A method for reducing reflectivity in a microwave heating process

Abstract

The application discloses a method for reducing reflectivity in a microwave heating process, and belongs to the technical field of microwave heating. The application adopts mechanical design combined with a numerical simulation scheme to build an improved strategy suitable for the case of large microwave reflection, so as to solve the problem that a microwave source is damaged due to excessive reflection, and further optimize the reflection of the microwave system to reduce the frequent breakdown phenomenon of high-power microwaves at the corners. The optimization parameters determined by the application include but are not limited to the microwave source phase under the optimal reflection condition, the dielectric constant of the content in the microwave resonant cavity, the microwave source position, the microwave waveguide shape and the placement position of the food in the resonant cavity; the microwave power and food processing parameters before optimization are adopted, and the reflection of the microwave equipment after optimization according to the technical scheme is recorded. The application can provide guidance for low-frequency, high-power microwave or other microwave systems that may have reflection problems.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of microwave heating technology, in particular to a method for reducing reflectivity in the process of microwave heating. BACKGROUND

[0002] Low-frequency microwave has low frequency and large penetration depth in liquid medium and food medium. This advantage can greatly improve the processing thickness and processing amount of food when the food is processed by microwave. At the same time, this characteristic also has a disadvantage. Since the penetration depth of microwave in the medium is large, the microwave source will detect a part of the reflected microwave, resulting in damage to the microwave source. SUMMARY

[0003] The purpose of the present application is to provide a method for reducing reflectivity in the process of microwave heating to make up for the shortcomings of the prior art.

[0004] The present application adopts mechanical design combined with numerical simulation scheme to build an improved strategy suitable for the case of large microwave reflection, so as to solve the problem of damage to the microwave source due to excessive reflection, and further optimize the reflection of the microwave system to reduce the frequent breakdown of high-power microwave at the corners.

[0005] On the basis of ensuring the optimal heating effect on food, the present application carries out the technical scheme of reducing the subsequent reflectivity according to the maximum reflectivity limit value that the microwave source can withstand. After numerical simulation of reflection optimization, the same condition parameters should be used for experimental comparison to determine the accurate value of the actual reducible reflectivity.

[0006] To achieve the above purpose, the present application adopts the following technical scheme:

[0007] A method for reducing reflectivity in the process of microwave heating, which reduces the reflectivity from two aspects of microwave device design and device use. First, the microwave device is geometrically modeled to reduce the reflectivity from the equipment design level, including the phase of the microwave source, the emission and composition of the microwave source, the design of the microwave transmission member, etc. Then, the actual use of the microwave device is designed, including changing the electrical and thermal parameters of the dielectric in the resonant cavity, the size of the food, the spacing between the front and rear arrangement of the food, the number of food arrangement, etc.

[0008] Further, the specific steps of the method are as follows:

[0009] S1: Geometric modeling of microwave device;

[0010] S2: Collecting reflectivity related data;

[0011] S3: Determining the type of microwave source;

[0012] S4: Optimize the microwave source: the position of the microwave source, the microwave source is single or double; the distance between the microwave sources should be 2.5-4.5 times the length of the rectangular waveguide at the frequency of the microwave source, so as to reduce the electric field interference between adjacent microwave sources; the microwave source can adopt single emission mode, which can reduce the reflectivity by about 15%-30% compared with double microwave sources, but the heating effect on food is reduced by about 20%, which should be considered comprehensively according to the actual food heating demand and the acceptable reflection limit value of the microwave source; the microwave source can also be double microwave sources, in which case other factors for reducing reflectivity should be optimized to obtain lower microwave reflectivity;

[0013] S5: The phase interval of the microwave source is 0-2π, and the phase difference interval of the microwave source designed symmetrically up and down is 0-2π;

[0014] S6: Reduce the reflectivity by changing the shape of the tapered waveguide; the shape of the tapered waveguide is preferably a right-angle tapered waveguide, which can reduce the reflectivity by about 10% based on the isosceles tapered waveguide; if the isosceles tapered waveguide is selected, other factors for reducing reflectivity should be combined to reduce reflectivity;

[0015] S7: Reduce the reflectivity by modifying the dielectric properties in the resonant cavity; the dielectric loss of the liquid in the microwave resonant cavity should be in the range of 0.5-5, so as to reduce the consumption of microwave energy by the liquid medium in the microwave resonant cavity, and more microwave energy is used for food heating;

[0016] S8: Control the size of the food in the use of the microwave device to reduce the reflectivity; the size, length, width and thickness of the food should be kept in the range of 50%-90% of the length, width and height of the microwave resonant cavity, so that more energy can be absorbed by the food, thereby reducing the reflectivity;

[0017] S9: Reduce the reflectivity by designing the front and rear spacing of single-row food; the front and rear spacing of single-row food should be 1.1-1.3 times, which can effectively reduce the reflectivity;

[0018] S10: Reduce the reflectivity by controlling the upper and lower spacing of double-row food; the upper and lower spacing of double-row food should be kept in the range of 1.1-1.6 times, which can reduce the reflectivity and increase the food heating area;

[0019] S11: Reduce the reflectivity by exhausting the liquid in the microwave resonant cavity.

[0020] S12: Reduce the reflectivity from the design level of the microwave device according to S4-S6, and reduce the reflectivity from the use level of the microwave equipment according to S7-S11.

[0021] The optimized parameters determined in the above steps include, but are not limited to, the phase of the microwave source, the dielectric constant of the contents of the microwave resonant cavity, the position of the microwave source, the shape of the microwave waveguide, and the placement position of the food in the resonant cavity. Subsequently, mechanical design and processing are performed according to these parameters. The reflection of the microwave equipment optimized according to the present technical solution is recorded using the microwave power and food processing parameters before optimization.

[0022] Further, the S1 specifically collects the component size and spatial position parameters of the microwave equipment with high reflectivity, including but not limited to the length, width, height, thickness (inner wall thickness), whether there are round corners, chamfers, etc. of each component; according to the collected data, electromagnetic field simulation software based on finite element or finite time domain difference is used to perform geometric modeling on each component of the microwave equipment one by one.

[0023] Further, in the S2, the collected reflectivity-related data should include the transmission power, reflected power, voltage, current, and VSWR value of each microwave source, and other key indicators; when performing reflectivity analysis, the reflectivity (percentage) or reflected dB value should be used for representation.

[0024] Further, in the S3, the types of microwave sources include, but are not limited to, rectangular, transverse electromagnetic (TEM), coaxial, circular, and periodic.

[0025] Further, in the S5, when it is a single-direction microwave source, the phase of the microwave source does not affect the heating mode and reflectivity, and 0-2π is selected; when it is an up-down microwave source, the phase difference between the upper and lower microwave sources should be π, at which time the heating effect of the middle layer of the food is good and the reflectivity is low; when there are two groups of microwave sources, the phase difference of the microwave sources in different groups can be combined, one group should be π to obtain a good microwave heating effect in the middle of the food, and the other group should be 0 to obtain a microwave heating effect on the upper and lower sides of the food; for multiple groups of microwave sources, the same way as for two groups of microwave sources is used for cyclic arrangement to ensure a good heating effect and low reflectivity.

[0026] Further, in the S7, when the loss of the liquid medium is in the range of 0.5-2, the reflection is large, and the reflectivity should be reduced from the design and use levels of the microwave equipment; when the dielectric loss of the liquid medium is in the range of 2-5, part of the microwave energy is absorbed by the liquid medium, and the overall reflectivity is low.

[0027] Further, in the S8, when the size of the heated food is reduced to 50% or less, the reflectivity should be reduced by combining other equipment design and use level methods.

[0028] Further, in the S9, the reflectivity is lowest when the front and rear spacing of the single row of food is 1.2 or 1.3 times the spacing.

[0029] Further, in the S11, the frequency of the exhaust should be at least once per hour.

[0030] Compared with the prior art, the present application has at least the following beneficial effects:

[0031] The present application designs a complete technical solution for the microwave aggregation effect caused by reflectivity and high power. It can provide an optimization strategy with low time cost, low processing cost and low economic cost for the current microwave equipment reflection problem, and can reduce the reflection by about 30% to 70%. For most microwave equipment, in addition to the requirement of low reflectivity, it is still required to achieve high food energy utilization rate. The top-down technical solution is designed, and different design scheme combinations can reduce reflectivity while preserving food microwave energy utilization rate as much as possible. The present application can provide guidance for low-frequency, high-power microwave or other microwave systems that may have reflection problems. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 The technical solution flowchart of the present application.

[0033] Figure 2 The single group of microwave sources and double group of microwave sources schematic diagram.

[0034] Figure 3 The initial phase change of the microwave source when there is no food and when there is food affects the reflectivity.

[0035] Figure 4 The heating mode in the microwave resonant cavity when the phase difference is π and the phase difference is 0.

[0036] Figure 5 The geometric model and microwave source reflection when the food ratio is 90% (microwave sources 1 and 2 are a group, and 3 and 4 are a group).

[0037] Figure 6 The combined model and microwave source reflection when the food ratio is 70% (microwave sources 1 and 2 are a group, and 3 and 4 are a group).

[0038] Figure 7 The combined model and microwave source reflection when the food ratio is 50% (microwave sources 1 and 2 are a group, and 3 and 4 are a group).

[0039] Figure 8 The combined model and microwave source reflection when the food ratio is 30% (microwave sources 1 and 2 are a group, and 3 and 4 are a group).

[0040] Figure 9For food ratio of 90%, the front and rear spacing of food is 1.1, 1.2, 1.3 times the spacing geometric model and reflection.

[0041] Figure 10 For food ratio of 90%, the front and rear spacing of food is 1.4, 1.5 times the spacing geometric model and reflection.

[0042] Figure 11 For double row food single width ratio of 45%, length and thickness ratio of 90%, the upper and lower center spacing of food is 1.05-1.20 of the upper and lower center distance.

[0043] Figure 12 For double row food single width ratio of 45%, length and thickness ratio of 90%, the upper and lower center spacing of food is 1.25-1.40 of the upper and lower center distance.

[0044] Figure 13 For the reflection dB value and food temperature change when the liquid water medium is pure water.

[0045] Figure 14 For the reflection dB value and food temperature change when the liquid water medium is tap water.

[0046] Figure 15 For the geometric modeling, electric field distribution, simulation and experimental verification of the reflection when using right-angle tapered waveguide.

[0047] Figure 16 For the geometric modeling, electric field analysis, simulation and experimental verification of the reflection when using isosceles tapered waveguide.

[0048] Figure 17 For the geometric modeling, electric field distribution, simulation and experimental verification of the reflection when using isosceles tapered waveguide single microwave source.

[0049] Figure 18 For the geometric modeling, reflection and temperature distribution of cylindrical food when exhaust treatment is carried out in the microwave resonant cavity.

[0050] Figure 19 For the geometric modeling, reflection and temperature distribution of cylindrical food when exhaust treatment is not carried out in the microwave resonant cavity. DETAILED DESCRIPTION

[0051] For the purposes of the present disclosure, the technical solutions and advantages are more clearly and obviously understood, the present disclosure is further described in detail below with reference to the accompanying drawings and in conjunction with specific embodiments. Obviously, the described embodiments are part of the embodiments of the present disclosure, rather than all the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present disclosure.

[0052] The following examples calculate the optimization effect: reflection reduction rate = pre-optimization reflectivity - post-optimization reflectivity.

[0053] Example 1, Influence of phase difference of microwave source on reflection

[0054] For different equipment and specific needs, the phase of the microwave source may be different. For the standing wave formed by the microwave in the metal resonant cavity, the positions of the nodes and the peaks in the resonant cavity are different. Generally speaking, people hope that the center of the food can be effectively and quickly heated, and the reflection optimization is carried out on the basis of meeting this goal. This embodiment designs a set of upper and lower symmetrical microwave source resonant cavities, and the specific geometric shape is as shown in Figure 2 A, in order to further analyze the influence of the food, two groups of models with and without food are designed, and the size of the food is 140 mm*200 mm*20 mm. Numerical simulation software is used for geometric modeling and study on the reflectivity of the phase difference of the upper and lower microwave sources.

[0055] Figure 3 A and Figure 3 B show the influence of the initial phase change of a single microwave source on the reflectivity when there is food and no food in a single set of upper and lower symmetrical microwave sources. The abscissa is the phase difference of the upper and lower microwave sources, and the coordinate 0 indicates that the phase difference is 0, and the coordinate 6*pi / 3 indicates that the phase difference is 2π (2*pi). Each adjacent abscissa value in the figure represents a phase difference of π / 3. Figure 3 It is shown that the change trend of the reflection with and without food is basically the same, and the difference between the two is that the difference between the reflectivities of the two microwave sources is smaller when there is food, and the sum of the reflectivities of the two microwave sources is larger. Taking the case where the phase of the reflectivity is π as an example, after adding the food, the reflectivity can be reduced from 0.6 to below 0.2.

[0056] In addition, in production and life, multiple microwave resonant cavities are often used in series. When there are two sets of upper and lower symmetrical microwave sources, people hope that the surface and the middle internal area of the food can obtain the best heating effect. Therefore, when considering the reflection of the phase of the microwave source, the expected heating effect of the microwave equipment needs to be considered.

[0057] This embodiment also designs a set of upper and lower symmetrical microwave source resonant cavities as shown in Figure 2Figure B shows the two groups of symmetrical microwave sources constitute a series of microwave resonant cavity. At this time, the two groups of microwave sources and a single group of microwave sources using the same design of the upper and lower symmetry, at this time for the group, the reflection of the upper and lower microwave sources is consistent with the single group (Figure 1B). Figure 3 The phase difference between the upper and lower microwave sources of the first group of microwave resonant cavities is π (pi), and the phase difference between the upper and lower microwave sources of the second group of microwave resonant cavities is 0. Among them, the first group is planned to quickly heat the food, and the second group is used to raise the temperature of the surface and the middle content of the food.

[0058] Figure 4 Figure A shows the heating mode distribution when the phase difference is π (pi), Figure 4 Figure B shows the heating mode distribution when the phase difference is 0. It can be seen that for the phase difference of pi, the place with high electric field intensity is in the middle of the cavity, and for the phase difference of 0, the place with maximum electric field intensity is in the center of the upper and lower parts. Two groups of microwave sources can be designed, one group of microwave sources with a phase difference of pi to have a better heating effect on the inside of the food, and the other group of microwave sources with a phase difference of 0 to quickly heat the upper and lower layers of the food. The combination of the two groups of microwave sources can achieve rapid heating of the whole food. Combined with the analysis, Figure 3 When there is no food, the reflectivity of the first group of microwave sources is about 0.6, and the reflectivity of the second group is about 0.65; when there is food, the reflectivity of the first group of microwave sources is about 0.15, and the reflectivity of the second group of microwave sources is about 0.95. This shows that the upper and lower symmetrical microwave sources with a phase difference of π (pi) are suitable for heating the center of the food and have a lower reflectivity. When the expected heating requirement requires precise heating of the upper and lower layers of the food, the reflectivity is larger, and the reflectivity should be optimized in combination with other microwave equipment design and use level technical solutions.

[0059] Example 2, the coverage area of the food is 50% to 90%

[0060] The test of the coverage area of the food is taken as an example. The thickness of the food is set to be the same 2 cm, and the microwave heating treatment is carried out when the food covers the cavity area by 70% and 50%, respectively, to explore the microwave heating effect and reflectivity. According to the parameterization scanning of the running of the food in the resonant cavity, it is checked whether the coverage area of the food affects the reflectivity. In order to more intuitively see the influence on the reflection, the display method of the reflectivity is changed from the proportion to the dB value.

[0061] The length, width and thickness of the resonant cavity are set to 1415 mm, 230 mm and 81 mm respectively. The length and width of food 1 are 90% of the length and width of the microwave resonant cavity, i.e. 1274 mm and 207 mm, and the thickness is selected to be 73 mm. The length and width of food 2 are 70% of the length and width of the microwave resonant cavity, i.e. 1015 mm and 160 mm, and the thickness is selected to be 56 mm. The length, width and thickness of food 3 are 50% of the length, width and thickness of the microwave resonant cavity, i.e. 710 mm, 115 mm and 40 mm.

[0062] The reflection when the size of the food is 90% of the size of the resonant cavity is shown in Figure 2. The reflection is minimum at -7 dB when the food center is at -0.2 m and 0.2 m. The reflection of one group of microwave sources is less than -1.5 dB at other time. Figure 5

[0063] The reflection when the size of the food is 70% of the size of the resonant cavity is shown in Figure 3. The reflection is minimum at -2 dB when the food center is at -0.8 m and 0.8 m. The food is almost in the middle of the first resonant cavity and the second resonant cavity at this time. Figure 6

[0064] The reflection when the size of the food is 50% of the size of the resonant cavity is shown in Figure 4. Similar to the case when the size of the food is 70%, the reflection is also minimum when the food center is at -0.8 m and 0.8 m. The reflection increases from -2 dB when the size of the food is 70% to -0.37 dB when the size of the food is 50%. This indicates that the area covered by the food in the resonant cavity has a great influence on the reflection of the microwave source. Figure 7

[0065] In addition, the following common phenomenon can be observed when the size of the food is 70% and 50%. It can be observed that the reflection coefficients of microwave sources 1 and 2 gradually decrease when the food passes through the first microwave source. The reflection of the second resonant cavity where microwave sources 3 and 4 are located is almost around 0 dB since there is no food in the resonant cavity. This indicates that the reflection will increase significantly when there is no food in the resonant cavity. In summary, the length, width and thickness of the food can be selected to be 50% to 90% of the size of the resonant cavity, and the microwave reflection is within an acceptable range at this time. If the actual situation permits, the size of the food should be controlled to be 70% to 90% as much as possible to obtain a lower reflection and reduce the damage to the microwave source.

[0066] In Comparative Example 2, the area covered by the food is less than 30%

[0067] ​​​Similar to Example 2, the length, width, and thickness of the resonant cavity are set to 1415 mm, 230 mm, and 81 mm, respectively. The length, width, and thickness of the food 4 are 30% of the length, width, and thickness of the microwave resonant cavity, i.e., 425 mm, 70 mm, and 25 mm, respectively.

[0068] The reflection situation when the food size is 30% of the resonant cavity size is as follows: Figure 8 As shown in the diagram, the overall reflection pattern changes. Microwave sources 1 and 2 show increased reflection at -0.8 m, 0.6 m, and 0.8 m, 0.6 m. The minimum values ​​are concentrated at -0.4 m and 0.4 m, approximately 0.09 dB. This change in reflection pattern may be due to the small size of the food, causing microwaves to enter the left and right surfaces of the food, resulting in some microwave refraction or reflection, thus altering the proportion of reflected microwaves received by the microwave sources. The above results indicate that when the size of the food within the resonant cavity decreases, reflection increases significantly. The food size should be limited to the range of 50%–90%, allowing as many microwaves as possible to pass through the food rather than penetrate the cavity to enter the opposing microwave source, thereby reducing reflection.

[0069] Example 3: Geometric models and reflections of food components with front-to-back spacing of 1.1, 1.2, and 1.3 times the food spacing when the food component accounts for 90% of the total volume.

[0070] Example 2 and Comparative Example 2 show that when there is only one food item, the second microwave source experiences significant reflection when the food item passes through the first microwave source because there is no food item present. Therefore, the location and spacing of food items should be rationally planned and allocated to achieve production continuity and effectively reduce reflectivity.

[0071] Based on the experience of Example 2, this example sets up multiple groups of food feeders to ensure that food is always passing through each microwave source. For example... Figure 9 As shown in Figure A, extended sections are added to the left and right sides of the microwave resonant cavity to enable continuous processing of food. Similar to Example 2, the length, width, and thickness of the resonant cavity are set to 1415 mm, 230 mm, and 81 mm, respectively.

[0072] The length and width of food group 4 are 90% of the length, width, and thickness of the microwave resonant cavity, respectively, which are 1274 mm, 207 mm, and 73 mm. The center distance between the front and rear food items is 1.1 times the length of the food item. Figure 9 B shows the reflection of a 90% sized food item at a center-to-center distance of 1.1 times the microwave's width. At this distance, the reflection from the four microwave sources is approximately the same, averaging around -1.7 dB. The minimum reflection of -3.3 dB is obtained when the food's center is at 0°. Figure 10As shown, when the distance between the food center and the food is less than 1.3 times, the reflection inside the food is small, and the reflection when the food runs to the 0 position is the smallest, which is -10.5 dB when the distance is 1.2 times and -9.5 dB when the distance is 1.3 times. The reflection at the center position 0 is smaller than that when the distance is 1.1 times. Therefore, for the food of this size, the distance between the front and back of the food should be selected as 1.2 times or 1.3 times to obtain smaller reflection.

[0073] Schematic diagram and reflection of the food group with 90% size, 1.4-1.5 times distance food group with continuous feeding of Example 3

[0074] The length and width of the single-row food group 6 are 90% of the length, width and thickness of the microwave resonant cavity, i.e. 1274 mm, 207 mm, and the thickness is selected as 73 mm. When the distance between the foods is greater than 1.4 times, the reflectivity of the middle part will increase. Figure 10 A and Figure 10 B shows the reflection when the distance between the foods is 1.4 and 1.5, respectively. When the distance is increased to 1.4, the reflectivity near the middle position increases rapidly to about -1.5 dB. When the distance is further increased to 1.5 times, the reflectivity at the food position 0 increases by about -1 dB. The above results show that when the distance between the foods is increased to 1.4 or more, the reflection increases, and the distance between the foods should be limited to within 1.3 times.

[0075] Example 4, when the single width of the double-row food is 45%, the length and thickness are 90%, and the distance between the upper and lower centers of the foods is 1.10-1.60, the reflection

[0076] The length and width of the double-row food group 5 are 45% of the length, width and thickness of the resonant cavity, and the two rows of foods together account for 90%, and the distance between the front and back of the foods is 1.1 times. The distance between the upper and lower rows of foods is set to be between 1.1 and 1.6 times of the width (based on 45% of the resonant cavity size) of the foods. Figure 11 The reflection and temperature distribution of the double-row foods are shown. When the distance between the upper and lower rows of foods is reduced from 1.1 times to 1.6 times, the reflection at the beginning of the food running and at the end of the food running is reduced from about 3.14 dB to 3.85 dB; the reflection when the food runs to ±-1 m to ±0.4 m is reduced from -3.25 dB to -4.35 dB; and the reflection when the food runs to the position 0 is reduced from -3.02 dB to -3.55 dB. The above results show that when the distance between the upper and lower rows of foods is increased from 1.1 to 1.6 times, the reflection value gradually decreases.

[0077] Figure 11The right column shows that the highest temperature of the double-row food increases from 106 ℃ to 133 ℃ when the upper and lower spacing of the double-row food is in the range of 1.1-1.6, and the area of the food with medium temperature gradually decreases, and the area that can be heated on both sides of the food gradually decreases. Therefore, when trying to control the reflection, the most suitable food arrangement should be selected by combining the temperature heating condition with the system analysis.

[0078] Comparative Example 4, double-row food covering resonant cavity ratio is 90%, the upper and lower spacing of the double-row food is 1.70-1.80 times

[0079] The length and width of the double-row food 6 are 90% and 45% of the length, width and thickness of the resonant cavity, respectively, and the resonant cavity width is 90% after the combination of the upper and lower rows. The front and rear spacing of the food group is 1.1 times the size in the x direction. The double-row food is placed in the upper and lower rows, and the spacing therebetween is 1.7-1.8 times the width of a single food (based on 45% of the resonant cavity size). Figure 12 The left column shows the reflection of the resonant cavity. When the spacing of the upper and lower rows of food is increased by 1.7-1.8 times, the reflection value is further increased from -3.75 dB to -3.6 dB at the beginning and end of the food running; when the food runs to ±1 m to ±0.5 m, the reflection value increases from -4.30 dB to -4.20 dB; when the food runs to 0 m, the reflection value increases from -3.45 dB to -3.35 dB. Figure 12 The right column shows the temperature distribution when the spacing between the upper and lower rows of food increases from 1.7 times to 1.8 times, and the highest temperature decreases from 133 ℃ to 132 ℃; at this time, the area that is heated on both sides of the food gradually decreases, and at 1.8 times, the food on both sides (especially outside the microwave source coverage) is almost not heated. Therefore, when controlling the reflection, the food heating condition should also be considered, especially when the temperature difference between the highest temperature and the lowest temperature of the food is required. In summary, the upper and lower spacing of the double-row food should be controlled in the range of 1.1-1.6 to obtain a lower reflection rate and a larger heating area.

[0080] Example 5, reflection when pure water and purified water are used as medium

[0081] The purified water by water purifier is used for dielectric property determination, and the dielectric constant of the purified water is 57.2, the dielectric loss is 1.9j, and the conductivity is 4.57*10-2 S / m. The dielectric properties and the electric properties of the purified water are used as the filling medium in the resonant cavity for research. The reflection and the food heating effect under the filling medium are analyzed in depth. Two groups of microwave sources are used for heating, and the power of each microwave source is 100 W, and the running time is 12 s. The dielectric properties of the food are 80-40j (the imaginary part is negative), the thermal conductivity is 0.50 W / (m*K), the density is 1210.3 kg / m³, the constant pressure heat capacity is 4.35 J / (kg*K), and the conductivity is 0.98 S / m.

[0082] The numerical simulation results of the reflection when the purified water is used as the filling medium of the resonant cavity are shown in Figure 13 A. The microwave source reflection value without food is -2.4 dB, and the lowest reflection with food is -3.7 dB. When the food enters the position 0, the reflection is -1.7 dB. The heating situation of the food when the purified water is used as the filling medium of the resonant cavity is shown in Figure 13 B. During the heating process, the temperatures on the left and right sides are relatively high, reaching 372 °C; while the temperatures on the upper and lower sides are relatively low, with the lowest temperature being 327 °C. When the purified water is used as the filling medium, the food is heated to a relatively optimal effect. At this time, the reflectivity is about -1.8 dB, and if it does not meet the requirements of the microwave source for reflectivity, the reflectivity should be reduced by combining other microwave equipment design and use level strategies.

[0083] The reflection when the tap water is used as the medium in Comparative Example 5

[0084] The tap water flowing out of the faucet is used for dielectric property determination, and the dielectric constant is 52, the dielectric loss is 6.5j, and the corresponding conductivity is 0.1555 S / m. The dielectric properties of the tap water are used as the filling medium in the resonant cavity for research. The reflection and the heating of the food are analyzed. The processing in the microwave source is the same as that in Example 4.

[0085] Two groups of microwave sources are used for heating, and the power of each microwave source is 100 W, and the running time is 12 s. The dielectric properties of the food are 80-40j, the thermal conductivity is 0.50 W / (m*K), the density is 1210.3 kg / m³, the constant pressure heat capacity is 4.35 J / (kg*K), and the conductivity is 0.98 S / m.

[0086] The numerical simulation results of the reflection when the tap water is used as the filling medium of the resonant cavity are shown in Figure 14The reflection values without food passing are -4.2 dB, the lowest reflection is -4.75 dB, and the reflection at position 0 is -3.6 dB. Compared with the purified water, the reflection values without food passing, the lowest reflection, and the reflection at position 0 are all smaller than those of the purified water. Therefore, when the water in the microwave resonant cavity is replaced from the purified water to the tap water, the reflection condition is weakened.

[0087] The heating condition of the tap water as the resonant cavity filling medium is shown in Figure 14 B. The left and right sides are heated well, the highest temperature of the upper and lower sides is 130 ℃, the lower temperature is concentrated on the upper and lower sides, and the lowest temperature is about 117 ℃. Compared with Example 5, although the reflectivity is reduced when the tap water is used, the heating temperature of the food is reduced by about 200 ℃ in total. In summary, for the condition that there is no other condition to improve the microwave source reflection condition and the temperature requirement after the microwave heated food is relatively low, the tap water can be used for the replacement of the heating medium to reduce the reflectivity so as to realize the continuous work of the microwave source.

[0088] Example 6, selection of a right-angle waveguide to reduce the reflectivity

[0089] Figure 15 A shows the combined modeling of the right-angle tapered waveguide, Figure 15 B shows the thermal-type electric field distribution of the microwave in the right-angle tapered waveguide. Figure 15 C shows the heating condition of the 140 mm×200 mm×15 mm food in the middle layer of the XY plane in the microwave resonant cavity, and the maximum electric field strength is 3.05*103 V / m. Figure 15 D shows the numerical simulation reflection condition and the reflectivity obtained by the experimental verification when the right-angle tapered waveguide is used. The reflectivity of the food at the beginning and the end of the movement is about 0.7, the reflectivity decreases to about 0.2 when the food runs to the middle, and the numerical simulation result is well fitted with the experimental result. In summary, the right-angle tapered waveguide should be preferred to obtain a lower microwave reflectivity.

[0090] Comparative Example 6, selection of a traditional isosceles waveguide for the reflectivity

[0091] Figure 16 A is the geometric modeling condition of the right-angle tapered waveguide, Figure 16 B is the electric field distribution condition when the isosceles tapered waveguide is used. Figure 16 C is the electric field distribution condition of the 140 mm×200 mm×15 mm food in the middle layer of the XY plane in the resonant cavity, and the maximum electric field strength is 2.84*103 V / m. Figure 16D shows the numerical simulation and experimental reflection of the traditional isosceles waveguide. The experimental results show that the reflectivity of the food at the beginning and end of the movement remains at about 0.7, and when the food moves to position 0, the reflectivity decreases significantly to about 0.3. The numerical simulation results have a high degree of fitting with the experimental data. At this time, the proportion that can be used by the food is about 0.4, which is reduced by 25% compared with the food energy utilization rate when using the right-angle tapered waveguide. In summary, in the case of using an isosceles tapered waveguide, if the limit value of the microwave source emission cannot be reached, other microwave design and use level strategies should be combined to further obtain a lower reflectivity.

[0092] Example 7, explore the reflection characteristics of the microwave source single shot

[0093] Figure 17 A shows the geometric modeling of the microwave source used. Figure 17 B shows the electric field distribution inside the microwave equipment when using the microwave source single shot from above. Figure 17 C is the electric field distribution of the XY plane of the resonant cavity where the food is located, and the maximum electric field is 2.3*103 V / m. Figure 17 D shows the reflection of the single shot microwave source. At the beginning and end of the food running stage, the reflectivity is about 0.56; and when the food moves to the middle, the reflectivity further decreases to 0.43. In summary, using a microwave source to irradiate in one direction can reduce the emission rate when there is no food passing through, but the proportion of energy that can be absorbed by the food decreases. Therefore, when other microwave equipment design or use level technical solutions cannot be used to reduce reflectivity, or when the food volume is small, the microwave source single shot method can be used to reduce reflectivity.

[0094] Example 8, use a degassing tower or other exhaust scheme to reduce other contents

[0095] The reflection when the water in the microwave resonant cavity is treated by exhaust is shown in Figure 18 . The dielectric properties are 57.5 and the dielectric loss is 1.9. Figure 18 A shows the geometric model of the cylindrical food (radius 50 mm, thickness 40 mm) and the microwave equipment used. Figure 18 B is the reflection of the four microwave sources obtained using numerical simulation software. The results show that the microwave source reflection at the beginning and end of the food running without food is about 2.4 dB, and the minimum reflection at ±0.8 m is -4.1 dB. Figure 18 C is the microwave reflection during experimental verification. The reflection without food at the beginning and end of the food running is about 2.5 dB, with some fluctuations. The minimum reflection at a distance of ±0.8 meters is about -4.25 dB. Figure 18D is the temperature distribution of XY plane at the end of heating, the temperature is higher on the left and right sides, with the left side being the highest at 147 °C and the upper and lower sides being the lowest at 131 °C. Figure 18 E is the temperature distribution of YZ plane, with the middle temperature being the highest at 134 °C and the two sides being the lowest at 130 °C. Figure 18 F is the temperature distribution of XZ plane, with the left and right sides being higher, the left side of the food being the highest at 147 °C, and the middle temperature being the lowest at 133 °C. In summary, exhaust devices should be equipped as much as possible to obtain stable microwave heating effect and lower reflectivity.

[0096] Comparative Example 8, without exhaust treatment

[0097] The reflection of water in the microwave resonant cavity without exhaust treatment is shown in Figure 19 The dielectric properties of the high-temperature hot water containing gas were measured, and the dielectric constant was 35 and the dielectric loss was 0.5. Figure 19 A shows the cylindrical food (radius 50 mm, thickness 40 mm) used in Comparative Example 8 and the geometric model of the microwave equipment used. Figure 19 B is the reflection of the four microwave sources obtained using numerical simulation software. The results show that the reflection of the microwave source at the food-free position at the beginning and end of food operation is about -1.4 d B, which indicates that when no exhaust treatment is performed, the reflection will increase from -2.4 dB to -1.4 dB, and no exhaust will greatly increase the reflectivity. Figure 19 C is the reflection of the four microwave sources during experimental verification. When there is no exhaust, the reflection of the four microwave sources is different from the simulation trend, with a large difference, which indicates that when there is no exhaust, the dielectric properties of the water are the weighted combination of water and air. The reflection can only use one dielectric property of water, and cannot simulate this situation. At this time, due to the continuous reflection and refraction of microwaves by the air in the water, the heating of the food becomes unpredictable. The average dielectric properties of the water without exhaust are 35-0.5j, and the temperature values obtained by numerical simulation with this value are shown in Figure 19 D, 19E, 19F. Figure 19 D shows the temperature distribution of the XY plane of the cylindrical food, with the highest temperature on the left and right sides being 74.2 °C and the lowest temperature on the upper and lower sides being 59.9 °C. Compared with the exhaust condition of Comparative Example 8, the highest temperature decreases by about 72 °C and the lowest temperature decreases by about 71.1 °C. This indicates that most of the energy is reflected back to the microwave source by the air bubbles in the water. Figure 19 E is the temperature distribution of YZ plane, with the middle temperature being the highest at 62.1 °C, which is 71.9 °C lower than the temperature when exhaust is used; the two sides are the lowest, which is 70.3 °C lower than the temperature when exhaust is used. Figure 19F is the temperature distribution of YZ plane, the temperature on both sides is higher, the left side temperature is the highest, 74.2 ℃, less than the exhaust temperature by 72.8 ℃; the middle temperature is the lowest, 61.9 ℃, less than the exhaust temperature by 71.1 ℃. In summary, without exhaust, the microwave heating effect will be greatly affected, and the reflectivity will be increased.

[0098] The above-described specific embodiments further illustrate the purposes, technical solutions and beneficial effects of the present disclosure, and it should be understood that the above-described is only a specific embodiment of the present disclosure and is not intended to limit the present disclosure. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present disclosure should be included in the protection scope of the present disclosure.

Claims

1. A method for reducing reflectivity in a microwave heating process, characterized by, The method reduces the reflectivity from two aspects of microwave device design and device use; first, the microwave device is geometrically modeled to reduce the reflectivity from the equipment design aspect, including the phase of the microwave source, the emission and composition of the microwave source, and the design of the microwave transmission member; Then, the design is made from the actual use of the microwave device, including changing the electrical and thermal parameters of the dielectric in the resonant cavity, the size of the food, the spacing between the front and rear of the food, and the number of food display rows; the specific steps of the method are as follows: S1: Geometric modeling of the microwave device; S2: Collecting reflectivity-related data; S3: Determining the type of microwave source; S4: Optimizing the microwave source: the position of the microwave source, whether the microwave source is single-shot or double-shot, the spacing between the microwave sources should be 2.5-4.5 times the long side of the rectangular waveguide at the frequency of the microwave source; the microwave source adopts a single emission mode, if the microwave source adopts a double-shot mode, it should be combined with other factors to reduce the reflectivity of the microwave; S5: The phase interval of the microwave source selected is 0-2π, and the phase difference interval of the microwave source designed symmetrically up and down is 0-2π; S6: Reduce the reflectivity by changing the shape of the tapered waveguide; if an isosceles tapered waveguide is selected, it should be combined with other factors to reduce the reflectivity of the microwave; S7: Reduce the reflectivity by modifying the dielectric properties in the resonant cavity; the dielectric loss of the liquid in the microwave resonant cavity should be within the range of 0.5-5; S8: Control the size of the food in the use of the microwave device to reduce the reflectivity; the size, length, width, and thickness of the food should be kept within 50%-90% of the length, width, and height of the microwave resonant cavity; S9: Reduce the reflectivity by designing the front and rear spacing of the single-row food; the front and rear spacing of the single-row food should be 1.1-1.3 times; S10: Reduce the reflectivity by controlling the upper and lower spacing of the double-row food; the upper and lower spacing of the double-row food should be kept within the range of 1.1-1.6 times; S11: Reduce the reflectivity by exhausting the liquid in the microwave resonant cavity.

2. The method for reducing reflectivity in a microwave heating process of claim 1, wherein, S1: For microwave equipment with high reflectivity, collect the component size and spatial position parameters of the microwave equipment, including the length, width, height, inner wall thickness, whether there is a round corner, chamfer, etc. of each component; based on the collected data, use electromagnetic field simulation software based on finite element or finite time domain difference principle to geometrically model each component of the microwave device.

3. The method for reducing reflectivity in a microwave heating process of claim 1, wherein, In S2, the collected reflectivity-related data includes the emission power, reflected power, voltage, current, and VSWR value of each microwave source; when analyzing the reflectivity, the emission rate or reflected dB value should be used to represent it.

4. The method for reducing reflectivity in a microwave heating process of claim 1, wherein, In S3, the type of microwave source includes: rectangular, transverse electromagnetic, coaxial, circular, and periodic.

5. The method for reducing reflectivity in a microwave heating process of claim 1, wherein, S5: When it is a single-direction microwave source, the microwave source phase does not affect the heating mode and reflectivity, and 0-2π is selected; when it is an up-down microwave source, the phase difference of the up-down microwave sources should be π; when there are two groups of microwave sources, the phase difference of the microwave sources of different groups is combined, wherein one group 1 should be π, and the phase difference of the other group should be 0; for multiple groups of microwave sources, the same way as the two groups of microwave sources is used for cyclic arrangement.

6. The method for reducing reflectivity in a microwave heating process of claim 1, wherein, S7: When the loss of the liquid medium is in the range of 0.5-2, the reflectivity should be reduced in combination with other factors for reducing the reflectivity of the microwave; when the dielectric loss of the liquid medium is in the range of 2-5, at this time part of the microwave energy is absorbed by the liquid medium, and the overall reflectivity is low.

7. The method for reducing reflectivity in a microwave heating process of claim 1, wherein, S8: When the size of the heated food is reduced to less than 50%, the reflectivity should be reduced in combination with other factors for reducing the reflectivity of the microwave.

8. The method for reducing reflectivity in a microwave heating process of claim 1, wherein, S9: The front and rear spacing of the single-row food is 1.2-1.3 times the spacing.

9. The method for reducing reflectivity in a microwave heating process of claim 1, wherein, S11: The frequency of the exhaust is at least once per hour.

Images