A method and device for heat transfer enhancement of magnetic molten salt nanofluid and sCO2 conjugation by synergistic effect of thermal magnetic coupling field

Abstract

This invention discloses a method and apparatus for enhancing heat transfer between magnetic molten salt nanofluid and sCO2 through thermomagnetic coupling. The method introduces magnetic molten salt nanofluid and manipulates it using an external magnetic field, constructing a heat transfer model synergistically integrating a velocity field, a temperature gradient field, a pressure gradient field, and a magnetic field. Numerical simulations are used to obtain the velocity, temperature, and pressure distributions of the fluid, allowing analysis of the directional aggregation behavior of magnetic nanoparticles in the wall region, forming a stable thermal conductivity network and significantly reducing wall thermal resistance. Furthermore, response surface methodology (RSM) combined with neural network technology is employed to optimize the angle parameters between the velocity field and the magnetic field, temperature gradient field, and pressure gradient field. α , β , gamma This method achieves optimal configuration of multi-physics coupling, further improving heat exchange efficiency. It provides new ideas and technical support for the design of high-efficiency thermal management systems.

Description

Technical Field

[0001] This invention belongs to the field of thermal energy engineering and heat and mass transfer technology, specifically relating to a method and device for enhancing heat transfer by magnetic molten salt nanofluid and sCO2 conjugate through thermomagnetic coupling field synergy. Background Technology

[0002] With the continued growth of global energy demand and the advancement of carbon neutrality goals, improving energy efficiency and reducing carbon emissions have become important development directions for various industries. Supercritical carbon dioxide (sCO2) and high-temperature molten salt, as novel high-efficiency heat storage and heat transfer media respectively, are widely used in high-temperature thermal energy systems such as solar thermal power generation, nuclear energy utilization, and industrial waste heat recovery. Compared with traditional heat transfer oil or steam, sCO2 and high-temperature molten salt exhibit superior overall performance in thermal energy systems. sCO2 has lower viscosity and higher density, resulting in higher efficiency in flow and heat exchange processes. It also has good thermal conductivity, especially exhibiting enhanced heat transfer near the critical point, which is conducive to rapid and efficient heat exchange. High-temperature molten salt, with its high specific heat capacity and energy density, excels in heat storage, storing more thermal energy per unit mass and volume. It also possesses a wide operating temperature range and good thermal stability, adapting to continuous operation under high-temperature conditions. However, the relatively low thermal conductivity of molten salt limits its performance in efficient heat exchange processes. To improve its thermal properties, researchers often enhance it by adding nanoparticles to molten salt, that is, by directly doping to improve thermal conductivity.

[0003] Currently, heat exchange between high-temperature molten salt and sCO2 mainly employs traditional heat exchanger structures such as printed circuit board type and shell-and-tube type. However, under high-temperature and high-pressure operating environments, these heat exchangers often face problems such as low heat transfer efficiency, complex structure, and easy material corrosion, which seriously restricts the long-term stable operation of the heat exchange system. Especially after introducing nanoparticles into molten salt to enhance its thermal conductivity, the stability of the system is further challenged. With the extension of operating time, the lack of effective control over the spatial distribution of nanoparticles in the fluid leads to problems such as particle agglomeration, deposition, or uneven distribution in the high-temperature liquid environment. This not only reduces the actual effect of enhanced heat conduction but may also lead to a decrease in the thermal stability of the system, an increase in flow resistance, and even adverse phenomena such as fluctuations in thermal conductivity and a decrease in heat storage efficiency, affecting the long-term stable operation of the system. Therefore, how to control the distribution state of nanoparticles is one of the key issues that urgently need to be addressed to improve the performance of molten salt composite materials.

[0004] Furthermore, the heat exchange between molten salt nanofluids and sCO2 is an indirect conjugate heat transfer process, meaning that the two media exchange heat through heat coupling via a solid wall. This heat transfer mechanism differs significantly from direct fluid-to-fluid heat transfer, and the effectiveness of enhanced heat transfer relies more heavily on wall thermal resistance control and heat transfer interface management. The uneven distribution and decreased thermal properties of nanoparticles will further increase wall thermal resistance, thereby inhibiting the improvement of the overall heat exchanger performance. Therefore, there is an urgent need to explore and propose an active thermophysical property control method that can dynamically adjust the distribution and thermal conductivity behavior of nanoparticles in molten salt during operation to suppress particle aggregation and sedimentation, improve fluid stability, and significantly enhance the actual thermal conductivity of molten salt nanofluids. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method and device for enhancing heat transfer by combining magnetic molten salt nanofluid with sCO2 in a thermomagnetically coupled field. This solves the problems in the prior art, such as the magnetic molten salt nanofluid being prone to agglomeration and instability when flowing in a heat exchanger, large fluctuations in thermal conductivity, and large thermal resistance of the heat exchange wall.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A method for enhancing heat transfer through thermomagnetic coupling field-assisted magnetic molten salt nanofluid and sCO2 conjugate, comprising the following steps: S1, obtain the physical property parameters of magnetic molten salt nanofluid and supercritical carbon dioxide in the heat exchanger, wherein the magnetic molten salt nanofluid is a fluid in which magnetic nanoparticles are added to molten salt, and a magnetic field is applied to the magnetic molten salt nanofluid side of the heat exchanger. S2, numerical simulations are performed on the dynamic model, heat transfer model, and turbulent flow model of the two types of fluids to obtain the velocity field, temperature gradient field, and pressure gradient field of the fluids; the two types of fluids include magnetic molten salt nanofluids and supercritical carbon dioxide; wherein, an external magnetic field is applied in the dynamic model, heat transfer model, and turbulent flow model of the magnetic molten salt nanofluids. S3, the velocity field, temperature gradient field and pressure gradient field of the magnetic molten salt nanofluid are coupled; for the magnetic molten salt nanofluid, the first angle between the velocity vector and the normal vector of the temperature gradient field, the second angle between the velocity vector and the normal vector of the pressure gradient field, and the third angle between the velocity vector and the direction of the applied magnetic field are obtained, and the optimal values ​​of the three angles when the heat exchange efficiency is the highest are determined by numerical simulation. S4. The key influencing factors of the three included angles are identified by response surface methodology, and the values ​​of each key influencing factor when the three included angles are optimal are obtained by neural network. S4 executes the heat exchanger by taking the values ​​of various influencing factors.

[0007] A further improvement of the present invention is that: Preferably, in S1, the physical properties of the magnetic molten salt nanofluid are obtained through microscale simulation methods.

[0008] Preferably, in S1, the magnetic nanoparticles include metal-based nanoparticles and metal oxide-based nanoparticles, wherein the metal-based nanoparticles include iron, cobalt, and nickel, and the metal oxide-based nanoparticles are ferrite nanoparticles.

[0009] Preferably, in S2, magnetic volume force terms, particle Brownian diffusion, thermophoretic migration, and magnetic drift terms are introduced into the fluid dynamics model, heat transfer model, and turbulent flow model of the magnetic molten salt nanofluid.

[0010] Preferably, in S3, when determining the optimal values ​​of the three included angles, the optimization objective is to maximize the Nusselt number and minimize the flow pressure drop or magnetic reluctance loss.

[0011] Preferably, in S3, when determining the optimal values ​​of the three included angles, the optimization objective is a coordination index, which is:

[0012] in, Indicates volume average. f ( Ha ) is the Hartmann number influence function. w i These are weighting coefficients. α The angle between the velocity vector and the normal vector of the temperature gradient field. β The angle between the velocity vector and the normal vector of the pressure gradient field. γ Let be the angle between the velocity vector and the direction of the applied magnetic field.

[0013] Preferably, in S4, the influencing factors include the flow rate, temperature gradient, and pressure gradient of the magnetic molten salt nanofluid and the supercritical carbon dioxide fluid, as well as the magnetic field strength of the magnetic molten salt nanofluid.

[0014] Preferably, in step S4, after determining the key influencing factors of the three included angles using the surface response analysis method, the values ​​of the key influencing factors and the response values ​​are input into the neural network to train the neural network.

[0015] A heat transfer enhancement device for magnetic molten salt nanofluid and supercritical carbon dioxide (SCO2) using the above-mentioned enhancement method includes a heat exchanger. The heat exchanger is divided into a magnetic molten salt nanofluid side and a supercritical carbon dioxide side. A magnetic field is applied to the magnetic molten salt nanofluid side. The magnetic molten salt nanofluid includes molten salt and magnetic nanoparticles. The magnetic nanoparticles are magnetic metal-based nanoparticles or metal oxide-based nanoparticles.

[0016] Preferably, the applied magnetic field causes the magnetic nanoparticles to aggregate directionally in the heat exchanger and move directionally in a chain-like or network structure.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a method for enhancing heat transfer between magnetic molten salt nanofluids and sCO2 through thermomagnetic coupling. This method is based on a novel four-field synergistic heat transfer approach using magnetic nanoparticles to enhance molten salt. Metal-based or metal oxide-based nanoparticles (such as ferrite nanoparticles Fe3O4 and Fe2O3) with paramagnetic and diamagnetic properties are used as molten salt additives. An external magnetic field is applied as a control mechanism in a high-temperature molten salt-sCO2 heat exchange system. During the flow of the molten salt nanofluid, the distribution and migration behavior of magnetic nanoparticles in the local flow field are controlled by the magnetic field, achieving synergistic control of the magnetic field, velocity field, temperature gradient field, and pressure gradient field within the heat exchanger. By coupling fluid dynamics, heat transfer, and turbulent flow models, and utilizing the angular relationships between the magnetic field, velocity field, temperature gradient field, and pressure gradient field, the heat exchange efficiency is significantly improved.

[0018] (1) This method takes into account the synergistic effect of magnetic field, velocity field, temperature gradient field and pressure gradient field, and optimizes the angle relationship between these fields through numerical simulation, which significantly improves the heat exchange efficiency between molten salt and sCO2, reduces the thermal resistance inside the heat exchanger and improves the thermal energy conversion efficiency.

[0019] (2) This invention utilizes an external magnetic field to induce the directional aggregation of magnetic nanoparticles in the wall region, forming a stable thermal conductivity network and reducing the wall thermal resistance. Through magnetic field control, the microscale thermal conductivity can be adjusted online, enhancing the overall thermal conductivity.

[0020] (3) The present invention uses response surface methodology (RSM) combined with neural network method to perform multi-factor optimization, analyze the interactive influence of different independent variables on heat exchange effect, further optimize heat exchange efficiency, ensure stable operation of the system under various working environments, and improve the adaptability and flexibility of the system.

[0021] (4) This invention has good adaptability and scalability, and is applicable to complex heat exchangers and thermal conditions in various high-temperature and high-pressure thermal energy systems, especially in the fields of solar thermal power generation, nuclear energy systems and industrial waste heat recovery. By improving heat exchange efficiency, this invention can effectively improve the overall performance of energy systems, reduce energy waste, and provide a more efficient and sustainable solution for energy conversion and utilization. Attached Figure Description

[0022] Appendix Figure 1 This is a flowchart illustrating the calculation process of the method of the present invention; Appendix Figure 2 This is a diagram of the oriented chain structure of magnetic nanoparticles induced by a magnetic field near the wall surface. Appendix Figure 3 This is a schematic diagram of the heat conduction path (thermal channel).

[0023] Appendix Figure 4 A magnetic molten salt nanofluid and sCO2 conjugate heat transfer device with thermomagnetic coupling field coordination Detailed Implementation Hereinafter, the terms "first," "second," "third," and "fourth" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first," "second," "third," or "fourth" may explicitly or implicitly include one or more of that feature.

[0024] The synchronization method provided in this application can be applied to mobile phones, tablets, wearable devices, in-vehicle devices, augmented reality (AR) / virtual reality (VR) devices, laptops, and ultra-mobile personal computers. In this application, the specific type of terminal device is not limited to terminal devices such as mobile personal computers (UMPCs), netbooks, and personal digital assistants (PDAs).

[0025] It should be noted that the terms "first," "second," etc., used in the specification and drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0026] This invention addresses the problems of unstable aggregation, large fluctuations in thermal conductivity, and high thermal resistance of magnetic molten salt nanofluids during heat exchanger flow. By applying a magnetic field to one side of the magnetic molten salt fluid and combining this with the control of velocity, temperature, and pressure fields, real-time adjustment of the wall heat flux density and nanoparticle distribution is achieved. This creates a thermal channel on the inner wall of the heat exchanger, improving the indirect conjugate heat transfer efficiency between the nanofluid and the sCO2 fluid on the other side. The model is used to predict the impact of multi-field interactions on heat transfer performance under different operating conditions, guiding subsequent parameter optimization. This approach can significantly improve heat transfer efficiency, reduce energy consumption, and enhance the system's operational stability and adaptability, showing broad prospects for engineering applications.

[0027] See Figure 1 The first aspect of this invention discloses a method for enhancing the conjugate heat transfer of magnetic molten salt nanofluid and sCO2 through thermomagnetic coupling field synergy. This method is a four-field synergistic heat transfer model that comprehensively considers the magnetic field, velocity field, temperature gradient field, and pressure gradient field. The specific steps are as follows: S1. Obtaining physical property parameters: Using microscale simulation methods based on molecular dynamics, the basic thermophysical property data of magnetic molten salt nanofluids and sCO2 are obtained respectively. For molten salt nanofluids, the variation of physical properties such as thermal conductivity, specific heat capacity, and viscosity with factors such as nanoparticle content and temperature is calculated. For sCO2, although its thermophysical property data have relatively mature empirical formulas at larger scales, microscale simulation can provide more accurate data, especially the behavior under extreme conditions (such as thermophysical property parameters under supercritical conditions), providing a physical basis for subsequent numerical simulations. Therefore, the thermophysical property parameters of sCO2 can be obtained through empirical formulas or microscale simulation.

[0028] S2, through finite element numerical simulation, analyzes the internal flow and heat transfer characteristics of the heat exchanger: using fluid dynamics model, heat transfer model and turbulent flow model, numerical simulation is performed to obtain the velocity field, temperature gradient field and pressure gradient field of the fluid.

[0029] In this process, the flow process of the fluid inside the heat exchanger is analyzed through numerical simulation to obtain the thicknesses of the velocity boundary layer, temperature boundary layer, and concentration boundary layer. Furthermore, the temperature gradient field and pressure gradient field inside the heat exchanger are calculated to provide a basis for optimizing heat exchange performance.

[0030] In the magnetic molten salt nanofluid model, in addition to considering the velocity, temperature, and pressure gradient fields, a magnetic field also needs to be introduced. Since the magnetic field influences the magnetic nanoparticles, the behavior of the magnetic nanoparticles and the construction of a thermal conductivity network are necessary during the overall model construction. An external magnetic field is applied to the magnetic molten salt fluid side to induce the directional aggregation of nanoparticles, forming a stable chain structure, constructing thermal conductivity bridging links, and reducing wall thermal resistance. In the numerical simulation, the influence of the external magnetic field is realized by adding magnetic field-related terms to the flow and heat conduction equations, introducing magnetic volume force terms (such as Lorentz force) to affect the motion of the magnetic particles. Through external magnetic field control, the microscale thermal conductivity can be adjusted online, improving the overall heat conduction efficiency.

[0031] Specifically, considering the influence of the magnetic field on the motion of nanoparticles, a magnetic volume force term and Brownian diffusion are introduced to enable the model to capture the dynamic behavior of particles in complex coupled fields. This regulation process mainly relies on the intervention of the Lorentz force on the flow path of magnetic nanoparticles. For neutral but magnetically responsive particles, the magnetic volume force form represents the force behavior, as shown in the following equation: (1) in, μ 0 is the permeability of free space. Vp is the magnetic susceptibility, and Vp is the particle volume. This indicates the effect of the magnetic field gradient.

[0032] See Figure 2 and Figure 3 In the presence of an external magnetic field, nanoparticles aggregate directionally due to magnetic force, forming chain-like or network structures. This structure significantly enhances the heat conduction path between the solid and liquid interfaces, thereby reducing interfacial thermal resistance. In this case, the effective thermal conductivity of nanofluids containing chain-like thermally conductive structures can no longer be expressed using the thermal conductivity of nanofluids when the particles are dispersed; it needs further modification and can be represented by a modified Maxwell model or Hamilton-Crosser model. (2) in, k eff For nanofluids containing chain-like thermally conductive structures, the effective thermal conductivity is... k f The thermal conductivity of molten salt, k p The thermal conductivity of the magnetic nanoparticles. φeff The volume fraction of the nanoparticles. n It is a geometric factor.

[0033] Therefore, a static magnetic field can not only dynamically control the microstructure of particles, but also stably form a heat-conducting network, improve thermal conductivity, and enhance the overall heat transfer efficiency and stability of the system.

[0034] S3, couples the velocity field, temperature gradient field, and pressure gradient field of the magnetic molten salt nanofluid; for the magnetic molten salt nanofluid, calculates the angle between the velocity vector and the normal vector of the temperature gradient field. α The angle between the velocity vector and the normal vector of the pressure gradient field β and the angle between the velocity vector and the direction of the applied magnetic field. γ Numerical optimization methods were used to adjust the multiphysics coupling and the angles between the three points mentioned above.

[0035] This invention considers the principle of four-field synergy, mainly referring to the geometric relationship between the applied magnetic field, the fluid velocity field, the temperature gradient field, and the pressure gradient field. During heat exchange, the angles between the applied magnetic field, the fluid velocity field, and the temperature and pressure gradient fields are key factors determining heat exchange efficiency.

[0036] The above process first establishes a fluid flow model, a temperature gradient field, and a pressure gradient field, and then couples the three together.

[0037] Establishment of the fluid flow model: The velocity field of the magnetic molten salt nanofluid during the flow process is described by solving the Navier-Stokes equations. The flow model considers the effects of inertial forces, viscous forces, pressure gradients, and applied magnetic fields on the fluid motion.

[0038] (3) in, v For fluid velocity vector, ρ Let be the fluid density, and be the dynamic viscosity of the fluid. F It is the magnetic force generated by an external magnetic field.

[0039] Establishment of the temperature gradient field: A temperature gradient field model is established based on the heat conduction equation. The temperature change of the fluid is affected by the flow velocity, thermal conductivity, and the effect of the applied magnetic field on the heat flow. The heat conduction equation considers the coupling effect of heat convection and heat conduction. (4) in, T For the temperature field, Cp For specific heat capacity, k Thermal conductivity,Q It serves as an external heat source.

[0040] Establishment of the pressure gradient field: The pressure gradient field is obtained by solving for the pressure distribution of the fluid in the channel. This distribution is determined by the flow velocity, temperature gradient, and the interaction between the fluid and the wall. Changes in the pressure gradient field directly affect the flow characteristics of the fluid, thereby affecting the heat exchange efficiency.

[0041] (5) To achieve coupling between flow, temperature, and pressure gradients, independent solutions for each physical field are first calculated in numerical simulations. Then, based on these independent solutions, they are coupled together, and a feedback mechanism is used to correct the interactions between the flow, temperature, and pressure fields. The specific process is as follows: the velocity field solution is calculated, and the velocity distribution is obtained through the fluid's kinematic characteristics; the temperature field changes are calculated based on the velocity field, and the temperature field is adjusted according to the flow velocity and external heat sources; the pressure gradient field is further calculated based on the temperature and velocity fields, and changes in the pressure field are fed back to the velocity field; finally, based on the kinematic characteristics of the magnetic nanoparticles and considering the influence of an applied magnetic field, the coupling between the various physical fields is adjusted.

[0042] Regarding the cooperative angle between the velocity field and the temperature gradient field: (6) Similarly, the cooperative angle between the velocity field and the pressure gradient field can be defined: (7) Applying a magnetic field B The Lorentz force generated in the magnetohydrodynamic fluid affects the velocity and flow field structure.

[0043] (8) Define the cooperative angle between the velocity field and the magnetic field: (9) If the direction of the magnetic field is different from the direction of the flow, the magnetic field also has a certain inhibitory effect on the flow. The degree of inhibition is expressed by the Hartmann number (…). M The characterization is represented by the expression: (10) The four-field synergy principle requires that during heat exchange, the angle between the flow velocity vector and the normal vector of the temperature gradient field ( ) α ) and the angle between the velocity vector and the normal vector of the pressure gradient field ( β ) and the angle between the velocity vector and the direction of the applied magnetic field ( γMaintaining specific relationships. These angles determine the efficiency of fluid flow and heat conduction. Ideally, the fluid velocity field should maintain an optimized angular relationship with the magnetic field, temperature gradient field, and pressure gradient field to enhance heat transfer during the heat exchange process.

[0044] To maximize heat exchange efficiency, the values ​​of these angles must be optimized. Studies have shown that, ideally, α and β The angle should be as close to 0° as possible to maximize the fluid's heat transfer efficiency. The optimal angle γ depends on the specific properties of the fluid and the strength and direction of the applied magnetic field. Therefore, the optimal angle value is not that all three angles are fixed at 0°, but rather that the heat exchange efficiency is maximized by adjusting these angles within an optimized range. Numerical simulations are used to calculate the heat exchange efficiency at different angle values, and the angles are gradually adjusted until the optimal angle value that maximizes the heat exchange efficiency is found. Specifically, in the optimization process, different... α , β The γ value was used to evaluate its impact on heat exchange efficiency, seeking the angle combination that minimizes heat loss or maximizes heat transfer rate. During the optimization process, the turbulent characteristics of the fluid, the aggregation behavior of magnetic particles, and the geometric design of the heat exchanger were taken into account. Finally, a set of optimal angle values ​​were determined that can reduce heat loss and improve overall heat exchange performance while ensuring flow and heat transfer efficiency.

[0045] In an ideal state, α and γ The velocity direction should be as close to 0° as possible, meaning the velocity direction should be highly aligned with both the heat flow direction and the magnetic field direction to reduce energy loss and magnetoresistance. Optimizing the structure or turbulence design can align the fluid flow direction with the temperature gradient direction, thereby improving convective heat transfer efficiency. By controlling the magnetic field direction, the velocity direction can be aligned as closely as possible with the magnetic field direction, reducing flow resistance caused by the Lorentz force, increasing flow velocity, and maintaining thermal boundary layer disturbance, further enhancing heat exchange performance. To achieve this goal, numerical simulation and multi-objective optimization methods are employed, iteratively optimizing based on the following objective function: maximizing the Nusselt number (Nu) to enhance heat transfer while minimizing the flow pressure drop (or magnetoresistance loss), achieving comprehensive optimization. α and γ The angle between the two is adjusted to make them as close to 0 as possible, achieving the optimal configuration.

[0046] As a preferred embodiment, the present invention constructs a weighted comprehensive synergy index, because... α and γ They are not entirely independent. In many practical systems, adjusting the direction of the magnetic field can alter the fluid flow path, thereby affecting... α Structural optimization, if it alters the velocity distribution, will also directly affect... γThe changes. Therefore, this invention adopts a synergistic adjustment strategy: dynamically optimizing through coordinated control of magnetic field direction and geometric structure design. α and γ The coupling relationship is considered to achieve globally optimal heat transfer performance. To achieve joint optimization of heat transfer performance, flow structure, and energy consumption cost, this invention constructs the following weighted comprehensive synergistic index: (11) Among them, GFSI is a weighted comprehensive synergy index. Indicates volume average. f ( Ha ) is the Hartmann number influence function. Nu ref With Δ P ref Used as a reference value for normalization. w i It is a weighting coefficient that can be adjusted according to different heat transfer / flow / magnetic control objectives. The larger the GFSI value, the more the system simultaneously achieves high heat transfer efficiency, good field synergy, and low flow energy consumption. The optimization process is to find the design parameter combination that maximizes this overall score.

[0047] The direction of the velocity field is dynamically controlled by an external magnetic field to minimize the equicooperative angle in real time. Electromagnetic coils are used in microchannel or magnetic nanofluidic systems to adjust the direction of the magnetic field in real time; the system monitors the local GFSI index and controls the direction or intensity of the magnetic field through a feedback mechanism; thus forming a "cooperative sensing-feedback control closed-loop system" to maximize the synergistic effect of the magnetothermal flow field.

[0048] S4. After optimizing the synergistic angle, response surface methodology (RSM) is used for multi-factor optimization. Combined with neural network methods and Design Expert software, dimensionality reduction experiments of independent variables are conducted to analyze the interactive effects of different independent variables (flow rate, fluid properties, external magnetic field strength, structural design, etc.) on the heat transfer effect, and further optimize the heat transfer efficiency.

[0049] Specifically, response surface methodology (RSM) combined with neural network methods is used to determine the levels of key factors (such as angle, flow velocity, fluid properties, external magnetic field strength, and structural design). Numerical simulations are then used to obtain response values ​​(such as Nusselt number and thermal resistance) at each test point. Based on the collected data, a polynomial response surface model is established to describe the relationship between the response values ​​and each factor. Analysis of variance is performed to evaluate the significance and goodness of fit of the model. Within the RSM framework, the expectation function or satisfaction function is used for multi-objective optimization to obtain the final key influencing factors.

[0050] A neural network model is trained using the collected final key factor values ​​and response values ​​to capture the nonlinear relationships between factors. Through cross-validation, the network structure and parameters are optimized to obtain the trained neural network model. In practical applications, key influencing factors can be directly input into the neural network model to determine the optimal factor values. This process combines response surface methodology and neural network modeling for multi-objective optimization, determining the optimal combination of factors to maximize heat exchange efficiency and minimize energy consumption.

[0051] For example, the neural network model can be a BP neural network or other neural networks.

[0052] S5 involves controlling the values ​​of various influencing factors within the heat exchanger for heat exchange. During this process, magnetic nanoparticles directionally aggregate: a magnetic field is applied to one side of the magnetic molten salt fluid. Under the influence of the magnetic field, the magnetic nanoparticles undergo directional migration, enrichment, or dispersion, thereby controllably altering the local thermal conductivity, specific heat capacity, and heat flux distribution. By adjusting the magnetic field strength, direction, and spatial distribution pattern, combined with the matching relationship between the velocity field and the temperature gradient direction, enhanced heat conduction paths, the so-called "thermal channels," can be formed in key heat exchange areas, reducing wall thermal resistance. This method achieves online adjustment of microscale thermal conductivity through external magnetic field control, improving overall heat conduction efficiency. Combining magnetic field direction control with geometric structure design, the angle between the velocity field and the magnetic field is dynamically optimized. γ This ensures that the fluid flow direction is consistent with the magnetic field direction, reduces flow hindrance caused by Lorentz force, increases flow velocity, and maintains thermal boundary layer disturbance.

[0053] Specifically, the process of adjusting the magnetic field strength, direction, and spatial distribution pattern described above is as follows: (1) Magnetic field strength parameters: the strength of the external magnetic source (such as the magnitude of the current) (2) Magnetic field direction: change the position or orientation of the magnetic source or use different magnetic field sources. (3) Spatial distribution pattern: Gradient magnetic field is adopted, that is, the magnetic field strength gradually changes in space. This can make magnetic particles have different aggregation behaviors in different regions, forming an orderly heat conduction path. It is also possible to create "heat channels" or enhance the heat conduction network by adjusting the local concentrated area of ​​the magnetic field.

[0054] See Figure 4The second aspect of this invention discloses a thermomagnetically coupled magnetic molten salt nanofluid and supercritical carbon dioxide conjugate heat transfer enhancement device. This device is based on an existing heat exchanger, which is divided into a magnetic molten salt nanofluid side and a supercritical carbon dioxide side. The magnetic molten salt nanofluid comprises molten salt and magnetic nanoparticles, wherein the magnetic nanoparticles are ferrite nanoparticles (Fe3O4) with a mass fraction of 1-10 wt.% and a particle size of 50 nm. Unlike existing heat exchanger devices, this invention designs a magnetic field excitation device on the magnetic molten salt nanofluid side that matches the heat exchanger structure. The direction of the magnetic field lines is as close as possible to or parallel to the flow direction of the magnetic molten salt nanofluid, allowing the magnetic nanoparticles to move under the influence of magnetic force, thereby moving the internal molten salt. For example, the magnetic field excitation device can be an adjustable coil electromagnetic field, or any device capable of applying magnetic force in the target direction. This allows for the formation of a controllable local magnetic field distribution during heat exchanger operation, providing an external driving means for subsequent control of particle behavior. The magnetic field applied by the magnetic field excitation device is in the same direction as the flow direction of the magnetic molten salt nanofluid. The specific arrangement of magnets and other components can be adjusted according to the actual situation. By adjusting the arrangement of magnets, the form of the chain-like or network-like structure formed by the magnetic nanoparticles in the internal fluid can be adjusted.

[0055] This invention introduces paramagnetic or diamagnetic metal-based or metal oxide nanoparticles into traditional molten salts to prepare magnetic molten salt nanofluids with magnetic responsiveness. By rationally controlling the particle size and concentration, its thermal stability and dispersibility under high-temperature environments are ensured. To improve its thermal conductivity, a magnetic control mechanism is innovatively introduced into the heat exchange system. A stable static magnetic field is applied by a control system, forming a magnetic field gradient within the flow channel. This magnetic field induces the magnetic nanoparticles to aggregate directionally in the wall region through the Lorentz force. The magnetic dipole interaction between particles promotes their aggregation into a chain-like structure along the magnetic field lines, thereby significantly improving their microstructure and heat conduction pathways. This structure helps reduce wall thermal resistance and improve overall thermal conductivity, thus enhancing the system's stability and heat transfer efficiency. Existing research has shown that the chain-like aggregation structure of nanoparticles significantly improves the thermal conductivity of nanofluids and has a significant effect on improving overall heat transfer efficiency. Therefore, this invention discloses a method for enhancing heat transfer by combining magnetic molten salt nanofluid with sCO2 in a thermomagnetic coupled field, which realizes active intervention in local thermal properties and enhancement of the heat transfer process. It is particularly suitable for high-temperature thermal energy conversion and utilization in fields such as solar thermal power generation, nuclear power generation and industrial waste heat recovery.

[0056] Furthermore, by rationally selecting particle size and concentration, the thermal stability and dispersibility in a high-temperature molten salt environment are ensured. Simultaneously, the goal of this step is to establish a working fluid system with magnetic response capabilities and the basic conditions for external field control, making it possible to actively intervene in local thermophysical properties.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for enhancing heat transfer of magnetic molten salt nanofluids and sCO2 through thermomagnetic coupling field synergy, characterized in that, Includes the following steps: S1, obtain the physical property parameters of magnetic molten salt nanofluid and supercritical carbon dioxide in the heat exchanger, wherein the magnetic molten salt nanofluid is a fluid in which magnetic nanoparticles are added to molten salt, and a magnetic field is applied to the magnetic molten salt nanofluid side of the heat exchanger. S2, numerical simulations are performed on the dynamic model, heat transfer model, and turbulent flow model of the two types of fluids to obtain the velocity field, temperature gradient field, and pressure gradient field of the fluids; the two types of fluids include magnetic molten salt nanofluids and supercritical carbon dioxide; wherein, an external magnetic field is applied in the dynamic model, heat transfer model, and turbulent flow model of the magnetic molten salt nanofluids. S3, the velocity field, temperature gradient field and pressure gradient field of the magnetic molten salt nanofluid are coupled; for the magnetic molten salt nanofluid, the first angle between the velocity vector and the normal vector of the temperature gradient field, the second angle between the velocity vector and the normal vector of the pressure gradient field, and the third angle between the velocity vector and the direction of the applied magnetic field are obtained, and the optimal values ​​of the three angles when the heat exchange efficiency is the highest are determined by numerical simulation. S4. The key influencing factors of the three included angles are identified by response surface methodology, and the values ​​of each key influencing factor when the three included angles are optimal are obtained by neural network. S4 executes the heat exchanger by taking the values ​​of various influencing factors.

2. The method for enhancing heat transfer of magnetic molten salt nanofluid and sCO2 through thermomagnetic coupling field synergy according to claim 1, characterized in that, In S1, the physical properties of the magnetic molten salt nanofluid are obtained through microscale simulation methods.

3. The method for enhancing heat transfer of magnetic molten salt nanofluid and sCO2 through thermomagnetic coupling field synergy according to claim 1, characterized in that, In S1, the magnetic nanoparticles include metal-based nanoparticles and metal oxide-based nanoparticles. The metal-based nanoparticles include iron, cobalt, and nickel, and the metal oxide-based nanoparticles are ferrite nanoparticles.

4. The method for enhancing heat transfer of magnetic molten salt nanofluid and sCO2 through thermomagnetic coupling field synergy according to claim 1, characterized in that, In S2, magnetic volume force, particle Brownian diffusion, thermophoretic migration, and magnetic drift terms are introduced into the fluid dynamics model, heat transfer model, and turbulent flow model of magnetic molten salt nanofluids.

5. The method for enhancing heat transfer of magnetic molten salt nanofluid and sCO2 through thermomagnetic coupling field synergy according to claim 1, characterized in that, In S3, when determining the optimal values ​​of the three included angles, the optimization objective is to maximize the Nusselt number and minimize the flow pressure drop or magnetic reluctance loss.

6. The method for enhancing heat transfer of magnetic molten salt nanofluid and sCO2 through thermomagnetic coupling field synergy according to claim 1, characterized in that, In S3, when determining the optimal values ​​of the three included angles, the optimization objective is a coordination index, which is: in, Indicates volume average. f ( Ha ) is the Hartmann number influence function. w i These are weighting coefficients. α The angle between the velocity vector and the normal vector of the temperature gradient field. β The angle between the velocity vector and the normal vector of the pressure gradient field. γ Let be the angle between the velocity vector and the direction of the applied magnetic field.

7. The method for enhancing heat transfer of magnetic molten salt nanofluid and sCO2 through thermomagnetic coupling field synergy according to claim 1, characterized in that, In S4, the influencing factors include the flow rate, temperature gradient, and pressure gradient of the magnetic molten salt nanofluid and the supercritical carbon dioxide fluid, as well as the magnetic field strength of the magnetic molten salt nanofluid.

8. The method for enhancing heat transfer of magnetic molten salt nanofluid and sCO2 through thermomagnetic coupling field synergy according to claim 1, characterized in that, In S4, after determining the key influencing factors of the three included angles through surface response analysis, the values ​​of the key influencing factors and the response values ​​are input into the neural network to train the neural network.

9. A device for enhancing heat transfer of magnetic molten salt nanofluid and sCO2 conjugate using the thermomagnetic coupling field synergy of the enhancement method described in claim 1, characterized in that, The device includes a heat exchanger, which is divided into a magnetic molten salt nanofluid side and a supercritical carbon dioxide side. A magnetic field is applied to the magnetic molten salt nanofluid side. The magnetic molten salt nanofluid includes molten salt and magnetic nanoparticles. The magnetic nanoparticles are magnetic metal-based nanoparticles or metal oxide-based nanoparticles.

10. The thermomagnetically coupled magnetic molten salt nanofluid and sCO2 conjugate heat transfer enhancement device according to claim 9, characterized in that, The applied magnetic field causes the magnetic nanoparticles to aggregate directionally in the heat exchanger and move in a chain-like or network-like structure.

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