Design method of multi-section thermionic energy converter based on multi-physical field coupling

Abstract

The application discloses a multi-physical field coupling-based multi-section thermionic energy converter design method, which comprises the following steps: (1) starting a multi-physical field coupling software, selecting a two-dimensional axisymmetric component about a multi-section thermionic energy converter geometric structure in the software, and adding a solid heat transfer module, a solid mechanics module, a surface-to-surface radiation module and a current module of the software into the component; (2) establishing a geometric model according to the geometric structure of the multi-section thermionic energy converter, and adding corresponding physical property parameters according to different materials in the geometric structure; (3) setting initial conditions and boundary conditions for the several modules respectively; (4) dividing a grid for the geometric model; (5) selecting a steady-state solver in the software to perform calculation, and performing optimization calculation on the reversing connector geometric structure in the multi-section thermionic fuel element, the length and position of the emitter and the receiver and the inter-electrode gap width through an optimization module in the software, so that the optimal thermoelectric conversion efficiency is obtained.

Description

Technical Field

[0001] This invention relates to the field of thermionic fuel element technology, and in particular to a design method for a multi-section thermionic energy converter based on multi-physics field coupling. Background Technology

[0002] The basic principle of thermionic energy converters, also known as thermionic conversion technology, is based on the Fermi-Dirac statistical theory. When the temperature of a metal increases, the average energy of electrons in the metal increases. Electrons with energy exceeding the metal's work function have the opportunity to escape from the metal and enter a vacuum. This technology was proposed in the 1950s and was primarily applied in the aerospace field for the next two decades. Later, the United States was the first to propose using thermionic energy converters in thermal power plant generator sets, achieving a power generation efficiency of up to 50%. Theoretically, the maximum efficiency of thermoelectric conversion using thermionic conversion technology is higher than that of thermoelectric power generation technology. However, due to the stringent environmental requirements of thermionic conversion technology, it is generally used in space nuclear reactor power supplies. In 1961, the Soviet Union developed four space reactor power systems: ROMASHIKA, BUK, TOPAZ-1, and TOPAZ-2. TOPAZ-1 and TOPAZ-2 both use thermal reactors as heat sources, employing thermionic energy converters for thermoelectric conversion. TOPAZ-1 uses a multi-stage thermionic energy converter, while TOPAZ-2 uses a single-stage thermionic energy converter.

[0003] Compared to single-stage thermionic converters, multi-stage thermionic converters employ a series circuit structure along the length of each fuel rod. Therefore, each thermionic converter segment is shorter than that of a single-stage converter. With a constant output voltage, the voltage of each segment in a multi-stage thermionic converter is approximately the total voltage divided by the number of segments. This reduces the current density in the circuit, thereby reducing Joule heat loss and improving thermoelectric conversion efficiency. However, the series circuit structure of multi-stage thermionic converters is more complex than that of single-stage converters, requiring careful design of structural parameters to extend their lifespan. The design of multi-stage thermionic fuel elements primarily involves experimentally measuring the thermoelectric conversion efficiency of the thermionic converter under different operating conditions and structures, and then making corresponding structural improvements. However, because the operation and testing of the thermionic converter occur in a high-temperature environment, and the interelectrode space needs to be maintained under vacuum or filled with cesium vapor, the work function and high-temperature resistance requirements of the materials are high, resulting in significant consumption of human, material, and financial resources. Summary of the Invention

[0004] To address the aforementioned problems, the present invention aims to provide a design method for a multi-segment thermo-ion energy converter based on multi-physics coupling, enabling numerical simulation of the temperature field, stress field, and potential field of the multi-segment thermo-ion energy converter, as well as structural and material design of the multi-segment thermo-ion energy converter, thereby achieving optimized thermoelectric conversion efficiency and the safest structure.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A design method for a multi-section thermionic energy converter based on multi-physics field coupling includes the following steps:

[0007] (1) Start the multiphysics coupling software, select the two-dimensional axisymmetric component about the geometry of the multi-section thermionic energy converter in the multiphysics coupling software, and add the solid heat transfer module, solid mechanics module, surface-to-surface radiation module and current module of the multiphysics coupling software to the component.

[0008] (2) Establish a geometric model of the multi-section thermionic energy converter based on the geometric structure of the multi-section thermionic energy converter, and add corresponding physical property parameters according to the different materials in the geometric structure;

[0009] (3) Set initial and boundary conditions for the solid heat transfer module, solid mechanics module, surface-to-surface radiation module and current module respectively;

[0010] (4) Mesh the geometric model of the multi-section thermionic energy converter;

[0011] (5) Select the steady-state solver in the multiphysics coupling software to perform calculations. The optimization module in the multiphysics coupling software performs optimization calculations on the commutation connection geometry, the length and position of the emitter and receiver, and the width of the inter-electrode gap of the multi-section thermo-ion energy converter, so as to obtain the best thermoelectric conversion efficiency.

[0012] Based on the above technical solution, the further improvement of the present invention is that: in step (1), the solid heat transfer module and the solid mechanical module achieve bidirectional coupling through thermal expansion and gap heat transfer process, and the solid heat transfer module and the current module achieve bidirectional coupling through electromagnetic heat and the thermal ion emission process of the emitter surface.

[0013] Based on the above technical solution, the further improvement of the present invention is as follows: In step (2), the established geometric model of the multi-section thermionic energy converter includes an emitter, a receiver, an emitter multilayer tube, a receiver multilayer tube, and a commutation connector. According to the specific heat transfer, mechanical and electromagnetic properties of different materials in the geometric structure, manuals and literature data are consulted to set the material property parameters.

[0014] Based on the above technical solution, the further improvement of the present invention is that: in step (3), the initial conditions include the initial temperature distribution, the initial displacement distribution and the initial potential distribution; the gap heat transfer model is used in the setting of the boundary conditions in the solid heat transfer module; and the local volt-ampere characteristics caused by thermionic emission and the electron cooling phenomenon are considered in the setting of the boundary conditions in the current module.

[0015] Furthermore, the inter-section heat transfer of the multi-section thermionic energy converter considers the influence of different gas compositions and inter-section width on the heat transfer coefficient; the inter-section heat transfer uses the gas inter-section heat transfer model proposed by Ross and Stoute to calculate the temperature drop h in the inter-section. gas :

[0016]

[0017] In the formula:

[0018] λ gas —The thermal conductivity of the gas;

[0019] d eff — Before the gap closes, it is (R1+R2), and after closing, it is exp(-0.00125P)(R1+R2), where R1 and R2 are the roughness of the two walls, respectively.

[0020] d — gap width;

[0021] (g1+g2) — the temperature jump distance of the wall surface;

[0022] The formula for calculating the wall temperature jump distance takes into account the influence of the mixed gas composition, and the formula is as follows:

[0023]

[0024] In the formula:

[0025] f i —The molar fraction of the i-th gas;

[0026] M i —The molar mass of the i-th gas;

[0027] a i —The matching coefficient of the i-th gas;

[0028] T gas —Temperature of the gas mixture;

[0029] P gas —The pressure of the mixed gas.

[0030] The local voltage-current characteristics caused by thermionic emission were established using data measured in an experiment on the voltage-current characteristics of a single-section thermon energy converter. For the emission process of an electron at a specific point on the emitter, due to electron cooling, the total energy density q carried away by the electron e from the emitter is... e for

[0031]

[0032] In the formula:

[0033] j—current density;

[0034] φ C —Emitting polar work function;

[0035] —Emitter potential;

[0036] —Receiving electrode potential;

[0037] k B — Boltzmann constant;

[0038] T E —Emitter temperature;

[0039] e — electron charge;

[0040] The energy density q brought in by electrons to the receiving electrode c for

[0041]

[0042] Since the circuit connections between the sections of the multi-section thermionic energy converter are in series, in order to determine the potential distribution of each emitter and receiver, a test potential is artificially introduced at the commutation connector. The difference in emission current of different emitters is calculated as the potential feedback coefficient to correct the test potential. The true potential of the commutation connector is obtained through iterative calculation, so that the potential can satisfy the current conservation on different plates.

[0043] Based on the above technical solution, the further improvement of the present invention is that: in step (4), since the geometric structure of the multi-section thermionic energy converter is complex, the mesh division of the geometric model of the multi-section thermionic energy converter adopts the method of first dividing the geometric model and then dividing each part into rectangular meshes, so as to speed up the calculation.

[0044] Based on the above technical solution, the further improvement of the present invention is as follows: In step (5), by setting thermoelectric conversion efficiency as the optimization target and setting the commutation connector geometry, the length and position of the emitter and receiver, and the inter-electrode gap width as constraints in the optimization module of the multiphysics coupling software, the parameters affecting thermoelectric conversion efficiency, namely the commutation connector geometry, the length and position of the emitter and receiver, and the inter-electrode gap width, are optimized.

[0045] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0046] (1) By introducing the solid heat transfer module, solid mechanics module, current module, surface-to-surface radiation module and current module in the multiphysics coupling software, as well as the gap heat transfer boundary conditions and potential feedback coefficient, the bidirectional coupling simulation between different physical fields was realized, and the problem of solving the potential distribution of each section of the multi-section thermion converter was solved.

[0047] (2) A geometric model of a multi-section thermionic energy converter is established in a multi-physics coupling software to simulate and calculate the overall performance of the multi-section thermionic energy converter. Furthermore, through optimization calculation, technical support can be provided for the optimal design scheme of the multi-section thermionic energy converter, which greatly saves R&D costs and time.

[0048] The method of this invention can analyze the thermoelectric conversion characteristics of multi-section thermionic energy converters used in reactors, explore the distribution laws of temperature field, displacement field and electric potential field of multi-section thermionic energy converters, and provide technical support for the optimal design scheme of multi-section thermionic energy converters. Attached Figure Description

[0049] Figure 1 This is a schematic diagram of a multi-section thermionic energy converter.

[0050] Figure 2 This is a schematic diagram of the mesh division of each layer of one section of a multi-section thermionic energy converter;

[0051] Figure 3 The current-voltage characteristics of a multi-section thermionic energy converter under different thermal power conditions;

[0052] Figure 4 It represents the thermoelectric conversion efficiency characteristics of a multi-section thermionic energy converter under different thermal power conditions. Detailed Implementation

[0053] To more clearly illustrate the purpose, technical solution, and advantages of the present invention, the following embodiments are provided, and the implementation of the present invention is specifically described in conjunction with the accompanying drawings.

[0054] The design method for multi-segment thermionic energy converters based on multi-physics field coupling involves the following multi-segment thermionic energy converter structures: Figure 1 As shown, heat is conducted layer by layer from the inside of the emitter multilayer tube outwards, converting thermal energy into electrical energy at the emitter surface. The multi-section thermionic energy converter's thermoelectric conversion structure, from the inside out, includes the emitter multilayer tube, each emitter section (…). Figure 1 The middle section consists of three parts, commutation connectors, inter-electrode gaps between the transmitter and receiver, and receiver sections ( Figure 1 The converter consists of three sections (emitter, receiver, and multilayer tube). Wires leading from the first emitter section and the last receiver section are connected to the load. The design method for a multi-section thermionic energy converter based on multiphysics coupling includes the following steps:

[0055] (1) Start the software and model initialization: Start the COMSOL Multiphysics coupling software. In the COMSOL Multiphysics coupling software, select the two-dimensional axisymmetric component about the geometry of the multi-section thermionic energy converter. Add the solid heat transfer module, solid mechanics module, surface-to-surface radiation module and current module of COMSOL Multiphysics coupling software to this component. Add thermal expansion, temperature coupling, surface-to-surface radiation heat transfer and electromagnetic heat nodes in the multiphysics nodes to realize the data transfer from the temperature field to the displacement field and from the electric potential field to the temperature field.

[0056] (2) Establish geometric and input material parameters: Based on the specific geometric structure parameters of the multi-section thermionic energy converter, establish the geometric model of the multi-section thermionic energy converter, and add corresponding physical property parameters according to the different materials in the geometric structure.

[0057] (3) Setting Boundary and Initial Conditions: Initial and boundary conditions were set for the solid heat transfer module, solid mechanics module, surface-to-surface radiation module, and current module, respectively. Initial conditions included initial temperature distribution, initial displacement distribution, and initial potential distribution. A gap heat transfer model was used in the boundary conditions of the solid heat transfer module, while the local voltage-current characteristics caused by thermionic emission and electron cooling were considered in the boundary conditions of the current module. The gap heat transfer of the multi-section thermionic energy converter considered the influence of different gas compositions and gap width on the heat transfer coefficient. The gap heat transfer used the gas gap heat transfer model proposed by Ross and Stoute to calculate the temperature drop h in the gap. gas :

[0058]

[0059] In the formula:

[0060] λ gas —The thermal conductivity of the gas;

[0061] d eff — Before the gap closes, it is (R1+R2), and after closing, it is exp(-0.00125P)(R1+R2), where R1 and R2 are the roughness of the two walls, respectively.

[0062] d — gap width;

[0063] (g1+g2) — the temperature jump distance of the wall surface;

[0064] The formula for calculating the wall temperature jump distance takes into account the influence of the mixed gas composition, and the formula is as follows:

[0065]

[0066] In the formula:

[0067] f i —The molar fraction of the i-th gas;

[0068] M i —The molar mass of the i-th gas;

[0069] a i —The matching coefficient of the i-th gas;

[0070] T gas —Temperature of the gas mixture;

[0071] P gas —The pressure of the mixed gas.

[0072] The local voltage-current characteristics caused by thermionic emission were established using data measured in an experiment on the voltage-current characteristics of a single-section thermon energy converter. For the emission process of an electron at a specific point on the emitter, due to electron cooling, the total energy density q carried away by the electron e from the emitter is... e for

[0073]

[0074] In the formula:

[0075] j—current density;

[0076] φ C —Emitting polar work function;

[0077] —Emitter potential;

[0078] —Receiving electrode potential;

[0079] k B — Boltzmann constant;

[0080] T E —Emitter temperature;

[0081] e — electron charge.

[0082] The energy density q brought in by electrons to the receiving electrode c for

[0083]

[0084] Since the circuit connections between the sections of the multi-section thermionic energy converter are in series, in order to determine the potential distribution of each emitter and receiver, a test potential is artificially introduced at the commutation connector. The difference in emission current of different emitters is calculated as the potential feedback coefficient to correct the test potential. The true potential of the commutation connector is obtained through iterative calculation, so that the potential can satisfy the current conservation on different plates.

[0085] (4) Mesh generation: The geometric model of the multi-section thermionic energy converter is meshed, such as... Figure 2 As shown, due to the complex geometry of the multi-section thermionic energy converter, the geometric model of the multi-section thermionic energy converter is first divided into regular shapes, and then each part is divided into rectangular meshes to speed up the calculation.

[0086] (5) Solver settings: Select the steady-state solver in COMSOL Multiphysics software for calculation. The uniformity of the temperature distribution of the plates has a significant impact on the thermoelectric conversion efficiency of the multi-section thermionic energy converter. Table 1 shows the temperature distribution of the emitter and receiver under different geometric structures calculated under different conditions of the relative z-axis positions of the heat source center and the emitter center and the heat source length, thus obtaining the geometric structure with the most uniform temperature distribution. The analysis process for other geometric structures in the multi-section thermionic energy converter is similar.

[0087] Table 1 Temperature distribution of emitter and receiver at different heat source center locations and lengths

[0088]

[0089] (6) Optimization Design: The thermoelectric conversion efficiency was set as the optimization target and the commutator geometry, emitter and receiver lengths and positions, and inter-electrode gap width were set as constraints in the optimization module of COMSOL Multiphysics software. This allowed for the optimization design of the parameters affecting thermoelectric conversion efficiency, namely the commutator geometry, emitter and receiver lengths and positions, and inter-electrode gap width. Based on the optimized geometric model, the volt-ampere characteristics of the multi-section thermionic energy converter under different thermal power conditions (e.g., ...) can be calculated. Figure 3 (as shown) and the thermoelectric conversion efficiency characteristics of a multi-section thermionic energy converter under different thermal power conditions ( Figure 4 As shown in the figure, this allows for the evaluation of the performance of multi-section thermionic energy converters and provides guidance for further improvements to them.

[0090] The above description is merely illustrative of the principles and effects of the present invention and is not intended to limit the invention. Those skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A design method for a multi-section thermionic energy converter based on multi-physics field coupling, characterized in that, Includes the following steps: (1) Start the multiphysics coupling software, select the two-dimensional axisymmetric component about the geometry of the multi-section thermionic energy converter in the multiphysics coupling software, and add the solid heat transfer module, solid mechanics module, surface-to-surface radiation module and current module of the multiphysics coupling software to the component; (2) Establish a geometric model of the multi-section thermionic energy converter based on its geometric structure, and add corresponding physical property parameters according to the different materials in the geometric structure; (3) Set initial and boundary conditions for the solid heat transfer module, solid mechanics module, surface-to-surface radiation module and current module respectively; (4) Mesh the geometric model of the multi-section thermionic energy converter; (5) Select the steady-state solver in the multiphysics coupling software to perform calculations. Through the optimization module in the multiphysics coupling software, perform optimization calculations on the commutation connection geometry, the length and position of the emitter and receiver, and the inter-electrode gap width of the multi-section thermo-ion energy converter, so as to obtain the best thermoelectric conversion efficiency. In step (2), the established geometric model of the multi-section thermionic energy converter includes an emitter, a receiver, an emitter multilayer tube, a receiver multilayer tube, and a switching connector. Based on the specific heat transfer, mechanical, and electromagnetic properties of different materials in the geometric structure, manuals and literature data are consulted to set the material property parameters. In step (3), the initial conditions include the initial temperature distribution, the initial displacement distribution and the initial potential distribution. The gap heat transfer model is used in the setting of the boundary conditions in the solid heat transfer module. The local volt-ampere characteristics caused by thermionic emission and the electron cooling phenomenon are considered in the setting of the boundary conditions in the current module. The gap heat transfer in the multi-section thermionic energy converter considers the influence of different gas compositions and gap width on the heat transfer coefficient; the gap heat transfer uses the gas gap heat transfer model proposed by Ross and Stoute to calculate the temperature drop in the gap. h gas : （1） In the formula: λ gas —The thermal conductivity of the gas; d eff —Before the gap closes ( R 1+ R 2), after closure, it becomes exp(-0.00125P) ( R 1+ R 2), R 1 and R 2 represents the roughness of the two walls, respectively; d —Gap width; ( g 1+ g 2) — Wall temperature jump distance; The formula for calculating the wall temperature jump distance takes into account the influence of the mixed gas composition, and the formula is as follows: （2） In the formula: f i —The molar fraction of the i-th gas; M i —The molar mass of the i-th gas; a i —The matching coefficient of the i-th gas; T gas —Temperature of the gas mixture; P gas —The pressure of the mixed gas.

2. The design method for a multi-section thermionic energy converter based on multi-physics field coupling according to claim 1, characterized in that, In step (1), the solid heat transfer module and the solid mechanics module achieve bidirectional coupling through thermal expansion and gap heat transfer processes, and the solid heat transfer module and the current module achieve bidirectional coupling through electromagnetic heat and the thermal ion emission process on the emitter surface.

3. The design method for a multi-section thermionic energy converter based on multi-physics field coupling according to claim 1, characterized in that, The local current-voltage characteristics caused by thermionic emission were set using data measured by experiments on the current-voltage characteristics of a single-section thermon energy converter.

4. The design method for a multi-section thermionic energy converter based on multi-physics field coupling according to claim 3, characterized in that, For the emission process of an electron at a certain point on the emitter, due to the electron cooling phenomenon, the electron... e Total energy density carried away from the emitter q e for （3） In the formula: —Current density; C —Emitting polar work function; φ E —Emitter potential; φ C —Receiving electrode potential; k B — Boltzmann constant; T E —Emitter temperature; e —Electron charge; Energy density brought into the receiving electrode by electrons q c for （4）。 5. The design method for a multi-section thermionic energy converter based on multi-physics field coupling according to claim 1, characterized in that, Since the circuit connections between the sections of the multi-section thermionic energy converter are in series, in order to determine the potential distribution of each emitter and receiver, a test potential is artificially introduced at the commutation connector. The difference in emission current of different emitters is calculated as the potential feedback coefficient to correct the test potential. The true potential of the commutation connector is obtained through iterative calculation, so that the potential can satisfy the current conservation on different plates.

6. The design method for a multi-section thermionic energy converter based on multi-physics field coupling according to claim 1, characterized in that, In step (4), since the geometric structure of the multi-section thermionic energy converter is complex, the meshing of the geometric model of the multi-section thermionic energy converter is carried out by first dividing the geometric model and then dividing each part into rectangular meshes in order to speed up the calculation.

7. The design method for a multi-section thermionic energy converter based on multi-physics field coupling according to claim 1, characterized in that, In step (5), by setting thermoelectric conversion efficiency as the optimization target and setting the commutation connector geometry, the length and position of the emitter and receiver, and the inter-electrode gap width as constraints in the optimization module of the multiphysics coupling software, the parameters affecting thermoelectric conversion efficiency, namely the commutation connector geometry, the length and position of the emitter and receiver, and the inter-electrode gap width, are optimized.

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