WO2013156875A2

WO2013156875A2 - A phase array in-line heater

Info

Publication number: WO2013156875A2
Application number: PCT/IB2013/001751
Authority: WO
Inventors: Ben ZICKEL
Original assignee: Goji Ltd.
Priority date: 2012-03-27
Filing date: 2013-03-26
Publication date: 2013-10-24
Also published as: WO2013156875A3

Abstract

Some of the disclosed embodiments include systems, apparatuses, and methods for heating a flowing material by applying radio frequency (RF) energy. In some embodiments, such an apparatus may include a conduit, in which the material flows; a cavity, encompassing at least a portion of the conduit; at least two radiating elements, configured to apply RF energy to the cavity at a common frequency and at a controllable phase difference; and a processor, configured to control the phase difference such that an electromagnetic field excited in the cavity has lower intensity near walls of the conduit than at the conduit center.

Description

A PHASE ARRAY IN-LINE HEATER

[001] This a pplication cla ims the benefit of priority to U.S. Provisional Patent Application No. 61/616147, filed on March 27, 2012, which is incorporated herein in its entirety.

Technical field

[002] This Patent Application relates to a device a nd method for applying RF energy to a flowing material.

Background

[003] Electromagnetic waves have been used in various applications to supply energy to objects. I n the case of radio frequency (RF) radiation, for exam ple, electromagnetic energy may be supplied using a magnetron, which is typically tuned to supply electromagnetic energy only at a single frequency, usually, via a single a ntenna. One exam ple of a commonly used device for supplying electromagnetic energy is a microwave oven. Typical microwave ovens supply electromagnetic energy at or about a single frequency of 2.45 GHz. Microwave applicators for heating a flowing fluid are also know.

Summary

[004] Some exempla ry aspects of the disclosure may be directed to an apparatus for heating a flowing material. The apparatus may incl ude a cond uit, in which the material flows, and a cavity, encom passing at least a portion of the cond uit. The apparatus may further include at least two radiating elements, configured to apply energy to the cavity. The appa ratus may include a n RF energy source, configured to supply RF energy to the radiating elements at a common frequency a nd at a predetermined phase difference, such that an electromagnetic field excited by the RF energy source has a lower intensity near the cond uit walls tha n at the cond uit center. The apparatus may a lso include a processor, configured to control the phase difference such that the electromagnetic field excited near the conduit walls has a lower intensity than at the conduit center.

[005] Some embodiments of the invention include a system for heating a flowing material by applying radio frequency ( RF) energy to the flowing material. The system may include: a conduit, in which the material flows during operation of the system; a cavity, encompassing at least a portion of the cond uit; at least two radiati ng elements, configured to a pply RF energy to the cavity at a common freq uency and at a controllable phase difference; and a processor, configured to control the phase difference such that an electromagnetic field excited in the cavity has lower intensity near walls of the conduit than at the conduit center.

[006] In some embodiments, the processor may be further configured to receive electromagnetic feedback from the cavity. The electromagnetic feedback may be indicative of the dielectric constant of the flowing material.

[007] In some embodiments, the processor may be configured to control the phase difference based on the electromagnetic feedback.

[008] In some embodiments, the system may include longitudinally spaced radiating elements decoupled from each other.

[009] In some embodiments, the system may include one or more arrays of radiating elements located in a volume defined between the conduit and the cavity.

[010] In some embodiments, the processor may be configured to control the frequency, and/or amplitude differences between radiation applied by the at least two radiating elements.

[Oil] In some embodiments, the system may include at least one RF energy source, configured to supply RF energy to the radiating elements.

[012] According to some embodiments of the invention, there is provided a method for heating a flowing material that flows in a conduit by application of radio frequency (RF) energy to the conduit through radiating elements. The method may include: determining a target energy delivery profile; receiving electromagnetic feedback in response to RF energy applied to the conduit at a plurality of excitation setups, each excitation setup comprising a set of values of variable parameters that affect a field pattern excited in the conduit; and selecting excitation setups based on the feedback, the application of the excitation setups resulting in the determined target energy delivery profile; and controlling at least one energy application unit to apply RF energy at the selected excitation setups.

[013] In some such methods, the electromagnetic feedback may be indicative of the dielectric constant of the flowing material. [014] In some embodiments, controlling at least one energy application unit to apply RF energy at the selected excitation setups may include controlling phase differences between RF radiations emitted by two or more radiating elements.

[015] In some embodiments, selecting excitation setups comprises selecting phase differences between RF radiations emitted by two or more radiating elements.

[016] In some embodiments, controlling at least one energy application unit to apply RF energy at the selected excitation setups comprises controlling energy application at amplitude differences between RF radiations emitted by two or more radiating elements.

[017] In some embodiments, controlling at least one energy application unit to apply RF energy at the selected excitation setups comprises controlling energy application at a common frequency by two or more of the radiating elements.

[018] In some embodiments, the target energy delivery profile includes application of more power near the center of the conduit than near the walls of the conduit.

[019] In some embodiments, the target energy delivery profile causes uniform heating of the flowing material.

[020] According to some embodiments of the invention, there is provided a system as described above, wherein the processor of the system is configured to carry out a method as described above.

[021] The present invention, in some embodiments thereof, may include an apparatus for heating a flowing material that flows in a conduit by applying radio frequency (RF) energy to the conduit through radiating elements. The apparatus may include a processor configured to: cause application of RF energy to the conduit; determine a target energy delivery profile; receive electromagnetic feedback in response to the application of the RF energy; select excitation setups based on the feedback, the application of the excitation setups resulting in the determined target energy delivery profile; and control at least one energy application unit to apply RF energy at the selected excitation setups.

[022] In some such apparatuses, the electromagnetic feedback is indicative of the dielectric constant of the flowing material. [023] In some embodiments, the processor may be configured to control the at least one energy application unit to apply RF energy at the selected excitation setups by controlling phase differences between RF radiations emitted by two or more radiating elements.

[024] In some embodiments, the processor may be configured to select excitation setups by selecting phase differences between RF radiations emitted by two or more radiating elements.

[025] In some embodiments, the processor may be configured to control the at least one energy application unit to apply RF energy at the selected excitation setups by controlling energy application at amplitude differences between RF radiations emitted by two or more radiating elements.

[026] In some embodiments, the processor may be configured to control the at least one energy application unit to apply RF energy at the selected excitation setups by controlling energy application at a common frequency by two or more of the radiating elements.

[027] In some embodiments, the target energy delivery profile includes application of more power near the center of the conduit than near the walls of the conduit.

[028] In some embodiments, the target energy delivery profile causes uniform heating of the flowing material.

[029] According to some embodiments of the invention, there is provided a method for applying radio frequency (RF) energy to a flowing material flowing in a conduit. The method may include: controlling a phase difference between RF radiation emitted by two or more radiating elements such that an electromagnetic field excited in a cavity encompassing at least a portion of the conduit has lower intensity near walls of the conduit than at the conduit center.

[030] Some such method may include receiving electromagnetic feedback in response to RF energy applied to the conduit and controlling the phase difference between RF radiation emitted by two or more radiating elements based on the electromagnetic feedback. Brief Description of the Drawings

[031] Some embodiments of the invention are herein described, by way of examples only, with reference to the accompanying drawings. With specific references now to the drawings in detail, it is contemplated that the particulars shown are exemplary and for purposes of illustrative discussion only. In this regard, the description of the drawings provides examples to those skilled in the art how embodiments of the invention may be practiced.

[032] Fig. 1 is a diagrammatic illustration of a system for heating a flowing material with RF energy according to some embodiments of the invention;

[033] Fig. 2 is a diagrammatic illustration of a cross section in a system according to some embodiments of the invention;

[034] Fig. 3A is a diagrammatic illustration of an apparatus for applying RF energy according to some embodiments of the invention; [035] Fig. 3B is a diagrammatic illustration of an RF source according to some embodiments of the invention;

[036] Fig. 4 shows exemplary simulation results of a field distribution obtained across a conduit according to some embodiments of the invention;

[037] Fig. 5A shows temperature distributions calculated for water heated according to some embodiments of the invention;

[038] Fig. 5B shows temperature distributions calculated for oil heated according to some embodiments of the Invention; and

[039] Fig. 6 is a flowchart of a method for heating a flowing material according to some embodiments of the invention. Detailed Description

[040] Reference will now be made in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. When appropriate, the same reference numbers are used throughout the drawings to refer to the same or like parts.

[041] In one respect, disclosed embodiments may involve apparatus and methods for heating a flow of material (also referred as flowing material) by RF energy. In some embodiments, the flow of material may include a flow of fluid (e.g., gas and/or liquid). In some embodiments, the flow of material may include a flow of solids (e.g., grains). In some embodiments, the flow of material may include a flow of a colloid, for example, an oil in water emulsion, a water in oil emulsion, a foam (or other gas in liquid colloid), fume (or other solid in gas colloid), etc. [042] In some embodiments, the material may flow inside a conduit. The conduit may include any channel, canal, duct, passage, or pipe through which the material can flow. In some embodiments, the flowing material may fill the conduit, such that substantially the entire inner volume of the conduit is full of the material and/or substantially the entire inner surface of the conduit is in contact with the material. The conduit, or a portion of it, may be placed inside a cavity. Radiating elements may be provided between the conduit and the cavity to feed RF energy into the conduit. The conduit may be wholly or partially RF transparent (e.g., the conduit may be constructed from a dielectric material capable of transferring at least a portion of the RF energy emitted from the radiating element, and a portion of it may be enclosed in an RF reflective cavity. [043] In some embodiments, the conduit may include one or more windows (or openings) made from RF transparent material, e.g., in one or more walls of the conduit. These windows or openings may allow RF energy to penetrate the inner volume of the conduit (e.g., the windows may be constructed from a dielectric material capable of transferring at least a portion of the RF energy emitted from the radiating element to the inner volume of the conduit). In some embodiments, wider sections of the conduit, or even substantially the entire conduit, may be constructed from RF transparent material. Consistent with some embodiments, RF transparent material may include any material capable of tra nsferring at least some EM energy in the RF range. Some examples of RF transparent materials may include: glass, such as tempered soda-lime glass (also known as PYREX), and heat resistant polymers, such as Silicone, Polycarbonate, etc. [044] In some embodiments, the flow of material may be heated by RF energy applied synchronously by two or more radiating elements. The radiating elements may excite, in the cavity, electromagnetic fields having a common frequency. In some embodiments, the electromagnetic signal applied by one of the radiating elements (which may also be referred to as a field excited by the one of the radiating elements) may be phase-shifted from the signal applied by another of the radiating elements.

[045] In some embodiments, phase differences between the signals applied by two or more of the radiating elements may be controlled to achieve a target field intensity distribution inside the conduit. For example, the target field intensity distribution may have higher field intensity at regions where the material flows more slowly (e.g., near the conduit walls), and lower field intensity at regions where the material flows faster (e.g., at the center of the conduit). In some embodiments, the target field intensity distribution may result in substantially uniform temperature distribution across a cross section of the flowing material perpendicular to the flow direction. [046] In some embodiments, a temperature distribution may be considered substantially uniform, if it is more uniform than a temperature distribution resulting from application of a uniform field intensity across the conduit or application of energy with random phase differences between the radiating elements. In some embodiments, a temperature distribution is considered sufficiently uniform, if the temperatures across the entire material processed are within a predetermined range. In some embodiments, the standard deviation of the temperatures across the entire material processed is within a predetermined threshold, within 1%, 2% or 5% of average temperature. The upper limit in the temperature range may be a temperature threshold, above which the material processed may be damaged. The lower limit in the temperature range may be a temperature threshold, below which the processing does not occur, or occurs too slowly.

[047] In some respects, the disclosure may involve apparatuses and methods for applying RF energy. The term RF energy, as used herein, includes energy deliverable by electromagnetic radiation in the RF portion of the electromagnetic spectrum, which includes radiation with a wavelength in free space of 100 km to 1 mm, or alternatively, with a frequency of 3 KHz to 300 GHz. In some examples, the applied RF energy may fall within frequency bands between 500 MHz to 1500 MHz or between 700 MHz and 1200 MHz or between 800 MHz and 1 GHz.

[048] As used herein, RF wave may refer to electromagnetic waves having a frequency in the RF portion of the electromagnetic spectrum. Microwave and ultra high frequency (UHF) energy, for example, are both within the RF range. In some other examples, the applied RF energy may fall only within one or more industrial, scientific and medical (ISM) frequency bands, for example, between 433.05 and 434.79 MHz, between 902 and 928 MHz, between 2400 and 2500 MHz, and/or between 5725 and 5875 MHz.

[049] An apparatus according to some embodiments of the disclosure may include one or more energy application units. An energy application unit may include one or more radiating elements and an RF energy source configured to supply RF energy to the radiating element(s). In some embodiments, the energy application unit may include two or more synchronized RF energy sources, which may be controlled to feed the radiating elements with signals having a common frequency, at a controlled phase difference, and/or at a controlled amplitude difference. In some embodiments, the energy application unit may include a single RF energy source, which may be controlled to feed the radiating elements with signals having a common frequency, at a controlled phase difference, and/or at a controlled amplitude difference. In some embodiments, energy may be applied from each of the energy application units sequentially. Alternatively, energy may be applied concurrently from two or more of the energy application units. In some embodiments, e.g., when two energy application units apply energy at different frequencies, sequential energy application from the energy application units may result in the same or similar processing effects as applying energy from two or more of the radiating elements concurrently. Therefore, embodiments disclosed in this disclosure may be implemented regardless of the number of energy application units included in the apparatus.

[050] An energy application unit according to some embodiments may apply energy at two or more different excitation setups. Applying energy at different excitation setups may result in excitation of different field patterns in an energy application zone. The excitation setups may each include a set of parameters that may affect the field pattern and may be controlled by components of the apparatus. Different setups may differ from one another by one or more values of these parameters. Such a parameter is referred to herein as a controllable field affecting parameter (c-FAP). In some embodiments, a value may be selected for each c-FAP, and the excitation setup may be defined by the selected values. Varying a selected value of a c-FAP varies the excitation setup, which, in turn, may vary the field pattern excited in the energy application zone. The selection may be among values available to the apparatus, for example, frequencies may be selected among frequencies in the frequency range at which the RF energy source is operable.

[051] The energy application zone may include any void, location, region, or a rea where electromagnetic energy may be applied. In some embodiments, an energy application zone may include an RF-transparent or partially RF-transparent conduit, which may be configured to receive a flowing material (e.g., fluid) to be heated. In some embodiments, an energy application zone may include a cavity, encompassing at least a portion of the conduit. The energy application zone may include an interior of an enclosure that allows existence, propagation, and/or resonance of RF waves. For purposes of this disclosure, all energy application zones may alternatively be referred to as cavities. [052] In some embodiments, varying values of c-FAPs may result in significant variations in the generated field patterns. In other instances, however, varying values of c-FAPs may cause little or no change in the generated field patterns (e.g., if the variation between the two values of the c-FAP is small).

[053] As an analogy, an excitation setup, and how it may be set with respect to the energy application unit of the disclosed embodiments, may be viewed as similar, or analogous, to setting the controls of a switchboard that includes a set of knobs, dials, switches, or other value-selectors. In the switchboard case, switching from one setup to another may be accomplished by manipulating one (or more) of the value-selectors. Each unique set of values associated with the value-selectors may result in a different control setup. In other words, the position of all the value selectors collectively (e.g., the positions of all the knobs, dials and switches collectively) may define a single control setup.

[054] Similarly, in the presently disclosed embodiments, an energy application unit may be configured to control one or more field affecting parameters, and an energy excitation setup of the energy application units may be defined by the field affecting parameters' values. Changing from one excitation setup to another may be accomplished by changing the values associated with one (or more) of the c-FAPs. Each unique set of c-FAP values may result in a unique excitation setup. In some embodiments, the energy application unit may be controlled by a processor, and the values of the field affecting parameters the unit is configured to control (which may also be referred to as c-FAPs available to the unit) may be set using micro-switches, transistors, electronic circuitry, or any other value selectors.

[055] Applying energy at a particular excitation setup may excite an electromagnetic field pattern in the energy application zone. For brevity, this excited electromagnetic field pattern may be referred to as an excitation. Thus, each excitation setup may correspond to an excitation. Accordingly, a reference to a supply, reception, absorption, leaking, etc. of an excitation setup may refer to a supply, reception, absorption, leaking, etc. of the corresponding excitation. Thus, for example, a statement that a given excitation or excitation setup is absorbed in the object may be interpreted as that energy associated with an electromagnetic field excited by the energy application unit at the given excitation setup is absorbed in the object. [056] Some apparatuses may be configured to control other field affecting parameters than others. For example, in some embodiments, an apparatus may include a processor that controls the frequency of an electromagnetic wave applied by an energy application unit to the energy application zone. In such an apparatus, the frequency may be available as a controllable field affecting parameter (c-FAP). In one example, such an apparatus may control the frequency to have any of two or more values, e.g. 800MHz, 800.5MHz, etc. By controlling the frequency and changing from one frequency value to another, the excitation setup may be changed, which, in turn, may change an electromagnetic field pattern excited in the energy application zone.

[057] In another example, an energy application unit may include two radiating elements that emit radiation at a controllable phase difference. The phase difference may be controlled to have two or more values, e.g., 0°, 90°, 180°, or 270°. The phase difference between electromagnetic fields emitted by the two radiating elements may be available as a c-FAP to an apparatus comprising the energy application unit. [058] In another example, a difference between intensities at which two radiating elements emit electromagnetic fields of the same frequency may be controlled, and thus may be available as a c-FAP.

[059] In another example, an energy application zone may include one or more conductive elements (e.g., rods), each of which may be controlled to be either in a parasitic state or in a connected state. The value of the state of each rod (i.e. parasitic or connected) may affect the electromagnetic field pattern excited in the energy application zone. In apparatuses having such rods, the state of each rod may constitute a c-FAP.

[060] In another example, an energy application zone may include a magnetizable element (e.g., at a wall of the energy application zone) and an electromagnet near the magnetizable element. The magnetizable element and the electromagnet may be arranged such that a field pattern excited in the energy application zone may be affected by current flowing in the electromagnet. In embodiments where this current is controllable, the value of the current (e.g., 1mA, 20mA, 500 mA, etc.) may be available as a c-FAP. [061] In another example, an energy application unit may include a plurality of radiating elements, and each radiating element may be connected to a power source or disconnected from the power source (e.g., turned on or off). In such embodiments, the status of each radiating element (i.e., on or off) may be available as a c-FAP. Additionally, or alternatively, the total number of radiating elements turned on may constitute a c-FAP. [062] Other examples of parameters that may serve as controllable field affecting parameters in some embodiments may include, but not limited to, the position of a radiating element, orientation of a radiating element, position and/or orientation of conducting elements in the energy application zone, cavity dimensions, or any other controllable parameter, the value of which may affect a field pattern excited in the energy application zone upon RF energy application to the zone.

[063] Excitation setups including only a single c-FAP may be referred to as one-dimensional excitation setups. An excitation setup including multiple c-FAPs may be referred to as a multi-dimensional excitation setup. For example, an apparatus configured to control the state of each of six rods to be either parasitic or connected may have a six-dimensional excitation setup. Two examples of such excitation setups may be: (parasitic, parasitic, parasitic, connected, connected, connected), and (parasitic, connected, connected, parasitic, parasitic, connected). In general, the number of c-FAPs available to an apparatus determines the dimension of an excitation setup available to the apparatus. The collection of all the excitations that may be excited by an apparatus (or the collection of all the excitation setups available to a n apparatus) may be referred to as the excitation space of the apparatus. The dimension of an excitation space of an apparatus may be the same as the dimension of each excitation setup available to that apparatus. For example, the excitation space of an apparatus that controls only frequency and phase shift between signals emitted simultaneously from all the available radiating elements is two dimensional, and the excitation space of an apparatus that controls only frequency, which radiating element is emitting at each instance, and phase shift between signals emitted simultaneously from all the emitting radiating elements may be three dimensional.

[064] In some embodiments, an energy application unit may be controlled by a processor configured to control energy application in accordance with feedback. The feedback may be indicative of, for example, the temperature, weight, position, moisture, volume, or any other characteristic of the object or the energy application zone (e.g., moisture level in the cavity). In some embodiments, the feedback may include electromagnetic feedback.

[065] As used herein, EM feedback may include any received signal or any value calculated based on one or more received signals, which may be indicative of the dielectric response of the cavity and/or the object to electromagnetic fields excited in the cavity. For example, EM feedback may include input and output power levels, network parameters, e.g., S parameters, Y parameters, reflection and transmission coefficients, impedances, etc, as well as values derivable from them. Examples of derivable values may include dissipation ratios, time or excitation setup derivatives of any of the above, etc. EM feedback may be excitation-dependent. For example, EM feedback may include signals, the values of which vary over different excitation setups. Therefore, EM feedback measured when energy is applied at various excitation setups may be used for controlling energy application.

[066] A dissipation ratio may be defined as a proportion between the power (or energy) supplied to the radiating elements and the power (or energy) absorbed in the cavity. The power absorbed in the cavity may be equated with the difference between the power supplied to the radiating elements and the power detected to output from the cavity (e.g., power reflected back to the transmitting radiating element and/or power coupled to the other radiating elements). Dissipation ration may be calculated from S-parameters or gamma parameters detected.

[067] Thus, in some embodiments, energy application may be controlled such that one or more aspects of energy application at a given excitation setup (e.g., amount of energy, power level at which energy is applied, time duration at which energy is applied etc.) may depend on electromagnetic (EM) feedback received. In some embodiments, the EM feedback received may be associated with one excitation or multiple excitations. In some embodiments, the EM feedback received may be associated with an excitation other than an excitation that caused the EM feedback.

[068] As used herein, if a machine (e.g., a processor) is described as being configured to perform a task (e.g., configured to cause application of a predetermined field pattern), it is to be understood that the machine includes the components or elements (e.g., parts, hardware, software, etc.) needed to make the machine capable of performing the described task during operation. In some embodiments, the machine may also perform the task during operation. Similarly, when a task is described as being done in order to establish a target result (e.g., in order to increase heating uniformity), such a discussion associates the task with the target result. In some embodiments, the target result may be fully or partially accomplished through performing the task. [069] As used herein, the term processor may include an electric circuit that performs a logic operation on input or inputs. For example, a processor may include one or more integrated circuits, microchips, microcontrollers, microprocessors, all or part of a central processing unit (CPU), graphics processing unit (GPU), digital signal processors (DSP), field- programmable gate array (FPGA) or other circuit suitable for executing instructions or performing logic operations.

[070] The instructions executed by the processor may, for example, be pre-loaded into the processor or may be stored in a separate memory unit such as a RAM, a ROM, a hard disk, an optical disk, a magnetic medium, a flash memory, other permanent, fixed, or volatile memory, or any other mechanism capable of storing instructions for the processor. The processor(s) may be customized for a particular use, or can be configured for general- purpose use and can perform different functions by executing different software.

[071] If more than one processor is employed, all may be of similar construction, or they may be of differing construction. Further, the processors may be electrically connected together or they may be electrically isolated from one another. They may be separate from one another or may be integrated together. When more than one processor is used, they may be configured to operate independently or collaboratively. They may be coupled electrically, magnetically, optically, acoustically, mechanically or by other means permitting them to interact. [072] Fig. 1 is a diagrammatic illustration of a system 100 for heating (or for other processing) a flowing material with RF energy according to some embodiments of the disclosure. Fig. 2 is a diagrammatic illustration of a cross section in system 100. In some embodiments, system 100 may include a conduit 102, in which the material may flow during heating. Conduit 102 may be of any shape that allows the material to flow inside, including, for example, a cylinder having its longitudinal axis along the flow direction of the material. The cylinder base may be, for example, circular, oval, or polygonal.

[073] In some embodiments, the conduit may include one or more windows (or openings) made from RF transparent material. These windows or openings may allow RF energy to penetrate the inner volume of the conduit. Optionally, wider sections of the conduit, or even substantially the entire conduit, may be constructed from RF transparent material. RF transparent material may include any material capable of transferring at least some of the RF energy applied to heat the flowing material. Some non-limiting examples of RF transparent materials may include: glass, such as tempered soda-lime glass (also known as PYREX), heat resistant polymers, such as Silicone, Polycarbonate, etc. [074] A portion of conduit 102 may be enclosed inside a cavity 104. Cavity 104 may be made of an RF reflective material, e.g., stainless steel, or any other electrically conductive material. Thus, in some embodiments, RF radiation does not leak out of cavity 104, or leaks only to a degree acceptable by the relevant regulatory authorities. Cavity 104 may encompass conduit 102 entirely, or it may encompass only a portion thereof. In the latter case, cavity 104 may have openings 106 and 108, through which conduit 102 may be inserted. In some embodiments, openings 106 and 108 may be equipped with sealing elements (not shown), such as choke systems and/or gaskets, for preventing or reducing leakage of RF energy from cavity 104 via the openings.

[075] In some embodiments, the dimension of cavity 104 along a direction transverse to the material flow may be larger than the dimension of conduit 102 in the same direction. The volume defined between conduit 102 and cavity 104 may include radiating elements 110, configured to apply RF energy to the inside of conduit 102 in order to heat the material (not shown) that flows therein. The larger the cavity is, the more space that may be available for radiating elements. For example, the larger a diameter of a cylindrical cavity is, the more space there may be for angularly spaced radiating elements. Similarly, the larger a height of a cylindrical cavity is, the more space there may be for longitudinally spaced radiating elements.

[076] Radiating elements spaced apart from each other along a longitudinal axis of conduit 102 (such as radiating elements A and B shown in FIG. 1) may be referred to as longitudinally spaced radiating elements. Longitudinally spaced radiating elements may be utilized for heating the flowing material at differing locations along the conduit. In some embodiments, the dielectric properties of the material may change during the flow along the conduit, for example, due to temperature increase.

[077] In some embodiments, longitudinally spaced radiating elements may be controlled to apply energy that is most efficiently absorbed by the flowing material in the vicinity of the radiating elements. Thus, in some embodiments, longitudinally spaced radiating elements may be controlled to apply RF energy at mutually different excitation setups, for example, at mutually different frequencies. The frequencies or other c-FAPs may be determined, for example, based on feedback indicative of the absorption efficiency of energy applied at differing frequencies. Additionally, or alternatively, the frequencies, phases, etc., may be determined based on feedback indicative of temperature, viscosity, or other physical properties of the material at the vicinity of the radiating elements.

[078] In some embodiments, feedback may be obtained by suitable sensors (152, 154, 156, and 158 illustrated in Fig. 2) inside conduit 102. The sensors may include, for example, temperature sensitive sensors (152, 154, 156) and flow sensors (158). In some embodiments, one or more field sensors may detect EM field intensity in the conduit. The measured field intensities may also be used as feedback. Additionally, or alternatively, data indicative of physical properties of the material may be inferred or derived from EM feedback, for example, by correlating the EM feedback values with physical properties of the material. In some embodiments, such correlation may be established, for example, based on experiments carried out beforehand, where the physical properties and the EM feedback were measured. This correlation between EM feedback and physical properties may be stored on a memory provided in system 100 or accessible to system 100.

[079] In some embodiments, excitation setups may be determined based on dielectric properties of the material, and these dielectric properties may be inferred or derived from EM feedback. For example, by measuring scattering parameter Sn and input impedance Z₀, one can calculate the complex dielectric constant of the material. For example, for a cylindrical conduit with an internal radius R_int placed in the middle of a cavity of radius R_ext and height H, with a wavelength significantly larger than the physical dimensions of the cavity, the complex dielectric constant, ε= ε'-i ε" at frequency F may be given by Equation (1):

(1) s = ^s₀H \n ^~l (R_E R_I ] / C

Where

(2) C

2mFZ, and

Z₀(l + S„)

(3) Z,

1 - S_U

[080] Thus, to calculate ε from Z₀ and S_n one may first calculate Z_c (according to Equation (3)), calculate C based on the calculated value of Z_c (according to Equation (2)), and then, calculate ε based on the value calculated for C (according to Equation (1)). Knowing the dielectric constant ε, one can determine which excitation setups to use for achieving a given field distribution, e.g., by running a simulation or an analytic calculation using the known dimensions R_int R_ext H and the dielectric constant ε. In some embodiments, the processor may select excitation setups to be applied to the conduit based on the calculated dielectric constant ε.

[081] In some embodiments, frequencies, amplitudes, and phases may be selected for each radiating element based on the dielectric properties of the material in the cavity (e.g., ε), and the electric field patterns generated within the material at each possible frequency, phase and amplitude. The selection may be made to achieve a target energy delivery profile (also referred to herein as target field intensity distribution). Target energy delivery profiles may be defined, for instance, in one of the following ways: matching energy delivery to a preset profile, maximizing energy delivery at a specific location inside the conduit, maximizing energy delivery at a specific location in the conduit while minimizing energy delivered to all other locations, etc. These targets may be included in simulations or analytical calculations to reveal which excitation setups may best provide the target condition. For example, field distributions may be simulated for a large number of excitation setups. And then, an optimization procedure may be used to find excitation setup combinations that provide a field distribution closest to the required one. The excitation setups found this way may be applied sequentially, for example, at a change rate between one excitation setup to another that is rapid enough in comparison to the material flow rate to allow water exposure to all the excitation setups in the combination.

[082] Radiating elements spaced apart from each other along a circumference of conduit 102 (like, for example, radiating elements A and C as shown in Fig. 2) may be referred to as angularly spaced radiating elements. In some embodiments, angularly spaced radiating elements may be utilized for generating a target field distribution across a cross section of conduit 102 perpendicular to the flow of material. Such an arrangement, for example, may create larger field intensity near the longitudinal axis of conduit 102 than near the circumference of conduit 102. In some embodiments, certain target field distributions may be generated by controlling phase and/or amplitude differences between fields excited by angularly spaced radiating elements.

[083] Angularly spaced radiating elements may form an array of radiating elements, for example, arrays 112, 114, and 116 illustrated in Fig. 1. In some embodiments, each array may consist of radiating elements equally distanced from an end of conduit 102 along a longitudinal axis of the conduit. In some embodiments, arrays that are longitudinally displaced from each other (e.g., arrays 112 and 114 or 112 and 116) may be decoupled from each other. For example, two arrays may be decoupled from each other by the distance between them. If the distance is sufficiently large, fields excited by one array may not interact (or may only nominally interact) with fields excited by the other. In some embodiments, adjacent arrays may be decoupled from each other by other ways, for example, in time, such that when one array excites fields, another one may be silent. In another example, two arrays may be decoupled by frequency: each array may emit at a different frequency, such that radiation from the two arrays do not interact or only nominally influence each other. The periods of silence may be shorter, for example, by a factor of 2, 10, 1000, etc. in comparison to the time the material flows from one array to the other. Using the above exemplary factors, if the material flows 10 cm per second, and the arrays are spaced 10 cm from one another, the two arrays may excite fields at duty cycle of 50% and rate of 2, 10, or 1000 Hz.

[084] In some embodiments, arrays that are longitudinally displaced from each other may be coupled to each other, and the excitation setups may be determined in consideration of such coupling. For example, in some embodiments, all the radiating elements compose a single large array, and the excitation setups may be defined by a frequency, common to all the radiating elements, and phase differences between each two of the radiating elements.

[085] Radiating elements and/or arrays of such elements may be fed by F sources. For example, RF source 118 may feed array 112, RF source 120 may feed array 116, and RF source 122 may feed array 114. In some embodiments, each radiating element may be fed by its own source. In some embodiments, two or more sources may be synchronized with each other, so coherency may be accomplished between radiating elements fed by different sources. [086] An RF source may include any component(s) that are suitable for generating and supplying RF energy to radiating elements. One example, out of many, of RF sources suitable for use in some embodiments is shown in Fig. 3B, and described below.

[087] In some embodiments, each array of radiating elements may be fed by an RF source of its own. In some embodiments, a single array may be fed from two or more RF sources. In some embodiments, an RF source may be common to two or more arrays. The RF sources may be controlled by one or more processors. For example, as shown in Fig. 1, all the sources may be controlled by processor 130. Alternatively, each array may be controlled by its own processor. The processors may or may not be commonly controlled by a central processor. [088] Fig. 3A is a diagrammatic illustration of an apparatus 200 for applying RF energy according to some embodiments of the invention. Apparatus 200 may be part of system 100. Apparatus 200 may include one or more energy application units 210, 260. An energy application unit may include one or more radiating elements (212, 262, 264) and an RF energy source (220, 270) configured to supply RF energy to the radiating element(s). Radiating elements of energy application unit may be provided in an array (as discussed in reference to Fig.l). Energy application zone 280 may or may not be part of apparatus 200. Each RF energy source may be structured similarly to source 300 shown in Fig. 3B. In some embodiments, an energy application unit may include two or more synchronized RF energy sources, which may be controlled to feed the radiating elements with signals having a common frequency, at a controlled phase difference, at a controlled amplitude difference, etc. In some embodiments, energy may be applied from each of the energy application units individually or, alternatively, energy may be applied concurrently from two or more of the energy application units. In some embodiments, application from the energy application units individually may result in the same or similar processing effects as applying energy from two or more of the radiating elements concurrently. Therefore, a similar discussion may be relevant both to apparatuses including one energy application unit and to apparatuses including a plurality of energy application units, and the invention may be implemented irrespective of the number of energy application units included in the apparatus. An energy application unit may be controlled by a processor 290. For example, processor 290 may set the value of each controllable field affecting parameter (c-FAP) to define excitation setups at which energy may be applied to energy application zone 280. In some embodiments, processor 290 may control the energy application units based on input the processor receives from detectors 295, 296, 297. Each of detectors 295, 296, and 297 may receive electromagnetic feedback from one of radiating elements 212, 262, and 264, respectively. In some embodiments, one or more of detectors 295, 296, 297 may include a dual directional coupler, configured to allow signals to flow from the RF source (e.g., source 220) to the radiating elements when the radiating elements radiate energy, and to allow signals to flow from the radiating elements to the detector when the radiating elements receive EM feedback, and to distinguish between forward and backward signals when they flow simultaneously. In some embodiments, processor 290 may control the energy application units based on input the processor receives from sensors, e.g., sensor 298. The sensors may be used to sense any information, including electromagnetic power, temperature, weight, humidity, motion, etc. The sensed information may be used for any purpose, including process control, verification, automation, authentication, safety, etc.

[089] Fig. 3B is a diagrammatic illustration of an RF source 300 according to some embodiments of the disclosure. In some embodiments, source 300 may be configured to feed two radiating elements, e.g., radiating elements A and C, and similar structures may be used for feeding a larger number of radiating elements, for example, all the radiating elements in array 112. In some embodiments, each radiating element may receive RF energy from a different source. In some embodiments, some or all of the sources may be synchronized with each other.

[090] Source 300 may include an oscillator 305, which may generate electromagnetic radiation oscillating at a radio frequency (e.g., the oscillator may generate an AC waveform oscillating at a predetermined frequency). This radiation may be split by splitter 310, such that one portion of the radiation is provided to phase shifter 315, and the remaining portion is provided to phase shifter 320. The phase shifter may be configured to cause a time delay in the AC waveform in a controllable manner, delaying the phase of an AC waveform anywhere from between 0-360 degrees. In some embodiments. . When the radiation is split only to two, a single phase shifter may suffice. On the other hand, when radiation is split towards a larger number of radiating elements, a larger number of phase and/or amplitude shifters may be used.

[091] The phase shifted electromagnetic signals may then be amplified by amplifiers 325 and 330, which may control amplitudes of the waves that will be transmitted by radiating elements A and C into cavity 104. Having two separate amplifiers may allow control of the amplitude of a wave transmitted by radiating element A independently from control of the amplitude of a wave transmitted by radiating element C. In some embodiments, the amplifiers may be connected to the radiating element via two way directional couplers, allowing for separate measurements of power going into the radiating elements and power returning to the radiating element, e.g., by coupling signals going in opposite directions to different detectors. For example, such measurements may allow measuring S parameters, input impedances, etc. [092] In some embodiments, phase difference between signals supplied to two or more radiating elements may be obtained directly from the power source - for example: the output frequency and the phase emitted from each radiating element may be determined by the source (for example, by using a Direct Digital Synthesizer).

[093] In some embodiments, processor 130 may control RF source 300 or any component of RF source 300 (e.g., processor may control phase shifter 315 or 320 to obtain a desired phase difference between radiating elements A and C). In some embodiments, processor 130 may control RF source 300 or any component thereof based on electromagnetic feedback.

[094] The present invention is not limited to a particular RF source. An RF source may include any component(s) that is suitable for generating and supplying EM energy. RF Source may include one or more of a power supply configured to generate EM waves that carry EM energy. For example, the power supply may include a semiconductor oscillator, e.g., a voltage controlled oscillator, configured to generate AC waveforms (e.g., AC voltage or current) with controllable frequency. The frequency may be controlled to be constant or to vary. Alternatively, or additionally, a source of EM energy may include any other power supply, e.g., EM field generator, EM flux generator, solid state amplifier or any mechanism for generating vibrating electrons.

[095] Fig. 4 shows simulation results of field distribution obtained in a conduit filled with a material having dielectric constant ε' = 8.25, and loss tangent tan5 - 0.33. The common frequency used for the excitation was lOOOMHz. The field distribution was obtained using eight radiating elements radiating at the common frequency and with predetermined phase differences between them. The phase differences were set to concentrate the field at half the radius to the right of the center of the conduit. As may be seen, the field indeed concentrated at that point. [096] In a similar manner, by using different excitation parameters, other field intensity distributions may be obtained. In addition, applying differing field intensity distributions over time may result in a heating effect similar to that achieved with a single field intensity distribution, equal to the sum of the differing field intensity distributions. This way, many differing heating patterns may be achieved, including, for example, a heating pattern that compensates for the differences in flow rate across a cross section of the conduit.

[097] Fig. 5A shows simulation results of a temperature distribution in water flowing inside a conduit in a flow rate of 0.1 liter/sec. The simulations included numerical solutions to equations 1-3 presented above. The simulated conduit has a diameter of 50mm, and a length of 1.2m. Heating was not applied to the first 0.1m (designated "inflow"), and to the last 0.1m (designated "outflow"). The water entered the conduit at 20°C (lower line). The power used was 10KW. The upper line shows temperature distribution along a cross section at the outflow, the middle line shows temperature distribution along a cross section at the middle of the conduit, and the lower line shows temperature distribution along a cross section at the inflow. As shown in Fig. 5A, the temperature at the outflow was between about 42°C near the conduit wall and 47°C at the center.

[098] Using conventional jacket heating at the same heating power and flow rate resulted in a temperature of about 35°C at the center and about 140°C at the conduit wall. At a midpoint between the inflow and the outflow (i.e., 50 cm from both), water heated with RF energy had a temperature of about 33°C at the center and about 31°C at the conduit wall.

[099] Fig. 5B shows simulation results similar to those of Fig. 5A, with the differences that the flowing material used in the simulation was engine oil, and the power used was 40KW. Like in Fig. 5A, the upper line shows temperature distribution along a cross section at the outflow, the middle line shows temperature distribution along a cross section at the middle of the conduit, and the lower line shows temperature distribution along a cross section at the inflow. As shown in Fig. 5B, the temperature was substantially the same across the diameters both at the outflow and at the middle. Completely uniform temperature at the inflow was used as a starting condition of the simulations shown in both figures 5A and 5B. These results show that RF heating according to the teachings of the present disclosure may result in uniform heating of flowing materials of vastly different properties. [0100] Fig. 6 is a flowchart of a method 600 for heating by RF energy a flowing material that flows in a conduit, according to some embodiments of the invention. The method may be carried out by a processor, for example, processor 290 of Fig. 3A or processor 130 of Fig. 1. Method 600 is not limited to heating a flowing material by RF energy and may be used for other processing by RF energy - for example: sterilization of a flowing material (e.g., milk pasteurization). Method 600 may include a step 602 of determining a target energy delivery profile. The target energy delivery profile may be designed to achieve a certain temperature profile in the flowing material. For example, the temperature profile to be achieved may be a uniform profile, e.g., such as that described in Figs. 5A and 5B. The target energy delivery profile may be designed to achieve a certain energy field distribution in the flowing material and/or a certain energy field distribution in a cross-section perpendicular to the flowing material. The target energy delivery profile may take into account flow dynamics of the flowing material. For example, for a laminar flow, where flow is faster near the center of the conduit, the target energy delivery profile may include application of more power near the center of the conduit than near the walls of the conduit. The target energy delivery profile may be determined by the processor, for example, in response to information received from a flow meter. The processor may be configured to calculate a target energy delivery profile taking into account the desired temperature profile and the flow rate. For example, faster flow may indicate that the rate differences between flow of the material at the conduit walls and at its center are larger than would be expected if the flow is slower. In some embodiments, the flow rate, or any other variable that may affect the desired energy deliver profile is determined by the processor based on input from a user. In some embodiments, the target energy delivery profile may be pre-programmed in the processor or saved in a memory of system 100, for example, in case the flow of the material is expected to be constant.

[0101] In some embodiments, method 600 may further include a step 604 of receiving electromagnetic feedback. The EM feedback may be indicative of the dielectric constant of the flowing material. Step 640 may include calculating the dielectric constant of the flowing material (e.g., ε) from the received electromagnetic feedback (as discussed above). This step may be useful, for example, if the system or apparatus (e.g., system 100) the processor of which may run method 600, is configured to flow various different materials, for example, different kinds of juices. In some embodiments, step 604 may be useful also to follow small changes in dielectric constant of the material, for example, changes that originate in temperature changes of the flowing material. For example, if the heated material is water, and it comes into the process from an open reservoir, the water's temperature may change quite significantly between day and night. The dielectric constant of the flowing material may change the tendency of the material to absorb the RF energy. For example, hot water absorbs RF less efficiently than cold water. If the target temperature profile includes also a target value for the temperature, and not only a target distribution of the temperature, the dielectric constant may affect the amount of energy required for heating. [0102] Method 600 may further include a step 606 of selecting excitation setups. The selection may be based on the feedback, and/or on the target energy delivery profile. In some embodiments, the selection may be such that the application of the selected excitation setups results in the determined target energy delivery profile. For example, if it is known that a certain excitation setup causes a hot spot at the center of the conduit it may be selected for application, and if the excitation setup causes a hot spot near the walls of the conduit it may be left unselected. In some embodiments, the selection may include selecting or setting a phase difference between two or more radiating elements (e.g., between radiating elements of array 112)

[0103] In some embodiments, method 600 may include determining amounts of energy to be applied at one or more selected excitation setups, for example, by determining a weight to be associated with each excitation setup. The weight associated with an excitation setup may indicate an amount of RF energy to be applied at the excitation setup. For example, the weight may be indicative of a power level to be applied to the excitation setup and/or the duration along which the RF energy may be applied at the excitation setup. The processor may determine amounts of RF energy (e.g., weights) to be applied at one or more excitation setups, based on the EM feedback. In some embodiments, the processor may determine weights to be applied at one or more excitation setups, based on the EM feedback associated with those excitation setups. Thus, excitation setups may be associated with weights, so some are selected to be applied for more time and/or at higher power level than others. [0104] Method 600 may also include a step 608, of controlling an energy application unit (e.g., unit 210 or 260 shown in Fig. 3A) to apply energy at the selected excitation setups. The excitation setups may include setups, wherein two or more radiating elements emit RF energy at the same frequency at overlapping time periods. In some embodiments, there may be two or more energy application units controlled to apply the energy at the selected excitation setups. For example, different energy application units may apply energy at different frequencies, non-coherently with each other. In some cases, each energy application unit may be coherent with itself, e.g., one energy application unit may emit RF at a first frequency through two radiating elements at a controlled phase difference, and at the same time, another energy application unit may emit RF energy at a different frequency through two radiating elements at a controlled phase difference between them. In some embodiment, different excitation setups may differ from one another in a phase difference between radiation radiated by two (or more) of the radiating elements. In such embodiments, the control of energy application may include control of the phase difference. Similarly, different excitation setups may differ from one another in an amplitude difference between radiation radiated by two (or more) of the radiating elements. In such embodiments, the control of energy application may include control of the amplitude difference.

[0105] In the foregoing Detailed Description, various features are grouped together in a single embodiment for purposes of streamlining the disclosure. It should not be interpreted to require that the claimed invention include more features than those expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment.

[0106] Moreover, it will be apparent to those skilled in the art from consideration of the specification and practice of the present disclosure that various modifications and variations can be made to the disclosed systems and methods without departing from the scope of the invention, as claimed. For example, one or more steps of a method and/or one or more components of an apparatus or a device may be omitted, changed, or substituted without departing from the scope of the invention. Thus, it is intended that the specification and examples be considered as exemplary only, with a true scope of the invention being indicated by the following claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. A system for heating a flowing material by applying radio frequency (RF) energy, the system comprising:

a conduit, in which the material flows;

a cavity, encompassing at least a portion of the conduit;

at least two radiating elements, configured to apply RF energy to the cavity at a common frequency and at a controllable phase difference; and

a processor, configured to control the phase difference such that an electromagnetic field excited in the cavity has lower intensity near walls of the conduit than at the conduit center.

2. The system of claim 1, wherein the processor is further configured to receive electromagnetic feedback from the cavity.

3. The system of claim 2, wherein the electromagnetic feedback is indicative of the dielectric constant of the flowing material.

4. The system of claim 2 or 3, wherein the processor is further configured to control the phase difference based on the electromagnetic feedback.

5. A system according to any one of the preceding claims, comprising longitudinally spaced radiating elements decoupled from each other.

6. A system according to any one of the preceding claims, comprising one or more arrays of radiating elements located in a volume defined between the conduit and the cavity.

7. A system according to any one of the preceding claims, wherein the processor is further configured to control a frequency of the applied RF energy.

8. A system according to any one of claims 2-7, wherein the processor is further configured to control amplitude differences between radiation applied by the at least two radiating elements based on the electromagnetic feedback.

9. A system according to any one of the preceding claims, further comprising at least one RF energy source, configured to supply RF energy to the at least two radiating elements.

10. A method for heating a flowing material that flows in a conduit by application of radio frequency (RF) energy to the conduit through radiating elements, the method comprising: determining a target energy delivery profile; receiving electromagnetic feedback in response to RF energy applied to the conduit at a plurality of excitation setups, each excitation setup comprising a set of values of variable parameters that affect a field pattern excited in the conduit;

selecting excitation setups based on the feedback, the application of the excitation setups resulting in the determined target energy delivery profile; and

controlling at least one energy application unit to apply RF energy at the selected excitation setups.

11. The method of claim 10, wherein the electromagnetic feedback is indicative of the dielectric constant of the flowing material.

12. The method according to claim 10 or 11, wherein controlling at least one energy application unit to apply RF energy at the selected excitation setups comprises controlling phase differences between RF radiations emitted by two or more radiating elements.

13. The method according to claim 10, 11, or 12, wherein selecting excitation setups comprises selecting phase differences between RF radiations emitted by two or more radiating elements.

14. The method according to any one of claims 10 to 13, wherein controlling at least one energy application unit to apply RF energy at the selected excitation setups comprises controlling energy application at amplitude differences between RF radiations emitted by two or more radiating elements.

15. The method according to any one of claims 10 to 14, wherein controlling at least one energy application unit to apply RF energy at the selected excitation setups comprises controlling energy application at a common frequency by two or more of the radiating elements.

16. A method according to any one of claims 10 to 15 wherein the target energy delivery profile includes application of more power near the center of the conduit than near the walls of the conduit.

17. A method according to any one of claims 10 to 16 wherein the target energy delivery profile causes uniform heating of the flowing material.

18. A system according to any one of claims 1 to 9, wherein the processor is configured to carry out a method according to any one of claims 9 to 17.

19. An apparatus for heating a flowing material that flows in a conduit by applying radio frequency (RF) energy to the conduit through radiating elements, the apparatus comprising a processor configured to:

cause application of RF energy to the conduit;

determine a target energy delivery profile;

receive electromagnetic feedback in response to the application of the RF energy; select excitation setups based on the feedback, the application of the excitation setups resulting in the determined target energy delivery profile; and

control at least one energy application unit to apply RF energy at the selected excitation setups.

20. The apparatus of claim 19, wherein the electromagnetic feedback is indicative of the dielectric constant of the flowing material.

21. The apparatus according to claim 19 or 20, wherein the processor is configured to control the at least one energy application unit to apply RF energy at the selected excitation setups by controlling phase differences between RF radiations emitted by two or more radiating elements.

22. The apparatus according to any one of claims 19 to 21, wherein the processor is configured to select excitation setups by selecting phase differences between RF radiations emitted by two or more radiating elements.

23. The apparatus according to any one of claims 19 to 22, wherein the processor is configured to control the at least one energy application unit to apply RF energy at the selected excitation setups by controlling energy application at amplitude differences between RF radiations emitted by two or more radiating elements.

24. The apparatus according to a ny one of claims 19 to 23, wherein the processor is configured to control the at least one energy application unit to apply RF energy at the selected excitation setups by controlling energy application at a common frequency by two or more of the radiating elements.

25. An apparatus according to any one of claims 19 to 24 wherein the target energy delivery profile includes application of more power near the center of the conduit than near the walls of the conduit.

26. An apparatus according to any one of claims 19 to 25 wherein the target energy delivery profile causes uniform heating of the flowing material.

27. A method for applying radio frequency (RF) energy to a flowing material flowing in a conduit, the method comprising:

controlling a phase difference between RF radiation emitted by two or more radiating elements such that an electromagnetic field excited in a cavity encompassing at least a portion of the conduit has lower intensity near walls of the conduit than at the conduit center.

28. The method according to claim 27, comprising receiving electromagnetic feedback in response to RF energy applied to the conduit and controlling the phase difference between

RF radiation emitted by two or more radiating elements based on the electromagnetic feedback.