Gradient-refractive-index efficient microwave heater and analogue simulation method

Abstract

The invention discloses a gradient refractive index efficient microwave heater and an analogue simulation method, and relates to the technical field of microwave heating devices.The heater comprises an asymmetric transmission circular waveguide, a transition part, a cylindrical medium and a heating load which are sequentially communicated, and the outer diameter of the cylindrical medium is smaller than that of the asymmetric transmission circular waveguide; the transition part is a connecting piece for connecting the asymmetric transmission circular waveguide and the cylindrical medium, the tail end part of the cylindrical medium extends into the heating load, and a metasurface structure with a gradient refractive index is arranged in the asymmetric transmission circular waveguide. One-way transmission of electromagnetic waves is achieved through the metasurface structure with the gradient refractive index, and then electromagnetic energy is concentrated into a medium used for plug-in heating. According to the heater, the cylindrical medium is inserted into the load to efficiently heat materials with different shapes, the dielectric constant range of the materials changes from 8 to 80, and a simulation result shows that the heating efficiency is always higher than 90% when the dielectric constant of the load is in the dynamic range of 8-80.

Description

Technical Field

[0001] The present invention relates to the technical field of microwave heating devices, and more particularly to the technical field of a gradient refractive index efficient microwave heater and a simulation method thereof. Background Art

[0002] Since the 1940s, microwave energy has been applied to heating. Under microwave irradiation, polar molecules in the material oscillate under the action of a high-frequency electric field, thereby converting microwave energy into heat. Compared with the traditional conduction heating method, microwave heating has the advantages of fast heating speed, high selectivity, and environmental friendliness. Therefore, it is widely used in various fields such as food processing, chemical reactions, medical treatment, and material synthesis. Traditional microwave heating requires placing the load in the cavity. The load is restricted by the shape and size of the cavity, and the microwave application scenario is single. Moreover, during microwave heating, different dielectric constants, volumes, and shapes of the load will cause unstable microwave heating efficiency. In addition, microwave heating is prone to uneven heating, hot spots, and thermal runaway, which will seriously affect the product quality and even cause combustion or explosion. These defects have greatly restricted the development of the industrial application of microwave energy.

[0003] To improve the heating efficiency, a method widely used in industrial applications is the three stub tuner. The three stub tuner needs to be dynamically adjusted as the dielectric constant of the load changes, and impedance mismatch is inevitable during the adjustment process. Mode stirrers and turntables are still important components for improving microwave heating performance and have been widely studied. Some researchers proposed placing metal sheets on a rotating turntable as a mode stirrer. Some researchers proposed studying the combined effect of mode stirrers and turntables by using implicit functions and level set methods. Another method to improve microwave heating performance is to change the microwave frequency during heating. Some researchers proposed three frequency transfer heating algorithms based on solid-state microwave sources. Some researchers proposed studying the influence of fixed-frequency heating and swept-frequency heating on the heating uniformity of samples. Some researchers proposed a direct heating model for multi-layer low-loss dielectric cylinders and studied the maximum electric field conditions for improving heating efficiency. Some researchers suggested inserting an impedance matching structure between the air and the heated sample to achieve efficient microwave heating. Some researchers proposed introducing a rectangular waveguide, where the straight tube is surrounded by metal rings. High heating efficiency is also maintained for loads with a wide dielectric constant range.

[0004] The above research is either not applicable to heating loads with a large range of dynamic dielectric constant changes or has strict restrictions on the shape and size of the load. Therefore, there is an urgent need to develop a microwave heating system to effectively and uniformly heat different shapes and sizes of loads with large dynamic dielectric constant changes. Summary of the Invention

[0005] The object of the present invention is to provide a gradient refractive index high-efficiency microwave heater and a simulation method to solve the above technical problems.

[0006] The present invention specifically adopts the following technical solutions to achieve the above object:

[0007] One aspect of the present invention provides a gradient refractive index high-efficiency microwave heater, which includes an asymmetric transmission circular waveguide, a transition part, a cylindrical dielectric, and a heating load connected in sequence. The outer diameter of the cylindrical dielectric is smaller than that of the asymmetric transmission circular waveguide. The transition part is a connecting member connecting the asymmetric transmission circular waveguide and the cylindrical dielectric. The end of the cylindrical dielectric extends into the heating load. An ultrathin surface structure with a gradient refractive index is arranged inside the asymmetric transmission circular waveguide to realize the unidirectional transmission of electromagnetic waves.

[0008] Specifically, a part of the cylindrical dielectric extends into the heating load. The inside of the asymmetric transmission circular waveguide is designed with an ultrathin surface structure with a gradient refractive index, so as to realize the asymmetric transmission of electromagnetic waves, reduce the reflection of electromagnetic waves, improve the energy utilization efficiency, realize the unidirectional transmission of electromagnetic waves, and then concentrate the electromagnetic energy into the dielectric for insert heating; in order to reduce the size of the waveguide and concentrate the microwave energy, a transition structure is designed to guide the electromagnetic waves into the cylindrical dielectric for continuous propagation. A part of the cylindrical dielectric is inserted into the heating load body for uniform and efficient heating, and different-shaped materials with a dielectric constant ranging from 8 to 80 can be efficiently heated; in the simulation model, only the heated material shows dielectric loss, which clearly illustrates how the microwave energy is absorbed by the material. The simulation results show that the heating efficiency is always higher than 90% in the dynamic range of the load dielectric constant from 8 to 80.

[0009] In one embodiment, the asymmetric transmission circular waveguide includes a cylindrical outer shell and an ultrathin surface structure sleeved inside the outer shell. The wall thickness of the ultrathin surface structure gradually changes along the axial direction. The thinnest end of the wall thickness of the ultrathin surface structure is the feeding end, and the thickest end of the wall thickness of the ultrathin surface structure is the scattering boundary. The transition part is located at the scattering boundary of the ultrathin surface structure. The microwave energy is transmitted unidirectionally along the direction from the feeding end to the scattering boundary. The ultrathin surface structure is obtained by rotating a wedge-shaped dielectric around its center.

[0010] In one embodiment, the transition part includes a transition outer shell, a hollow conical dielectric lining, and a solid cylindrical dielectric lining. The hollow conical dielectric lining and the solid cylindrical dielectric lining are connected into one body and are located inside the transition outer shell. One end of the hollow conical dielectric lining is connected to the scattering boundary of the ultrathin surface structure. The square size of the hollow conical dielectric lining gradually decreases along the waveguide transmission direction, and the hollow part of the hollow conical dielectric lining extends into the inside of the solid cylindrical dielectric lining.

[0011] In one embodiment, the electromagnetic field transmission mode in the asymmetric transmission circular waveguide is TM 01 mode.

[0012] Specifically, the electromagnetic field transmission mode in the asymmetric transmission circular waveguide is TM 01 mode, and the axisymmetric characteristic of the electromagnetic field distribution of this mode can improve the uniformity of microwave heating. In one embodiment, the axial length of the asymmetric transmission circular waveguide is greater than the axial length of the metasurface structure.

[0013] In one embodiment, the asymmetric transmission circular waveguide is a C22 circular waveguide.

[0014] The second aspect of the present invention provides a simulation method for a gradient refractive index efficient microwave heater, which is used to simulate a gradient refractive index efficient microwave heater as described above, and includes the following steps:

[0015] S1. Use COMSOL multi-physics simulation software to simulate the microwave heating process of the microwave heater:

[0016] S11. The simulation of the microwave heater involves the coupling of the electromagnetic field and the solid heat transfer field:

[0017] The distribution of the electromagnetic field is calculated using the following Helmholtz equation:

[0018]

[0019] Among them, is the electric field strength, k0 represents the free space wave number, ω represents the angular frequency, σ is the conductivity, μ r is the relative magnetic permeability, ε r represents the relative permittivity, ε0 is the relative permittivity of vacuum, μ0 represents the vacuum permeability, j represents the imaginary part of a complex number, represents the differential operator;

[0020] The temperature field distribution is calculated using the following thermodynamic equation:

[0021]

[0022] Among them, ρ, C p , T, Q and K t respectively represent the density, constant pressure heat capacity, thermodynamic temperature, heat source and thermal conductivity of the material; t represents the heating time;

[0023] The electromagnetic field and the heat transfer field are coupled through the electromagnetic loss equation:

[0024]

[0025] Among them, ε” represents the imaginary part of the relative permittivity, Qe is the electromagnetic energy loss, which is used as the heat source causing temperature change during the heating process;

[0026] S12. Setting of boundary conditions: During the simulation, the feeding end of the asymmetric transmission circular waveguide is set as the excitation port with TM 01 mode, and the outer walls of the asymmetric transmission circular waveguide and the heating load are set as perfect electric conductors;

[0027]

[0028] Among them, is the unit normal vector of the interface;

[0029] Conduct heat transfer and reaction analysis on the reactants, set the thermal boundary as adiabatic, and satisfy the following equation:

[0030]

[0031] Among them, represents the heat flux, the initial temperature of heating is set to 293.15 K. Except for the part in contact with the inserted medium, the outer wall of the heating load is an adiabatic boundary, and there is no heat exchange between the load and the external air during the heating process;

[0032] TM 01 mode is rotationally symmetric, and its electromagnetic field distribution is symmetric about the axis of the asymmetric transmission circular waveguide. This symmetric characteristic shows high uniformity when applied to microwave heating, and the TM 01 mode has low loss, is easy to be excited and converted, and improves the flexibility and efficiency of the heater;

[0033] S2. The wedge-shaped dielectric of the metasurface structure realizes a gradient refractive index metasurface:

[0034] The metasurface presents a gradient refractive index in the microwave propagation direction, which determines the unidirectional propagation of microwaves in the waveguide. Assuming that the electromagnetic wave propagates along the positive x-axis, the relationship between the relative permittivity of the metasurface structure and the x-axis coordinate can be described as follows:

[0035] ε'(x) = n(x) 2 = [α(x - x0) + β] 2 , (7)

[0036] Among them, x0 is the starting point on the x-axis of the metasurface structure on the left side, n(x) refers to the refractive index at the x position on the metasurface structure, ε'(x) represents the relative permittivity of the metasurface structure, α represents the refractive index gradient factor, and β is the adjustment factor;

[0037] The equivalent relative permittivity of the air and metasurface structure mixing region can be expressed as follows:

[0038]

[0039] Among them, ε r0 is the dielectric constant of the metasurface structure. The ratio of the air area to the entire hybrid area is denoted as δ. Then the proportion of the metasurface structure area is 1 - δ, and the equivalent relative dielectric constant of the hybrid area is set to ε equ , and the relative dielectric constant of the metasurface structure is ε r0 . The wedge-shaped dielectric of the metasurface structure is used to achieve continuous variation of the relative dielectric constant. As shown in Figure 3 , the relative dielectric constant distribution pattern of the metasurface structure is compared with that of the graded-index material. The equivalent relative dielectric constant of the metasurface structure in the region of coordinate x with a length of Δx is set to ε2. When Δx approaches 0, ε2 is expressed as:

[0040]

[0041] The relative dielectric constant of the graded-index material at x is expressed as:

[0042]

[0043] The above equations have the same form and are all related to the coordinate position x, indicating that the wedge-shaped dielectric of the metasurface structure can replace the graded-index material;

[0044] S3. Simulate the microwave transmission in the asymmetric transmission circular waveguide when the thickness d of the wedge-shaped dielectric of the metasurface structure is different to evaluate the microwave energy utilization efficiency for different d values.

[0045] In one embodiment, in step S3, microwave energy is fed into the feeding port of the asymmetric transmission circular waveguide, and the other end is set as the scattering boundary condition; the microwave energy reaching the scattering boundary radiates into space and is not reflected, so that the reflection coefficient of the feeding port can be used as a reliable measure of the microwave energy utilization efficiency.

[0046] Specifically, Figure 5 shows the variation of the reflection coefficient of the feeding port of the conventional cavity asymmetric transmission circular waveguide and the asymmetric transmission circular waveguide when the d value is different. It can be seen that the wedge-shaped dielectric of the metasurface structure can significantly improve the microwave energy transmission efficiency; according to the simulation results, d = 14 mm is selected as the width of the right end of the wedge-shaped dielectric of the metasurface structure; the internal electric field distribution of the asymmetric transmission circular waveguide is as shown in Figure 6 , and it can be seen that the microwave energy is unidirectionally transmitted along the metasurface from left to right.

[0047] The beneficial effects of the present invention are as follows:

[0048] The present invention is reasonably designed. The metasurface structure can control the propagation of electromagnetic waves and has been applied in many fields. Research shows that the metasurface with a periodic groove structure can make microwaves propagate asymmetrically in a rectangular waveguide and effectively heat continuous fluids with different dielectric constants. In this paper, an asymmetric transmission circular waveguide based on a graded-index metasurface is designed, and electromagnetic waves propagate in the waveguide in the TM 01 mode, and the energy is concentrated on the cylindrical medium for flexible, efficient, and uniform plug-in heating. The metasurface structure is designed using wedge-shaped ceramics, which has the advantages of low cost and easy processing. A multi-physics simulation model is established to evaluate the efficiency and temperature uniformity of the heater. Brief Description of the Drawings

[0049] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0050] Figure 1 is a schematic structural diagram of a high-efficiency microwave heater with a graded refractive index.

[0051] Figure 2 is the field distribution of the TM01 mode in the circular waveguide; where — is the electric field, --- is the magnetic field.

[0052] Figure 3 is the relative dielectric constant distribution diagram, where (a) is the ladder-graded refractive index material and (b) is the wedge-shaped dielectric.

[0053] Figure 4 is the two-dimensional cross-section of the asymmetric transmission circular waveguide.

[0054] Figure 5 is the variation of the reflection coefficient of the circular waveguide with the d value.

[0055] Figure 6 is the electric field distribution in the asymmetric transmission circular waveguide when d = 14 mm.

[0056] Figure 7 is the schematic diagram of the two-dimensional cross-section of the heater.

[0057] Figure 8 is the schematic diagram of the energy utilization efficiency at different contraction lengths and radii.

[0058] Figure 9 is the schematic diagram of the variation of the heating efficiency with the insertion length of the medium.

[0059] Figure 10It is the electric field distribution in the heating area when g = 30 mm; (a) is the z - x plane, and (b) is the y - z plane.

[0060] Figure 11 It is the graph of the reflection coefficient of the heating system varying with the load dielectric constant.

[0061] Figure 12 It is the comparison of the heating efficiency varying with the load dielectric constant.

[0062] Figure 13 It is the electric field distribution diagram when dielectrics of different lengths are inserted into the load.

[0063] Figure 14 It is the variation of the reflection coefficient with the insertion length g of the dielectric.

[0064] Figure 15 It is the variation of the heating efficiency with the insertion length g of the dielectric.

[0065] Figure 16 It is the heating efficiency graph of the load with shape a.

[0066] Figure 17 It is the heating efficiency graph of the load with shape b.

[0067] Figure 18 It is the heating efficiency graph of the load with shape c.

[0068] Figure 19 It is the temperature distribution diagram of the internal section of the load after heating for 30 s. Detailed implementation manners

[0069] To make the technical problems, technical solutions, and technical effects of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Usually, the components of the embodiments of the present invention described and shown in the accompanying drawings here can be arranged and designed in various different configurations.

[0070] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed present invention, but merely represents the selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0071] It should be noted that like reference numerals and letters denote like items in the following figures, and thus, once an item is defined in one figure, it need not be further defined or explained in subsequent figures. In addition, the terms "first", "second", etc. are used for descriptive distinction only and should not be construed as indicating or implying relative importance.

[0072] In the description of the embodiments of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "inner", "outer", "upper", etc. is based on the orientation or positional relationship shown in the figures, or the orientation or positional relationship in which the inventive product is customarily placed during use. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation of the present invention.

[0073] Embodiment 1

[0074] As Figure 1 shown, this embodiment provides a gradient refractive index high-efficiency microwave heater, which includes an asymmetric transmission circular waveguide, a transition part, a cylindrical dielectric, and a heating load that are connected in sequence. The outer diameter of the cylindrical dielectric is smaller than the outer diameter of the asymmetric transmission circular waveguide. The transition part is a connecting piece connecting the asymmetric transmission circular waveguide and the cylindrical dielectric. The end part of the cylindrical dielectric extends into the heating load. An ultrathin surface structure with a gradient refractive index is arranged inside the asymmetric transmission circular waveguide to achieve unidirectional transmission of electromagnetic waves.

[0075] Specifically, a part of the cylindrical dielectric extends into the heating load. The inside of the asymmetric transmission circular waveguide is designed to have an ultrathin surface structure with a gradient refractive index, so as to achieve asymmetric transmission of electromagnetic waves, reduce reflection of electromagnetic waves, improve energy utilization efficiency, realize unidirectional transmission of electromagnetic waves, and then concentrate electromagnetic energy into the dielectric for plug-in heating; in order to reduce the size of the waveguide and concentrate microwave energy, a transition structure is designed to guide electromagnetic waves into the cylindrical dielectric for continuous propagation. A part of the cylindrical dielectric is inserted into the heating load body for uniform and efficient heating, and different-shaped materials with a dielectric constant ranging from 8 to 80 can be efficiently heated; in the simulation model, only the heated material exhibits dielectric loss, which clearly shows how microwave energy is absorbed by the material. The simulation results show that the heating efficiency is always higher than 90% within the dynamic range of the load dielectric constant from 8 to 80.

[0076] Embodiment 2

[0077] This embodiment is a further optimization based on Embodiment 1. Specifically:

[0078] The asymmetric transmission circular waveguide includes a cylindrical outer housing and a metasurface structure sleeved inside the outer housing. The wall thickness of the metasurface structure gradually changes along the axial direction. The end with the thinnest wall thickness of the metasurface structure is the feeding end, and the end with the thickest wall thickness of the metasurface structure is the scattering boundary. The transition part is located at the scattering boundary of the metasurface structure. Microwave energy is unidirectionally transmitted along the direction from the feeding end to the scattering boundary. The metasurface structure is obtained by rotating a wedge-shaped dielectric around its center.

[0079] The transition part includes a transition outer housing, a hollow conical dielectric lining, and a solid cylindrical dielectric lining. The hollow conical dielectric lining and the solid cylindrical dielectric lining are connected as a whole and are located inside the transition outer housing. One end of the hollow conical dielectric lining is connected to the scattering boundary of the metasurface structure. The square size of the hollow conical dielectric lining gradually decreases along the waveguide transmission direction. The hollow part of the hollow conical dielectric lining extends into the interior of the solid cylindrical dielectric lining.

[0080] The axial length of the asymmetric transmission circular waveguide is greater than the axial length of the metasurface structure.

[0081] The asymmetric transmission circular waveguide is a C22 circular waveguide.

[0082] The electromagnetic field transmission mode in the asymmetric transmission circular waveguide is the TM 01 mode.

[0083] Specifically, the electromagnetic field transmission mode in the asymmetric transmission circular waveguide is the TM 01 mode. The axisymmetric characteristic of the electromagnetic field distribution in this mode can improve the uniformity of microwave heating. Example 3

[0084] This embodiment provides a simulation method for a gradient refractive index high-efficiency microwave heater, which is used to simulate a gradient refractive index high-efficiency microwave heater in Example 2, and includes the following steps:

[0085] S1. Use COMSOL multi-physics simulation software to simulate the microwave heating process of the microwave heater:

[0086] S11. The simulation of this microwave heater involves the coupling of the electromagnetic field and the solid heat transfer field:

[0087] The distribution of the electromagnetic field is calculated using the following Helmholtz equation:

[0088]

[0089] Among them, is the electric field strength, k0 represents the free space wave number, ω represents the angular frequency, σ is the conductivity, μ r is the relative magnetic permeability, ε r represents the relative permittivity, ε0 is the relative permittivity of vacuum, μ0 represents the vacuum magnetic permeability, and j represents the imaginary part of a complex number. represents the differential operator;

[0090] The temperature field distribution is calculated using the following thermodynamic equation:

[0091]

[0092] where ρ, C p , T, Q, and K t respectively represent the density, specific heat capacity at constant pressure, thermodynamic temperature, heat source, and thermal conductivity of the material; t represents the heating time;

[0093] The electromagnetic field and the heat transfer field are coupled through the electromagnetic loss equation:

[0094]

[0095] where ε” represents the imaginary part of the relative permittivity, and Q e is the electromagnetic energy loss, which is used as the heat source causing temperature changes during the heating process;

[0096] S12, Setting of boundary conditions: During the simulation, the feeding end of the asymmetric transmission circular waveguide is set as the excitation port with the TM 01 mode, and the outer walls of the asymmetric transmission circular waveguide and the heating load are set as ideal electric conductors;

[0097]

[0098] where is the unit normal vector of the interface;

[0099] Heat transfer and reaction analysis are performed on the reactants, and the thermal boundary is set as adiabatic, satisfying the following equation:

[0100]

[0101] where represents the heat flux, the initial temperature of heating is set to 293.15 K, and except for the part in contact with the inserted medium, the outer wall of the heating load is an adiabatic boundary, and there is no heat exchange between the load and the external air during the heating process;

[0102] The TM 01 mode is rotationally symmetric, and its electromagnetic field distribution is symmetric about the axis of the asymmetric transmission circular waveguide. This symmetric characteristic is applied to microwave heating to exhibit high uniformity, and the TM 01 mode has low loss, is easy to be excited and converted, and improves the flexibility and efficiency of the heater;

[0103] S2, The wedge-shaped dielectric of the metasurface structure realizes a gradient refractive index metasurface:

[0104] The metasurface exhibits a gradually varying refractive index in the microwave propagation direction, which determines the unidirectional propagation of microwaves in the waveguide. Assuming that the electromagnetic wave propagates along the positive x-axis, the relationship between the relative permittivity of the metasurface structure and the x-axis coordinate can be described as follows:

[0105] ε'(x) = n(x) 2 = [α(x - x0) + β] 2 , (7)

[0106] where x0 is the starting point on the x-axis of the metasurface structure on the left side, n(x) refers to the refractive index at the x position on the metasurface structure, ε'(x) represents the relative permittivity of the metasurface structure, α represents the refractive index gradient factor, and β is the adjustment factor;

[0107] The equivalent relative permittivity of the air and metasurface structure mixed region can be expressed as follows:

[0108]

[0109] where ε r0 is the permittivity of the metasurface structure. The ratio of the air area to the entire mixed area is expressed as δ, then the proportion of the metasurface structure area is 1 - δ. The equivalent relative permittivity of the mixed area is set as ε equ , and the relative permittivity of the metasurface structure is ε r0 . The wedge-shaped dielectric of the metasurface structure is used to achieve a continuous change in the relative permittivity. As Figure 3 shown, the relative permittivity distribution pattern of the metasurface structure is compared with the graded-index material. The equivalent relative permittivity of the metasurface structure in the region of coordinate x and length Δx is set as ε2. When Δx approaches 0, ε2 is expressed as:

[0110]

[0111] The relative permittivity of the graded-index material at x is expressed as:

[0112]

[0113] The above equations have the same form and are all related to the coordinate position x, indicating that the wedge-shaped dielectric of the metasurface structure can replace the graded-index material;

[0114] S3. Simulate the microwave transmission in an asymmetric transmission circular waveguide when the wedge dielectric thickness d of the metasurface structure is different to evaluate the microwave energy utilization efficiency at different d values. Microwave energy is fed into the feeding port of the asymmetric transmission circular waveguide, and the other end is set as the scattering boundary condition; the microwave energy reaching the scattering boundary radiates into space and is not reflected, so that the reflection coefficient of the feeding port can be used as a reliable measure of the microwave energy utilization efficiency.

[0115] Specifically, Figure 5 shows the variation of the reflection coefficient at the feeding port of the conventional cavity asymmetric transmission circular waveguide and the asymmetric transmission circular waveguide when the d value is different. It can be seen that the wedge dielectric of the metasurface structure can significantly improve the microwave energy transmission efficiency; according to the simulation results, d = 14 mm is selected as the width of the right end of the wedge dielectric of the metasurface structure; the internal electric field distribution of the asymmetric transmission circular waveguide is as Figure 6 shown, and it can be seen that the microwave energy is transmitted unidirectionally from left to right along the metasurface.

[0116] In this embodiment, the simulation and design of the size of the asymmetric transmission circular waveguide are as follows:

[0117] According to the above theory, by rotating the wedge dielectric of the metasurface structure around its center, an asymmetric transmission circular waveguide is obtained. The material of the metasurface structure is a ceramic with a relative dielectric constant of 9.9. Figure 4 is the two-dimensional cross-section of the designed asymmetric transmission circular waveguide;

[0118] The total length of the asymmetric transmission circular waveguide is 310 mm, the length of the metasurface section is 270 mm, and the thickness at the starting point of the left end of the metasurface is 3 mm.

[0119] Simulate the microwave transmission in an asymmetric transmission circular waveguide when the wedge dielectric thickness d of the metasurface structure is different to evaluate the microwave energy utilization efficiency at different d values. Microwave energy is fed into the feeding port of the asymmetric transmission circular waveguide, and the other end is set as the scattering boundary condition; the microwave energy reaching the scattering boundary radiates into space and is not reflected, so that the reflection coefficient of the feeding port can be used as a reliable measure of the microwave energy utilization efficiency; Figure 5 shows the variation of the reflection coefficient at the feeding port of the conventional cavity asymmetric transmission circular waveguide and the asymmetric transmission circular waveguide when the d value is different. It can be seen that the wedge dielectric of the metasurface structure can significantly improve the microwave energy transmission efficiency; according to the simulation results, d = 14 mm is selected as the width of the right end of the wedge dielectric of the metasurface structure; the internal electric field distribution of the asymmetric transmission circular waveguide is as Figure 6 shown, and it can be seen that the microwave energy is transmitted unidirectionally from left to right along the metasurface.

[0120] Adopt the simulation and optimization with a heating load:

[0121] In order to reduce the size of the heater and concentrate the electric field energy for flexible and efficient plug-in heating, the rear end of the asymmetric transmission circular waveguide needs to be shrunk into a section of cylindrical dielectric. The optimization process of the dimensions of the transition part and the radius and length of the inserted dielectric section. As Figure 7 shown, the transition part is gradually shrunk by a wedge-shaped dielectric of the metasurface structure. The heater is simulated and optimized by adjusting the length c of the air cavity in the transition part and the radius h1 of the cylindrical dielectric after contraction. The end of the transition part is set as the scattering boundary condition. The range of c is 50 mm to 100 mm, with a step size of 10 mm, and the range of h1 is 20 mm to 33 mm, with a step size of 1 mm.

[0122] The simulation results are as Figure 8 shown. Considering the high efficiency and stability of the heater, the length c of the air cavity in the transition part is set to 90 mm, and the radius h1 of the cylindrical dielectric is set to 28 mm, that is, the diameter is 56 mm. On this basis, the influence of the change in the length g of the cylindrical dielectric inserted into the load on the heating efficiency is analyzed. It is assumed that the relative dielectric constant of the load is 80 and the loss tangent is 0.1, and it is installed in a cube metal container with a side length of 120 mm. Figure 9 The simulation results show that when the inserted dielectric length is greater than 15 mm, the heating efficiency always remains above 90%. To save costs while ensuring the high efficiency of the heater, the dielectric insertion length is set to 30 mm. The electric field distribution in the heating area is as Figure 10 shown. The cross-sectional electric field perpendicular to the propagation direction shows the characteristic of axisymmetric distribution, and it shows good uniformity when applied to heating.

[0123] Analysis of the heating efficiency of the heater for different loads: The influence of the load dielectric constant, the inserted length of the heating dielectric, and the changes in the size and shape of the load on the heating efficiency is discussed through multi-physics simulation.

[0124] 1) Influence of dielectric constant change on heating efficiency:

[0125] The heating efficiency of materials with different dielectric constants is calculated using the above model, and the heating effects with and without the gradient refractive index metasurface are compared. The real part ε' of the relative dielectric constant of the heated load ranges from 8 to 80, and the loss tangent ε" ranges from 0.1 to 1. This range includes the most common materials.

[0126] Figure 11The heating efficiency of the heating device for loads with dielectric constants varying within the above range is shown. When the real part of the dielectric constant is 12 or 18, the lowest S11 value is observed. S11 increases as the loss tangent of the load increases, which may be due to insufficient penetration depth of electromagnetic waves into the material caused by a high loss tangent, thus limiting the effective energy absorption. For any load with a dielectric constant in the range of 8 to 80, S11 always remains below -10 dB. Figure 12 The simulation results show that the change in the dielectric constant of the material to be heated has little effect on the microwave heating efficiency, only affecting the energy utilization rate by about 10%. In terms of energy utilization efficiency, there is a significant difference between the microwave heater with a gradient refractive index metasurface and without a metasurface. The heating efficiency of the system without a metasurface is lower, and after adding a metasurface in the asymmetric transmission circular waveguide, the heating efficiency always remains above 90%, an increase of 20% - 25%.

[0127] In summary, the microwave heater can maintain an energy utilization efficiency of over 90% for materials with different dielectric constants within a large dynamic range. It can initiate different chemical reactions and achieve efficient microwave heating.

[0128] 2) Influence of the dielectric insertion length on the heating efficiency:

[0129] The change range of the length g of the cylindrical dielectric extending into the load (heating load) is 10 - 60 mm (the loss tangent angle of the load is 0.1). Figure 13 and Figure 14 show the electric field distribution and S11 inside the load of dielectrics with different lengths inserted into the load. Figure 14 It shows that compared with the case where the cylindrical dielectric is not inserted, when the cylindrical dielectric is inserted into the load, the heating efficiency can be significantly improved. In addition, when the load dielectric constant varies between 8 and 80, S11 always remains below -10 dB. As Figure 15 shown, within a certain range, the heating efficiency increases with the increase in the length of the dielectric insertion. When the insertion length is large enough, the heating efficiency remains above 90% with respect to the length change, showing good robustness.

[0130] 3) Influence of the change in load shape on the heating efficiency:

[0131] To analyze the influence of different load shapes on the system heating efficiency, three different load shapes, namely cube a, cuboid b, and cylinder c, were designed, and the heating efficiency of different dielectric constant materials under each load shape was simulated by electromagnetic simulation. The simulation results of heating these samples under the three different load shapes are as Figure 16 , Figure 17 and Figure 18As shown. When heating loads with different dielectric constants using three load shapes, the microwave heating efficiency remains above 90%, indicating that the change in load shape has little effect on the heating efficiency. The above results show that the heater can flexibly and effectively heat loads with varying shapes, sizes, and dielectric constants.

[0132] Analysis of heating uniformity: Examine the uniformity of the temperature distribution when the designed heater is used for heating. According to Figure 2 and Figure 10 the axisymmetric characteristics of the electromagnetic field distribution in the TM 01 mode, it can be inferred that the heater will heat the load very uniformly in the direction perpendicular to the microwave propagation direction. The temperature distribution at different positions after heating for 30 seconds when the load dielectric constant is 80 (loss tangent is 0.1) is simulated, and the simulation results are as Figure 19 shown, showing good uniformity of the temperature distribution in the Z-Y plane.

Claims

1. A high-efficiency microwave heater with a gradient refractive index, characterized in that: The invention comprises an asymmetric transmission circular waveguide, a transition part, a cylindrical medium and a heating load which are connected in sequence, wherein the outer diameter of the cylindrical medium is smaller than the outer diameter of the asymmetric transmission circular waveguide, the transition part is a connector connecting the asymmetric transmission circular waveguide and the cylindrical medium, the end part of the cylindrical medium extends into the heating load, and a metasurface structure with a gradient refractive index is arranged inside the asymmetric transmission circular waveguide to realize unidirectional transmission of electromagnetic waves.

2. The high-efficiency microwave heater with a gradient refractive index according to claim 1, characterized in that: The asymmetric transmission circular waveguide includes a cylindrical outer shell and a metasurface structure sleeved in the outer shell, the wall thickness of the metasurface structure gradually changes along the axial direction, the end of the metasurface structure with the thinnest wall thickness is the feeding end, and the end of the metasurface structure with the thickest wall thickness is the scattering boundary, the transition part is located at the scattering boundary of the metasurface structure, and microwave energy is transmitted unidirectionally from the feeding end to the scattering boundary. The metasurface structure is obtained by rotating a wedge-shaped dielectric around its center.

3. A gradient refractive index high-efficiency microwave heater according to claim 2, characterized in that: The transition part includes a transition shell, a hollow conical dielectric inner village and a solid cylindrical dielectric lining. The hollow conical dielectric inner village and the solid cylindrical dielectric inner village are connected as a whole and are located in the transition shell. One end of the hollow conical dielectric inner village is connected to the scattering boundary of the metasurface structure. The square size of the hollow conical dielectric inner village gradually decreases along the waveguide transmission, and the hollow part of the hollow conical dielectric inner village extends into the interior of the solid cylindrical dielectric inner village.

4. The high-efficiency microwave heater with a gradient refractive index according to claim 1, characterized in that: The electromagnetic field transmission mode in the asymmetric transmission circular waveguide is TM 01 mold.

5. The high-efficiency microwave heater with a gradient refractive index according to claim 1, characterized in that: The axial length of the asymmetric transmission circular waveguide is greater than the axial length of the metasurface structure.

6. The high-efficiency microwave heater with a gradient refractive index according to claim 1, characterized in that: The asymmetric transmission circular waveguide is a C22 circular waveguide.

7. A simulation method for a gradient refractive index high-efficiency microwave heater, characterized in that: Used to simulate a gradient refractive index high-efficiency microwave heater as described in any one of claims 1 to 6.

8. The simulation method of a gradient refractive index high-efficiency microwave heater according to claim 7, characterized in that: The steps include: S1. Use COMSOL multi-physics simulation software to simulate the microwave heating process of microwave heater: S11. The simulation of the microwave heater involves the coupling of the electromagnetic field and the solid heat transfer field: The distribution of the electromagnetic field is calculated using the Helmholtz equation as follows: in, is the electric field intensity, k0 is the free space wave number, ω is the angular frequency, σ is the conductivity, μ r is the relative magnetic permeability, ε r represents the relative permittivity, ε0 ​​is the relative permittivity of vacuum, μ0 represents the magnetic permeability of vacuum, j represents the imaginary part of the complex number, represents a differential operator; The temperature field distribution is calculated using the following thermodynamic equation: Among them, ρ, C p , T, Q and K t They represent the density, constant pressure heat capacity, thermodynamic temperature, heat source and thermal conductivity of the material respectively; t represents the heating time; The electromagnetic field and heat transfer field are coupled through the electromagnetic loss equation: Among them, ε” represents the imaginary part of the relative dielectric constant, Q e It is the electromagnetic energy loss, which is used as the heat source causing the temperature change during the heating process; S12, Boundary condition setting: During the simulation, the feed end of the asymmetric transmission circular waveguide is set to have TM 01 The excitation port of the mode, the asymmetric transmission circular waveguide and the outer wall of the heating load are set as ideal electrical conductors; in, is the unit normal vector of the interface; Perform heat transfer and reaction analysis on the reactants and set the thermal boundaries to be adiabatic to satisfy the following equations: in, represents the heat flow. The initial temperature of heating is set to 293.15K. Except for the part in contact with the inserted medium, the outer wall of the heated load is an adiabatic boundary. There is no heat exchange between the load and the external air during the heating process. S2. The wedge-shaped dielectric of the metasurface structure realizes the gradient refractive index metasurface: The metasurface presents a gradient refractive index in the direction of microwave propagation, which determines the unidirectional propagation of microwaves in the waveguide. Assuming that the electromagnetic wave propagates along the positive x-axis, the relationship between the dielectric constant of the metasurface structure and the x-axis coordinate can be described as follows: Wherein, x0 is the starting point on the x-axis of the metasurface structure on the left, n(x) refers to the refractive index at the x position on the metasurface structure, ε'(x) represents the relative dielectric constant of the metasurface structure, α represents the refractive index gradient factor, and β is the adjustment factor; The equivalent relative dielectric constant of the mixed area of ​​air and metasurface structure can be expressed as follows: Among them, ε r0 is the dielectric constant of the metasurface structure, the ratio of the air area to the entire mixed area is expressed as δ, then the metasurface structure area ratio is 1-δ, and the equivalent relative dielectric constant of the mixed area is set to ε equ , and the relative dielectric constant of the metasurface structure is ε r0 The wedge-shaped dielectric of the metasurface structure is used to achieve continuous change of relative permittivity, and the relative permittivity distribution pattern of the metasurface structure is compared with that of the gradient refractive index material. The equivalent relative permittivity of the metasurface structure in the region of coordinate x and length Δx is set to ε2. When Δx is close to 0, ε2 is expressed as: The relative dielectric constant of a graded-index material at x is expressed as: The above equations have the same form and are all related to the coordinate position x, thus indicating that the wedge-shaped dielectric of the metasurface structure can replace the graded refractive index material; S3. The microwave transmission in the asymmetric transmission circular waveguide with different thickness d of the wedge-shaped dielectric of the metasurface structure is simulated to evaluate the microwave energy utilization efficiency with different d values.

9. The simulation method of a gradient refractive index high-efficiency microwave heater according to claim 8, characterized in that: In step S3, microwave energy is fed into the feed port of the asymmetric transmission circular waveguide, and the other end is set to a scattering boundary condition; The microwave energy reaching the scattering boundary radiates into space without being reflected, so that the reflection coefficient of the feed port can be used as a reliable measure of the microwave energy utilization efficiency.

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