change material melting flow field is conducive to maximizing heat storage. Figure 4The cloud map analysis of the temperature field and velocity field of the casing phase change thermal storage system using different fractals is shown. The left side of the figure represents the liquid phase fraction, and the right side represents the flow rate. As time increases, the liquid phase fraction of each operating condition increases, indicating that the thermal storage material continuously absorbs heat and undergoes phase change. Case-D has a higher liquid phase fraction at 400s, with most areas appearing red and a higher degree of phase change. However, Case-H still has more blue areas within the same time, with a lower liquid phase fraction and a relatively slow phase change. In addition, a higher flow rate facilitates heat transfer and can accelerate the heat storage and phase change process. For example, Case-D has relatively more flow rate areas in some areas at the beginning, which may be one of the reasons for its faster growth in liquid phase fraction. However, Case-H has a generally lower flow rate (more blue areas), which is not conducive to rapid heat transfer, resulting in a slower phase change.

[0098] from Figure 5 (a) It can be seen that within 800s, Case-D starts to heat up quickly, indicating that the heat transfer is rapid and can quickly introduce heat into the phase change material (PCM). In contrast, Case-H heats up slowly and has a low heat transfer efficiency, mainly because its structure makes the PCM volume larger, which requires it to absorb more heat. Figure 5 (b) It can be seen that the liquid phase fraction of Case-D increases rapidly, while the liquid phase fraction of Case-H increases laggingly. However, when the liquid phase fraction is above 0.9, the melting is very slow. This is mainly due to the influence of natural convection, and the lower half of the casing phase change thermal storage system is difficult to melt. Figure 5 (c) It can be seen that at the initial moment, the Nusselt numbers of Case-D and Case-F are high, which means that the convective heat transfer effect is strong and the heat transfer effect driven by fluid flow is good, which is also due to the large heat transfer area in the front. Figure 5 Analysis of the data in (d) reveals that Case-F offers the best overall heat transfer performance. Compared to the unsimplified Case-A, the average temperature change of the phase change material is minimal, and the complete melting time increases by 4 seconds. However, the increased volume of the phase change material leads to a 4.84% increase in both heat storage and heat storage rate. Therefore, Case-F can be used as a simplified result for further study.

[0099] Furthermore, in the step of "determining the eccentric angle and eccentric spacing of the multi-stage tube unit", as shown in the attached Figure 6 The eccentric angle is set to 28.5°-31.5°, and the attached Figure 7 Comparison data of eccentric angles, attached Figure 8 The figure shows the comparison data of the eccentric distance when the eccentric distance is 37.5-42.5mm.

[0100] In this embodiment, Figure 6The liquid fraction and flow rate distribution of multiple tubes with different eccentric angles at different times (100s-400s) from Case-F-1 to Case-F-5 are shown. At 300 seconds, the distribution of different eccentric angles can be clearly seen. Case-F-1 with the smallest eccentric angle still has unmelted phase change materials in the working area. This is mainly because there are fewer pipes arranged in the upper half of the area. However, due to the existence of natural convection, from the velocity cloud map, the smaller the eccentric angle, the more severe the velocity disturbance of the casing phase change thermal storage system, and the faster the casing phase change thermal storage system melts. However, all casing phase change thermal storage systems have difficulty melting the bottom of the lower half of the area. This is because the heat source is too far away and the natural convection of the phase change material causes heat transfer. From Figure 7 A study on the data found that in the early 0-200s, the temperatures at 27° and 28.5° rose faster. Since there was no pipe in front to inhibit its natural convection, it shows that the heat transfer efficiency at this angle is relatively high, which can quickly increase the temperature of the phase change material. At 33°, the temperature rise is relatively slow, and there is a certain obstacle to heat transfer. In the later period, the final temperatures of each angle tend to be similar, but the time to reach similar temperatures is different. 27° and 28.5° take a shorter time, indicating that in the overall heat storage process, these two angles can reach thermal equilibrium faster, and the heat storage speed has an obvious advantage. As time goes on, the liquid phase rate of each angle approaches 1, but 27° and 28.5° approach complete phase change earlier, indicating that they can complete phase change faster during the heat storage process and have better heat storage performance. In addition, the Nusselt numbers of 27° and 28.5° are relatively higher in the early stage, indicating that the fluid flow drives better heat transfer. Comprehensive thermal storage analysis revealed that the 28.5° angle exhibited the highest thermal storage capacity and heat storage rate, along with a short complete melting time and high thermal storage efficiency, resulting in the best overall thermal storage performance. The 33° angle performed poorly on these indicators, resulting in suboptimal thermal storage performance. Overall, the angle of the circular tubes around the center significantly impacted the thermal storage performance of the multi-tube configuration, with an optimal value near 28.5° for the same eccentricity.

[0101] from Figure 8 (a) It was found that in the early stage of 0-200s, the temperature rise of 37.5mm and 40mm eccentricity was relatively fast, the heat transfer efficiency was high, and the temperature of the phase change material could be quickly increased; the temperature rise of 42.5mm eccentricity was slow because the heat transfer from the phase change material was more dispersed. However, in the later stage, since the bottom area was difficult to melt, the larger the eccentricity, the faster the bottom area of the sleeve phase change thermal storage system could melt, making the overall sleeve phase change thermal storage system melt faster. Figure 8 (b) It is found that the liquid phase rate of the 30mm eccentricity increases the slowest and the heat storage capacity is the weakest. This is because the multi-tube distribution is too biased towards the inside of the shell and tube phase change thermal storage system, and is far away from the bottom phase change material. In addition, due to the influence of natural convection, the phase change material in the bottom area of the shell and tube phase change thermal storage system is more difficult to melt, and the phase change process is slow, which affects the overall heat storage efficiency. Figure 8(c) It is found that at the initial moment, the Nusselt number of each eccentricity is relatively high, reflecting the significant effect of convective heat transfer. The Nusselt number of 40mm eccentricity is relatively higher, and the fluid flow drives the heat transfer effect better; the Nusselt number of 30mm eccentricity is low, and the convective heat transfer is weak, which is not conducive to heat transfer. As the time changes, the Nusselt number of all eccentricities decreases with time. This is because after the average temperature of the phase change material increases, the heat transfer gradually decreases. It is best to conduct a comprehensive analysis Figure 8 (d) The results show that an eccentricity of 42 mm achieves optimal overall heat transfer. Compared to a 30 mm eccentricity, the average temperature (500 s) of the phase change material increases by approximately 1.27%, the heat storage capacity (500 s) increases by approximately 6.05%, the complete melting time decreases by approximately 15.15%, and the heat storage rate increases by approximately 9.16%. Overall, a 40 mm eccentricity offers significant advantages in heat storage performance.

[0102] In summary, the optimal parameter combination of the multi-stage tube unit is an eccentricity angle of 29.95° and an eccentricity spacing of 41.59 mm.

[0103] Furthermore, the specific steps of the step “Using response surface methodology and NSGA-II algorithm for multi-objective optimization” are:

[0104] First, the target program is set, and the sample points are set using the CCD method. Then the sample amount is input into the program, and then the response surface is obtained. The objective function of the response surface is analyzed, and the objective function is brought into the genetic algorithm for optimization to obtain the parameter combination.

[0105] In this embodiment, the input is the sample data of the response surface method, and the sample data table is shown in Table 3:

[0106] Table 3. Sample data

[0107]

[0108] Furthermore, in the step “multi-objective optimization using response surface methodology and NSGA-II algorithm”, the objective functions of the multi-objective optimization are maximizing heat storage and minimizing melting time.

[0109] Furthermore, the optimized objective equation relationship fitting formula is:

[0110] Heat storage capacity=-2230.46401+125.7517A+73.7684B-0.079191AB-1.51058A 2 -1.17946B 2

[0111] (30mm≤A≤42.5mm, (27°≤B≤33°)

[0112] Complete melting time=30205.38672-1017.03565A-546.46514B

[0113] -0.973333AB+12.2944A 2 +9.77778B 2

[0114] (30mm≤A≤42.5mm, 27°≤B≤33°).

[0115] In summary, this technical solution focuses on multi-stage tube thermal storage technology, aiming to address the low thermal storage efficiency of current multi-stage tube latent heat storage systems. Through structural design and performance analysis of the multi-stage tube thermal storage system, computational fluid dynamics software was used to deeply investigate the internal heat and mass transfer processes. The results demonstrate that the multi-stage tube structure significantly improves thermal storage density. The optimal fractal simplified structure, with the same volume coefficient, is found to have approximately 12 tubes. Compared to a conventional non-fractal thermal storage structure, the heat storage capacity and heat storage rate increased by 127.32%, while the melting time decreased by 85.47%. A single-factor comprehensive thermal storage analysis revealed that an eccentricity of 28.5° exhibited the highest heat storage capacity and heat storage rate. Furthermore, a larger eccentricity resulted in the best overall heat transfer. Finally, a multi-objective optimization of the multi-stage tube latent heat storage system yielded the optimal heat storage and melting time values of 1439.39 J and 364.44 s, respectively, corresponding to spacing and angle values of 41.59 mm and 29.95°, respectively.

[0116] The above disclosure is only a preferred embodiment of the present invention, and certainly cannot be used to limit the scope of the rights of the present invention. Ordinary technicians in this field can understand that all or part of the processes of the above embodiment and equivalent changes made in accordance with the claims of the present invention are still within the scope of the invention.

Claims

1. A multi-tube latent heat storage device based on fractal optimization, characterized in that: It includes an outer shell and a multistage tube unit. The multistage tube structure is arranged inside the outer shell. The multistage tube unit is eccentrically arranged inside the outer shell as a whole. The eccentric angle of the multistage tube unit is 28.5°-31.5°, and the eccentric spacing of the multistage tube unit is 37.5-42.5mm. The multistage tube unit includes a central tube and multiple peripheral tubes. The multiple peripheral tubes are evenly arranged on the periphery of the central tube. The center spacing between the multiple peripheral tubes and the central tube is 35mm. The pipe diameters of the central tube and the multiple peripheral tubes are both 13.86mm. The interiors of the central tube and the multiple peripheral tubes are filled with phase change material, and the phase change material is preferably paraffin RT55.

2. The multi-tube latent heat storage system based on fractal optimization according to claim 1, characterized in that: The number of the plurality of peripheral tubes is specifically 12, and the 12 peripheral tubes are evenly arranged around the periphery of the central tube, and a bifurcation angle between every two adjacent peripheral tubes and the central tube is 30°.

3. The multi-tube latent heat storage system based on fractal optimization according to claim 1, characterized in that: The number of the plurality of peripheral tubes is specifically 11, the bifurcation angle between the two adjacent peripheral tubes located at the inner top of the outer shell and the central tube is 60°, and the bifurcation angle between each of the remaining two adjacent peripheral tubes and the central tube is 30°.

4. A multi-tube latent heat storage method based on fractal optimization, applied to the multi-tube latent heat storage device based on fractal optimization as claimed in claim 3, characterized in that: The steps include: Establish a physical model of a multi-tube latent heat system, and perform a three-dimensional simulation on the physical model to obtain a three-dimensional model of the multi-tube latent heat system; Conduct simulation analysis on a three-dimensional model of a multi-tube latent heat system, simulate the melting and solidification process of phase change materials, and analyze the system's heat transfer performance and energy storage efficiency; Designing the specific structure of the multi-stage tube unit in the multi-tube latent heat system to keep the volume of the phase change material constant; performing fractal optimization on the structure of the multi-stage tube unit to determine the fractal structure of the multi-stage tube unit; Simplifying the fractal structure of the multi-stage tube unit to determine the optimized structure of the multi-stage tube unit; Determining the eccentric angle and eccentric spacing of the multi-stage tube unit; Response surface methodology and NSGA-II algorithm were used for multi-objective optimization to provide guidance for the optimal design of multi-tube latent heat system.

5. The multi-tube latent heat storage method based on fractal optimization according to claim 4, characterized in that: In the step "Simulating and analyzing the three-dimensional model of the multi-tube latent heat system to simulate the melting and solidification process of the phase change material", the enthalpy porosity method is used to simulate the melting process of the phase change material in the multi-tube latent heat system. The control equation is as follows: Energy equation: Where ρ is the density of the phase change material, H is the enthalpy of the phase change material, is the vector differential operator, k is the thermal conductivity; Continuity equation: Where, is the velocity vector; Momentum equation: Where t is time; ρ is density; p is pressure; μ is the dynamic viscosity; ξ is the thermal expansion coefficient; T ref is the reference temperature, the source term S is the damping term of Darcy's law and can be expressed as: The liquid phase rate at different times is expressed as follows: Among them, is T solid Solidus temperature, T liquid is the liquidus temperature of the phase change material; The total enthalpy consists of sensible enthalpy and latent enthalpy: ΔH=λL H=λL+h Where ΔH is the latent heat enthalpy and h is the sensible heat enthalpy.

6. The multi-tube latent heat storage method based on fractal optimization according to claim 5, characterized in that: In the step "Simulate and analyze the three-dimensional model of the multi-tube latent heat system," the temperature of the heat transfer fluid is simplified to the pipe wall temperature. At the initialization moment t = 0, the temperature of the phase change material is 293K, and it is in a solidified state. When t > 0, the temperature of the heat transfer fluid is set to 363K, causing the phase change material to melt. The formula is as follows:

7. The multi-tube latent heat storage method based on fractal optimization according to claim 6, characterized in that: The specific steps of the step "Multi-objective optimization using response surface methodology and NSGA-II algorithm" are: First, the target program is set, and the sample points are set using the CCD method. Then the sample amount is input into the program, and then the response surface is obtained. The objective function of the response surface is analyzed, and the objective function is brought into the genetic algorithm for optimization to obtain the parameter combination.

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