Method for reducing reflectivity in microwave heating process

Abstract

The invention discloses a method for reducing reflectivity in a microwave heating process, and belongs to the technical field of microwave heating. According to the method, mechanical design is combined with a numerical simulation scheme to build a set of improvement strategies suitable for the condition of large microwave reflection, so that the problem that a microwave source is damaged due to too large reflection is solved, reflection of a microwave system is further optimized, and the breakdown phenomenon frequently occurring at corners of high-power microwaves is reduced. Optimization parameters determined by the method include but not limited to microwave source phase under the optimal reflection condition, dielectric constant of microwave resonant cavity content, microwave source position, microwave waveguide shape and placing position of food in the resonant cavity. The reflection condition of the microwave equipment optimized according to the technical scheme is recorded by adopting the microwave power and the food processing parameters before optimization. According to the invention, guidance can be provided for low-frequency and high-power microwaves or other microwave systems which may have the problem of reflection.

Description

Technical Field

[0001] This invention relates to the field of microwave heating technology, and more specifically to a method for reducing reflectivity during microwave heating. Background Technology

[0002] Low-frequency microwaves have a low frequency, resulting in a greater penetration depth in liquid and food media. This advantage can significantly increase the processing thickness and volume of food during microwave processing. However, this characteristic also presents a drawback: due to the greater penetration depth of microwaves in the medium, the microwave source may detect some of the reflected microwaves, potentially causing damage to the microwave source. Summary of the Invention

[0003] The purpose of this invention is to provide a method for reducing reflectivity during microwave heating, thereby overcoming the shortcomings of the prior art.

[0004] This invention employs a combination of mechanical design and numerical simulation to develop an improved strategy suitable for situations with significant microwave reflection, aiming to address the problem of microwave sources being damaged due to excessive reflection. Furthermore, it optimizes the reflection of the microwave system to reduce the frequent breakdown phenomenon of high-power microwaves at corners.

[0005] This invention, while ensuring superior heating effect on food, proposes a technical solution to reduce reflectivity based on the maximum reflectivity limit that the microwave source can withstand. Furthermore, after conducting numerical simulations for reflectivity optimization, experimental comparisons should be performed using the same conditions and parameters to determine the precise value of the actual reflectivity reduction.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for reducing reflectivity during microwave heating is proposed, which achieves reflectivity reduction from two levels: microwave device design and device use. First, the microwave device is geometrically modeled to reduce reflectivity from the equipment design level, including the phase of the microwave source, the emission and composition of the microwave source, and the design of microwave transmission components. Second, the design is based on the actual use of the microwave device, including changing the electrothermal parameters of the dielectric in the resonant cavity, food size, the spacing between food items, and the number of food display rows.

[0007] Furthermore, the specific steps of the method are as follows: S1: Geometric modeling of microwave device; S2: Collect reflectance-related data; S3: Determine the type of microwave source; S4: Optimize the microwave source: Consider the microwave source's location and whether it's a single-emission or two-way transmission. The spacing between microwave sources should be 2.5-4.5 times the longer side of the rectangular waveguide at the microwave source's frequency to reduce electric field interference between adjacent sources. A single-emission microwave source can reduce reflectivity by approximately 15%-30% compared to two-way transmission, but its heating effect on food decreases by about 20%. This should be considered in conjunction with actual food heating requirements and the acceptable reflectivity limits of the microwave source. Two-way transmission is also possible; in this case, other factors that reduce reflectivity should be optimized to achieve a lower microwave reflectivity. S5: The phase range of the microwave source is 0-2π, and the phase difference range of the microwave source with symmetrical design is 0-2π. S6: Reduce reflectivity by changing the shape of the tapered waveguide; right-angled tapered waveguides are preferred, which can reduce reflectivity by about 10% compared to isosceles tapered waveguides; if isosceles tapered waveguides are used, other microwave reflectivity-reducing factors should be used in conjunction to further reduce reflectivity. S7: Reduce reflectivity by modifying the dielectric properties within the resonant cavity; the dielectric loss of the liquid in the microwave resonant cavity should be in the range of 0.5~5 to reduce the consumption of microwave energy by the liquid medium within the microwave resonant cavity, allowing more microwave energy to be used for food heating. S8: Control the size of food to reduce reflectivity when using microwave devices; the size of food, including length, width, and thickness, should be kept at 50% to 90% of the length, width, and height of the microwave resonant cavity. This allows more energy to be absorbed by the food, thereby reducing reflectivity. S9: Reduce reflectivity by designing the front-to-back spacing of a single row of food items; the front-to-back spacing of a single row of food items should be 1.1 to 1.3 times the food's density, which can effectively reduce reflectivity. S10: Reduce reflectivity by controlling the vertical spacing between the two rows of food; the vertical spacing between the two rows of food should be kept within the range of 1.1-1.6 times, at which point the microwave reflectivity is low and the area of ​​the food that can be heated is large; S11: The reflectivity is reduced by venting the liquid inside the microwave resonant cavity.

[0008] S12: Reduce reflectivity from the microwave device design level according to steps S4-S6, and reduce reflectivity from the microwave equipment usage level according to steps S7-S11.

[0009] The optimized parameters determined through the above steps include, but are not limited to, the microwave source phase under optimal reflection conditions, the dielectric constant of the contents of the microwave resonant cavity, the microwave source position, the microwave waveguide shape, and the placement of the food within the resonant cavity. Subsequently, mechanical design and processing are performed based on these parameters. Using the original microwave power and food processing parameters, the reflection characteristics of the microwave equipment optimized according to this technical solution are recorded.

[0010] Furthermore, S1 specifically involves: for microwave equipment with high reflectivity, collecting the component dimensions and spatial position parameters of the microwave equipment, including but not limited to the length, width, height, thickness (inner wall thickness), and whether there are rounded corners or chamfers of each component; based on the collected data, using electromagnetic field simulation software based on the finite element or finite time domain difference principle, geometrically modeling each component of the microwave equipment one by one.

[0011] Furthermore, in S2: the collected reflectivity-related data should include key indicators such as the transmit power, reflective power, voltage, current, and VSWR value of each microwave source; when performing reflectivity analysis, it should be expressed in terms of emissivity (percentage) or reflectance dB value.

[0012] Furthermore, in S3, the type of microwave source includes, but is not limited to, rectangular, transverse electromagnetic (TEM), coaxial, circular, and periodic.

[0013] Furthermore, in S5: when it is a unidirectional microwave source, the microwave source phase does not affect the heating mode and reflectivity, and 0-2π is selected; when it is an upper and lower opposing 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 food is better and the reflectivity is lower; when there are two sets of opposing microwave sources, the phase differences of different sets of microwave sources can be combined, one set should be π to obtain a better microwave heating effect in the middle of the food, and the other set should have a phase difference of 0 to obtain microwave heating effects on the upper and lower parts of the food; for multiple sets of opposing microwave sources, they are arranged in a cycle in the same way as the two sets of microwave sources to ensure a better heating effect and a lower reflectivity.

[0014] Furthermore, in S7, when the loss of the liquid medium is in the range of 0.5 to 2, the reflection is relatively large, and the reflectivity should be reduced by combining the design and usage aspects of microwave equipment; when the dielectric loss of the liquid medium is in the range of 2 to 5, some of the microwave energy is absorbed by the liquid medium, and the overall reflectivity is low.

[0015] Furthermore, in S8, when the size of the heated food is reduced to less than 50%, reflectivity should be reduced in conjunction with other equipment design and usage methods.

[0016] Furthermore, in S9, the reflectivity is lowest when the front-to-back spacing of a single row of food items is 1.2 or 1.3 times the spacing.

[0017] Furthermore, in S11, the exhaust frequency should be at least once per hour.

[0018] Compared with the prior art, the present invention has at least the following beneficial effects: This invention presents a complete technical solution addressing the issues of reflectivity and microwave focusing effects at high power. It offers an optimized strategy with low time, processing, and economic costs to solve the current problem of microwave equipment reflection, reducing reflection by approximately 30% to 70%. For most microwave devices that require both low reflectivity and high energy utilization in food processing, a top-down technical solution is designed. Different combinations of design schemes minimize reflectivity while maximizing microwave energy utilization in food. This invention can provide guidance for low-frequency, high-power microwave systems or other microwave systems that may suffer from reflection problems. Attached Figure Description

[0019] Figure 1 This is a flowchart of the technical solution of the present invention.

[0020] Figure 2 The diagram shows a single microwave source and a dual microwave source.

[0021] Figure 3 The effect of the initial phase change of the microwave source on reflectivity is shown when there is no food and when there is food.

[0022] Figure 4 The heating modes in the microwave resonant cavity are defined as having a phase difference of π and a phase difference of 0.

[0023] Figure 5 The geometric model and microwave source reflection are shown when the food content is 90% (microwave sources 1 and 2 are in one group, and 3 and 4 are in another group).

[0024] Figure 6 The combined model and microwave source reflection situation when the food content is 70% (microwave sources 1 and 2 are in one group, and 3 and 4 are in another group).

[0025] Figure 7 The combined model and microwave source reflection are shown when the food content is 50% (microwave sources 1 and 2 are in one group, and 3 and 4 are in another group).

[0026] Figure 8 The combined model and microwave source reflection are shown when the food content is 30% (microwave sources 1 and 2 are in one group, and 3 and 4 are in another group).

[0027] Figure 9 Geometric models and reflection patterns of food items with front-to-back spacing of 1.1, 1.2, and 1.3 times the spacing when the food content is 90%.

[0028] Figure 10 Geometric models and reflections of food items with a front-to-back spacing of 1.4 and 1.5 times the food spacing when the food content is 90%.

[0029] Figure 11The reflection of food items in a double row, where the width accounts for 45% and the length and thickness account for 90%, is calculated when the center-to-center distance between the top and bottom of the food is 1.05-1.20 mm.

[0030] Figure 12 The reflection of food items in a double row, where the width accounts for 45% and the length and thickness account for 90%, is calculated when the center-to-center distance between the top and bottom of the food is 1.25-1.40 mm.

[0031] Figure 13 The data includes the reflection dB value when the liquid medium is pure water and the temperature change of the food.

[0032] Figure 14 The data includes the reflection dB value when the liquid medium is tap water and the temperature change of the food.

[0033] Figure 15 This document presents the geometric modeling, electric field distribution, simulation, and experimental verification of reflection when using a right-angled tapered waveguide.

[0034] Figure 16 The reflection is shown in the geometric modeling, electric field analysis, simulation and experimental verification when using an isosceles conical waveguide.

[0035] Figure 17 This document describes the geometric modeling, electric field distribution, simulation, and experimental verification of a single microwave source using an isosceles conical waveguide.

[0036] Figure 18 This study provides geometric modeling, reflection characteristics, and temperature distribution of cylindrical food samples during exhaust processing within a microwave resonant cavity.

[0037] Figure 19 This study aims to model the geometry, reflection, and temperature distribution of cylindrical food within a microwave resonant cavity without venting. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the following detailed description, in conjunction with specific embodiments and accompanying drawings, further illustrates the invention. Obviously, the described embodiments are only a portion, not all, of the embodiments disclosed in this invention. All other embodiments obtained by those skilled in the art based on the embodiments disclosed in this invention without inventive effort are within the scope of protection of this invention.

[0039] The following examples calculate the optimization effect: Reflection reduction rate = Reflection before optimization - Reflection after optimization.

[0040] Example 1: The Influence of Phase Difference of Microwave Source on Reflection The phase setting of the microwave source may vary depending on the equipment and specific requirements. The positions of the nodes and crests of the standing wave formed by microwaves within the metal resonant cavity will also differ. Generally, it is desirable for the center of food to be effectively and rapidly heated; therefore, reflection optimization is performed while meeting this objective. This embodiment designs a set of vertically symmetrical microwave source resonant cavities, with the specific geometry as shown below. Figure 2 As shown in Figure A, to further analyze the impact of food presence, two models were designed: one with food and one without. The food dimensions were 140 mm * 200 mm * 20 mm. Numerical simulation software was used for geometric modeling, and the reflectivity of the phase difference between the upper and lower microwave sources was studied.

[0041] Figure 3 A and Figure 3 B shows the effect of the initial phase change of a single microwave source on reflectivity when there is food and when there is no food in a single symmetrical microwave source group. The horizontal axis represents the phase difference between the upper and lower microwave sources, with 0 indicating a phase difference of 0 and 6*pi / 3 indicating a phase difference of 2π (2*pi). Each adjacent horizontal axis value in the figure represents a phase difference of π / 3. Figure 3 The trends in reflectivity are generally consistent with those without food. The difference lies in the presence of food: the difference in reflectivity between the two microwave sources is smaller, and the sum of their reflectivity is larger. For example, when the reflectivity phase is π, adding food can reduce the reflectivity from 0.6 to below 0.2.

[0042] Furthermore, multiple microwave resonant cavities are often used in series in production and daily life. When two microwave sources with symmetrical designs are present, it is desirable for both the surface and the internal regions of the food to achieve optimal heating. Therefore, when considering reflection when using the phase of the microwave source, the expected heating effect of the microwave equipment must also be taken into account.

[0043] This embodiment also includes the following design: Figure 2 As shown in Figure B, a series microwave resonant cavity is composed of two sets of symmetrical microwave sources. In this case, the dual-set microwave sources and the single-set microwave sources employ the same vertically symmetrical design. Within each set, the reflection characteristics of the vertical microwave sources are the same as those of the single-set source. Figure 3 The situation is consistent with that of the previous case. The phase difference between the upper and lower microwave sources in the first microwave resonant cavity is π (pi), and the phase difference between the upper and lower microwave sources in the second microwave resonant cavity is 0. The first group is planned to be used for rapid heating of food, and the second group is planned to be used for raising the temperature of the surface and the contents of the food.

[0044] Figure 4 A shows the heating mode distribution when the phase difference is π (pi). Figure 4B shows the heating pattern distribution when the phase difference is 0. It can be seen that for a phase difference of pi, the electric field strength is higher in the center of the cavity; for a phase difference of 0, the maximum electric field strength is located at the center of the upper and lower parts. Two sets of opposing microwave sources can be designed. One set has a phase difference of pi for better heating of the food's interior, while the other set has a phase difference of 0, enabling rapid heating of the upper and lower layers of the food. The combination of these two microwave sources allows for rapid heating of the entire food product. Figure 3 Analysis showed that without food, the reflectivity of the first group of microwave sources was approximately 0.6, while that of the second group was approximately 0.65. With food present, the reflectivity of the first group of microwave sources was approximately 0.15, and that of the second group was approximately 0.95. This indicates that the phase difference π (pi) between the upper and lower symmetrical microwave sources is suitable for heating the center of food and has low reflectivity. When precise heating of the upper and lower layers of food is required for the desired heating effect, the reflectivity is higher, and optimization of reflectivity should be combined with other microwave equipment design and application-level technical solutions.

[0045] Example 2: The food covers a large area, 50%–90%. The coverage area of ​​the food was tested. Taking a rectangular food piece as an example, with the food thickness set to a constant 2 cm, microwave heating was performed when the food covered 70% and 50% of the cavity area to investigate the microwave heating effect and reflectivity. Parametric scanning was performed according to the food's movement within the resonant cavity to examine whether the food's coverage area affected the reflectivity. To more intuitively illustrate the impact on reflectivity, the reflectivity display method was changed from a scale to dB values.

[0046] The resonant cavity is set with a length, width, and thickness of 1415 mm, 230 mm, and 81 mm, respectively. Food item 1 has a length and width that are 90% of the microwave resonant cavity's length, width, and thickness, respectively, i.e., 1274 mm and 207 mm, with a thickness of 73 mm. Food item 2 has a length and width that are 70% of the microwave resonant cavity's length, width, and thickness, respectively, i.e., 1015 mm and 160 mm, with a thickness of 56 mm. Food item 3 has a length, width, and thickness that are 50% of the microwave resonant cavity's length, width, and thickness, respectively, i.e., 710 mm, 115 mm, and 40 mm.

[0047] The reflection situation when the food size is 90% of the resonant cavity size is as follows: Figure 5 As shown, the reflection is minimal at -0.2m and 0.2m from the center of the food, at -7dB. At other times, the microwave source for one group is less than -1.5dB.

[0048] The reflection situation when the food size is 70% of the resonant cavity size is as follows: Figure 6As shown, when the center of the food is at -0.8m and 0.8m, the food is almost in the middle of the first and second resonant cavities, and the reflectivity is at its minimum of about -2 dB.

[0049] The reflection situation when the food size is 50% of the resonant cavity size is as follows: Figure 7 As shown, similar to the case where the food size is 70%, when the food center is at -0.8m and 0.8m, the food is also located in the middle region between the first and second resonant cavities. At this point, the reflectivity increases from -2dB to -0.37dB compared to when the size is 70%. This indicates that the area covered by the food within the resonant cavity has a significant impact on the reflection of the microwave source.

[0050] Furthermore, a common phenomenon was observed when the food size was 70% and 50%. It was observed that the reflection coefficients of microwave sources 1 and 2 gradually decreased as the food passed through the first microwave source. Since there was no food in the second resonant cavity where microwave source 34 was located, its reflectivity was almost 0 dB. This indicates that the reflectivity increases 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 resonant cavity size, at which point the microwave reflectivity is within an acceptable range. If practical conditions permit, the food size should be controlled within 70% to 90% to obtain a lower reflectivity and reduce damage to the microwave sources.

[0051] Comparative Example 2: The food's coverage area is less than 30%. 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.

[0052] 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 reflection is 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% to ensure that as many microwaves as possible pass through the food rather than penetrating the cavity to enter the opposing microwave source, thereby reducing reflection.

[0053] 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. 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.

[0054] 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.

[0055] 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 10 As shown, when the center-to-center distance of the food is less than 1.3 times the particle size, the internal reflection is relatively small, with the smallest reflection occurring when the food reaches position 0. The reflection is -10.5 dB at 1.2 times the particle size and -9.5 dB at 1.3 times the particle size. The reflection at the center position 0 is even smaller than at 1.1 times the particle size. Therefore, for food of this size, a front-to-back distance of 1.2 or 1.3 times the particle size should be chosen to minimize reflection.

[0056] Comparative Example 3: Schematic diagram and reflection of continuous sample introduction of food groups at 90% size and 1.4-1.5 times spacing. The length and width of the single-row food group 6 are 90% of the length, width, and thickness of the microwave resonant cavity, respectively, which are 1274 mm, 207 mm, and 73 mm. When the spacing between the food items is greater than 1.4 times, the reflectivity of the middle part will increase. Figure 10 A and Figure 10B shows the reflectance when the food spacing is 1.4 and 1.5 dB respectively. When the spacing increases to 1.4, the reflectance near the center position increases rapidly to about -1.5 dB. When the spacing is further increased to 1.5 times, the reflectance at the emissivity when the food position is 0 dB increases by about -1 dB. The above results indicate that when the spacing between the food items increases to more than 1.4, the reflectance increases, and the spacing between the food items should be controlled to be limited to within 1.3 times the emissivity.

[0057] Example 4: Reflection of food items in double rows when the width accounts for 45% and the length and thickness account for 90%, with a center-to-center distance of 1.10-1.60 mm. The length and width of the double-row food group 5 are 45% of the length, width, and thickness of the resonant cavity, respectively. The two rows of food together account for 90% of the total area, and the front-to-back spacing between the food items is 1.1 times. The vertical spacing between the two rows of food items is set to be between 1.1 and 1.6 times their width (based on 45% of the resonant cavity size). Figure 11 The reflection and temperature distribution of the double-row food were displayed. When the vertical spacing between the two rows of food decreased from 1.1 times to 1.6 times, the reflection at the beginning and end of the food movement decreased from approximately 3.14 dB to 3.85 dB; when the food moved from ±1 m to ±0.4 m, the reflection decreased from -3.25 dB to -4.35 dB; and when the food reached position 0, the reflection decreased from -3.02 dB to -3.55 dB. These results indicate that the reflection value gradually decreases as the spacing between the two rows of food increases from 1.1 to 1.6 times.

[0058] Figure 11 The right column shows that for double-row food with a vertical spacing of 1.1-1.6, the highest temperature increases from 106 ℃ to 133 ℃, with the area of ​​food with moderate temperature gradually decreasing, and the areas on the top and bottom of the food that can be heated also gradually decreasing. Therefore, when trying to control reflection, a systematic analysis should be conducted in conjunction with temperature heating to select the most suitable food arrangement.

[0059] Comparative Example 4: The proportion of double-row food covering the resonant cavity is 90%, and the vertical spacing between the double-row food is 1.70-1.80 times. The length and width of the double-row food items 6 are 90%, 45%, and 90% of the length, width, and thickness of the resonant cavity, respectively. After the two rows are combined, the width of the resonant cavity is 90%. The front-to-back spacing of the food items is 1.1 times the dimension in the x-direction. The double-row food items are placed in two rows, one above the other, with a spacing of 1.7-1.8 times the width of a single food item (based on 45% of the resonant cavity dimension). Figure 12The left column shows the reflection of the resonant cavity. When the spacing between the upper and lower rows of food increases by 1.7-1.8 times, the reflection value further increases from -3.75dB to -3.6dB in the early and late stages of food movement; when the food moves from ±1m to ±0.5m, the reflection value increases from -4.30dB to -4.20dB; and when the food moves to 0m, the reflection value increases from -3.45dB to -3.35dB. Figure 12 The right column shows the temperature distribution as the distance between the top and bottom rows of food increases from 1.7 times to 1.8 times, with the highest temperature decreasing from 133°C to 132°C. At this point, the heated area on the top and bottom sides of the food gradually decreases; at 1.8 times the distance, the food is almost completely heated on both sides (especially outside the microwave source's coverage area). Therefore, when controlling reflection, the heating of the food should be considered simultaneously, especially when there are requirements for the temperature difference between the highest and lowest temperatures of the food. In conclusion, the vertical distance between the top and bottom rows of food should be controlled between 1.1 and 1.6 times to obtain lower reflectivity and a larger heating area.

[0060] Example 5: Reflection of pure water and purified water as media The dielectric properties of water purified by a water purifier were measured. The dielectric constant of the purified water was 57.2, the dielectric loss was 1.9 J, and the conductivity was 4.57 × 10⁻² S / m. This dielectric and electrical property of the purified water was used as the filling medium in a resonant cavity for investigation. The reflection and food heating effect under this filling medium were analyzed in detail. Two microwave sources were used for heating, each with a power of 100 W and a running time of 12 s. The dielectric properties of the food were 80-40 J (imaginary part is negative), the thermal conductivity was 0.50 W / (m·K), the density was 1210.3 kg / m³, the constant-pressure heat capacity was 4.35 J / (kg·K), and the conductivity was 0.98 S / m.

[0061] Numerical simulation results of reflection when purified water is used as the medium filling the resonant cavity are as follows: Figure 13 As shown in Figure A, the microwave source reflection value is -2.4 dB when no food is present, and the lowest reflection value is -3.7 dB when food is present. When the food enters at position 0, the reflection value is -1.7 dB. The heating effect on food when using purified water as the resonant cavity filling medium is as follows. Figure 13 As shown in Figure B, during the heating process, the temperature on the left and right sides is higher, reaching 372 ℃; while the temperature on the top and bottom sides is relatively lower, with a minimum temperature of 327 ℃. When purified water is used as the filling medium, the food achieves a better heating effect. The reflectivity at this point is approximately -1.8 dB. If this does not meet the reflectivity requirements of the microwave source, reflectivity reduction strategies should be implemented in conjunction with other microwave equipment design and usage strategies.

[0062] Comparative Example 5: Reflection of tap water as a medium The dielectric properties of tap water were measured, yielding a dielectric constant of 52, a dielectric loss of 6.5 J, and a corresponding conductivity of 0.1555 S / m. This water with these dielectric properties was used as the filling medium within the resonant cavity for further investigation. Its reflection characteristics and its effect on food heating were analyzed. The processing within the microwave source was the same as in Example 4.

[0063] Two microwave sources were used for heating, each with a power of 100 W and a running time of 12 s. The food had a dielectric property of 80-40 J, a thermal conductivity of 0.50 W / (m·K), a density of 1210.3 kg / m³, a constant-pressure heat capacity of 4.35 J / (kg·K), and an electrical conductivity of 0.98 S / m.

[0064] Numerical simulation results of reflection when tap water is used as the medium filling the resonant cavity are as follows: Figure 14 As shown in Figure A, the reflection value is -4.2 dB when no food is present, the lowest reflection is -4.75 dB, and the reflection at position 0 is -3.6 dB. Compared with purified water, the reflection values ​​at the no-food, lowest, and position 0 positions are all lower than those of purified water. Therefore, replacing the water in the microwave resonant cavity with tap water reduces the reflection.

[0065] The heating process when tap water is used as the filling medium of the resonant cavity is as follows: Figure 14 As shown in B, the left and right sides received relatively good heating, with the highest temperature on the top and bottom sides reaching 130 ℃. Lower temperatures were concentrated on the top and bottom sides, with the lowest temperature around 117 ℃. Compared to Example 5, although the reflectivity decreased when using tap water, the overall heating temperature of the food decreased by approximately 200 ℃. In conclusion, when there are no other conditions to improve microwave source reflection and the required temperature after microwave heating of food is relatively low, tap water can be used as the heating medium to reduce reflectivity and thus allow the microwave source to continue operating.

[0066] Example 6: Using a right-angle waveguide to reduce reflectivity Figure 15 A shows the combined modeling of a right-angled tapered waveguide. Figure 15 B shows the electric field distribution of the thermal pattern of microwaves in a right-angled conical waveguide. Figure 15 C shows the heating of a 140 mm × 200 mm × 15 mm food sample in the XY plane intermediate layer within a microwave resonant cavity, with a maximum electric field strength of 3.05 × 10³ V / m. Figure 15Figure D shows the numerical simulation of reflection using a right-angled tapered waveguide and the experimentally verified reflectivity. The reflectivity of the food is approximately 0.7 at the beginning and end of its movement, decreasing to around 0.2 when the food reaches the middle stage. The numerical simulation results and experimental results fit well. In conclusion, a right-angled tapered waveguide should be preferred to obtain a lower microwave reflectivity.

[0067] Comparative Example 6: Reflectivity of a Traditional Isosceles Waveguide Figure 16 A represents the geometric modeling case using a right-angled tapered waveguide. Figure 16 B represents the electric field distribution when using an isosceles conical waveguide. Figure 16 C represents the electric field distribution of a 140 mm × 200 mm × 15 mm food sample in the middle layer of the XY plane within the resonant cavity, with a maximum electric field strength of 2.84 × 10³ V / m. Figure 16 Figure D shows the numerical simulation and experimental reflection results using a traditional isosceles waveguide. The experimental results show that the reflectivity of the food remains at approximately 0.7 in both the initial and later stages of movement, while significantly decreasing to around 0.3 when the food moves to position 0. The numerical simulation results fit the experimental data well. At this point, the proportion of energy usable by the food is approximately 0.4, a 25% reduction in energy utilization compared to when using a right-angled conical waveguide. In conclusion, when using an isosceles conical waveguide, if the microwave source emission limit cannot be met, other microwave design and usage strategies should be combined to further achieve lower reflectivity.

[0068] Example 7: Investigating the reflection characteristics of a microwave source during single-shot operation. 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 a single microwave source from above. Figure 17 C represents the electric field distribution in the XY plane of the resonant cavity where the food intermediate layer is located, with a maximum electric field value of 2.3*103 V / m. Figure 17 D illustrates the reflection characteristics of a single-shot microwave source. During the initial and final stages of food handling, the reflectivity is approximately 0.56; however, as the food moves to the middle, the reflectivity further decreases to 0.43. In summary, unidirectional microwave irradiation can reduce emissivity when no food is passing through, but the proportion of energy absorbed by the food is reduced. Therefore, when other microwave equipment design or usage-level technical solutions cannot be used to reduce reflectivity, or when the food volume is small, a single-shot microwave source can be used to reduce reflectivity.

[0069] Example 8: Using a degassing tower or other exhaust methods to reduce other contents. The reflection situation when the water in the microwave resonant cavity is vented is as follows: Figure 18 As shown. The dielectric property is 57.5, and the dielectric loss is 1.9. Figure 18 A shows a geometric model of a cylindrical food (50 mm in radius and 40 mm in thickness) and the microwave equipment used in its preparation. Figure 18 B represents the reflection of four microwave sources obtained using numerical simulation software. The results show that the microwave source reflection at the beginning and end of the food handling process, where there is no food, is approximately 2.4 dB, and the reflection is the lowest at ±0.8 m, which is -4.1 dB. Figure 18 C represents the microwave reflection during the experimental verification. The reflection was approximately 2.5 dB at the beginning and end of the food handling process, when there was no food, and showed some fluctuation. The minimum reflection at a distance of ±0.8 meters was approximately -4.25 dB. Figure 18 D represents the temperature distribution value in the XY plane at the end of heating. The higher temperatures are on the left and right sides, with the highest temperature on the left side being 147 ℃ and the lowest temperature on the top and bottom sides being 131 ℃. Figure 18 E represents the temperature distribution in the YZ plane, with the highest temperature in the middle of the food being 134 ℃ and the lowest temperature on both sides being 130 ℃. Figure 18 F represents the temperature distribution in the XZ plane. The temperatures are higher on the left and right sides, with the highest temperature (147 ℃) on the left side of the food and the lowest temperature (133 ℃) in the middle. In conclusion, an exhaust system should be installed to achieve stable microwave heating and low reflectivity.

[0070] Comparative Example 8: No exhaust treatment performed. The reflection of water inside the microwave resonant cavity without venting is as follows: Figure 19 As shown, the dielectric properties of the gas-containing high-temperature hot water were measured, and the dielectric constant was found to be 35, and the dielectric loss was 0.5. Figure 19 A shows the cylindrical food sample (50 mm radius, 40 mm thickness) used in Comparative Example 8 and the geometric model of the microwave equipment used. Figure 19 B represents the reflection of the four microwave sources obtained using numerical simulation software. The results show that the reflection value of the microwave sources at the point without food at the beginning and end of the food processing is approximately -1.4 dB. This indicates that without venting, the reflection will increase from -2.4 dB to -1.4 dB, and not venting will significantly increase the reflectivity. Figure 19C represents the reflection characteristics of the four microwave sources during the experimental verification. When there is no venting, the reflection characteristics of the four microwave sources show a different trend from the simulated situation, with a significant difference. This indicates that when there is no venting, the dielectric properties of the water are a weighted combination of water and air. However, the reflection characteristics can only use the dielectric properties of water alone, making it impossible to simulate this situation. In this case, due to the continuous reflection and refraction of microwaves by the air in the water, the heating of food becomes unpredictable. The average dielectric property of water without venting is 35-0.5J. The temperature value used in the numerical simulation is as follows: Figure 19 As shown in D, 19E, and 19F. Figure 19 D shows the temperature distribution in the XY plane of the cylindrical food sample. The highest temperature is 74.2℃ on the left and right sides, and the lowest temperature is 59.9℃ on the top and bottom sides. Compared with the case of exhaust in Comparative Example 8, the highest temperature decreased by approximately 72℃, and the lowest temperature decreased by approximately 71.1℃. This indicates that most of the energy was reflected back to the microwave source by the bubbles in the water. Figure 19 E represents the temperature distribution in the YZ plane. The highest temperature is in the middle, at 62.1 ℃, which is 71.9 ℃ lower than the temperature during exhaust. The lowest temperatures are on both sides, which are 70.3 ℃ lower than the temperature during exhaust. Figure 19 F represents the temperature distribution in the YZ plane. The temperatures are higher on both sides, with the highest temperature on the left side at 74.2 ℃, which is 72.8 ℃ lower than during exhaust. The lowest temperature is in the middle at 61.9 ℃, which is 71.1 ℃ lower than during exhaust. In conclusion, not exhausting the gas will significantly affect the microwave heating effect and lead to increased reflectivity.

[0071] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for reducing reflectivity during microwave heating, characterized in that, This method reduces reflectivity from two levels: microwave device design and device use. First, the microwave device is geometrically modeled to reduce reflectivity from the equipment design level, including the phase of the microwave source, the emission and composition of the microwave source, and the design of microwave transmission components. Then, the design is based on the actual use of the microwave device, including changing the electrothermal parameters of the dielectric in the resonant cavity, the size of the food, the spacing between the front and back of the food, and the number of rows of food.

2. The method for reducing reflectivity during microwave heating as described in claim 1, characterized in that, The specific steps of the method are as follows: S1: Geometric modeling of microwave device; S2: Collect reflectance-related data; S3: Determine the type of microwave source; S4: Optimize the microwave source: the location of the microwave source, whether the microwave source is single-emission or two-way emission; the spacing between 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 should use a single emission mode. If the microwave source uses two microwave sources in a two-way emission mode, other factors that reduce microwave reflectivity should be used to reduce reflectivity. S5: The phase range of the microwave source is 0-2π, and the phase difference range of the microwave source with symmetrical design is 0-2π. S6: Reduce reflectivity by changing the shape of the tapered waveguide; the tapered waveguide shape should be a right-angled tapered waveguide. If an isosceles tapered waveguide is used, it should be combined with other microwave reflectivity-reducing factors to further reduce reflectivity. S7: Reduce reflectivity by modifying the dielectric properties within the resonant cavity; the dielectric loss of the liquid in the microwave resonant cavity should be in the range of 0.5~5. S8: Control the size of food to reduce reflectivity when using microwave devices; the size of food, including length, width, and thickness, should be kept at 50% to 90% of the length, width, and height of the microwave resonant cavity; S9: Reduce reflectivity by designing the front-to-back spacing of a single row of food items; the front-to-back spacing of a single row of food items should be 1.1 to 1.3 times the reflectivity. S10: Reduce reflectivity by controlling the vertical spacing between two rows of food; the vertical spacing between two rows of food should be maintained within the range of 1.1-1.6 times. S11: The reflectivity is reduced by venting the liquid inside the microwave resonant cavity.

3. The method for reducing reflectivity during microwave heating as described in claim 2, characterized in that, Specifically, S1 involves: for microwave equipment with high reflectivity, collecting the component dimensions and spatial position parameters of the microwave equipment, including: the length, width, height, inner wall thickness, and whether there are rounded corners or chamfers for each component; based on the collected data, using electromagnetic field simulation software based on the finite element method or finite time domain difference principle, geometrically modeling each component of the microwave equipment one by one.

4. The method for reducing reflectivity during microwave heating as described in claim 2, characterized in that, In S2: the collected reflectivity-related data includes the transmit power, reflective power, voltage, current, and VSWR value of each microwave source; when performing reflectivity analysis, the reflectivity or reflectance dB value should be used.

5. The method for reducing reflectivity during microwave heating as described in claim 2, characterized in that, In S3, the types of microwave sources include: rectangular, transverse electromagnetic, coaxial, circular, and periodic.

6. The method for reducing reflectivity during microwave heating as described in claim 2, characterized in that, In S5: when it is a unidirectional microwave source, the microwave source phase does not affect the heating mode and reflectivity, and is selected as 0-2π; when it is an up-and-down microwave source, the phase difference between the up-and-down microwave sources should be π; when there are two sets of up-and-down microwave sources, the phase differences of the microwave sources of different groups are combined, one of which should be π and the other should have a phase difference of 0; for multiple sets of up-and-down microwave sources, they are arranged in a cyclic manner in the same way as the two sets of microwave sources.

7. The method for reducing reflectivity during microwave heating as described in claim 2, characterized in that, In step S7, when the loss of the liquid medium is in the range of 0.5 to 2, other factors that reduce microwave reflectivity should be used to reduce reflectivity. When the dielectric loss of the liquid medium is in the range of 2 to 5, some microwave energy is absorbed by the liquid medium, resulting in a lower overall reflectivity.

8. The method for reducing reflectivity during microwave heating as described in claim 2, characterized in that, In step S8, when the size of the heated food is reduced to less than 50%, other microwave reflectivity-reducing factors should be used to further reduce reflectivity.

9. The method for reducing reflectivity during microwave heating as described in claim 2, characterized in that, In S9, the front-to-back spacing of a single row of food items is 1.2 or 1.3 times the spacing.

10. The method for reducing reflectivity during microwave heating as described in claim 2, characterized in that, In S11, the exhaust frequency is at least once per hour.

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