MICROWAVE IRRADIATION DEVICE

A microwave irradiation device including: a holder that holds an irradiated object; a power feeder configured to conduct with an oscillator; and a directional antenna configured to emit microwaves from an irradiation source in an irradiation surface by power feed through conduction via the power feeder.

TECHNICAL FIELD

The disclosure relates to a microwave irradiation device.

BACKGROUND

In general, a heating device that dielectrically heats an irradiated object by irradiating the irradiated object with microwaves is known. In the dielectric heating, there is a case that the irradiated object is not uniformly heated for various reasons. Thus, various efforts for uniform heating have been made.

For example, JP 2020-21678 A discloses a microwave heating device in which a microwave reaction container where an irradiated object is placed is arranged in a conductive storage container, and a plurality of dipole antennas are uniformly arranged so as to surround the microwave reaction container. In this microwave heating device, each part is arranged such that the distance between each antenna and the inner wall of the conductive storage is approximately ¼ of the wavelength of the microwave to be irradiated, in the direction of viewing each dipole antenna from the microwave reaction container. With such arrangement, microwaves emitted from the dipole antenna are directed toward the microwave reaction container. As a result, the irradiated object in the microwave reaction container is uniformly heated.

SUMMARY

The above is an example, and there can be various methods for suppressing uneven heating in dielectric heating. An object of the disclosure is to suppress uneven heating in a microwave irradiation device.

According to one aspect of the disclosure, a microwave irradiation device includes: a holder configured to hold an irradiated object; a power feeder configured to conduct with an oscillator; and a directional antenna configured to emit microwaves from an irradiation source in an irradiation surface by power feed through conduction via the power feeder.

According to the disclosure, it is possible to suppress uneven heating in a microwave irradiation device.

DESCRIPTION OF EMBODIMENTS

First Embodiment

The first embodiment will be described with reference to the drawings. The present embodiment relates to a microwave irradiation device. The microwave irradiation device of the present embodiment is configured to irradiate an irradiated object with microwaves so as to internally heat the irradiated object. The irradiated object is, though not limited to, a food, for example. Thus, this microwave irradiation device and the microwave irradiation method using the microwave irradiation device can be used for production of food including packaged food, for example. The microwave irradiation device includes a conveyance device, and a plurality of objects to be irradiated are conveyed sequentially and heated sequentially. A plurality of directional antennas that emit microwaves are arranged side by side along the conveyance direction.

Device Configuration

FIG.1Ais a front view schematically illustrating an outline of a configuration example of a microwave irradiation device1according to the present embodiment, andFIG.1Bis a plan view schematically illustrating an outline of a configuration example of the microwave irradiation device1according to the present embodiment. As illustrated in these figures, the microwave irradiation device1includes a conveyance device60that conveys an irradiated object90that is a heat target object and is irradiated with microwaves. The conveyance device includes, for example, a belt61and a roller62. The belt61is hung on the roller62. The roller62is rotated by a motor (not illustrated) to move the belt61in the longitudinal axis direction. The irradiated object90is placed on the belt61and conveyed in a conveyance direction91by the movement of the belt61. A supply device84for sequentially supplying the irradiated object onto the belt61is provided at an upstream in the conveyance direction91of the conveyance device60. A carry-out device86that carries out, from the belt61, the irradiated object90having been conveyed is provided at a downstream in the conveyance direction91of the conveyance device60.

The microwave irradiation device1includes an antenna group30having a plurality of antennas40configured to irradiate the irradiated object90to be conveyed by the conveyance device60with microwaves. The plurality of antennas40are arranged along the conveyance direction91. Each of the antennas40is, for example, a directional antenna such as a loop antenna or a patch antenna. That is, each of the antennas40has an irradiation surface42, and is configured to emit microwaves from an irradiation source44in the irradiation surface42in the direction of a directional irradiation axis45. The direction of the directional irradiation axis45of each of the antennas40is directed to the irradiated object90to be conveyed by the conveyance device60. Each of the antennas40is fed from an oscillator10conducted through a power feeder such as a coaxial cable.

The periphery of the antenna group30is covered with metal for shielding of microwaves. That is, the conveyance device60is provided so as to pass through a metal housing82or in the metal housing82, and the antenna group30is arranged in the metal housing82.

The antenna40will be described using a loop antenna as an example.FIG.2is a schematic view illustrating an outline of a configuration example of a loop antenna51. The loop antenna51includes a conductive wire52formed in an annular shape and having a length equivalent of, for example, one wavelength of the microwave to be irradiated. Both ends of the conductive wire52are power feed points53. For example, a coaxial cable21as the power feeder20is connected to the power feed point53. The coaxial cable21connects and conducts the oscillator10and the loop antenna51. The oscillator10supplies high-frequency power to the loop antenna51through the coaxial cable21. When power is fed, a current is generated in the conductive wire52as an element, and the loop antenna51emits a radio wave to form an electric field.

In the loop antenna51having an annular shape, an opening surface54formed by the conductive wire52serves as the irradiation surface42, and the center of the opening surface54serves as the irradiation source44. The directional irradiation axis45is formed in a direction through the irradiation source44and perpendicular to the opening surface54, and microwaves are emitted in both directions along the directional irradiation axis45. Note that the shape formed by the conductive wire52is not limited to an annular shape, and may be a loop of another shape such as a quadrangle.

The orientation of the antenna40will be further described.FIGS.3A and3Bare schematic views for explaining the orientation of the antenna40. In the present embodiment, for example, as illustrated inFIG.3A, the antenna40is arranged such that the directional irradiation axis45is parallel to the surface of the belt61of the conveyance device60on which the irradiated object90is placed. Alternatively, as illustrated at least inFIG.3B, the antenna40is arranged such that the directional irradiation axis45does not intersect a structure that reflects microwaves among structures constituting the conveyance device60.

Although the microwaves emitted from the directional antenna40spread to some extent as indicated by a diffusion irradiation axis46inFIGS.3A and3B, the irradiation angle thereof is relatively narrow, and an electric field having the strongest intensity along the directional irradiation axis45is formed. Since the directional irradiation axis45does not intersect a structure that reflects microwaves, a strong reflection wave is not generated. As a result, a standing wave that can be generated by interference between the incident wave and the reflection wave is not generated. If a standing wave is generated by interference between a strong incident wave and a reflection wave, the electric field intensity is largely different between the position of antinodes and the position of nodes of the standing wave in particular, and uneven heating can occur in the irradiated object90. In the microwave irradiation device1of the present embodiment, since such standing wave is not generated, uneven heating is prevented from occurring in the irradiated object90.

Note that the structure that reflects microwaves described above means a structure that reflects microwaves to such extent as to generate standing waves that cause the uneven heating described above.

Operation

The operation of the microwave irradiation device1of the present embodiment will be described. The oscillator10outputs high-frequency power according to the frequency of the microwave. The frequency is, though not limited to, 2.45 GHz, 915 MHz, or 450 MHz, for example. The high-frequency power output from this oscillator10is supplied to the antenna40through the power feeder20. The antenna40emits microwaves in the direction of the directional irradiation axis45based on this power feed.

The conveyance device60rotates the belt61by the rotation of the roller62. The supply device84supplies the irradiated object90onto the belt61of the conveyance device60, for example, at regular intervals. The conveyance device60conveys the supplied irradiated object90in the conveyance direction91and causes the supplied irradiated object90to pass in front of the plurality of antennas40in the metal housing82. The irradiated object90passing in front of the antenna40is irradiated with microwaves from the antenna40. By the microwaves, the irradiated object90is dielectrically heated. The heated irradiated object90is conveyed to the outside of the metal housing82by the conveyance device60. The carry-out device86carries out the heated irradiated object90from the conveyance device60.

As described above, in the present embodiment, the directional antenna40is used for the antenna group30, and the directional irradiation axis45is designed not to intersect the structure that reflects the microwave of the conveyance device60. For this reason, a standing wave derived from the reflection wave is not generated in the microwave to be irradiated. As a result, the irradiated object90is uniformly heated.

As a heating device by dielectric heating, for example, a multimode heating device that reflects microwaves in a metal housing to heat a heated object is known. There is also known a single-mode heating device in which a heated object is arranged in a waveguide that conveys microwaves. With such device, reflection of microwaves is intentionally used. That is, a standing wave due to reflection is intentionally created, and dielectric heating is performed by this standing wave. However, in such standing wave, a difference in electric field intensity occurs depending on the location as is remarkable between the antinode position and the node position. This unevenness in electric field intensity causes uneven heating of the heated object. Since the microwave irradiation device1of the present embodiment is adjusted so as not to generate a standing wave, uniform heating can be achieved.

A heating device using a waveguide tends to increase in size, such as an increase in size of the waveguide particularly when the frequency is low. When a plurality of types of heating devices are combined for uniform heating, the entire device tends to increase in size. On the other hand, in the microwave irradiation device1of the present embodiment, no waveguide is used, and there is no need for a plurality of types of devices to be combined, thus making it easy to downsize the device. Since no waveguide is used, it is easy to use a microwave having a relatively low frequency. By lowering the frequency, it is possible to lower the half power depth.

Note that in the above-described embodiment, the case where the irradiated object90is held on the belt61of the conveyance device60at the time of irradiation with microwaves has been described as an example, but the disclosure is not limited to this example. The irradiated object90may be configured to be held on a stopped holding table. Also in this case, the antenna40is preferably provided such that the directional irradiation axis45does not intersect the structure constituting the holding table. Also in this case, generation of a standing wave due to a reflection wave is suppressed, and uneven heating due to dielectric heating is suppressed.

The microwave irradiation device1according to the present embodiment can be incorporated in processing devices for various uses or configured in an appropriate form. For example, in a case of being used for heat sterilization of a hermetically packaged food, the microwave irradiation device1is incorporated in a device configured to pressurize the irradiated object90that is a hermetically packaged food or to keep the irradiated object warm for a time required for sterilization. Alternatively, in order to be used for a material reaction treatment or the like, the irradiated object90, which is a treatment target object, may be accommodated in an appropriate reaction container, or the conveyance device60may be configured as a tube or the like through which the treatment target object flows.

Second Embodiment

The second embodiment will be described. Here, differences from the first embodiment will be described, and the same parts will be denoted by the same reference signs, and the description thereof will be omitted.

FIG.4is a plan view schematically illustrating an outline of a configuration example of a microwave irradiation device2of the second embodiment. In this figure, illustration of the oscillator10, the power feeder20, and the like is omitted. In the microwave irradiation device1of the first embodiment illustrated inFIG.1B, the antenna40is arranged on one side of the conveyance device60, whereas in the microwave irradiation device2of the second embodiment illustrated inFIG.4, the antennas40are arranged on both sides of the conveyance device60. Thus, in the microwave irradiation device1of the first embodiment illustrated inFIG.1B, the irradiated object90is irradiated with microwaves from one side, whereas in the microwave irradiation device2of the second embodiment illustrated inFIG.4, the irradiated object90is irradiated with microwaves from both sides. In particular, in the microwave irradiation device2of the second embodiment, the antennas40are provided to face each other across the conveyance device60, and the directional irradiation axes45of the antennas40facing each other overlap each other.

FIG.5is a schematic view illustrating the magnitude of the electric field effective value depending on the position along the directional irradiation axis45of the antennas40provided to face each other. Each of the antennas40facing each other are arranged at a first position P1and a second position P2. Thus, the conveyance device60passes through between the first position P1and the second position P2, and the irradiated object90passes therethrough. As illustrated inFIG.5, the microwave irradiation device2of the present embodiment is configured such that an electric field effective value becomes substantially constant between the first position P1and the second position P2.

According to the present embodiment, since the microwave is irradiated from both sides in the direction crossing the conveyance direction91, even if the size of the irradiated object90in the direction crossing the conveyance direction91is large to some extent, the irradiated object is heated from both sides, and uniform heating can be achieved. Since such effect can be obtained as long as the irradiated object90is irradiated with microwaves from both sides, the antenna40and the antenna40need not necessarily face each other.

In the present embodiment, the antenna40and the antenna40face each other, and the microwave irradiation device2is configured such that the electric field intensity by the microwave becomes substantially constant in the direction crossing the conveyance direction91. Such configuration can perform heating of the irradiated object90more uniformly. The electric field intensity between the antenna40and the antenna40facing each other being constant means the electric field intensity being constant to such extent as to satisfy a requirement regarding uniformity of heating of the irradiated object90. According to the present embodiment, heating of the irradiated object90can be performed more uniformly.

Modification of Heating Method

An example has been described above in which, using the microwave irradiation device2of the present embodiment, the irradiated object90is uniformly irradiated with microwaves from both sides by the two antennas40facing each other to heat the irradiated object90, but the heating method is not limited to this.

Through an experiment, it has been found that when the antennas40arranged on both sides of the irradiated object90are simultaneously used to uniformly irradiate both sides of the irradiated object90with microwaves, particularly a center part of the irradiated object90may be heated, and when one side of the irradiated object90is non-uniformly irradiated with microwaves, particularly an outer peripheral part of the irradiated object90may be heated. Thus, by combining uniform irradiation from both sides and non-uniform irradiation from one side, each of the center part and the outer peripheral part of the irradiated object90may be heated, and the entire irradiated object90may be uniformly heated, or may be intentionally non-uniformly heated.

For example, uniform microwave irradiation may be performed as the first irradiation, and non-uniform microwave irradiation may be performed as the second irradiation and the third irradiation. That is, the following irradiation can be performed in a state where the irradiated object90is at a position equidistant from the two antennas40between the pair of antennas40. In the first irradiation, the irradiated object90can be irradiated with the microwave having equal irradiation intensity from the two antennas40. In the second irradiation, the irradiated object90can be irradiated with the microwave from one of the antennas40. In the third irradiation, the irradiated object90can be irradiated with the microwave from the other of the antennas40. The irradiated object90can be uniformly heated by the combination of the first irradiation, the second irradiation, and the third irradiation. Alternatively, only the first irradiation and the second irradiation may be performed.

The first irradiation, the second irradiation, and the third irradiation may be performed by the same pair of antennas40as described above, or may be performed by a plurality of pairs of antennas40. In a case where the irradiation is performed by the plurality of pairs of antennas40, for example, in the plurality of pairs of antennas40arranged side by side on both sides of the conveyance device60, microwaves with equal irradiation intensity can be emitted from the two antennas40facing each other in a part, microwaves can be emitted from one of the antennas40in a part, and microwaves can be emitted from the other of the antennas40in a part. In this case, the first irradiation, the second irradiation, and the third irradiation can be performed due to the conveyance device60conveying the irradiated object90between these antennas40. In this case, the part where the first irradiation is performed may be provided with the pair of antennas40facing each other, whereas the part where the second irradiation and the third irradiation are performed may be provided with the antennas40only on one side as in the microwave irradiation device1of the first embodiment.

Furthermore, in the second irradiation and the third irradiation, it is sufficient to perform non-uniform microwave irradiation, and thus the irradiation intensity may be different between one and the other of the pair of antennas40facing each other. Alternatively, between the pair of antennas40having equal or unequal irradiation intensities, the irradiated object90may be brought close to one or to the other.

In order to change the distance between the irradiated object90and the antenna40, the conveyance device60may be configured to move the irradiated object90also in a direction orthogonal to the conveyance direction. Alternatively, the conveyance device60and the antenna40may be arranged such that the distance between the conveyance device60and the antenna40is different for each of the antennas40.

The modification of the heating method described here may be performed using an irradiation device different from the microwave irradiation device2of the second embodiment. For example, the antenna needs not be arranged along the conveyance device. For example, the irradiation device may have only antennas facing each other and no conveyance device may be provided. The conveyance device may also be configured to move the irradiated object between two antennas facing each other.

Third Embodiment

The third embodiment will be described. Here, differences from the first embodiment will be described, and the same parts will be denoted by the same reference signs, and the description thereof will be omitted.

FIG.6is a plan view schematically illustrating an outline of a configuration example of a microwave irradiation device3of the third embodiment. In this figure, illustration of the oscillator10, the power feeder20, and the like is omitted. In the microwave irradiation device3of the present embodiment, the loop antenna51is used as the antenna40constituting the antenna group30. As described above, in the loop antenna51, both front and back sides of the opening surface54serve as the irradiation surface42, and microwaves are emitted in both directions along the directional irradiation axis45. In the microwave irradiation device3of the third embodiment, both sides of a plurality of the loop antennas51arranged side by side are provided respectively with a first conveyance device71and a second conveyance device72corresponding to the conveyance device60of the first embodiment, and a conveyance device group70is formed.

According to the present embodiment, the microwave emitted from one loop antenna51to both sides irradiates the irradiated object90conveyed by the first conveyance device71and the irradiated object90conveyed by the second conveyance device72, respectively, and thus the energy efficiency of the microwave irradiation device3is good even with a simple configuration.

Also in the present embodiment, similarly to the second embodiment, the antennas may be provided on both sides of the conveyance device. The loop antennas51may be provided on both sides of the conveyance device, and a large number of conveyance devices may be arranged side by side in parallel.

Fourth Embodiment

The fourth embodiment will be described with reference to the drawings. The present embodiment relates to a microwave irradiation device. The microwave irradiation device of the present embodiment is configured to irradiate an irradiated object with microwaves so as to internally heat the irradiated object. The irradiated object is, though not limited to, a food, for example.

FIG.7is a view schematically illustrating an outline of a configuration example of a microwave irradiation device4according to the present embodiment. As illustrated in this figure, the microwave irradiation device4includes a holder66that holds the irradiated object90that is a heat target object and is irradiated with microwaves. The holder66can be a table on which, for example, the irradiated object90is held. The microwave irradiation device4includes the antenna40configured to irradiate the irradiated object90held by the holder66with microwaves. The antenna40is a directional antenna such as, for example, a loop antenna or a patch antenna. That is, the antenna40has the irradiation surface42, and is configured to emit microwaves from the irradiation source44in the irradiation surface42in the direction of the directional irradiation axis45. The direction of the directional irradiation axis45of the antenna40is along the holding surface of the holder66. The antenna40is fed from the oscillator10conducted through the power feeder20such as a coaxial cable. The periphery of the antenna40is covered with metal for shielding of microwaves. That is, the holder66and the antenna40are arranged in the metal housing82.

The orientation of the antenna40will be described.FIG.8is a schematic view for explaining the orientation of the antenna40. In the present embodiment, for example, as illustrated inFIG.8, the antenna40is arranged such that the directional irradiation axis45becomes parallel to the surface of the holder66on which the irradiated object90is placed. Alternatively, the antenna40is arranged such that at least the directional irradiation axis45does not intersect a structure that reflects microwaves among structures constituting the holder66.

Although the microwaves emitted from the directional antenna40spread to some extent as indicated by the diffusion irradiation axis46inFIG.8, the irradiation angle thereof is relatively narrow, and an electric field having the strongest intensity along the directional irradiation axis45is formed. Since the directional irradiation axis45does not intersect a structure that reflects microwaves, a strong reflection wave is not generated. As a result, a standing wave that can be generated by interference between the incident wave and the reflection wave is not generated.

FIG.9is a schematic view illustrating a comparative example. As illustrated inFIG.9, when the directional irradiation axis45intersects with the surface of the holder66, the microwave is reflected by the surface of the holder66. As a result of generation of a strong reflection wave, the incident wave and the reflection wave interfere with each other, and a standing wave is generated. When the standing wave is generated, the electric field intensity is largely different between the position of antinodes and the position of nodes of the standing wave in particular, and uneven heating can occur in the irradiated object90.

In the microwave irradiation device4of the present embodiment, as described with reference toFIG.8, since a standing wave due to interference between the incident wave and the reflection wave is not generated, uneven heating is prevented from occurring in the irradiated object90. Note that the structure that reflects microwaves described above means a structure that reflects microwaves to such extent as to generate standing waves that cause the uneven heating described above.

The operation of the microwave irradiation device4of the present embodiment will be described. The oscillator10outputs high-frequency power according to the frequency of the microwave. The frequency is, though not limited to, 2.45 GHz, 915 MHz, or 450 MHz, for example. The high-frequency power output from this oscillator10is supplied to the antenna40through the power feeder20. The antenna40emits microwaves in the direction of the directional irradiation axis45based on this power feed. The irradiated object90arranged on the holder66in front of the antenna40is irradiated with microwaves from the antenna40. By the microwaves, the irradiated object90is dielectrically heated.

As described above, in the present embodiment, the directional antenna40is used as an antenna that emits microwaves, and the directional irradiation axis45is designed not to intersect the structure that reflects the microwave of the holder66. For this reason, a standing wave derived from the reflection wave is not generated in the microwave to be irradiated. As a result, the irradiated object90is uniformly heated.

As a heating device by dielectric heating, for example, a multimode heating device that reflects microwaves in a metal housing to heat a heated object is known. There is also known a single-mode heating device in which a heated object is arranged in a waveguide that conveys microwaves. With such device, reflection of microwaves is intentionally used. That is, a standing wave due to reflection is intentionally created, and dielectric heating is performed by this standing wave. However, in such standing wave, a difference in electric field intensity occurs depending on the location as is remarkable between the antinode position and the node position. This unevenness in electric field intensity causes uneven heating of the heated object. Since the microwave irradiation device4of the present embodiment is adjusted not to generate a standing wave, uniform heating can be achieved.

A heating device using a waveguide tends to increase in size, such as an increase in size of the waveguide particularly when the frequency is low. When a plurality of types of heating devices are combined for uniform heating, the entire device tends to increase in size. On the other hand, in the microwave irradiation device4of the present embodiment, no waveguide is used, there is no need for a plurality of types of devices to be combined, thus making it easy to downsize the device. Since no waveguide is used, it is easy to use a microwave having a relatively low frequency. By lowering the frequency, it is possible to lower the half power depth.

For more uniform heating, the holder66may move back and forth along the directional irradiation axis45of the antenna40or rotate in a plane parallel to the directional irradiation axis By moving the irradiated object90in such direction, the irradiated object90can be more uniformly heated. The same may be performed by moving not the holder66but the antenna40.

Fifth Embodiment

The fifth embodiment will be described. Here, differences from the fourth embodiment will be described, and the same parts will be denoted by the same reference signs, and the description thereof will be omitted. The microwave irradiation device of the present embodiment is configured to irradiate an irradiated object, such as food, with microwaves to internally heat the irradiated object. The microwave irradiation device includes a conveyance device, and a plurality of objects to be irradiated are conveyed sequentially and heated sequentially.

FIG.10Ais a front view schematically illustrating an outline of a configuration example of a microwave irradiation device5according to the second present embodiment.FIG.10Bis a plan view schematically illustrating an outline of a configuration example of the microwave irradiation device5according to the second present embodiment.FIG.10Cis a view schematically illustrating an outline of a configuration example of an end surface viewed from a side at a position of the antenna40of the microwave irradiation device5according to the second present embodiment.

As illustrated in these figures, the microwave irradiation device5includes the conveyance device60as a holder that conveys the irradiated object90that is a heat target object and irradiated with microwaves. The conveyance device60includes, for example, a belt61and a roller62. The belt61is hung on the roller62. The roller62is rotated by a motor (not illustrated) to move the belt61in the longitudinal axis direction. The irradiated object90is placed on the belt61and conveyed in a conveyance direction91by the movement of the belt61. A supply device84for sequentially supplying the irradiated object90onto the belt61is provided at an upstream in the conveyance direction91of the conveyance device60. A carry-out device86that carries out, from the belt61, the irradiated object90having been conveyed is provided at a downstream in the conveyance direction91of the conveyance device60.

The microwave irradiation device5includes the antenna40configured to irradiate the irradiated object90conveyed by the conveyance device60with microwaves. The antenna40is, for example, a loop antenna that is a type of directional antenna. The loop antenna is as described with reference toFIG.2. The antenna40is fed from the oscillator10conducted through the power feeder20such as a coaxial cable.

Referring back toFIGS.10A to10C, the description will be continued. In the microwave irradiation device5of the present embodiment, the belt61of the conveyance device60is provided so as to penetrate the irradiation surface42. The irradiation surface42is the opening surface54of the antenna40which is the loop antenna51. That is, the irradiated object90is conveyed in the conveyance direction91so as to pass through the antenna40. For example, the irradiation surface42of the antenna40is perpendicular to the conveyance direction91, and the directional irradiation axis45of the antenna40is parallel to the conveyance direction91.

The periphery of the antenna40is covered with metal for shielding of microwaves. That is, the conveyance device60is provided so as to pass through the metal housing82, and the antenna40is arranged in the metal housing82.

Also in the microwave irradiation device5of the present embodiment, the directional irradiation axis45of the antenna40does not intersect a structure that reflects microwaves among the structures constituting the conveyance device60. As a result, a standing wave that can be generated by interference between the incident wave and the reflection wave is not generated. Also in the microwave irradiation device5of the present embodiment, since such standing wave is not generated, uneven heating is prevented from occurring in the irradiated object90.

Furthermore, in the microwave irradiation device5of the present embodiment, the irradiated object90passes through the irradiation source44of the antenna40, thereby achieving efficient and uniform heating of the irradiated object90.FIG.11schematically illustrates the electric field intensity depending on the location with the amplitude of a wavy line92. As illustrated in this figure, in the microwave irradiation device5of the present embodiment, the center part of the irradiated object90passes through the irradiation source44of the antenna40having high electric field intensity. Therefore, the irradiated object90generates heat at its center part, and is efficiently heated from the inside of the irradiated object90. That is, it is not necessary to consider the half power depth unlike when the microwave is emitted from the outside of the irradiated object90. Since the irradiated object90placed on the belt61of the conveyance device60moves in the conveyance direction91, the heat generation position changes in the irradiated object90and is uniformly heated.

A comparative example is illustrated inFIG.12.FIG.12schematically illustrates a heating device in which the antenna40is arranged laterally with respect to the conveyance direction91of the conveyance device60generally known. As the electric field intensity is schematically indicated by the wavy line92, in the arrangement as illustrated inFIG.12, the irradiated object90is away from the irradiation source44of the antenna40, and the power hardly reaches the center part of the irradiated object90. Thus, the center part may be difficult to heat. On the other hand, in the microwave irradiation device5of the present embodiment, the electric field intensity increases at the center part of the irradiated object90.

The operation of the microwave irradiation device5of the present embodiment will be described. The oscillator10outputs high-frequency power according to the frequency of the microwave. The frequency is, though not limited to, 2.45 GHz, 915 MHz, or 450 MHz, for example. The high-frequency power output from this oscillator10is supplied to the antenna40through the power feeder20. The antenna40emits microwaves in the direction of the directional irradiation axis45based on this power feed.

The conveyance device60rotates the belt61by the rotation of the roller62. The supply device84supplies the irradiated object90onto the belt61of the conveyance device60, for example, at regular intervals. The conveyance device60conveys the supplied irradiated object in the conveyance direction91and causes the supplied irradiated object90to pass through the opening surface54of the antenna40, which is the loop antenna51in the metal housing82. The irradiated object90passing through the opening surface54of the antenna40is irradiated with microwaves from the antenna40. By the microwaves, the irradiated object90is dielectrically heated. The heated irradiated object90is conveyed to the outside of the metal housing82by the conveyance device60. The carry-out device86carries out the heated irradiated object90from the conveyance device60.

As described above, in the present embodiment, the directional antenna40is used, and the directional irradiation axis45is designed so as not to intersect the structure that reflects the microwave of the conveyance device60. For this reason, a standing wave derived from the reflection wave is not generated in the microwave to be irradiated. As a result, the irradiated object90is uniformly heated. The irradiated object90passes through the irradiation source44of the antenna40. For this reason, a strong electric field is generated inside the irradiated object and the irradiated object90is efficiently heated from the inside. In addition, effects similar to those of the fourth embodiment can be obtained.

The microwave irradiation device5according to the present embodiment can be incorporated in processing devices for various uses or configured in an appropriate form. For example, in a case of being used for heat sterilization of a hermetically packaged food, the microwave irradiation device4is incorporated in a device configured to pressurize the irradiated object90that is a hermetically packaged food or to keep the irradiated object warm for a time required for sterilization. Alternatively, in order to be used for a material reaction treatment or the like, the irradiated object90, which is a treatment target object, may be accommodated in an appropriate reaction container, or the conveyance device60may be configured as a tube or the like through which the treatment target object flows.

Sixth Embodiment

The sixth embodiment will be described. Here, differences from the fourth embodiment will be described, and the same parts will be denoted by the same reference signs, and the description thereof will be omitted.

FIG.13is a view schematically illustrating an outline of a configuration example of a microwave irradiation device6according to the sixth embodiment. As illustrated in this figure, the microwave irradiation device6includes the antenna group30including the two antennas40. The antenna40is a directional antenna such as, for example, a loop antenna or a patch antenna. Although not limited to this, it is assumed here that the antenna40is the loop antenna51. The two antennas40of the antenna group30are arranged in parallel such that the irradiation surfaces42face each other. The holder66that holds the irradiated object90is provided so as to penetrate the opening surface54forming this irradiation surface42. Thus, the irradiated object90held by the holder66is sandwiched between the two antennas40. The directional irradiation axes45of the two antennas40are parallel to the surface of the holder66on which the irradiated object90is held so as not to generate strong reflection waves. The two antennas40are configured to irradiate the irradiated object90with microwaves from opposite sides. The microwaves emitted from the respective antennas40overlap each other.

FIG.14schematically illustrates the electric field intensity depending on the location with the amplitude of the wavy line92. As illustrated in this figure, the microwave irradiation device6of the present embodiment irradiates the irradiated object90with microwaves from both sides.

A schematic view illustrating the magnitude of the electric field effective value depending on the position along the directional irradiation axis45of the antennas40provided to face each other is asFIG.5described above. Each of the antennas40facing each other are arranged at a first position P1and a second position P2. Thus, the irradiated object90is arranged between the first position P1and the second position P2. As illustrated inFIG.5, the microwave irradiation device4of the present embodiment is configured such that an electric field effective value becomes substantially constant between the first position P1and the second position P2.

According to the present embodiment, since the irradiated object90is irradiated with microwaves from both sides, and the electric field intensity formed by the microwaves is substantially equal at even different positions, uniform heating of the irradiated object90can be achieved. The electric field intensity between the antenna40and the antenna40facing each other being constant means the electric field intensity being constant to such extent as to satisfy a requirement regarding uniformity of heating of the irradiated object90. According to the present embodiment, heating of the irradiated object90can be performed more uniformly. In addition, effects similar to those of the microwave irradiation device4of the fourth embodiment can be obtained.

Here, the case where the directional irradiation axes45of the two antennas40are parallel to the surface of the holder66, on which the irradiated object90is placed, so as not to generate strong reflection waves has been described as an example. The directional irradiation axis45is preferably parallel to the placement surface of the holder66, but is not limited to this. However, it is preferable that the two antennas40are arranged such that at least the directional irradiation axes45do not intersect a structure that reflects microwaves and constitutes the holder66provided between the opening surfaces54and54facing each other of the two antennas40. In this way, a standing wave derived from the reflection wave is not generated in the microwave to be irradiated, and as a result, the irradiated object90is uniformly heated.

Seventh Embodiment

The seventh embodiment will be described. Here, differences from the fifth embodiment will be described, and the same parts will be denoted by the same reference signs, and the description thereof will be omitted.

FIG.15Ais a front view schematically illustrating an outline of a configuration example of a microwave irradiation device7according to the seventh embodiment, andFIG.15Bis a plan view schematically illustrating an outline of a configuration example of the microwave irradiation device7according to the seventh embodiment. As illustrated in these figures, the microwave irradiation device7according to the seventh embodiment includes the conveyance device60that conveys the irradiated object90that is a heat target object and is irradiated with microwaves, similarly to microwave irradiation device5according to the fifth embodiment.

The microwave irradiation device7of the seventh embodiment includes the antenna group30having the plurality of antennas40configured to irradiate the irradiated object90conveyed by the conveyance device60with microwaves. The plurality of antennas40are arranged along the conveyance direction91. Each of the antennas40is, for example, the loop antenna51. The belt61of the conveyance device60is arranged to pass through each of the antennas40. Each of the antennas40is fed from the oscillator10conducted through the power feeder20such as a coaxial cable. The plurality of antennas40are arranged such that the electric field effective value becomes substantially constant between the adjacent antennas40and40, similarly to the antenna40of the microwave irradiation device6of the sixth embodiment. The periphery of the antenna group30is covered with the metal housing82for shielding of microwaves.

According to the microwave irradiation device7of the seventh embodiment, the plurality of antennas40make the electric field intensity by the microwave substantially constant along the belt61. The irradiated object90conveyed by the conveyance device60moves in the electric field having this constant intensity. The irradiated object90is configured to pass through the irradiation source44of each of the antennas40. Due to these, the microwave irradiation device7can efficiently and uniformly heat the irradiated object90. In addition, effects similar to those of the microwave irradiation devices of the first to sixth embodiments can be obtained.

Eighth Embodiment

The eighth embodiment will be described. Here, differences from the seventh embodiment will be described, and the same parts will be denoted by the same reference signs, and the description thereof will be omitted.FIG.16is a plan view schematically illustrating an outline of a configuration example of a microwave irradiation device8according to the eighth embodiment. As illustrated in this figure, the microwave irradiation device8according to the eighth embodiment is similar to the microwave irradiation device7according to the seventh embodiment, but differs from the microwave irradiation device7according to the seventh embodiment in that the orientations of some of the antennas40are changed, and the directional irradiation axes45of the antennas40are not along the conveyance direction91.

In heating of the irradiated object90, it is not necessarily preferable that power is uniformly supplied. For example, when the irradiated object90has a region that is likely to be heated and a region that is less likely to be heated, more power is supplied to the region that is less likely to be heated, whereby the entire irradiated object90is uniformly heated.

The example illustrated inFIG.16is an example in which the lower side and the upper side of the irradiated object90in the figure are the regions that are less likely to be heated. The antennas40are arranged such that one of the directional irradiation axes45, which is facing each other, of the antennas40on the supply device84side is biased on the lower side of the figure with respect to the conveyance direction91, and the other is biased on the upper side in the figure, whereby the electric fields are formed toward the lower side and the upper side in the figure, respectively, and power is applied from different directions with respect to the irradiated object90passing therethrough. As a result, the entire irradiated object90is uniformly heated. In addition, effects similar to those of the microwave irradiation device7of the seventh embodiment can be obtained.

Experiment Example 1

The uniformity of heating by the microwave irradiation device according to the above-described embodiment was evaluated with potato salad packed in a container as a heat target object.

Method

For the evaluation, a test device having the similar configuration to that of the microwave irradiation device6according to the sixth embodiment described with reference toFIG.13was used.FIG.17illustrates an outline of a configuration example of this test device100. The test device100includes an oscillator110, and two loop antennas140and a food holding table166arranged in a metal housing182.

The oscillation frequency of the oscillator110was 450 MHz. As the loop antenna140, a square loop antenna made of an aluminum material and having a circumferential length corresponding to one wavelength (λ=666 mm) was used. The two loop antennas140were arranged such that their opening surfaces faced each other and the directional irradiation axis145became parallel to the food holding table166. The interval between the two loop antennas140was λ/4=166.5 mm. Power feed to the loop antenna140was in-phase power feed. A plate made of polyethylene (PE) having a thickness of 5 mm was used for the food holding table166. The food holding table166was arranged so as to penetrate the two loop antennas140.

A heat target object190was 150 g of potato salad served on a polypropylene (PP) material tray having a length of 115 mm, a width of 80 mm, and a depth of 20 mm. The heat target object190was arranged centrally between the two loop antennas140on the food holding table166. The heat target object190was arranged in two ways, that is, arranged such that the length direction of the tray became perpendicular to the directional irradiation axis145(vertical placement) and arranged such that the length direction of the tray became parallel to the directional irradiation axis145(horizontal placement). The temperature measurement was performed by attaching a plurality of thermolabels (manufactured by NiGK Corporation) to the surface of the potato salad. The temperature measurement was performed after heating at an output of 150 W for 5 minutes.

Numerical analysis of the electric field intensity formed between the two loop antennas140was performed.

Results

As a result of the numerical simulation of the electric field intensity, a uniform electric field as illustrated inFIG.5was obtained between the two loop antennas140.

FIG.18illustrates a test result of heating the heat target object190placed vertically. Thermolabels (a), (b), and (c) arranged side by side along the directional irradiation axis145, that is, along the line connecting the midpoints of the two long sides of the tray all indicated 90° C. On the other hand, both thermolabels (d) and (e) arranged at a position away from the directional irradiation axis145, that is, near the center of the short sides of the tray indicated less than 50° C.

FIG.19illustrates a test result of heating the heat target object190placed horizontally. Thermolabels (f), (g), and (h) arranged side by side along the directional irradiation axis145, that is, along the line connecting the midpoints of the two short sides of the tray all indicated 100° C. On the other hand, both thermolabels (i) and (j) arranged at a position away from the directional irradiation axis145, that is, near the center of the long sides of the tray indicated 80° C.

All the results illustrated inFIGS.18and19made it clear that heating can be uniformly performed in a short time along the directional irradiation axis145. In any case, a temperature gradient occurred such that the temperature decreased as the distance from the directional irradiation axis145increased, and the heating efficiency was higher on the directional irradiation axis145than that on the diffusion irradiation axis. By arranging the directional irradiation axis145having high heating efficiency so as not to intersect structures such as the conveyance device and the food holding table166, it has been found to be possible to perform heating with suppressed energy loss due to generation and absorption of standing waves due to reflection of microwaves.

Experiment Example 2

The heating characteristics by the microwave irradiation device according to the above embodiment were further evaluated with a thermal indicator gel that is a food model as a heat target object.

Method

FIG.20illustrates an outline of a configuration example of a test device200used for evaluation. This test device200corresponds to the configuration of the part including the pair of antennas40facing each other in the microwave irradiation device2of the second embodiment described with reference toFIG.4and the configuration of the microwave irradiation device6of the sixth embodiment described with reference toFIG.13. This situation corresponds to a state in which the irradiated object90is positioned in the middle of the two antennas40by the conveyance device60in the seventh embodiment described with reference toFIGS.15A and15B. The configuration of the test device200was as follows.

The test device200includes a metal housing282that shields electromagnetic waves. The metal housing282was formed of an aluminum material, and had dimensions of a width of 500 mm, a length of 350 mm, and a height of 400 mm. A holding table266was horizontally provided in the metal housing282. The holding table266was made of a glass epoxy material, and had dimensions of a width of 331 mm and a thickness of 5 mm. One end of the holding table266in the width direction is provided with a first loop antenna240avia a first bracket249a, and the other end of the holding table266in the width direction is provided with a second loop antenna240bvia a second bracket249b. The first bracket249aand the second bracket249bwere each made of polyethylene (PE) material. Each of the first loop antenna240aand the second loop antenna240bwas formed in a square shape with an aluminum material, and had outer dimensions of a length of 214 mm, a height of 111 mm, and a thickness of 2 mm. The first loop antenna240aand the second loop antenna240bwere arranged so as to face each other, and installed such that the directional irradiation axes of the microwaves to be emitted became parallel to the holding table266. The interval between the first loop antenna240aand the second loop antenna240bwas 333 mm.

The material of the metal housing282is not limited to aluminum, and may be other metal materials such as iron and stainless steel. The material of the holding table266, the first bracket249a, and the second bracket249bmay be another material having a low dielectric constant and a low loss, such as a resin material such as polypropylene, polyethylene terephthalate, or polycarbonate, for example.

A microwave oscillator (not illustrated) was connected to a first feed port223aand the second feed port223bprovided in the metal housing282via a coaxial cable not illustrated. This coaxial cable is branched in the middle, and power output from the oscillator is fed in parallel to the first feed port223aand the second feed port223b. The first feed port223ais connected to a first power feed point253aof the first loop antenna240a. The second feed port223bis connected to a second power feed point253bof the second loop antenna240b. By partway branching from one oscillator and feeding power to each antenna in parallel, it is possible to perform simultaneous irradiation without the output from one antenna being erroneously recognized as reflection by the other antenna.

The frequency of the output power of the microwave oscillator was 450 MHz. The microwave power output from the microwave oscillator is fed in phase to the first loop antenna240aand the second loop antenna240b. Microwaves are emitted from the first loop antenna240aand the second loop antenna240b. Here, the interval between the first loop antenna240aand the second loop antenna240bis 333 mm as described above, which is equivalent of ½ wavelength of the output wavelength λ=666 mm.

As a food model290, a thermal indicator gel was used. This thermal indicator gel contains xylose and glycine, and is configured to change color to brown when the temperature reaches approximately 70° C. or higher due to the Maillard reaction. Electrical characteristics such as permittivity and electrical conductivity of the thermal indicator gel were adjusted to be generally equal to the electrical characteristics of commercially available potato salad by adjusting the concentration of oil, salt, or the like to be added. The food model290was prepared by filling a cup of polypropylene (PP) material with 150 g of the thermal indicator gel. No sealing was performed after filling.

This food model290was placed at an intermediate position between the first loop antenna240aand the second loop antenna240bon the holding table266. That is, the distance from the first loop antenna240aand the second loop antenna240bto the center of food model290was 166.5 mm. The food model290was heated at an output of 150 W.

As a comparative experiment, the food model290was heated in a professional-use microwave oven (manufactured by Panasonic Corporation, with output of 250 W).

Results

FIG.21illustrates a photograph of the food model290after heating with an output of 150 W using the test device200. InFIG.21, the upper part illustrates a case where the heating time is 4 minutes, and the lower part illustrates a case where the heating time is 6 minutes. InFIG.21, a left column illustrates a state of the surface of the food model290photographed from above. In this figure, the left-right direction is the direction of the directional irradiation axes of the first loop antenna240aand the second loop antenna240b. InFIG.21, the right column illustrates a state of a longitudinal cross section of the food model290cut by a line indicated by a one-dot chain line in the left column.

As illustrated inFIG.21, the center part of the food model290was uniformly discolored to brown, which indicates that the center part was uniformly heated.

FIG.22illustrates a photograph of the food model290after heating with an output of 250 W using a professional-use microwave oven as a comparative experiment. InFIG.22, the upper part illustrates a case where the heating time is 3 minutes, and the lower part illustrates a case where the heating time is 5 minutes. InFIG.22, a left column illustrates a state of the surface of the food model290photographed from above. InFIG.22, the right column illustrates a state of a longitudinal cross section of the food model290cut by a line indicated by a one-dot chain line in the left column.

When the professional-use microwave oven was used, the outer peripheral part of the food model290was discolored to dark brown, which indicates that overheating occurred at the outer peripheral part. It is assumed that the outer peripheral part of the food model290was continuously irradiated with microwaves while the microwaves were multiple-reflected in the oven. Furthermore, the heat generation generated in the outer peripheral part of the food model290was not equal along the container periphery, and a heat generation defect not generating heat occurred in a part surrounded by a circle299in the figure. This indicates that there is no uniformity in the standing wave distribution formed by reflection of microwaves in the metal housing. This suggested that there was no reproducibility of heating.

On the other hand, in the case of using the test device200according to the present embodiment, it was confirmed that the irradiation method suppressing the standing wave was capable of selectively heating the food center part.

Experiment Example 3

In Experiment Example 2 described above, the heating situation at positions equidistant from the first loop antenna240aand the second loop antenna240bwas examined. In the present experiment example, a heating situation at a position biased to any one of the first loop antenna240aand the second loop antenna240bwas examined. This situation corresponds to the state of the first embodiment described with reference toFIG.1Band the like or the third embodiment described with reference toFIG.6, or the state in which the irradiated object90is conveyed by the conveyance device60to a position biased with respect to the two antennas40in the seventh embodiment described with reference toFIG.15Band the like.

Method

FIG.23is a view illustrating an outline of an implementation situation of the present experiment example. In the present experiment example, the test device200illustrated inFIG.20was used. The food model290was placed at a position where the distance from the first loop antenna240ato the center of the food model290was 56.5 mm. The output of the test device200was 150 W, and heating was performed for 5 minutes.

Results

FIG.24illustrates a photograph of the food model290after heating. InFIG.24, the left photograph illustrates a state of the surface of the food model290photographed from above. In this figure, the left-right direction is the direction of the directional irradiation axes of the first loop antenna240aand the second loop antenna240b, the left side is the first loop antenna240aside close to the food model290, and the right side is the second loop antenna240bside away from the food model290. InFIG.24, the right photograph illustrates a state of a longitudinal cross section of the food model290cut at the position of the one-dot chain line illustrated in the left photograph. Similarly, the left side is the first loop antenna240aside close to the food model290, and the right side is the second loop antenna240bside away from the food model290.

FIG.24made clear that the outer peripheral part of the food model290generated heat in this case. It was found that the heat generation range on the antenna vicinity side was wider than the heat generation range on the antenna far side, and the degree of heat generation was also large. It has been found that when the heat target object is arranged asymmetrically with respect to the pair of antennas while being close to one side of the pair of antennas facing each other, and the heat target object is irradiated with microwaves at different distances from the respective antennas, diffraction of the electric field to the outer peripheral part of the heat target object is increased, and the outer peripheral part can be efficiently heated without causing heat generation defects.

Experiment Example 4

The situation of Experiment Example 2 described above was analyzed by numerical simulation.

Method

Coupled analysis of heat and electromagnetic field was performed using CST STUDIO SUITE (manufactured by Dassault Systems), which is thermal coupled analysis software. An analytical model of the test device200illustrated inFIG.20was constructed. The heat target object was an imitation of 150 g of commercially available potato salad packed in a cup made of polypropylene (PP) material. The electrical characteristics of the heat target object were relative permittivity εr=51, electrical conductivity ρ=1.2 s/m, and dissipation factor tan δ=0.95, based on actually measured values of commercially available potato salad.

As illustrated inFIG.25Aillustrating the analysis result, this heat target object390was placed at an intermediate position between a first loop antenna340aand a second loop antenna340bfacing each other. That is, the distance from the center of the heat target object390to each antenna was 166.5 mm.

In the present analysis experiment example, a holding table366on which the heat target object390is placed is arranged so as to penetrate the first loop antenna340aand the second loop antenna340b. In this respect, this model is closer to the microwave irradiation device6according to the sixth embodiment illustrated inFIG.13rather than the test device200illustrated inFIG.20. On the other hand, since the physical property values of the holding table366are set to simulate a resin with a low dielectric constant and a low loss, it can be assumed that this model substantially reproduces both the device configuration of the test device200illustrated inFIG.20and the configuration of the microwave irradiation device6according to the sixth embodiment illustrated inFIG.13.

Using the above model, the temperature distribution in the case of heating for 5 minutes with the output of 150 W was analyzed.

Results

FIGS.25A and25Billustrate thermal coupled analysis results.FIG.25Ais a perspective view illustrating the analysis result, andFIG.25Billustrates a cross section perpendicular to the directional irradiation axes of first loop antenna340aand second loop antenna340bpassing through the center of the heat target object390. Similarly to the result of Experiment Example 2 illustrated inFIG.21, the center part of the heat target object390strongly generated heat and reached a high temperature. The results of the present numerical analysis were in good agreement with the experimental results. This numerical analysis was confirmed to be reliable.

Experiment Example 5

Analysis by numerical simulation was performed on the device configuration corresponding to the microwave irradiation device1of the first embodiment described with reference toFIG.1Band the like, the microwave irradiation device4of the fourth embodiment described with reference toFIG.7, and the microwave irradiation device5of the fifth embodiment described with reference toFIG.10Aand the like.

Method

The same analysis as in Experiment Example 4 was performed. As illustrated inFIG.26Aillustrating the analysis result, a model was constructed and analyzed, the model corresponding to a part including one antenna40in the microwave irradiation device1of the first embodiment described with reference toFIG.1B, or the microwave irradiation device4of the fourth embodiment described with reference toFIG.7and the microwave irradiation device5of the fifth embodiment described with reference toFIG.10Aand the like. That is, in this model, a loop antenna440was arranged only on one side of a heat target object490placed on a holding table466. The heat target object490was similar to the heat target object390of Experiment Example 4.

In the analysis whose result is illustrated inFIG.26A, the distance from the loop antenna440to the center of the heat target object490was 166.5 mm. In the analysis whose result is illustrated inFIG.26B, the distance from the loop antenna440to the center of the heat target object490was 56.5 mm. The output was 150 W, and the temperature distribution in the case of heating for 5 minutes was analyzed.

Results

FIGS.26A and26Billustrate thermal coupled analysis results. As illustrated inFIG.26A, it has been found that, when the distance from the loop antenna440to the center of the heat target object490is 166.5 mm, heat is generated particularly on the side close to the loop antenna440in the outer peripheral part of the heat target object490. It has been found that the outer peripheral part of the heat target object on the antenna side can be selectively heated by arranging the antenna on only one side with respect to the heat target object and asymmetrically irradiating the heat target object with microwaves.

FIG.26Billustrates a result of a case where the distance between the heat target object490and the loop antenna440is reduced as compared with the case ofFIG.26A, and the distance from the loop antenna440to the center of the heat target object490is 56.5 mm. By reducing the distance between the heat target object490and the loop antenna440, the temperature in a wider range of the outer peripheral part of the heat target object490increased more than that in the case ofFIG.26A. This result was consistent with the result of Experiment Example 3 illustrated inFIG.24.

It has been found that a region where heat is generated in the heat target object can be adjusted by asymmetrically emitting the heat target object with microwaves and further adjusting the distance between the heat target object and the antenna.

Experiment Example 6

An experiment to examine a heating method was conducted.

Method

Using the test device200illustrated inFIG.20, an experiment was conducted using, as a heat target object, a sample of 140 g of commercially available potato salad filled in a cup made of polypropylene (PP) material, the sample being not sealed after filling. Heating conditions were as follows. First, the heat target object was placed at an intermediate position between the first loop antenna240aand the second loop antenna240b, and heated with an output of 150 W for 2.5 minutes. After heating, the heat target object was left as an interval for 1.5 minutes. Subsequently, the heat target object was placed at a position where the distance from the first loop antenna240ato the center of the heat target object was 56.5 mm, and the heat target object was heated with an output of 150 W for 2.5 minutes. The temperature of this heat target object during this time was measured using an optical fiber thermometer.

Results

FIG.27illustrates results of temperature measurement of the center part (solid line) and the outer peripheral part (broken line) of the heat target object. When the heat target object was placed at an intermediate position between the first loop antenna240aand the second loop antenna240b, the center part of the heat target object was heated more than the outer peripheral part, and the temperature after heating for 2.5 minutes was 70° C. at the center part and 45° C. at the outer peripheral part. There was not much temperature decrease in the interval period of 1.5 minutes, and the temperature at the end of the interval period was 67° C. in the center part and in the outer peripheral part. Thereafter, when the heat target object was brought close to the first loop antenna240aside and heated, the outer peripheral part of the heat target object was heated more than the center part, and the temperature after heating for 2.5 minutes became 73° C. in the center part and 100° C. in the outer peripheral part.

It has been found that the center part and the outer peripheral part can be heated and the entire heat target object can be uniformly heated by combining the arrangement of the heat target object at the intermediate position between the first loop antenna240aand the second loop antenna240band the uniform emission of microwaves from both antennas with the arrangement of the heat target object close to the first loop antenna240aand the non-uniform emission of microwaves from both antennas. That is, as in the microwave irradiation device5of the fifth embodiment and the microwave irradiation device7of the seventh embodiment, it has been found that by moving the irradiated object90by the conveyance device60to change the positional relationship between the antenna40and the irradiated object90, it is possible to heat the center part and the outer peripheral part of the irradiated object90and possible to uniformly heat the entire irradiated object90.

Experiment Example 7

A test device corresponding to the microwave irradiation device7of the seventh embodiment described with reference toFIG.15Band the like was produced, and the heating characteristics in the case of heating the heat target object while conveying it were evaluated.

Method

FIG.28is a view illustrating an outline of a configuration example of a test device500according to the present experiment example. The test device500includes a first metal housing582a, a second metal housing582b, and a third metal housing582cthat shield electromagnetic waves. The first metal housing582awas formed of an aluminum material, and had dimensions of a length of 440 mm along the conveyance direction, a width of 350 mm orthogonal to the conveyance direction, and a height of 400 mm. A conveyor561made of resin and having a width of 140 mm was provided so as to pass through the inside of the first metal housing582a. The inlet part of the conveyor561of the first metal housing582awas provided with the second metal housing582bso as to be connected to the first metal housing582a, and the outlet part of the conveyor561of the first metal housing582awas provided with the third metal housing582cso as to be connected to the first metal housing582a. Each of the second metal housing582band the third metal housing582cwas formed of an aluminum material, and had dimensions of a length of 220 mm, a width of 248 mm, and a height of 80 mm.

In the first metal housing582a, a circular first loop antenna540aand a circular second loop antenna540bwere provided such that the conveyor561penetrates. Each of the first loop antenna540aand the second loop antenna540bwas formed of an aluminum material, and had an inner diameter of 232 mm and a thickness of 2 mm. The first loop antenna540aand the second loop antenna540bwere arranged so as to face each other, and installed such that the directional irradiation axis of the emitted microwave was parallel to the holding surface of the conveyor561. The interval between the first loop antenna540aand the second loop antenna540bwas 333 mm.

An oscillator510was connected to the first loop antenna540aand the second loop antenna540bvia a coaxial cable521. The frequency of the output power of the oscillator510was 450 MHz. The output power of the oscillator510is fed to the first loop antenna540aand the second loop antenna540bin parallel and in phase via the coaxial cable521. The interval between the first loop antenna540aand the second loop antenna540bis 333 mm as described above, which is equivalent of ½ wavelength of the output wavelength λ=666 mm. By partway branching from one oscillator and feeding power to each antenna in parallel, it is possible to perform simultaneous irradiation without the output from one antenna being erroneously recognized as reflection by the other antenna.

A sample of 140 g of potato salad filled in a cup made of polypropylene (PP) material, the sample being not sealed after filling, was a heat target object590. Three heat target objects590, i.e., a first heat target object591, a second heat target object592, and a third heat target object593, were prepared, and arranged on the conveyor561at predetermined intervals. The conveying speed of the conveyor561was 1 mm/sec, and the output was 300 W. The surface temperature of the potato salad after heating was measured by thermography installed at the outlet of the third metal housing582c.

As a comparative experiment, a similar cup-filled potato salad was heated with an output of 150 W for 5 minutes using a professional-use microwave oven, and the surface temperature was measured by thermography.

Results

FIG.29illustrates images obtained by thermography of the first heat target object591, the second heat target object592, and the third heat target object593heated using the test device500.FIG.30illustrates, as a comparative experiment, an image obtained by thermography of the heat target object590heated using a professional-use microwave oven.

As illustrated inFIG.30, when a microwave oven is used for heating, it is assumed that microwaves are constantly emitted from the outer peripheral part of the heat target object590while being multiple-reflected in the oven, and overheating occurred in the outer peripheral part of the heat target object590. In this comparative experiment, the temperature difference between the outer peripheral part and the center part was 33° C. Furthermore, the heat generation in the outer peripheral part of the heat target object590does not exhibit an equal temperature distribution along the periphery, and a low-temperature part occurred as in the part surrounded by a circle599in the figure. This indicates that there is no uniformity in the standing wave distribution formed by reflection of microwaves in the metal housing. This suggested that there was no reproducibility of heating.

On the other hand, as illustrated inFIG.29, in each of the first heat target object591, the second heat target object592, and the third heat target object593heated using the test device500, the outer peripheral part was slightly higher than the center part, but this temperature difference was about 7° C., and uniformity was high. As described above, it has been shown that the present heating method that suppresses power reflection can achieve highly uniform heating as compared with a heating method that actively uses power reflection as represented by a microwave oven.

Experiment Example 8

The microwave irradiation device7of the seventh embodiment was analyzed by numerical simulation.

Method

A model of the microwave irradiation device7according to the seventh embodiment illustrated inFIG.31was produced. In this model, a metal housing682was formed of an aluminum material, and had dimensions of a length of 1320 mm, a width of 350 mm, and a height of 400 mm. A conveyor661made of resin was provided so as to pass through the inside of the metal housing682. In the metal housing682, a first loop antenna640a, a second loop antenna640b, and a third loop antenna640c, which were three loop antennas, were provided such that the conveyor661penetrates. Each of these loop antennas was made of an aluminum material, and had outer dimensions of 214 mm×111 mm. The frequency of power fed to each of a first feed unit653aof the first loop antenna640a, a second feed unit653bof the second loop antenna640b, and a third feed unit653cof the third loop antenna640cwas 450 MHz (wavelength λ=666 mm). The distance between the loop antennas was 333 mm (λ/2).

The position of the second loop antenna640barranged at the center was set as the coordinate origin, and the electric field intensity formed between the antennas was analyzed.

As comparative analysis, the electric field intensity was calculated with respect to the irradiation distance from the oscillator when microwave oscillation was performed at a frequency of 450 MHz and an output electric field of 1 v/m in the waveguide.

Using the model illustrated inFIG.31, analysis was performed with a sample prepared by filling a polypropylene (PP) material cup with 150 g of potato salad as a heat target object. Coupled analysis of heat and electromagnetic field was performed using CST STUDIO SUITE (manufactured by Dassault Systems), which is thermal coupled analysis software. The electrical physical property values of potato salad that is a heat target object were relative permittivity εr=51, electrical conductivity ρ=1.2 s/m, and dissipation factor tan δ=0.95, based on actually measured values. The heat generation distribution of the heat target object when microwave heating was performed with an output of 150 W for a heating time of 5 minutes while conveying one heat target object was analyzed.

Results

FIG.32illustrates a result of analyzing the electric field intensity formed between the antennas in the model illustrated inFIG.31. In this figure, positions of −333 mm, 0 mm, and 333 mm indicated by broken lines indicate positions where loop antennas are arranged.FIG.33illustrates an analysis result of the electric field intensity with respect to the irradiation distance from an oscillator in a waveguide performed as comparative analysis.

As illustrated inFIG.33, as is known, antinodes and nodes of the standing wave due to reflection from the metal housing are alternately formed in the waveguide. At the antinode position, the electric field intensity is twice the oscillator output, and at the node position, the electric field intensity becomes zero. That is, the difference in electric field intensity depending on the location is large. On the other hand, as illustrated inFIG.32, in the microwave irradiation device according to the seventh embodiment, the electric field intensity distribution had high uniformity without nodes.

FIG.34illustrates the analysis result of the heat generation distribution of a heat target object690when the heat target object690, which was 150 g of potato salad filled in a cup of polypropylene (PP) material, was heated. The left figure illustrates a cross section along the conveyance direction by the conveyor661, and the right figure illustrates a cross section in a direction orthogonal to the conveyance direction by the conveyor661. A uniform heat generation distribution in the heat target object690was confirmed.

Although the present disclosure has been described above with reference to the preferred embodiment, the present disclosure is not limited only to the embodiment described above, and various modifications can be made within the scope of the present disclosure.

The contents of the documents described in this description and the description of the Japanese application that is the basis of Paris priority of the present application are all incorporated herein.