Electrical heating system for heating a fluid stream

Abstract

An electric heating system for heating a fluid stream, the system comprising: a housing having an inlet for a fluid stream to be heated by the electric heating system and an outlet for a fluid stream heated by the electric heating system; and a resistance heater mounted within the housing, the resistance heater comprising: a fluid input and a fluid output fluidly coupled to the inlet and outlet, respectively; a plurality of annular walls constructed from an electrically conductive material, the walls nested to define a plurality of annular flow channels arranged concentrically in series about a longitudinal axis, the walls extending between first and second opposite ends of the resistance heater spaced apart from one another along the longitudinal axis; and first and second electrical terminals for connecting to a source of electrical energy to heat the walls of the resistance heater, the first and second electrical terminals electrically connected to respective first and second walls comprising an outer pair of walls adjacent to one another and positioned radially outward of the resistance heater. The plurality of annular walls are mechanically connected to one another, whereby adjacent flow channels have opposite fluid flow directions and are connected at adjacent ends of each channel to define an alternating serpentine flow path, the serpentine flow path having an input end at a fluid input and an output end at a fluid output, the input end and output end being located at radially outer and inner positions, respectively, relative to the longitudinal axis. The plurality of annular walls are electrically connected to one another to define a continuous conductive path extending between first and second electrical terminals, the conductive path having a first portion extending from the first wall to a center of the resistive heater and a second portion extending from the center of the resistive heater to the second wall. A method of generating a high temperature fluid flow using an electrical heating system is also disclosed.

Description

[Technical Field]

[0001] The present invention relates to an electrical heating system for heating a fluid flow and to a method of using such a system to heat a fluid flow, and has particular application to the use of such an electrical heating system to provide, for example, a heater for a resistojet thruster or a flow heater in a spacecraft. [Background technology]

[0002] An electrothermal thruster is a type of electric propulsion thruster for satellites that converts electrical energy into thermal energy, increasing the stagnation temperature of a gaseous propellant. The higher the stagnation temperature, the better the performance. A resistojet is a technology within the electrothermal thruster class that heats the gas by Joule (or resistive) heating of a solid heating element. High-temperature resistojets are attractive because they offer high performance, but the gas temperature is limited by the operating temperature of the heater.

[0003] Conventional resistojets use electric heaters to directly or indirectly heat the gaseous propellant. The gas flows through a heat exchanger, which is used to maximize the thermal efficiency of the resistojet by limiting the external temperature of the device. High-temperature resistojets have been achieved in the past using direct heating of the propellant with a concentric tubular heat exchanger, which successfully demonstrated the operation of this design concept. However, the fabrication of this configuration involved a combination of two manufacturing techniques: chemical vapor deposition (CVD) and electron beam (EB) welding. As a result, the assembly procedure was a long and complicated process.

[0004] Known high-temperature resistjets comprise concentric tubular heaters consisting of a series of long tubular elements fabricated by chemical vapor deposition arranged concentrically. The tubes are joined at their ends by small struts attached by electron beam welding. Such tubular heater structures have suffered from premature failure due to a combination of significant thermal expansion caused by extreme heating and manufacturing imperfections resulting from the numerous weld joints required by the heater assembly.

[0005] (1) "Manufacturing of a High-Temperature Resistojet Heat Exchanger by Selective Laser Melting" by F. Romei, ANGrubisic, and D. Gibbon, Acta Astronaut. 138 (2017) 356-368. doi:10.1016 / j.actaastro.2017.05.020, (2) "Validation of an additively manufactured resistojet through experimental and computational analysis" by F. Romei and ANGrubisic, Acta Astronaut. (2020). doi:10.1016 / j.actaastro.2019.10.046, and (3) "Endurance testing of the additively manufactured STAR As disclosed in the paper "Resistojet" by M. Robinson, A. Grubisic, G. Rempelos, F. Romei, C. Ogunlesi, and S. Ahmed, Mater. Des. (2019) 107907. doi:10.1016 / J.MATDES.2019.107907, a concentric tubular heater was fabricated as a single monolithic part using metal additive manufacturing. However, the tubular heater structure also suffered from premature failure issues due to thermal expansion caused by cyclic heater operation.

[0006] Further aspects and features of additively manufactured resistojets are described in the following presentations at the 36th International Electric Propulsion Conference, held September 15-20, 2019, at the University of Vienna in Vienna, Austria: (i) a presentation entitled "The development and qualification of the STAR resistojet system for telecommunications applications" by Romei, F., Robinson, MD, Ogunlesi, C., Gibbon, D., and Grubisic, AN; (2) a presentation entitled "Lifetime investigations of an additively manufactured high-temperature resistojet heat exchanger from tantalum" by Robinson, MD, Grubisic, AN, Romei, F., and Ogunlesi, C.; and (3) a presentation entitled "Structural effects on the high temperature performance of the Super High Temperature Additive Manufactured This is disclosed in a presentation by Ogunlesi, C., Romei, F., Robinson, MD, Grubisic, AN and Gibbon, D. entitled "Resistojet (STAR)."

[0007] Despite these extensive efforts to produce resistjets with structural designs that can overcome the problems previously discussed for monolithic heater designs, there nevertheless remains a need in the art for an electrical heating system for heating a fluid stream that can be used as a resistojet, has a high level of reliability, and preferably can also be additively manufactured, for example, by 3D printing. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] The paper by F. Romei, ANGrubisic, and D. Gibbon entitled "Manufacturing of a High-Temperature Resistojet Heat Exchanger by Selective Laser Melting," Acta Astronaut. 138 (2017) 356-368. doi:10.1016 / j.actaastro.2017.05.020 [Non-patent document 2] "Validation of an additively manufactured resistojet through experimental and computational analysis," by F. Romei and ANGrubisic, Acta Astronaut. (2020). doi:10.1016 / j.actaastro.2019.10.046 [Non-patent document 3] The paper, entitled "Endurance testing of the additively manufactured STAR resistojet," by M. Robinson, A. Grubisic, G. Rempelos, F. Romei, C. Ogunlesi and S. Ahmed, Mater. Des. (2019) 107907. doi:10.1016 / J.MATDES.2019.10790 Summary of the Invention [Problem to be solved by the invention]

[0009] It is therefore an object of the present invention to provide an electric heater configuration that can operate reliably at temperatures as high as, for example, 3,500K, and achieve increased reliability compared to known resistjets.

[0010] It is also an object of the present invention to provide an electric heater configuration that is compact and can be manufactured at low cost, for example using an additive manufacturing process known in the art as "3D printing" that is capable of producing otherwise monolithic electric heaters. A preferred additive manufacturing process for use in the present invention is known in the art as selective laser melting (SLM).

[0011] It is also an object of the present invention to provide an electric heater configuration that exhibits high thermal efficiency.

[0012] It is also an object of the present invention to provide an electric heater configuration that can be used in other applications in addition to use in resistjets to provide a heated fluid stream. [Means for solving the problem]

[0013] The present invention provides an electric heating system for heating a fluid stream, comprising: a housing having an inlet for a fluid flow to be heated by the electric heating system and an outlet for a fluid flow heated by the electric heating system; a resistive heater mounted within the housing; The resistive heater comprises: a fluid input and a fluid output fluidly coupled to the inlet and outlet, respectively; a plurality of annular walls constructed from an electrically conductive material, the walls nested to define a plurality of annular flow channels arranged concentrically in series about the longitudinal axis, the walls extending between first and second ends on either side of the resistive heater spaced apart from one another along the longitudinal axis; first and second electrical terminals for connecting to a source of electrical energy to heat the walls of the resistive heater, the first and second electrical terminals being electrically connected to respective first and second walls comprising an outer pair of walls adjacent to each other and positioned radially outward of the resistive heater; Equipped with the plurality of annular walls are mechanically connected to one another, whereby adjacent flow channels have opposite fluid flow directions and are connected at adjacent ends of each channel to define an alternating serpentine flow path, the serpentine flow path having an input end at a fluid input and an output end at a fluid output, the input end and the output end being located at radially outer and radially inner positions, respectively, relative to the longitudinal axis; the plurality of annular walls are electrically connected to one another to define a continuous conductive path extending between the first and second electrical terminals, the conductive path having a first portion extending from the first wall to a center of the resistive heater and a second portion extending from the center of the resistive heater to the second wall; An electric heating system is provided.

[0014] The present invention further provides a method of heating a fluid stream using such a system method for producing a high temperature fluid stream, the method comprising: a) providing an electric heating system according to the present invention; b) supplying a fluid to be heated to a fluid input of a resistive heater and causing it to flow along an alternating serpentine flow path to a fluid output of the resistive heater, the supplied fluid having a pressure greater than an external gas pressure surrounding the exterior of the housing; c) applying an electrical potential across the first and second terminals to heat the fluid flow in the alternating serpentine flow paths by the resistive heater; d) discharging the heated fluid stream from an outlet of the housing; Includes:

[0015] Preferred features of the system and method of the present invention are defined in the respective dependent claims.

[0016] The systems and methods of the present invention can be used in any application where a fluid needs to be heated to high temperatures, for example, up to 3,500K.

[0017] Preferred embodiments of the present invention can provide an electric heating system for heating a fluid flow, comprising a resistive heater that can be manufactured in a compact and low-cost manner, particularly by using an additive manufacturing process such as selective laser melting (SLM) that can produce a monolithic resistive heater.

[0018] A preferred embodiment of the present invention can provide a resistive heater configuration that allows the high temperature element of the resistive heater to freely expand and contract under thermal load. This improvement is fundamental to enabling an electric heater system to meet typical life requirements for space missions when the resistive heater is used as a resistojet in a spacecraft. Laboratory environmental life testing has shown that a preferred embodiment of the present invention can provide a resistive heater configuration that can exhibit a heater life of over 6,000 heating / cooling cycles, exceeding typical mission requirements for spacecraft such as satellites.

[0019] Preferred embodiments of the present invention can provide a resistive heater constructed from a refractory metal in pure or alloy form, which can enable heating of a fluid to temperatures as high as 3,500 K. Alternatively, for applications where maximum temperature is not the only requirement, for example, when corrosion or oxidation is a potentially significant issue, the resistive heater can be fabricated using a non-refractory metal, such as a nickel alloy or steel. The resistive heater of preferred embodiments of the present invention can be designed to achieve any given outlet temperature within this range while maintaining high thermal efficiency and demonstrating heater integrity over thousands of heating cycles. Furthermore, when the resistive heater has a one-piece monolithic form, the resistive heater can offer greater design freedom and faster and cheaper manufacture compared to traditional assembly methods using multiple parts and materials. Furthermore, fabricating a resistive heater in a one-piece monolithic form by additive manufacturing techniques has the advantage that future costs of additive manufacturing (AM) are predicted to decrease while print quality and material selection are predicted to increase, enhancing the benefits and advantages of the resistive heater of preferred embodiments of the present invention.

[0020] Preferred embodiments of the present invention can improve heat transfer to a fluid by using a wall structure in which a first set of walls defines an annular flow path and a second set of walls is located inside the annular flow path formed by the first set of walls.

[0021] A preferred embodiment of the present invention can provide electrical terminals that are located external to the resistive heater where the temperature is lower, thereby simplifying the mechanical connection of the electrical terminals, reducing conductive heat loss from the resistive heater, and improving thermal efficiency.

[0022] Preferred embodiments of the present invention allow the resistive heater to be manufactured without welding, without forming part of the housing that functions as the pressure envelope for the electric heater system when in use.

[0023] Preferred embodiments of the present invention may enable the resistive heaters to be manufactured by additive manufacturing processes such as selective laser melting (SLM), which are capable of producing monolithic electric heaters. Such processes may be used to manufacture large quantities of resistive heaters in a single printing process, and the resistive heaters may be made from high-melting-point materials such as nickel alloys and heat-resistant alloys, known to those skilled in the art from the publications mentioned above.

[0024] The resistive heater of preferred embodiments of the present invention utilizes a hybrid of different heat exchanger and heater concepts, especially when fabricated as a single component in an additive manufacturing (AM) process. The resistive heater has two primary functions: it generates heat using electrical resistance when an electric current is passed through it, and it also convectively heats the fluid flowing through it. The resistive heater of preferred embodiments of the present invention can be easily configured to achieve high fluid temperatures with high thermal efficiency in a compact, low-cost package. High temperatures are achieved through a circulating flow geometry, whereby the fluid makes a series of passes through the resistive heater, extending the fluid heating time.

[0025] Using additive manufacturing processes, the resistive heaters of preferred embodiments of the present invention can easily incorporate such features and meshes to increase heat transfer efficiency, which can maximize fluid temperature for a given structural temperature. The circulating flow geometry allows for high thermal efficiency by introducing the cold fluid to be heated at the outer diameter of the resistive heater, where the cold fluid captures radiant heat lost from the resistive heater as the fluid circulates inward toward the center where it is in fluid communication with the fluid outlet of the resistive heater. Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings: [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a schematic diagram showing the overall structure of an electric heater system incorporating a resistive heater according to the present invention; [Figure 2] 2 is a schematic diagram of the electric heater system of FIG. 1 , but modified to provide a housing or pressure envelope that causes the fluid heated through the resistive heater to expand and remain in the subsonic / sonic flow region, and the exiting fluid to maintain a high static temperature, according to an embodiment of the present invention. [Figure 3] 2 is a schematic diagram of the electric heater system of FIG. 1 but modified to provide a housing or pressure envelope in which fluid heated via a resistive heater is expanded and accelerated to a supersonic / hypersonic flow region, resulting in an exiting fluid having a low static temperature; [Figure 4] FIG. 10 is a perspective side view, partially cut away, from the outlet end of a monolithic resistive heater according to a further embodiment of the present invention. [Figure 5] 5 is a longitudinal cross-section through half of the monolithic resistive heater of FIG. 4, showing a schematic diagram of fluid flow and electrical connections in the monolithic resistive heater. [Figure 6] FIG. 10 is a perspective side view, partially cut away, from the outlet end of a monolithic resistive heater according to a further embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] Referring to FIG. 1 , which illustrates the general configuration of an electric heating system according to a preferred embodiment of the present invention, the present invention provides an electric heating system, generally designated 100, for heating a fluid stream. The fluid stream is supplied from a fluid source 101 by a supply system 102. The fluid may include a single fluid component or a mixture of multiple fluid components. System 100 includes a housing 104, otherwise known in the art as a pressure envelope, having an inlet 110 for a fluid inflow 103 to be heated by electric heating system 100 and an outlet 111 for a fluid outflow 105 heated by electric heating system 100. A resistive heater 108 is mounted within housing 104 by a mounting mechanism (not shown). Resistive heater 108 includes a fluid input 112 and a fluid output 113 fluidly coupled to inlet 110 and outlet 111, respectively. Fluid is supplied to inflow 103 by supply system 102 at a desired pressure and flow rate.

[0028] System 100 further comprises power electronics 107 located external to housing 104 for supplying electrical energy from electrical energy source 106 to resistive heater 108. In the illustrated embodiment of the invention, resistive heater 108 is configured to be electrically connected to electrical energy source 106 by a wired connection.

[0029] The electrical energy source 106 is configured to provide direct current or alternating current of any desired frequency to heat the resistive heater 108 .

[0030] The fluid flows through a resistance heater 108 which releases thermal energy to heat the fluid, increasing the total enthalpy of the outflow 105 relative to the enthalpy of the inflow 103 .

[0031] In the illustrated embodiment of the present invention, a single resistive heater 108 is located within the housing 104. However, in other embodiments of the present invention, multiple resistive heaters 108 are located within the housing 104, and the resistive heaters 104 may be arranged in series or parallel with respect to the inlet 103 and outlet 105. Additionally, the multiple resistive heaters 104 may be arranged electrically in series or parallel.

[0032] In a preferred embodiment of the present invention, electric heating system 100 is a resistojet and is configured for installation on a spacecraft for use in moving the spacecraft through space.

[0033] 2 shows a first preferred embodiment of the present invention in which the outlet 211 for fluid outflow 205 has a converging shape defined by a frustoconical wall 201 of a housing 204. In use, such an outlet 211 discharges heated fluid through an orifice 202 at a velocity below the speed of sound in the fluid, maintaining a high static temperature in the fluid. The degree of convergence of the frustoconical wall 201 can be varied to vary the outflow velocity and static temperature.

[0034] 3 shows a second preferred embodiment of the present invention, in which an outlet 311 for a fluid outflow 305 is integrated into the housing 304 or achieved by additional components. The outlet 311 is formed with a converging section 301, a throat 302, and a diverging section 303. When operating with a back pressure in the housing sufficiently higher than downstream or ambient pressure, such an outflow 305 discharges heated fluid at a velocity greater than the speed of sound in the fluid. Thus, a cooler, high-velocity flow is produced. The divergence of the duct can be varied to change the outflow velocity and static temperature.

[0035] 2 and 3, the outlet 211, 311 may be integral with the body 220, 230 of the housing 204, 304 or may consist of an additional component that is fitted to the body 220, 230 of the housing 204, 304. The use of such an additional component may increase the robustness of the electric heating system against thermal stresses and allow operation at higher temperatures for longer periods of time compared to the use of an integral housing that includes an outlet for the hot fluid exit.

[0036] In yet another embodiment not shown, the outlet for fluid outflow may be linear with an orifice having a diameter smaller than the diameter of the body of the housing, without any converging or diverging shape.

[0037] Next, the structure of a resistance heater 108 according to a preferred embodiment of the present invention, which can be used in any of the configurations shown in FIGS. 1 to 3, will be described with reference to FIGS.

[0038] The resistive heater 108 includes a plurality of annular walls 120 constructed of an electrically conductive material. As used herein, the term "annular" means "generally ring-shaped" and is not limited to a geometrically circular shape, but includes circular and non-circular shapes, such as ellipses, polygons, etc. In the illustrated embodiment, the annular walls 120 have a circular cross-section. However, the annular walls may have any desired cross-sectional shape, which may be any polygonal shape, such as a square, rectangle, triangle, hexagon, etc., or a curved or rounded shape, such as a circle, ellipse, etc.

[0039] The annular walls 120 are nested to define a plurality of annular flow channels 122 arranged concentrically in series about the longitudinal axis LL. The annular walls 120 extend between first and second opposite ends 124, 126 of the resistive heater 108 that are spaced apart along the longitudinal axis LL.

[0040] The multiple annular walls 120 are mechanically connected to one another such that adjacent flow channels 122 have opposite fluid flow directions and are connected at adjacent ends 128 of each channel 122 to define an alternating serpentine flow path 130 having an input end 132 at the fluid input 112 and an output end 134 at the fluid output 113. The input and output ends 132, 134 are located at radially outer and inner positions, respectively, relative to the longitudinal axis LL. The fluid input 112 and fluid output 113 are located at first and second ends 124, 126, respectively, of the resistive heater 108. The alternating serpentine flow path 130 has first and second annular closed sides 131, 133 located at the first and second ends 124, 126, respectively, of the resistive heater 108. The closed sides 131 and 133 are provided with turns 137 and 139, respectively, that connect adjacent flow channels 122.

[0041] In the illustrated embodiment, the outermost wall 120f is cylindrical, and apart from the outermost wall 120f, the other walls 120a-120e inside it include a cylindrical portion 136 and an adjacent conical portion 138. A free end portion 140 of the cylindrical portion 136 is located at the second end 126 of the resistive heater 108, and the conical portion 138 faces the first end 124 of the resistive heater 108. The conical portion 138 closest to the first end 124 of the resistive heater 108 includes a solid layer 142 that defines a closed end portion 144 of the plurality of annular walls 120.

[0042] Thus, one annular wall 120e inwardly adjacent the outermost wall 120f is configured to form a closed end portion 144 of the plurality of annular walls 120 at the first end 124 of the resistive heater 108. The closed end portion 144 closes the end 128 of the annular flow channel 122 at the first end 124 of the resistive heater 108, providing a change in direction for the alternating serpentine flow path 130 at the first end 124 of the resistive heater 108.

[0043] Each annular wall 120 is constructed either from a solid layer of conductive material, as shown schematically in FIG. 5 by solid lines, or from a perforated layer of conductive material, as shown schematically in FIG. 5 by dashed lines.

[0044] 4 and 5, the resistive heater 108 comprises alternating annular walls 120 comprised of solid or perforated layers of conductive material. The innermost wall 120a is comprised of a solid layer 125 of conductive material, the adjacent wall 120b is comprised of a perforated layer 127 of conductive material, the next wall 120c is comprised of a solid layer 125 of conductive material, the next wall 120d is comprised of a perforated layer 127 of conductive material, the next wall 120e is comprised of a solid layer 125 of conductive material, and the outermost wall 120f is comprised of a perforated layer 127 of conductive material.

[0045] Thus, each annular wall 120a, 120c, 120e comprised of solid layer 125 of conductive material is adjacent on at least one or both of its radially outer and inner sides to an annular wall 120b, 120d, 120f comprised of perforated layer 127 of conductive material, and each annular wall 120b, 120d, 120f comprised of perforated layer 127 of conductive material is adjacent on at least one or both of its radially outer and inner sides to an annular wall 120a, 120c, 120e comprised of solid layer 125 of conductive material. Perforated layer 127 comprises a perforated mesh, but may comprise any other type of perforation.

[0046] In another embodiment shown in FIG. 6, each annular wall 120a-120f is constructed from a solid layer 125 of conductive material.

[0047] The annular walls 120a-120f comprise n walls nested to form a series of annular walls 120a-120f. The series has a radially innermost wall 120a, with n=1, and a radially outermost wall 120f, with n=n. The series also has at least one radially intermediate wall 120b-120e between the radially innermost wall 120a and the radially outermost wall 120f. Each radially intermediate wall 120b-120e has a respective value n from 1 to n.

[0048] First and second electrical terminals 150, 152 are provided for connection to the electrical energy source 106 to heat the walls 120 of the resistive heater 108. The first and second electrical terminals 150, 152 are electrically connected to respective first and second walls 120f, 120e, comprising the outer pair of walls 120f, 120e that are adjacent to one another and positioned radially outward of the resistive heater 108. Thus, the electrical terminals 150, 152 are connected to the outermost wall 120f and the wall 120e inwardly adjacent thereto. In a preferred embodiment, the walls 120 to which the terminals 150, 152 are connected comprise the outer pair of walls 120f, 120e, although the terminals 150, 152 themselves need not be positioned radially outward or toward the resistive heater 108. The first terminal 150 may be located radially outward of or toward the resistive heater 108, while the second terminal 152 may be located elsewhere in any suitable position spaced from the first terminal 150, such as in the center of the resistive heater 108 below the closed end portion 144 of the wall 120. However, the terminals 150, 152 may be located in any desired location. As previously mentioned, the first and second electrical terminals 150, 152 are configured to be electrically connected to the source of electrical energy 106 by a wired connection.

[0049] The multiple annular walls 120 are electrically connected to one another to define a continuous conductive path 156 extending between the first and second electrical terminals 150, 152. The conductive path 156 has a first portion 158 extending from the first wall, i.e., the outermost wall 120f, to the center C of the resistive heater 108, and a second portion 160 extending from the center C of the resistive heater to the second wall, i.e., the wall 120e inwardly adjacent to the outermost wall 120f.

[0050] The annular walls 120a-120f are electrically connected to one another by a first electrical connection 162 that electrically connects the walls 120 having n as an even number to form a first portion 158 of the conductive path 156, and by a second electrical connection 164 that electrically connects the walls 120 having n as an odd number to form a second portion 160 of the conductive path 156.

[0051] In a preferred embodiment, the first and second electrical connections 162, 164 are integral with the walls 120 that are electrically interconnected by the respective electrical connections 162, 164. Each of the first and second electrical connections 162, 164 is parallel or perpendicular to the longitudinal axis LL. At least some of the walls 120 are provided with openings 168 extending therethrough, and at least one of the first and second electrical connections 162, 164 extends through the respective openings 168. The first and second electrical connections 162, 164 also include mechanical connections 170 that mechanically connect the walls 120 to one another.

[0052] In a preferred embodiment, the first and second electrical connections 162, 164, which also comprise the mechanical connection 170 perpendicular to the longitudinal axis LL, comprise radially oriented lateral struts 172, and a plurality of circumferentially spaced radially oriented lateral struts 172 are provided around the periphery of the walls 120 to interconnect n even or n odd walls 120. The struts 172 are provided towards both the first and second ends 124, 126 of the resistive heater 108. The struts 172 electrically and mechanically connect the annular walls 120, providing structural rigidity and a current path.

[0053] In a preferred embodiment, the first and second electrical connections 162, 164, which also have a mechanical connection 170 parallel to the longitudinal axis LL, have longitudinally oriented wall portions 174, and to provide openings 168, a plurality of circumferentially spaced longitudinally oriented wall portions 174 are provided around the periphery of the walls 120 to interconnect n even or n odd walls 120, or a single longitudinally oriented wall portion 174 is provided around the periphery of the walls 120 to interconnect n even or n odd walls 120, and the single longitudinally oriented wall portion 174 is provided with one or more openings 168.

[0054] The resistive heater 108 further includes an electrical connector 176 constructed from a conductive material located at the center C of the resistive heater 108, which electrically connects the inner pair of annular walls 120a, 120b to one another. The electrical connector 176 is shown in Figure 4, but for clarity is not shown in Figure 5, where it is instead represented by a hatched area.

[0055] In the illustrated embodiment, the electrical connector 176 takes the form of an inner coil 177 of the resistive heater 108. However, in alternative embodiments of the present invention, any other shape and configuration of electrical connector may be used to electrically connect the inner pair of annular walls 120a, 120b. For example, in another embodiment shown in FIG. 6, the electrical connector 276 takes the form of a ring member 278 of conductive material having a central orifice 280 through which the heated fluid stream is emitted.

[0056] In each embodiment, the electrical connector 176, 276 is preferably integral with the annular wall 120 of the resistive heater 108, which is formed as a one-piece monolithic body by an additive manufacturing process such as selective laser melting (SLM).

[0057] The electrical connector 176 includes a pair of elongated helical elements 178a, 178b concentrically arranged about the longitudinal axis LL and surrounded by the innermost wall 120a. A first end 180a, 180b of each helical element 178a, 178b is connected to a respective wall 120a, 120b of the inner pair of walls 120a, 120b, and opposite second ends 182a, 182b of the helical elements 178a, 178b are connected to each other by a connecting member 184 of the electrical connector 176. The connecting member 184 includes an annular ring. The first ends 180a, 180b of the helical elements 178a, 178b are positioned at the second end 126 of the resistive heater 108.

[0058] Therefore, the outermost annular wall 120f is connected to the spiral element 178a of the inner coil 177, the inwardly adjacent annular wall 120e is connected to the spiral element 178b of the inner coil 177, and the spiral elements 178a, 178b are connected to the lower end 179 of the inner coil 177 by the connecting member 184.

[0059] In the illustrated embodiment, solid annular walls 120a, 120c, and 120e generate heat and form fluid flow channels. Support posts 172 pass through openings 168 in annular wall 120, allowing electrical current to pass from electrical terminal 150 through all of perforated annular walls 120f, 120d, and 120b in sequence, through central heating coil 177, and then through all of solid annular walls 120a, 120c, and 120e in sequence to electrical terminal 152 (or vice versa). Terminal 150 provides both an electrical terminal (positive or negative) and a means for mechanically connecting resistive heater 108 to the housing. Terminal 152 provides an electrical terminal (positive or negative) connected to an electrical conductor that penetrates the wall of housing 104 and forms the pressure envelope boundary, and is sealed to the housing by an electrically insulating seal (not shown).

[0060] In a preferred embodiment of the present invention, the resistive heater 108 comprises a one-piece, monolithic body 186. In other words, the annular wall 120, the first and second electrical connections 162, 164, the mechanical connection 170, the electrical connector 176, and the first and second electrical terminals 150, 152 are all contained within the single, one-piece, monolithic body 186.

[0061] The one-piece monolithic body 186 can be fabricated by an additive manufacturing technique that may be selected from selective laser melting (SLM), selective laser sintering (SLS), direct metal laser sintering (DMLS), neutral beam melting (NBM), electron beam welding (EBW), laser deposition welding (LDW), laser beam melting (LBM), laser metal deposition (LMD), electron beam melting (EBM), direct energy deposition (DED), rapid prototyping (RP), and rapid manufacturing (RM). Preferably, the one-piece monolithic body 186 is fabricated by selective laser melting (SLM), producing successive horizontal slices or layers that progressively form the resistive heater 108, starting at the second end 126 of the resistive heater 108 and terminating at the first end 124 of the resistive heater 108.

[0062] The electric heating system 100 further comprises an annular closure member 188 positioned at the second end 126 of the resistive heater 108. The annular closure member 188 is shown highly diagrammatically in Figure 5, but is not shown in Figure 4 for clarity of illustration.

[0063] The annular closure member 188 is composed of an electrically insulating material, such as a ceramic material. The closure member 188 is typically an electrically insulating perforated disk with one or more holes 190 corresponding to the fluid outlets 113 and plugs the second end 126 of the resistive heater 108 to close the sides of the alternating serpentine flow path 130. The closure member 188 closes the end of the annular flow channel 122 at the second end 126 of the resistive heater 108 to cause a change in direction in the alternating serpentine flow path 130 at the second end 126 of the resistive heater 108. The closure member 188 may be in contact with the resistive heater 108 or may be spaced a small distance from the resistive heater 108, for example.

[0064] In some embodiments of the present invention, the housing 104 is not electrically connected to the resistive heater 108. However, in other embodiments of the present invention, the housing 104 may be electrically connected to one of the first and second electrical terminals 150, 152 of the resistive heater 108, such that each electrical terminal 150, 152 is connectable to the source of electrical energy 106 through the housing 104. Each electrical terminal 50, 52 may be integral with or separate from a mechanical connection (not shown) between the resistive heater 108 and the housing 104.

[0065] The present invention further provides a method for generating a high temperature fluid flow. In a preferred embodiment of the present invention, the electric heating system 100 is a resistojet and is installed within a spacecraft, and the method is for moving the spacecraft through space.

[0066] The method includes providing an electric heating system 100 as described above. A fluid to be heated is supplied to a fluid input 112 of a flow resistor 108, thereby flowing along a serpentine flow path 130 to a fluid output 113 of the flow resistor 108. The supplied fluid has a pressure greater than the external gas pressure surrounding an exterior 192 of the housing 104 (see FIG. 1 ).

[0067] An electrical potential is applied across first and second terminals 150, 152 to heat the fluid flow in the alternating serpentine flow path 130 by the resistive heater 108. The fluid flows from the bottom, circulates serially through the flow channel 122 via the upper turn 139 and the lower turn 137, and finally reaches the inner coil 177 and the outlet. The heated fluid flow then exits the outlet 112 of the housing 104. In the embodiment of FIG. 2, the heated fluid exits the housing outlet at subsonic or sonic speeds via a converging-diverging nozzle. In the embodiment of FIG. 3, the heated fluid exits the housing outlet at supersonic or hypersonic speeds via a converging-diverging nozzle.

[0068] The solid annular walls 120 define flow channels 122 therebetween. The perforated annular walls 120, which are generally in the form of a perforated mesh, have increased electrical resistance and increased surface area compared to solid annular walls of the same size. Therefore, providing alternating perforated annular walls 120 adjacent to solid annular walls 120, particularly within flow channels 122, can improve heat transfer to the fluid for a given electrical current.

[0069] Furthermore, in the illustrated embodiment, the solid annular wall 120 has smooth inner and outer cylindrical surfaces. However, alternatively, either or both of the inner and outer cylindrical surfaces may be provided with relief surfaces, which may increase surface area and improve heat transfer. Such surface features may be readily achieved using additive manufacturing processes such as selective laser melting (SLM). Additionally, the annular wall 120 may vary in thickness as well as cross-sectional shape and dimensions.

[0070] A primary application of the electric heater system of the preferred embodiment of the present invention is as a high temperature resistojet for space applications. Such high temperature resistojet can be used on small to large platforms, and the resistojet can be used as a thruster to provide primary or secondary thrust, respectively.

[0071] The electric heater system of the preferred embodiment of the present invention provides a higher propellant utilization efficiency (specific impulse - I) compared to any other current resistojet. sp ) can be provided. Thus, the electric heater system of the preferred embodiment of the present invention can provide a very cost-effective propulsion system for small satellites, including constellations.

[0072] The resistive heater in the electric heater system of the preferred embodiment of the present invention can convert electrical energy into thermal energy that is transferred to the fluid very efficiently. The resistive heater releases energy directly into the flow, creating a recirculation path, resulting in the highest temperature being generated at the center of the resistive heater, achieving thermal efficiency of up to 95%. When constructed with a high-melting-point metal, the resistive heater can heat the fluid to temperatures as high as 3,500 K without relying on combustion or other chemical reactions. Furthermore, the resistive heater in the electric heater system of the preferred embodiment of the present invention can be manufactured by an additive manufacturing process, such as selective laser melting (SLM), to provide a monolithic resistive heater. This additive manufacturing process can be used as a single manufacturing process to reduce cost and complexity during manufacturing, as well as minimize or eliminate thermal expansion stresses, thereby providing a resistive heater with high in-service reliability. This additive manufacturing process can also produce a compact, low-cost monolithic resistive heater.

[0073] Additionally, electrical terminals can be located or connected to the exterior of the resistive heater and can be connected to the radially outermost pair of annular walls of the resistive heater, so that the outermost annular walls of the resistive heater remain cool and a thermal gradient of temperature rise generally extends from the periphery of the resistive heater toward the center of the resistive heater. Such a thermal gradient of temperature rise also generally extends from the input to the output of the fluid flow through the resistive heater. This improves thermal efficiency and allows for high fluid outlet temperatures to be achieved.

[0074] In the satellite industry, the electric heater system of the preferred embodiment of the present invention can be used to replace chemical propulsion systems that use harmful propellants, thereby significantly reducing the cost of satellite assembly integration and testing activities and simplifying spacecraft design.

[0075] Furthermore, the electric heater system of a preferred embodiment of the present invention can be used in a variety of other heat flow applications beyond satellite propulsion. For example, the present invention can be used as a generator of superheated steam for antibacterial disinfection on biofilms and hard surfaces; food processing; epoxy resin curing; stripping or cleaning purposes in the refining and hydrocarbon industries; an electric replacement for gas torches for localized heat treatment; precision glass and jewelry processing; preheating of equipment in metal casting housings; start-up heating for solid oxide fuel cells; ignition of combustion engines; an electric heating source for hot gas welding of plastics and most metal alloys; a generator of high-energy non-ionizing airflow in hypersonic wind tunnels for testing the flight characteristics of aircraft, launchers, and satellite re-entries; and a heat gun with an increased temperature range exceeding the typical 600°C airflow temperature for wiring harnesses, soldering, and desoldering of circuit boards for electronic equipment.

[0076] Various improvements and modifications to the preferred embodiments of the invention will be apparent to those skilled in the art and are encompassed by the invention as defined in the appended claims.

Claims

1. 1. An electric heating system for heating a fluid stream, comprising: a housing having an inlet for a fluid flow to be heated by the electric heating system and an outlet for the fluid flow heated by the electric heating system; a resistive heater mounted within the housing; The resistive heater comprises: a fluid input and a fluid output fluidly coupled to the inlet and the outlet, respectively; a plurality of annular walls constructed from an electrically conductive material, the walls nested to define a plurality of annular flow channels arranged concentrically in series about a longitudinal axis, the walls extending between first and second opposite ends of the resistive heater spaced apart along the longitudinal axis; first and second electrical terminals for connecting to a source of electrical energy to heat the walls of the resistive heater, the first and second electrical terminals being electrically connected to respective first and second walls comprising an outer pair of the walls adjacent to each other and positioned radially outward of the resistive heater; Equipped with the plurality of annular walls are mechanically connected to one another, whereby adjacent flow channels have opposite fluid flow directions and are connected at adjacent ends of the respective channels to define alternating serpentine flow paths, the serpentine flow paths having an input end at the fluid input and an output end at the fluid output, the input ends and the output ends being located at radially outer and inner positions, respectively, relative to the longitudinal axis; the plurality of annular walls are electrically connected to one another to define a continuous conductive path extending between the first and second electrical terminals, the conductive path having a first portion extending from the first wall to a center of the resistive heater and a second portion extending from the center of the resistive heater to the second wall; Electric heating system.

2. The system of claim 1 , wherein the fluid input and the fluid output are located at the first and second ends of the resistive heater, respectively.

3. 3. The system of claim 1 or 2, wherein each annular wall is constructed from either a solid layer of the conductive material or a perforated layer of the conductive material.

4. 4. The system of claim 3, wherein the resistance heater comprises alternating annular walls comprised of solid or perforated layers of the conductive material, each annular wall comprised of the solid layer of conductive material being adjacent to at least one or both of its radially outer and radially inner sides with an annular wall comprised of the perforated layer of conductive material, and each annular wall comprised of the perforated layer of conductive material being adjacent to at least one or both of its radially outer and radially inner sides with an annular wall comprised of the solid layer of conductive material.

5. The system of claim 3 , wherein each annular wall is constructed from a solid layer of said conductive material.

6. 6. The system of claim 1, wherein the walls comprise n walls nested together to form a series of walls, the series of walls having a radially innermost wall with n=1, a radially outermost wall with n=n, and at least one radially intermediate wall therebetween, each wall having a respective value of n from 1 to n, and the walls are electrically connected to each other by a first electrical connection electrically connecting walls having n as an even number to form the first portion of the conductive path, and by a second electrical connection electrically connecting walls having n as an odd number to form the second portion of the conductive path.

7. The system of claim 6 , wherein the first and second electrical connections are integral with the wall electrically interconnected by the respective electrical connections.

8. The system of claim 7 , wherein each of the first and second electrical connections is parallel or perpendicular to the longitudinal axis.

9. 9. The system of claim 6, wherein at least some of the walls are provided with openings extending therethrough, and at least one of the first and second electrical connections extends through a respective opening.

10. 10. The system of claim 6, wherein the first and second electrical connections also comprise mechanical connections by which the walls are mechanically connected to one another.

11. 11. The system of claim 1, wherein at least one of the walls comprises a cylindrical portion and an adjacent conical portion, a free end portion of the cylindrical portion being positioned at the second end of the resistive heater and the conical portion being directed toward the first end of the resistive heater.

12. The system of claim 11 , wherein the conical portion of the resistive heater closest to the first end comprises a solid layer defining a closed end portion of the plurality of annular walls.

13. 13. The system of claim 1, wherein the electrical heating system further comprises an annular closure member constructed of an electrically insulating material positioned at the second end of the resistive heater, the closure member closing an end of the annular flow channel at the second end of the resistive heater to cause a change in direction in the alternating serpentine flow path at the second end of the resistive heater.

14. The system of claim 13 , wherein the at least one electrically insulating closure member is in contact with or spaced apart from the resistive heater.

15. 15. The system of claim 1, wherein one annular wall is configured to form a closed end portion of the plurality of annular walls at the first end of the resistive heater, the closed end portion closing an end of the annular flow channel at the first end of the resistive heater to provide a change in direction in the alternating serpentine flow path at the first end of the resistive heater.

16. 16. The system of claim 1, wherein the resistive heater further comprises an electrical connector constructed of a conductive material located at the center of the resistive heater that electrically connects the inner pair of annular walls to one another.

17. 17. The system of claim 16, wherein the electrical connector comprises a pair of elongated helical elements concentrically arranged about the longitudinal axis and surrounded by the innermost wall, with a first end of each helical element connected to a respective wall of the inner pair of walls and opposite second ends of the helical elements connected to each other by a connecting member of the electrical connector.

18. The system of claim 17 , wherein the connecting member comprises an annular ring.

19. 19. The system of claim 17 or 18, wherein the first end of the helical element is located at the second end of the resistive heater.

20. 20. The system of claim 1, wherein the resistive heater comprises a one-piece monolithic body.

21. 21. The system of claim 20, wherein the one-piece monolithic body is manufactured by an additive manufacturing technique optionally selected from selective laser melting (SLM), selective laser sintering (SLS), direct metal laser sintering (DMLS), neutral beam melting (NBM), electron beam welding (EBW), laser deposition welding (LDW), laser beam melting (LBM), laser metal deposition (LMD), electron beam melting (EBM), direct energy deposition (DED), rapid prototyping (RP), or rapid manufacturing (RM).

22. 22. The system of claim 1, wherein the housing is electrically connected to one of the first and second electrical terminals of the resistive heater, whereby the respective electrical terminal is connectable to the source of electrical energy through the housing.

23. 23. The system of claim 22, wherein the respective electrical terminals are integral with or separate from the mechanical connection between the resistive heater and the housing.

24. 24. The system of claim 1, wherein the first and second electrical terminals are configured to be electrically connected to the source of electrical energy by a wired connection.

25. 25. The system of claim 1, further comprising an electrical energy source connected to the first and second electrical terminals, the electrical energy source configured to provide direct current or alternating current.

26. 26. The system of claim 1, wherein a plurality of the resistive heaters are positioned within the housing, the resistive heaters being arranged in series or parallel with respect to the inlet and outlet for fluid flow.

27. 27. The system of claim 1, wherein the electrical heating system is a resistojet and is configured for installation on a spacecraft.

28. 1. A method for producing a high temperature fluid stream, comprising: a) providing an electric heating system according to any one of claims 1 to 27; b) supplying a fluid to be heated to the fluid input of a flow resistor, thereby causing it to flow along the alternating serpentine flow path to the fluid output of the flow resistor, the supplied fluid having a pressure greater than an external gas pressure surrounding the exterior of the housing; c) applying an electrical potential across the first and second terminals to heat the fluid flow in the alternating serpentine flow paths with the resistive heater; d) discharging the heated fluid stream from the outlet of the housing; A method comprising:

29. 30. The method of claim 28, wherein the heated fluid is emitted from the outlet of the housing at subsonic or sonic speeds through a converging nozzle.

30. 30. The method of claim 28, wherein the heated fluid is discharged from the outlet of the housing at supersonic or hypersonic speeds through a converging-diverging nozzle.

31. 31. The method of any one of claims 28 to 30, wherein the electrical heating system is a resistojet and is located within a spacecraft, and the method is for moving the spacecraft in space.

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