Abstract:
An apparatus includes a thermoelectric (TE) device, a gas flow conduit proximate to one side of the thermoelectric device, a plurality of flexible tubes proximate to a second side of the thermoelectric device, and a spring to control contact force between the flexible tubes and the thermoelectric device. The spring comprises a coil spring at least partially circumscribing the gas flow conduit. The thermoelectric device converts a temperature differential between the flexible tubes and the gas flow conduit into electrical energy.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority to U.S. Provisional Pat. App. Ser. No. 61/211,013 entitled FUEL-COOLED HEAT EXCHANGER WITH THERMOELECTRIC DEVICE COMPRESSION filed Mar. 25, 2009, which is hereby incorporated by reference in its entirety. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The Government may have certain rights in this invention pursuant to Contract No. FA8650-07-C-7721 awarded by the United States Air Force. 
    
    
     BACKGROUND 
     Hypersonic vehicles hold potential for future military application by shortening the time-to-target and thereby extending global reach. These vehicles are anticipated to be powered by scramjet (supersonic combustion ramjet) engines during hypersonic flight conditions. The structure which forms the hypersonic flow path in a scramjet engine is referred to in the art as a heat exchanger (HEX), which is a reference to the dual use of the flow conduit structure as a heat exchanger. Hypersonic HEXs are commonly fuel-cooled because air-cooling is not practical in hypersonic flight conditions. Fuel cooling also serves to preheat the combustion fuel, thereby adding energy to the fuel for combustion. In conventional jet engines, fuel pumps, on-board electric systems, and other accessory systems parasitically draw power from the engine&#39;s main power plant to function. However, unlike conventional jet engines, scramjet engines have no rotating mechanical elements. Hypersonic vehicles are therefore currently envisioned to rely on auxiliary power units (APUs) and/or batteries to meet the vehicle power requirements. However, both APUs and battery systems add significant weight, volume and system complexity. 
     SUMMARY 
     An apparatus according to the present invention includes a thermoelectric (TE) device, a gas flow conduit proximate to one side of the thermoelectric device, a plurality of flexible tubes proximate to a second side of the thermoelectric device, and a spring to control contact force between the flexible tubes and the thermoelectric device. The spring comprises a coil spring at least partially circumscribing the gas flow conduit. The thermoelectric device converts a temperature differential between the flexible tubes and the gas flow conduit into electrical energy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of vehicle including hybrid gas turbine and ramjet engine. 
         FIG. 2  is a schematic view of the hybrid gas turbine and ramjet engine of  FIG. 1 . 
         FIG. 3  is a schematic axial section view of the ramjet of  FIG. 2  including a fuel-cooled heat exchanger. 
         FIGS. 4A-4C  show details of the isolator section of ramjet of  FIG. 2  including a portion of the heat exchanger of  FIG. 3 . 
         FIG. 5  is a schematic section view of a portion of an alternative heat exchanger. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic view of vehicle  10  including fuselage  12 , wing  14 , tail assembly  16 , engine  18 , and cowl  20 . Vehicle  10  may be, for example, a manned aircraft. Alternative vehicles may be unmanned and may be reusable or may be one-way vehicles (e.g., missiles or disposable launch vehicles). Although this description is made with reference to a vehicle, embodiments of the present invention are applicable to any platform that includes demanding thermal management and power generation needs. In  FIG. 1 , wing  14  and tail assembly  16  are supported by fuselage  12 . Engine  18  is located in cowl  20  on an underside of fuselage  12 . Air flow path  24  carries a flow  26  through engine  18  between a forward inlet/intake  28  and an aft outlet  30  (e.g., an exhaust nozzle). 
       FIG. 2  is a schematic view of engine  18  located in cowl  20 . Engine  18  includes gas turbine  32  and ramjet  34 . An exemplary ramjet is a dual mode (i.e., subsonic and supersonic combustion) ramjet engine (i.e., a dual mode scramjet). A ramjet generally comprises a constricted tube through which inlet air is compressed by the high speed of the vehicle, a combustion chamber where fuel and compressed air are combusted, and a nozzle through which the exhaust jet leaves at higher speed than the inlet air, thereby generating thrust to power a vehicle in flight. There are few or no moving parts in a ramjet. In particular, there is no high-speed turbine, as in a turbofan or turbojet engine, that is expensive to produce and maintain. A ramjet requires airflow through the engine (in a scramjet the airflow must be supersonic), and therefore has a minimum functional speed. For example, in the hybrid vehicle shown in  FIGS. 1-3 , turbine  32  may be used to power vehicle  10  up to an appropriate speed beyond which ramjet  34  may augment or replace turbine  32  to power vehicle  10 . 
     In  FIG. 2 , a portion  26   a  of air flow  26  can be directed along flow path  36  into turbine  32 , while another portion  26   b  of flow  26  can be directed along flow path  38  into ramjet  34 . Flow path  38  carries a flow  26   b  through ramjet  34  between forward inlet/intake  28  and aft outlet  30 . Along flow path  26   b , ramjet  34  may include a forebody  34   a , an isolator  34   b  (often integrated therewith), and a combustor  34   c . During operation, air is scooped into ramjet  34  through forebody  34   a  and compressed along isolator  34   b  before entering combustor  34   c . The compressed air is mixed with fuel in combustor  34   c  and ignited. The products of combustion are exhausted through outlet  30  to produce useful thrust used to power vehicle  10  in flight. As shown in  FIG. 2 , engine  18  may also include control system  40  configured to control operation of combustor  34   c  in response to one or more of sensor input, operator input, and the like. Control system  40  may optionally be included as a portion of the avionics of vehicle  10 . 
     Gas turbine  32  is located along air flow path  36  carrying flow  26   a  between a forward inlet/intake  42  and an aft outlet  44  inboard of ramjet flow path  26   b  (e.g., partially recessed into fuselage  12  above cowl  20 ). Ramjet and turbine inlet flaps  46  and  48 , respectively, can selectively block ramjet and turbine inlets  28 ,  42  and flow paths  38 ,  36  when ramjet  34  or turbine  32 , respectively, is not in operation. Similarly, turbine outlet flap  50  may selectively block turbine flow path  36  when turbine  32  is not in use so as to provide an efficient nozzle for ramjet  34 . 
       FIG. 3  is a schematic axial section view showing further details of the ramjet engine  34  and flow  26   b . At least a portion of flow  26   b  is largely surrounded by heat exchanger (or conduit assembly)  60  for transferring heat from the air and combustion gases in ramjet  34  to pre-combustion ramjet fuel. A radially inward face of heat exchanger (or HEX)  60  forms a gas flow conduit through which flow  26   b  of ramjet  34  passes. Heat exchanger  60  can be formed as a generally rectangular conduit surrounding flow  26   b , sometimes referred to as a 2-D configuration, or as an annular conduit circumscribing flow  26   b , sometimes referred to as a 3-D configuration. For an exemplary hydrocarbon-based fuel, heat exchanger  60  is a liquid-fuel-cooled heat exchanger. An alternative fuel used to cool heat exchanger  60  is a hydrogen gas. Heat exchanger  60  can have an upstream fuel inlet  62  and a downstream fuel outlet  64 . In the illustrated embodiment, the inlet  62  is upstream of combustor  34   c  along flow path  26   b . Heat exchanger  60  can thereby be used to pre-heat the fuel used in combustor  34   c  using the hot air and fuel mixture exiting combustor  34   c . Fuel flow  66  of ramjet  34  can extend from storage tank  68  to fuel pump  70  and then to inlet  62 . After exiting outlet  64 , heated fuel may pass along flow path  72  to a fuel distribution valve network  74  and then to combustor  34   c . The valves of network  74  distribute the fuel to various combustor locations for various purposes (e.g., piloting v. main combustion) and to achieve desired staging. 
     In addition to pre-heating combustion fuel, heat exchanger  60  thermoelectrically generates electricity. Accordingly, exchanger  60  can be coupled to an electrical power conditioning, storage, and distribution system, such as system  76  shown schematically in  FIG. 3 . System  76  can receive raw electrical input from heat exchanger  60  and output appropriate electricity (e.g., of a constant and proper voltage) to drive, for example, control system  40 , fuel pump  70 , distribution valves of the network  74 , similar components associated with turbine  32 , and additional loads schematically shown as  78 . 
       FIGS. 4A-4C  show details of a portion of ramjet  34 , including isolator  34   b  and a portion of heat exchanger  60 .  FIG. 4A  is a perspective view of isolator  34   b  including heat exchanger  60 .  FIGS. 4B and 4C  are cut-away perspective views showing details of heat exchanger  60 . Heat exchanger  60  includes gas flow conduit  80 , thermoelectric (TE) devices  82 , fuel-cooled tubes  84 , one or more springs  86 , manifolds  88 , and casing  90 . In  FIGS. 4A-4C , gas flow conduit  80  surrounds a gas flow, such as flow  26   b  shown in  FIGS. 2 and 3 , and is formed as annular or 3-D type conduit. In order to withstand the extreme operating temperatures of hypersonic flight, gas flow conduit  80  can be manufactured from, for example, high temperature alloys or ceramics, or a ceramic matrix composite (CMC). CMC is approximately one third the density of metal and therefore provides a significant weight savings over a metal conduit. In some applications, a metal may tend to overheat because TE devices  82  act as a thermal insulator between the material of conduit  80  and fuel-cooled tubes  84 . CMC can operate at higher temperatures than metal, which makes it less likely to overheat in such applications. The CMC material, as well as other components of the heat exchanger  60 , can optionally include suitable coatings as desired for particular applications. Conduit  80  is arranged between the gas flow and TE devices  82 . Adjacent to (e.g., radially outward from) TE devices  82  are fuel-cooled tubes  84 . TE devices  82  are therefore arranged between the relatively hot gas flow conduit  80  and the relatively cool fuel-cooled tubes  84 , to enable generation of electricity from the thermal differential therebetween. Individual fuel-cooled tubes allow an opening through which electrical leads for TE devices  82  can pass, which facilitates assembly of heat exchanger  60 . 
     Generally speaking, TE devices produce a voltage in the presence of a temperature difference between two different electrically conductive materials. The voltage causes a continuous electrical current to flow in the conductors if they form a complete loop. The electrical current generated may be used to, for example, power accessory systems on an aircraft as discussed with reference to  FIG. 3  above. TE devices function best with optimal thermal contact, and thereby thermal conduction, between the TE device and, for example, a gas flow conduit of a fuel-cooled heat exchanger. However, manufacturing and assembly tolerances, variations in position and size in components during operation, and other factors may degrade contact between the TE device and the conduit. Therefore, embodiments of the present invention employ one or more springs  86  to bias TE devices  82  into contact with the relatively hot gas flow conduit  80  and the relatively cool fuel-cooled tubes  84  between which the TE devices  82  are arranged. The load placed on TE devices  82  by the springs  86 , directly or indirectly, helps ensure substantially continuous physical contact, while remaining below the structural limits of TE devices  82 . For example, a functional range for TE devices  82  used in embodiments of the present invention is approximately 140 to 350 kPa (20 to 50 psi). 
     In practice, TE devices  82  exhibit dimensional variations caused by both manufacturing tolerances and operational effects, e.g. thermal expansion during flight. For instance, dimensional variations in TE devices  82  may adversely affect thermal conduction by varying the amount of contact between fuel-cooled tubes  84  and TE devices  82  over which tubes  84  are arranged. Embodiments of the present invention therefore employ individual flexible fuel-cooled tubes  84 , as opposed to, for example, sets of multiple interconnected rigid tubes, that can better accommodate dimensional variations in TE devices  82 . Furthermore, the individual fuel-cooled tubes  84  can have a width dimension that is smaller that a corresponding dimension of each TE device  82 , thereby allowing for compensation in dimensions across a single TE device  82 . 
     In  FIGS. 4A-4C , gas flow conduit  80  has an annular, generally cylindrical shape through which gas flow  26   b  can pass. In the illustrated embodiment, conduit  80  provides primary structural support for heat exchanger  60 . Conduit  80  is surrounded by TE devices  82 , which are in turn surrounded by fuel-cooled tubes  84 . As shown in  FIGS. 4B and 4C , heat exchanger  60  can include many individual TE devices  82  and fuel-cooled tubes  84  arranged in combination to substantially cover gas flow conduit  80 . In the illustrated embodiment, fuel-cooled tubes  84  extend generally axially from a first to a second end of the isolator  34   b  section of heat exchanger  60 . The first and second ends of fuel-cooled tubes  84  are each fluidically connected to one of two annular manifolds  88 , which are configured to carry pre-combustion fuel to and from tubes  84 . Fuel-cooled tubes  84  can be arranged substantially perpendicular to each manifold  88 , and can have a substantially rectangular cross-sectional profile to provide increased surface area exposure for thermal energy transfer. Manifolds can each have a substantially rectangular cross-sectional profile. Fuel-cooled tubes  84  are flexible, in part, because they are individual tubes with a high length to width ratio. Thermo-structural analysis predicts that a wall thickness of 0.38 mm (0.015 inches) will be sufficient for tubes  84  made of INCONEL alloy (available from Special Metals Corporation, Huntington, West Va.) at 6.9 MPa (1 ksi) internal pressure operating in a temperature range of approximately 20-650° C. (68-1202° F.) which are typical of the conditions in a hypersonic HEX application. 
     At least partially circumscribing gas flow conduit  80 , TE devices  82 , and fuel-cooled tubes  84  are one or more springs  86  spaced from one another. In the illustrated embodiment, the springs  86  extend circumferentially to at least partially circumscribe the flow path  26   b , and are axially spaced from one another. Casing  90  encases and helps compress springs  86  in order to help keep TE devices  82  in contact with fuel-cooled tubes  84  and gas flow conduit  80 . In that way, inwardly-directed compressive loading is provided. Controlled pressure can be applied to the back of each individual fuel-cooled tube  84  by springs  86 , which are in compression between tubes  84  and casing  90 . Canted coil springs can be selected for the springs  86 , as shown in the illustrated embodiment, because they can provide a relatively constant load over a large displacement. A relatively constant load over a large displacement reduces a risk of overloading the TE devices as dimensional variations in the heat exchanger occur during operation. Custom designed Canted-Coil™ springs suitable for use as springs  86  are available from Bal Seal Engineering, Inc. of Foothill Ranch, Calif. 
     Casing  90  can be made of a metallic material. In the illustrated embodiment, casing  90  has a generally corrugated configuration that defines circumferentially-extending channels to accommodate springs  86 . 
     Embodiments of the present invention can also be applied to a 2-D type heat exchanger application as shown in  FIG. 5 , which is a schematic section view of a portion of alternative heat exchanger  100  including gas flow conduit  102 , thermoelectric (TE) devices  104 , fuel-cooled tubes  106 , one or more springs  108  (e.g., canted coil springs), supports  110 , casing  112 , fasteners  114 , and insulation  116 . As shown in  FIG. 5 , gas flow conduit  102  surrounds a gas flow path and is formed as a 2-D conduit, i.e., a conduit with a generally rectangular cross-section. In order to withstand the extreme operating temperatures of hypersonic flight, gas flow conduit  102  can be manufactured from, for example, high temperature alloys or ceramics, or a CMC material. Conduit  102  is arranged between gas flow path  26   b  and TE devices  104 . Adjacent to (e.g., radially outward from) TE devices  104  are fuel-cooled tubes  106 . TE devices  104  are therefore arranged between the relatively hot gas flow conduit  102  and the relatively cool fuel-cooled tubes  106  to enable generation of electricity from a thermal differential therebetween. Supports  110  extend between casing  112  and gas flow conduit  102 . Springs  108  are arranged between fuel-cooled tubes  106  and casing  112 . Casing  112  is attached to supports  110  by fasteners  114 , which are configured to help compress springs  108  against fuel-cooled tubes  106  to bring TE devices  104  into contact with fuel-cooled tubes  106  and gas flow conduit  102 . In that way an inwardly-directed compressive force is provided. At corners of heat exchanger  100 , insulation  116  can be provided to form a junction between banks of TE devices  104 , fuel-cooled tubes  106 , and springs  108  that are arranged generally perpendicular to each other. It should be noted that as used herein, springs  108  are described as circumscribing the flow path  26   b  in the embodiment of  FIG. 6 , even though springs  108  do not have a circular arrangement. 
     Those of ordinary skill in the art will recognize that embodiments of the present invention provide numerous advantages over prior heat exchangers employing TE devices. For example, heat exchangers according to the present invention help increase thermal conduction of the TE device by employing one or more generally circumferentially extending coil springs to bias the TE device into contact with the hot gas flow conduit and the cool fuel-cooled tubes between which the TE device is arranged. The load placed on the TE device by the springs is sufficient to ensure substantially continuous physical contact, while remaining below the structural limits of the TE device. Dimensional variations in the TE devices can be tolerated with the present invention because a width of the individually loaded fuel-cooled tubes can be significantly smaller than a corresponding width of the TE devices and therefore the pressure load will dynamically adjust to dimensional changes. The coil springs and flexible fuel-cooled tubes employed in embodiments of the present invention can also accommodate differential thermal growth between the various components in both steady state and transient conditions. Assembly time and manufacturing complexity is also relatively low. 
     Embodiments of the present invention employing the annular or 3-D type configuration have additional benefits. Weight, complexity, and part count are reduced by employing a sealed casing that can help reduce or eliminate a need for fastener hardware, which can provide a weight savings of approximately 4 kg/m 2  according to inventor calculations. Furthermore, in prior art heat exchangers TE devices can suffer from oxidative degradation. With the present invention, the flow of oxidizing gases around the TE devices can be minimized by sealing them between the continuous gas flow conduit and the casing. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims. For example, the present invention can be utilized with a variety of types of engines for electrical power generation. Moreover, the springs of the heat exchanger can be arranged in a helical pattern.