CN104797029A - Resonant cavity for verifying wood microwave pretreatment temperature distribution - Google Patents

Abstract

The invention discloses a resonant cavity for verifying the temperature distribution of wood microwave pretreatment, which comprises a horizontally arranged main resonant cavity, at least one feed-in waveguide is arranged on the side of the main resonant cavity, and the gap between the main resonant cavity and the feed-in waveguide is The feed-in waveguide is arranged horizontally along the outward direction from the center of the main resonator, and both end faces of the feed-in waveguide are hollow openings; the cross-section of the feed-in waveguide is rectangular, and the length of the rectangle is along the horizontal The direction is arranged, the width of the rectangle is arranged along the vertical direction, and the distance between the hollow opening at the end away from the center of the main resonant cavity on the feeding waveguide and the center of the main resonant cavity is greater than 1.1 times the length of the rectangle. The invention has the advantages of high utilization rate of wood microwave energy, uniform dispersion of microwave energy, good temperature distribution uniformity and low wood cracking rate.

Description

Resonant cavity for verification of temperature distribution in microwave pretreatment of wood

technical field

The invention relates to the technical field of microwave pretreatment of wood, in particular to a resonant cavity for verifying the temperature distribution of microwave pretreatment of wood.

Background technique

The area and stock volume of fast-growing plantations in my country rank first in the world, but fast-growing wood generally has defects such as poor permeability, drying and post-corrosion, and difficulty in flame-retardant treatment, so that the high value-added utilization of solid wood has not made breakthrough progress.

High-intensity microwave pretreatment is a new technology that has appeared in the field of wood modification in recent years. Its basic principle is to use high-intensity microwaves to treat wet wood instantaneously, so that the moisture in the wood can obtain enough energy in a short time to produce phase change. And gas thermal pressure effect, driven by steam expansion power, destroys the internal structure of wood, improves fluid migration ability, and creates extremely favorable preconditions for later drying and impregnation treatment of wood, and even new material preparation.

At present, countries such as the United States, Australia, and China have begun to trial-produce special equipment for microwave pretreatment of high-strength wood, and have studied the influence of microwave pretreatment on wood volume expansion rate, permeability and drying rate. The preliminary research results show that the optimized high-intensity microwave pretreatment can increase the volume of wood by more than 10%, increase the penetration of preservatives by 10-14 times, increase the drying rate by 5-10 times, and significantly improve the drying quality. The University of Melbourne in Australia even used this technology to dismantle and reconstruct wood, and trial-produced new wood with high permeability and low density and new wood composite materials with high strength and high surface hardness. This research has opened up a new path for the high-quality utilization of fast-growing plantation wood resources.

However, during the research process of wood microwave pretreatment, researchers found that after microwave pretreatment, the distribution of micro- and macro-pores inside the wood was not uniform enough, which brought difficulties to the post-impregnation treatment of wood and the preparation of high-performance new materials. Very difficult. These inhomogeneities are related to the nature of the wood itself and the uneven distribution of the dielectric constant. However, we cannot change the properties of the wood itself. We can only improve its heating external conditions to improve the uniformity of the microwave field and finally obtain an ideal wood. The effect of microwave treatment. The main factors related to the heating conditions are the microwave frequency and the microwave heating cavity, which together determine the temperature distribution of the wood during heating. There are two types of microwave frequencies used for microwave treatment of wood, 0.915Ghz and 2.45Ghz. Microwaves with a frequency of 2.45Ghz are more beneficial to the uniformity of temperature distribution of thinner wood, and this frequency is used in the design. After the frequency is selected, the center of the problem shifts to the design of the microwave resonant cavity. Therefore, how to develop high-efficiency microwave pretreatment equipment by developing a microwave resonant cavity with uniform microwave energy distribution has become a problem that restricts the development of high-intensity microwave pretreatment technology for wood. core and critical issues.

The most frequently used method for optimizing the design of microwave resonators is computer simulation technology. In order to ensure the reliability of the simulation results, it is necessary to compare the computer simulation temperature with the actual test results, verify the accuracy of the model simulation, and correct the model, and finally obtain Optimized mathematical model and size parameters of microwave resonator. In addition, different feed-in resonators have a great influence on wood microwave energy utilization, temperature distribution uniformity, and wood cracking rate. It is necessary to compare and analyze wood microwave pretreatment of different feed-in resonators during the test. . At present, the most commonly used microwave heating resonant cavity is the traveling wave heater, that is, there is an obliquely fed waveguide on the rectangular resonant cavity. Sex research is not enough. Since the wood moves in the cavity, the uniformity of heating throughout the wood depends on the cross-section. Although the microwave of the traveling wave heater can be absorbed by the wood through multiple reflections, the temperature distribution on the cross-section of the wood cannot be improved. Because the microwave energy of the traveling wave heater is more concentrated at the feed port, the localized high temperature produces a wood cracking rate. Although the microwave energy utilization rate of the traveling wave heater is high, the temperature distribution uniformity is not ideal and the wood cracking rate is high, so it is not the best choice.

Contents of the invention

The technical problem to be solved by the present invention is: aiming at the above-mentioned problems existing in the prior art, to provide a wood microwave preheating device with high utilization rate of wood microwave energy, uniform dispersion of microwave energy, good temperature distribution uniformity, and low wood cracking rate. Resonant cavities that handle temperature distribution.

In order to solve the problems of the technologies described above, the technical solution adopted in the present invention is:

A resonant cavity for verifying the temperature distribution of wood microwave pretreatment, comprising a horizontally arranged main resonant cavity, at least one feed-in waveguide is provided on the side of the main resonant cavity, and the gap between the main resonant cavity and the feed-in waveguide is communicate with each other, the feed-in waveguide is arranged horizontally along the direction outward from the center of the main resonant cavity, and both end faces of the feed-in waveguide are hollow openings.

Preferably, the cross-section of the feeding waveguide is rectangular, and the length of the rectangle is arranged along the horizontal direction, the width of the rectangle is arranged along the vertical direction, and the end of the feeding waveguide away from the center of the main resonant cavity The distance L between the hollow opening and the center of the main resonant cavity is greater than 1.1 times the length of the rectangle.

Preferably, both the main resonant cavity and the feeding waveguide are in the shape of a cuboid, the number of the feeding waveguide is one, and the feeding waveguide is arranged on one side of the main resonant cavity. Further, the length and width of the main resonant cavity are both 0.197m and the height is 0.155m, and the length and width of the rectangular cross section of the feeding waveguide are 0.0953m and 0.0546m.

Or preferably, both the main resonant cavity and the feeding waveguides are in the shape of a cuboid, the number of the feeding waveguides is two, and the two feeding waveguides are respectively symmetrically arranged on the side surfaces of the main resonant cavity. Further, the length and width of the main resonant cavity are both 0.197m and the height is 0.155m, and the length and width of the rectangular cross-section of the feeding waveguide are 0.0953m and 0.0546m.

Or preferably, the shape of the main resonant cavity is cylindrical, the shape of the feeding waveguide is cuboid, the number of the feeding waveguides is three, and the three feeding waveguides are symmetrically arranged in the main side of the resonator. Further, the diameter of the main resonant cavity is 0.197m, and the height is 0.155m, and the length of the rectangular cross section of the feeding waveguide is 0.0953m, and the width is 0.0546m.

Or preferably, the shape of the main resonant cavity is cylindrical, the shape of the feeding waveguide is cuboid, the number of the feeding waveguides is four, and the four feeding waveguides are symmetrically arranged on the main side of the resonator. Further, the diameter of the main resonant cavity is 0.197m, and the height is 0.155m, and the length of the rectangular cross section of the feeding waveguide is 0.0953m, and the width is 0.0546m.

The resonant cavity of the present invention for verifying the temperature distribution of wood microwave pretreatment has the following advantages: the resonant cavity of the present invention for verifying the temperature distribution of wood microwave pretreatment includes a main resonant cavity arranged horizontally, and the side of the main resonant cavity is provided with at least A horizontally arranged feed-in waveguide, the main resonant cavity and the feed-in waveguide are connected to each other, the feed-in waveguide is arranged horizontally along the direction outward from the center of the main resonant cavity, and the two end faces of the feed-in waveguide are hollow openings , based on the above structure, when verifying the temperature distribution of wood microwave pretreatment, the microwave energy can be evenly distributed on the wood without concentrating at the feeding inlet, so as to prevent the local high temperature of the wood from causing the wood cracking rate, and at the same time have wood microwave It has the advantages of high energy utilization rate, uniform dispersion of microwave energy, good temperature distribution uniformity, and low wood cracking rate.

Description of drawings

FIG. 1 is a schematic perspective view of the three-dimensional structure of Embodiment 1 of the present invention.

Fig. 2 is a simulated temperature distribution diagram of the wood surface when heated by Embodiment 1 of the present invention.

Fig. 3 is a simulated temperature distribution diagram of the central section of wood when heated by Embodiment 1 of the present invention.

FIG. 4 is a schematic diagram of a three-dimensional structure of Embodiment 2 of the present invention.

Fig. 5 is a simulated temperature distribution diagram of the wood surface when heated by the second embodiment of the present invention.

Fig. 6 is a simulated temperature distribution diagram of the central section of wood when heated using Example 2 of the present invention.

FIG. 7 is a schematic diagram of a three-dimensional structure of Embodiment 3 of the present invention.

Fig. 8 is a simulated temperature distribution diagram of the wood surface when heated using Example 3 of the present invention.

Fig. 9 is a simulated temperature distribution diagram of the central section of wood when heated using Example 3 of the present invention.

Fig. 10 is a schematic diagram of a three-dimensional structure of Embodiment 4 of the present invention.

Fig. 11 is a simulated temperature distribution diagram of the wood surface when heated using Embodiment 4 of the present invention.

Fig. 12 is a simulated temperature distribution diagram of the central section of wood when heated using Example 4 of the present invention.

Detailed ways

Embodiment one:

As shown in Figure 1, the resonant cavity used in this embodiment to verify the temperature distribution of wood microwave pretreatment includes a horizontally arranged main resonant cavity A, at least one feed-in waveguide is provided on the side of the main resonant cavity A, the main resonant cavity and the feed-in waveguide The input waveguides communicate with each other, the feed-in waveguides are arranged horizontally along the direction outward from the center of the main resonant cavity A, and both end faces of the feed-in waveguides are hollow openings. Referring to FIG. 1 , W represents the wood placed in the center of the main resonator A in this embodiment.

As shown in Figure 1, the cross-section of the feed-in waveguide is rectangular, and the length of the rectangle is arranged along the horizontal direction, and the width of the rectangle is arranged along the vertical direction. The distance L between the centers of the main resonant cavity A (see the mark in Figure 3) is greater than 1.1 times the length of the rectangle. Based on the above dimensions, the structure between the main resonant cavity A and the feeding waveguide in the group of microwave resonant cavities can be realized. The size matches the microwave with a frequency of 2.45Ghz; no doubt, those skilled in the art can also adjust the above structure to adapt to microwaves with other frequencies for heating wood, such as microwaves with a frequency of 0.915Ghz.

In this embodiment, the shape of the main resonator A is a cylinder, the shape of the feed-in waveguide is a cuboid, and the number of feed-in waveguides is four (see B, C, D, E in the figure), and the four The feed-in waveguides B, C, D, and E are symmetrically arranged on the side of the main resonant cavity A. What needs to be said is that in order to achieve uniform distribution of microwaves, the wood W is placed at the center of the main resonant cavity A. The specific shape of the main resonant cavity A corresponds to the shape of the wood W. Therefore, the shape of the wood W in this embodiment is Cylinder (as shown in Figure 1).

In this embodiment, the diameter of the main resonant cavity A is 0.197m, the height is 0.155m, the length of the rectangle feeding into the waveguide cross section is 0.0953m, and the width is 0.0546m. Based on the above dimensions, the group of microwave resonant cavities can be realized The structure and size between the main resonant cavity A and the feeding waveguide are matched with the microwave; and the size of the resonating cavity A and the feeding waveguides B, C, D, and E is small, and the feeding of the main resonating cavity A and the feeding waveguide The input is superimposed on the center of the main resonant cavity A, so that the central energy of the main resonant cavity A is higher, so it can prevent heat from gathering at the feeding port, and make the heating of the wood more uniform.

In this embodiment, the distance between the hollow opening at the end of the feeding waveguide away from the center of the main resonant cavity A and the center of the main resonant cavity A is 0.1485m, which is greater than 1.1 times the length of the rectangle (0.0953m*1.1=0.10483m).

In addition, in this embodiment, a temperature measurement hole is provided on the top of the main resonant cavity A for inserting a temperature measurement sensor. During the experiment, the temperature distribution of wood W during microwave treatment is measured by means of optical fiber sensing temperature measurement or infrared temperature measurement, which can be compared and analyzed with the simulation results to verify the temperature distribution of wood microwave heating in different resonant cavities, and analyze the simulation results and experiments. The reason for the temperature difference, and improve the model, in order to more accurately simulate and predict the temperature distribution.

In order to verify the temperature distribution uniformity of this embodiment, this embodiment uses the finite element analysis software COMSOLMultiphysics to simulate. In this simulation calculation, wood with a moisture content of 20% is selected, and its thermal conductivity λ is 0.14*(1- 0.72*(25-T)/100)[w/(m°C)] (T is the temperature at this time), specific heat C is 2130*(1+T/100)^0.2(J/kg°C), density ρ is 427 (kg/m3), the dielectric constant ε is 2.94-0.21j, the initial temperature T ₀ is 15°C, the fed microwaves are all TE10 waves, the frequency is 2.45GHz, and the single-port fed power is 1.5kw. To calculate the temperature at each point of the wood, it is necessary to divide the wood into many tiny parts, study the energy and temperature relationship of each part separately (of course, the heat transfer with the rest of the parts needs to be considered), and then integrate the results of each step Together, thus completing the entire calculation, this is the basic idea of finite element analysis. In the simulation calculation of this embodiment, the specific heat and thermal conductivity of wood vary with temperature. Since the temperature varies with time, the specific heat, thermal conductivity and temperature are all functions of time. Both of them are values at the same moment, so the calculation time of each step must be divided into quite small, which is obviously very cumbersome and unnecessary. Such a calculation method can be adopted, first assuming that the temperature of the wood is constant in a short period of time, so the specific heat and thermal conductivity are also constant, and these two constants can be used to calculate the energy absorbed by the wood during this period, At the same time, the temperature change of the wood is obtained, and the new temperature value is substituted into the specific heat and thermal conductivity to continue the calculation cycle until the predetermined time.

In this embodiment, the finite element analysis software COMSOL Multiphysics is used to simulate and verify the temperature distribution of wood microwave pretreatment. When simulating and verifying the temperature distribution of wood microwave pretreatment, it is necessary to establish an electromagnetic field and microwave energy distribution model and to calculate the heat transfer rate inside the wood. Migration model.

1) Establish the electromagnetic field and microwave energy distribution model:

When theoretically simulating the distribution of the electromagnetic field and temperature field in the wood W placed in the main resonator A during the high-intensity microwave heating process, in order to simplify the analysis and theoretical calculation, the following reasonable assumptions are first made on the heating model of the wood W: 1. The initial temperature and moisture content of the wood W are evenly distributed; 2. The volume of the wood W remains unchanged during microwave heating; 3. The cylindrical wood W is coaxial with the main resonant cavity A; 4. The distance between the wood surface and the air The heat exchange satisfies the convection and heat transfer boundary conditions.

According to Maxwell's equations, the electromagnetic field distribution in wood during microwave heating satisfies formulas (1)-(4).

$<span\; class="notranslate">\; \&dtri;</span><span\; class="notranslate">\; \&times;</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{<span\; class="notranslate">\; \&PartialD;</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; B</span>}}}{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

$<span\; class="notranslate">\; \&dtri;</span><span\; class="notranslate">\; \&times;</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; =</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; J</span>}}<span\; class="notranslate">\; +</span>\frac{<span\; class="notranslate">\; \&PartialD;</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; D.</span>}}}{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

$<span\; class="notranslate">\; \&dtri;</span><span\; class="notranslate">\; \&Center\; Dot;</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; B</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

$<span\; class="notranslate">\; \&dtri;</span><span\; class="notranslate">\; \&Center\; Dot;</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; D.</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

In formula (1)～(4), represents the electric field strength, Indicates the magnetic induction intensity, represents the magnetic field strength, represents the current density, represents the electric displacement, ρ _c represents the free charge density, Indicates the divergence operation, Represents the curl operation. in, σ is the electrical conductivity, ε is the permittivity, and μ is the magnetic permeability.

According to the formulas (1)-(4), the distribution of the electromagnetic field inside the wood W can be calculated. In this embodiment, the microwave excitation source uses a time-harmonic electromagnetic field membrane, i.e. electric field strength and magnetic field strength The value is: in, Indicates the electric field strength the amplitude of Indicates the magnetic field strength The amplitude of , i represents the imaginary unit, ω represents the angular frequency, and t represents the time. Magnetic induction and electric displacement can be expressed as electric field strength and magnetic field strength function, specifically as shown in formulas (5) and (6).

$<span\; class="notranslate">\; \&dtri;</span><span\; class="notranslate">\; \&times;</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; i\&omega;\&mu;</span>}\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

$<span\; class="notranslate">\; \&dtri;</span><span\; class="notranslate">\; \&times;</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&sigma;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; i\&omega;\&epsiv;</span>}<span\; class="notranslate">\; )</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; i\&omega;</span>}{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}^{<span\; class="notranslate">\; *</span>}\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

In formulas (5) and (6), Represents the curl operation, Indicates the electric field strength the amplitude of Indicates the magnetic field strength , i represents the imaginary number unit, ω represents the angular frequency, μ represents the magnetic permeability, σ represents the conduction coefficient, ε represents the permittivity, ε′ represents the real part of the complex permittivity, ε″ represents the imaginary part of the complex permittivity, ε ^* is the complex permittivity, and ε ^* can be expressed as ε ^* =ε′+iε″.

(Ayappa, 1997) formula (7) can be obtained from formulas (5) and (6).

$<span\; class="notranslate">\; \&dtri;</span><span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\frac{<span\; class="notranslate">\; \&dtri;</span>{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}^{<span\; class="notranslate">\; *</span>}}{{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}^{<span\; class="notranslate">\; *</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>{<span\; class="notranslate">\; \&dtri;</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; k</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

In formula (7), Indicates the divergence operation, Represents the electric field strength, k is the dielectric property parameter of wood W, ε represents the permittivity, ε′ represents the real part of the complex permittivity, ε″ represents the imaginary part of the complex permittivity, ε ^* is the complex permittivity, ε ^* It can be expressed as ε ^* =ε'+iε". The electric field distribution in wood W can be obtained by solving equation (7). In this embodiment, it is assumed that the dielectric constant of wood is constant along the direction of the electric field, so the first term of formula (7) is zero, which can be simplified to formula (8).

${<span\; class="notranslate">\; \&dtri;</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; k</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 8</span>}<span\; class="notranslate">\; )</span>$

In formula (8), Indicates the divergence operation, Indicates the electric field strength, and k is the dielectric property parameter of wood W. Among them, the expression of the dielectric property parameter k of wood W is shown in formulas (9) to (11).

k=α+iβ (9)

$\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;f</span>}}{\mathrm{<span\; class="notranslate">\; c</span>}}\sqrt{\frac{{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}^{<span\; class="notranslate">\; \text{'}</span>}<span\; class="notranslate">\; (</span>\sqrt{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; the\; tan</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; \&delta;</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; )</span>$

$\mathrm{<span\; class="notranslate">\; \&beta;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;f</span>}}{\mathrm{<span\; class="notranslate">\; c</span>}}\sqrt{\frac{{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}^{<span\; class="notranslate">\; \text{'}</span>}<span\; class="notranslate">\; (</span>\sqrt{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; the\; tan</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; \&delta;</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 11</span>}<span\; class="notranslate">\; )</span>$

In formulas (9) to (11), i represents the imaginary number unit, f represents the frequency of microwave radiation, c represents the speed of light, tanδ represents the wood loss factor, and ε' represents the real part of the complex permittivity. The expression of wood loss factor tanδ is shown in formula (12).

$\mathrm{<span\; class="notranslate">\; the\; tan</span>}\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}^{<span\; class="notranslate">\; \text{'}</span><span\; class="notranslate">\; \text{'}</span>}}{{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}^{<span\; class="notranslate">\; \text{'}</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 12</span>}<span\; class="notranslate">\; )</span>$

In formula (12), ε' represents the real part of the complex permittivity, and ε" represents the imaginary part of the complex permittivity.

In simulating the electric field distribution in the wood W, Equation (8) is solved by the boundary conditions of the following Equations (13) and (14).

$\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; \&times;</span><span\; class="notranslate">\; [</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 13</span>}<span\; class="notranslate">\; )</span>$

$\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; \&times;</span><span\; class="notranslate">\; [</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; h</span>}}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; h</span>}}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 14</span>}<span\; class="notranslate">\; )</span>$

In formulas (13) and (14), n represents the surface normal unit vector, Indicates the electric field strength at the air boundary position the amplitude of Indicates the electric field strength at the position of the wood boundary the amplitude of Indicates the magnetic field strength at the air boundary location the amplitude of Indicates the magnetic field strength at the position of the wood boundary The amplitude of , where subscript 1 represents air and subscript 2 represents wood.

Equation (14) can also be expressed as electric field strength by the expression shown in Equation (15) The function.

$\frac{<span\; class="notranslate">\; \&PartialD;</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span>}{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; i</span>}{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}\mathrm{<span\; class="notranslate">\; \&omega;</span>}\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; (</span>\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 15</span>}<span\; class="notranslate">\; )</span>$

In formula (15), Indicates the electric field strength the amplitude of Indicates the magnetic field strength The amplitude of , i represents the imaginary number unit, μ ₀ represents the permeability of the medium, and ω represents the angular frequency.

The solution of equation (8) is composed of two wave functions propagating in opposite directions as shown in equation (16).

E＝A ₁ e ^ikr +B ₁ e ^-ikr (16)

In Equation (16), E represents the electric field, i represents the imaginary number unit, k represents the wave number, r represents the radius, _A1 and _B1 represent the boundary parameters determined by the boundary conditions Equation (13) and Equation (14), respectively. According to the obtained electric field E in the wood W, Poynting's theorem can be used to obtain the electromagnetic field energy density Q at any position in the wood W, as shown in Equation (17), so as to complete the establishment of the electromagnetic field and microwave energy distribution model.

$\mathrm{<span\; class="notranslate">\; Q</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; \&omega;</span>}{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}^{<span\; class="notranslate">\; \text{'}</span><span\; class="notranslate">\; \text{'}</span>}\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; E.</span>}}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 17</span>}<span\; class="notranslate">\; )</span>$

In formula (17), Q represents the electromagnetic field energy density at any position in the wood W, ω represents the angular frequency, ε ₀ represents the vacuum permittivity, ε represents the permittivity, ε″ represents the imaginary part of the complex permittivity, and ε ^* is the complex permittivity, ε ^* can be expressed as ε ^* = ε'+iε", represents the electric field strength, Indicates the electric field strength complex conjugate.

2) Establish a thermal migration model.

During microwave pretreatment, the microscopic heat balance inside wood W can be described by Equation (18) (Ayappa, 1997).

$\mathrm{<span\; class="notranslate">\; \&rho;</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; P</span>}}\frac{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; T</span>}}{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; \&dtri;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; \&dtri;</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; Q</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 18</span>}<span\; class="notranslate">\; )</span>$

In formula (18), ρ represents the density of wood, C _P represents the specific heat of wood, k _T represents the thermal conductivity of wood, Represents the divergence operation, T represents the temperature, and Q represents the energy density of the electromagnetic field in the wood.

The boundary conditions of formula (18) are shown in formulas (19) and (20).

t=0, T=T _ini , 0≤r≤R (19)

t > 0, $-k_T\frac{\&PartialD;T}{\&PartialD;r}=h(T-T_a)+L_{\mathrm{vap}}{k}_m^{\text{'}}(C_{w,\mathrm{the\; s}}-C_{\mathrm{equi}}),$ r = R (20)

In formulas (19) and (20), t represents time, T represents temperature, T _ini represents the initial temperature of wood, k _T represents the thermal conductivity of wood, h represents the thermal conductivity coefficient in wood, T _a represents the initial temperature of air, L _vap is the vaporization heat of water, k′ _m is the water exchange coefficient, C _w,s is the moisture content of the wood surface, C _equi is the air equilibrium moisture content, and R is the radius of the cylindrical wood W.

Finally, after simulation and calculation by the finite element analysis software COMSOL Multiphysics, the simulated temperature distribution diagram of the wood W surface when heated by the resonant cavity of this embodiment is shown in Figure 2. In the temperature comparison bar on the right, the lower limit value is 29.641 Celsius, the upper limit is 99.657 degrees Celsius. Referring to FIG. 2, it can be known that the microwave energy is mostly absorbed by the central region of the wood W. Therefore, the representative central section of wood W is selected as the research object, and the simulated temperature distribution map of the central section of wood W is extracted separately to obtain the simulated temperature distribution map shown in Figure 3. In the temperature comparison bar on the right, The lower limit is 29.641 degrees Celsius and the upper limit is 98.43 degrees Celsius.

Embodiment two:

The basic structure of this embodiment is the same as that of the first embodiment, and the main difference is that the number of feed-in waveguides is different.

As shown in Figure 4, the number of feed-in waveguides in this embodiment is three (see B, C, D in the figure), and the three feed-in waveguides B, C, and D are symmetrically arranged in the main resonant cavity A on the side.

In this embodiment, the finite element analysis software COMSOL Multiphysics is used for simulation, and the establishment of the electromagnetic field and microwave energy distribution model and the thermal migration model used to calculate the heat transfer inside the wood are exactly the same as those in the first embodiment. Finally, the simulated temperature distribution diagram of the surface of the wood W when heated by the resonant cavity of this embodiment is obtained after simulation and calculation by the finite element analysis software COMSOL Multiphysics, as shown in FIG. 5 . Referring to FIG. 5, it can be known that the microwave energy is mostly absorbed by the central region of the wood W. Therefore, the representative central section of wood W is selected as the research object, and the simulated temperature distribution map of the central section of wood W is extracted separately to obtain the simulated temperature distribution map shown in Fig. 6 .

Embodiment three:

The basic structure of this embodiment is the same as that of the first embodiment, and the main differences are: the shape of the main resonant cavity A and the number of feeding waveguides are different. And generally speaking, the wood W is placed at the center of the main resonant cavity A, and in order to achieve uniform distribution of microwaves, the shape of the wood W is related to the shape of the main resonant cavity A, for example, the shape of the wood W in this embodiment is a cuboid, It is different from the shape of the wood W in the first embodiment which is a cylinder.

As shown in Figure 7, the shapes of the main resonant cavity A and the feed-in waveguide in this embodiment are both cuboid, the number of feed-in waveguides is two (see B, C in the figure), and the two feed-in waveguides B , C are respectively symmetrically arranged on the side of the main resonant cavity A, that is: the feed-in waveguide B is arranged on one side of the main resonant cavity A, and the feed-in waveguide C is symmetrically arranged on the other side of the main resonant cavity A opposite to the aforementioned side on one side.

In this embodiment, the length and width of the main resonator A are both 0.197m and 0.155m in height, and the length and width of the rectangular cross section of the feeding waveguide are 0.0953m and 0.0546m.

This embodiment adopts the finite element analysis software COMSOL Multiphysics to carry out the simulation, when establishing the electromagnetic field and microwave energy distribution model and the thermal migration model used to calculate the heat transfer inside the wood, because the shape of the wood W in this embodiment is a cuboid (and the implementation The cylinder in Example 1 is different), so the boundary conditions in the electromagnetic field and the microwave energy distribution model and the thermal migration model will be slightly different, but the basic principle of its modeling is the same as that of Example 1, so here No longer. Finally, the simulated temperature distribution diagram of the surface of the wood W when heated by the resonant cavity of this embodiment is obtained after simulation and calculation by the finite element analysis software COMSOL Multiphysics, as shown in FIG. 8 . Referring to FIG. 8, it can be known that the microwave energy is mostly absorbed by the central region of the wood W. Therefore, the representative central section of wood W is selected as the research object, and the simulated temperature distribution map of the central section of wood W is extracted separately to obtain the simulated temperature distribution map shown in FIG. 9 .

Embodiment four:

The basic structure of this embodiment is the same as that of the first embodiment, and the main differences are: the shape of the main resonant cavity A and the number of feeding waveguides are different. Same as the third embodiment, the shape of the wood W in this embodiment is different from that of a cylinder.

As shown in Figure 10, the shapes of the main resonant cavity A and the feed-in waveguide in this embodiment are both cuboid, and the number of feed-in waveguides is one (see B in the figure), and the feed-in waveguide B is located in the main resonant cavity A on one side of the

In this embodiment, the length and width of the main resonator A are both 0.197m and the height is 0.155m, and the length and width of the rectangular cross-section of the feeding waveguide B are 0.0953m and 0.0546m.

This embodiment adopts the finite element analysis software COMSOL Multiphysics to carry out the simulation, when establishing the electromagnetic field and microwave energy distribution model and the thermal migration model used to calculate the heat transfer inside the wood, because the shape of the wood W in this embodiment is a cuboid (and the implementation The cylinder in Example 1 is different), so the boundary conditions in the electromagnetic field and the microwave energy distribution model and the thermal migration model will be slightly different, but the basic principle of its modeling is the same as that of Example 1, so here No longer. Finally, the simulated temperature distribution diagram of the surface of the wood W when heated by the resonant cavity of this embodiment is obtained after simulation and calculation by the finite element analysis software COMSOL Multiphysics, as shown in FIG. 11 . Referring to FIG. 11 , it can be known that the microwave energy is mostly absorbed by the central region of the wood W. Therefore, the representative central section of wood W is selected as the research object, and the simulated temperature distribution map of the central section of wood W is extracted separately to obtain the simulated temperature distribution map shown in FIG. 12 .

Comprehensively comparing the simulated temperature distribution diagrams of the surface of the wood W in Examples 1 to 4 and the simulated temperature distribution diagrams of the central section of the wood W, it can be seen that the microwave energy in Examples 1 to 4 will be more dispersed on the wood W rather than It will be concentrated at the feeding entrance, so it can effectively reduce the wood cracking rate caused by local high temperature, and can effectively analyze the influence of various feeding methods on wood microwave energy utilization, temperature distribution uniformity, and wood cracking rate. Moreover, for the one-port feeding method of Embodiment 4 (the microwaves are concentrated on one side of the wood), the two-port feeding method of Embodiment 3 will make the microwave energy form a symmetrical distribution, and further, the three-port feeding method of Embodiment 2 will make The distribution of microwave energy is more uniform. The four-port feed-in method in Embodiment 1 will make the distribution of microwave energy relatively uniform. Therefore, it can be determined that the side of the main resonator A is provided with a larger number of horizontally arranged feed-in waveguides. , the wood microwave energy utilization rate is higher, the microwave energy is more uniform and more dispersed, the temperature distribution is more uniform, and the wood cracking rate is lower.

The above descriptions are only preferred implementations of the present invention, and the protection scope of the present invention is not limited to the above-mentioned embodiments, and all technical solutions under the idea of the present invention belong to the protection scope of the present invention. It should be pointed out that for those skilled in the art, some improvements and modifications without departing from the principle of the present invention should also be regarded as the protection scope of the present invention.

Claims (10)

1. A resonator for verifying the temperature distribution of wood microwave pretreatment, characterized in that: it comprises a horizontally arranged main resonator, the side of the main resonator is provided with at least one feed-in waveguide, and the main resonator communicate with the feed-in waveguide, the feed-in waveguide is arranged horizontally along the direction outward from the center of the main resonant cavity, and both end faces of the feed-in waveguide are hollow openings. 2. The resonant cavity for verifying the temperature distribution of wood microwave pretreatment according to claim 1, characterized in that: the cross-section of the feed-in waveguide is a rectangle, and the length of the rectangle is arranged along the horizontal direction, so The width of the rectangle is arranged along the vertical direction, and the distance L between the hollow opening at the end away from the center of the main resonant cavity on the feeding waveguide and the center of the main resonant cavity is greater than 1.1 times the length of the rectangle. 3. the resonant cavity for verifying the temperature distribution of wood microwave pretreatment according to claim 2, characterized in that: the shapes of the main resonant cavity and the feed-in waveguide are cuboid, and the number of the feed-in waveguide is One, the feed-in waveguide is arranged on one side of the main resonant cavity. 4. The resonant cavity for verifying the temperature distribution of wood microwave pretreatment according to claim 3, characterized in that: the length and width of the main resonant cavity are 0.197m, and the height is 0.155m, and the feed-in waveguide The length of the rectangle of the cross section was 0.0953 m, and the width was 0.0546 m. 5. the resonant cavity for verifying the temperature distribution of wood microwave pretreatment according to claim 2, characterized in that: the shapes of the main resonant cavity and the feed-in waveguide are cuboid, and the number of the feed-in waveguide is Two, and the two feeding waveguides are respectively symmetrically arranged on the side surfaces of the main resonant cavity. 6. The resonant cavity for verifying the temperature distribution of wood microwave pretreatment according to claim 5, characterized in that: the length and width of the main resonant cavity are both 0.197m and 0.155m high, and the feed-in waveguide The length of the rectangle of the cross section was 0.0953 m, and the width was 0.0546 m. 7. The resonant cavity for verifying the temperature distribution of wood microwave pretreatment according to claim 2, characterized in that: the shape of the main resonant cavity is a cylinder, and the shape of the feed-in waveguide is a cuboid. The number of the feed-in waveguides is three, and the three feed-in waveguides are symmetrically arranged on the side of the main resonant cavity. 8. The resonant cavity for verifying the temperature distribution of wood microwave pretreatment according to claim 7, characterized in that: the diameter of the main resonant cavity is 0.197m, the height is 0.155m, and the cross-section of the feed-in waveguide The rectangle has a length of 0.0953m and a width of 0.0546m. 9. The resonant cavity for verifying the temperature distribution of wood microwave pretreatment according to claim 2, characterized in that: the shape of the main resonant cavity is a cylinder, and the shape of the feed-in waveguide is a cuboid. The number of the feed-in waveguides is four, and the four feed-in waveguides are symmetrically arranged on the side of the main resonant cavity. 10. The resonant cavity for verifying the temperature distribution of wood microwave pretreatment according to claim 9, characterized in that: the diameter of the main resonant cavity is 0.197m, the height is 0.155m, and the cross-section of the feed-in waveguide The rectangle has a length of 0.0953m and a width of 0.0546m.