Abstract:
A system for managing fuel temperature in an engine includes a source of hot pressurized air and a turbine for converting hot pressurized air into cool expanded air. The system further includes a fuel tank for storing fuel, a fuel conduit fluidly connected to the fuel tank, and a first heat exchanger located on the fuel conduit. The first heat exchanger places the cool expanded air from the turbine in a heat exchange relationship with the fuel, thereby cooling the fuel.

Description:
BACKGROUND 
       [0001]    The present disclosure relates generally to fuel systems and more specifically, to methods and systems for managing fuel temperature. 
         [0002]    Gas turbine engines typically include an inlet, a fan, a low pressure compressor, a high pressure compressor, a combustor, and at least one turbine. Air is pulled through the inlet and into the engine by the fan. Air is then compressed by the compressors and sent to the combustor, where the compressed air is mixed with fuel. The air/fuel mixture is ignited to generate combustion gases, which are channeled to one or more turbines. The turbine(s) extract energy from the combustion gases to power the compressors, as well as produce useful work (e.g. propel an aircraft). 
         [0003]    Specific fuel consumption in a gas turbine engine is inversely proportional to the fuel temperature. Heat is commonly dumped from the engine oil system into the fuel system in order to cool the oil. An additional benefit is raising the temperature of the fuel and improving fuel efficiency for the engine. The fuel system also has a maximum temperature limit, which is often defined by coking in small fuel passages. A return to fuel tank conduit is typically employed to allow more fuel than required for combustion to flow and absorb heat from the system, thereby lowering the bulk temperature below the max allowable. 
       SUMMARY 
       [0004]    A system for managing fuel temperature includes a fuel tank for storing fuel and a return-to-tank fuel conduit fluidly connecting an outlet of the fuel tank with an inlet of the fuel tank. The system further includes a turbine for converting pressurized air into expanded air and a first heat exchanger located on the return-to-tank fuel conduit. The first heat exchanger places the expanded air from the turbine in a heat exchange relationship with the fuel, thereby cooling the fuel. 
         [0005]    A system for managing fuel temperature in an engine includes a source of hot pressurized air and a turbine for converting hot pressurized air into cool expanded air. The system further includes a fuel tank for storing fuel, a fuel conduit fluidly connected to the fuel tank, and a first heat exchanger located on the fuel conduit. The first heat exchanger places the cool expanded air from the turbine in a heat exchange relationship with the fuel, thereby cooling the fuel. 
         [0006]    A method for managing fuel temperature includes expanding a first portion of pressurized air to form expanded air, rejecting heat from fuel into the expanded air, thereby cooling the fuel, and flowing the cooled fuel into a fuel tank for later use by an engine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic flow chart depicting a fuel system in accordance with the prior art. 
           [0008]      FIG. 2  is a schematic flow chart depicting a first embodiment of a fuel system in accordance with the present disclosure. 
           [0009]      FIG. 3  is a schematic flow chart depicting a second embodiment of the fuel system in accordance with the present disclosure. 
           [0010]      FIG. 4  is a schematic flow chart depicting a third embodiment of the fuel system in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  is a schematic flow chart depicting fuel system  10  in accordance with the prior art. Shown in  FIG. 1  are gas turbine engine  12 , engine fan  14 , and components of fuel system  10 : tank  16 , first heat exchanger  18 , second heat exchanger  20 , third heat exchanger  22 , tank-to-engine conduit  24 , and return-to-tank conduit  26 . Fuel flows from tank  16  along tank-to-engine conduit  24  to provide fuel to a combustor of engine  12 . A portion of fuel is diverted upstream of engine  12  and flows along return-to-tank conduit  26  back to tank  16  in order to manage fuel temperature in fuel system  10 . 
         [0012]    Fuel system  10  is representative of the prior art for thermal management of fuel in gas turbine engine  12  having fan  14 . Tank  16 , first heat exchanger  18 , and second heat exchanger  20  are positioned along tank-to-engine conduit  24  in flow series. In other embodiments, first heat exchanger  18  and second heat exchanger  20  represent multiple heat exchangers and/or multiple heat loads. First heat exchanger  18  is located downstream of tank  16  and upstream of second heat exchanger  20  on tank-to-engine conduit  24 . Second heat exchanger  20  is located downstream of first heat exchanger  18  and upstream of engine  12  on tank-to-engine conduit  24 . Fuel for use by engine  12  is stored in tank  16 . Fuel flows out of tank  16 , along tank-to-engine conduit  24 , and into first heat exchanger  18 . First heat exchanger  18  transfers a heat load from an aircraft source (e.g. electronics, hydraulics, generators, environmental control system (ECS), fuel pumps, etc.) to the fuel, thereby increasing fuel temperature. Fuel exits first heat exchanger  18 , flows along tank-to-engine conduit  24 , and enters second heat exchanger  20 . Second heat exchanger  20  transfers a heat load from an engine source (e.g. oil/lubrication, fueldraulic actuation, fuel pumps, etc.) to the fuel, thereby increasing fuel temperature. Fuel exits second heat exchanger  20 , flows along tank-to-engine conduit  24 , and enters engine  12  for use by a combustor. Accordingly, fuel temperature increases as it traverses tank-to-engine conduit  24  and approaches engine  12 . 
         [0013]    Third heat exchanger  22  and tank  16  are positioned along return-to-tank conduit  26  in flow series. Fuel flows from fuel-to-engine conduit  24  and into return-to-tank conduit  26  upstream of second-heat exchanger  20  and downstream of engine  12 . Fuel flowing along return-to-tank conduit  26  enters third heat exchanger  22 . Third heat exchanger  22  transfers a heat load from the fuel to an air source (e.g. ram air), thereby decreasing fuel temperature. Fuel exits third heat exchanger  22 , flows along return-to-tank conduit  26 , and back into tank  16 . Accordingly, fuel temperature decreases as it traverses return-to-tank conduit  26  and approaches tank  16 . 
         [0014]    Each component of fuel system  10 , as well as engine  12 , specifies a maximum allowable fuel temperature. The maximum allowable fuel temperature determines the location of the specific component within fuel system  10 . For example, first heat exchanger  18  has a lower maximum allowable fuel temperature than second heat exchanger  20  and therefore, first heat exchanger  18  is located upstream of second heat exchanger. As the temperature of fuel in fuel system  10  approaches a maximum allowable fuel temperature for any component, more fuel is pumped from tank  16  through fuel-to-engine conduit  24  to return-to-tank conduit  26 . An increase in the amount of fuel diverted along return-to-tank conduit  26  can result in an increase in the average fuel temperature in tank  16 , and therefore, the temperature of fuel throughout fuel system  10 . This “passive” management of fuel temperature is problematic because high fuel temperatures induce more fuel recirculation, which results in ever increasing amounts of energy present within fuel system  10 . 
         [0015]      FIG. 2  is a schematic flow chart depicting a first embodiment of fuel system  28  in accordance with the present disclosure. Fuel system  28  contains many of the same components as fuel system  10  described above, and like numerals designate like components. Depicted in  FIG. 2  are gas turbine engine  12 , fan  14 , and components of fuel system  28 : tank  16 , first heat exchanger  18 , second heat exchanger  20 , third heat exchanger  22 , tank-to-engine conduit  24 , and return-to-tank conduit  26 . Also shown are turbine  30 , energy absorber  32 , shaft  34 , pressurized air conduit  36 , and expanded air conduit  38 . Third heat exchanger  22  cools fuel with expanded air from turbine  30 . In the example, the turbine  30  is independent from a turbine section within the gas turbine engine  12 . 
         [0016]    A portion of fuel system  28  is similar to fuel system  10  described above with respect to  FIG. 1 . Tank  16 , first heat exchanger  18 , and second heat exchanger  20  are positioned along tank-to-engine conduit  24  in flow series. Third heat exchanger  22  and tank  16  are positioned along return-to-tank conduit  26  in flow series. For the sake of brevity, the location and function of these similar components is not repeated here. In contrast to fuel system  10  described above, fuel system  28  includes turbine  30  for providing expanded air to third heat exchanger  22 . 
         [0017]    Turbine  30  is attached to energy absorber  32  (e.g. generator, compressor, fan, etc.) by shaft  34 . Pressurized air conduit  36  connects a source of pressured air (e.g. fan, ram, bleed, etc.) to an inlet of turbine  30 , and expanded air conduit  38  connects an outlet of turbine  30  to third heat exchanger  22 . Pressurized air is conducted from its source to turbine  30  along pressurized air conduit  36 . Within turbine  30 , the hot pressurized air is expanded and cooled. The expansion of air within turbine  30  generates rotational energy, which is transferred to energy absorber  32  by shaft  34 . Expanded air is conducted from turbine  30  to third heat exchanger  22  by expanded air conduit  38 . Within third heat exchanger  22 , heat is dumped from the fuel to the expanded air, which is then dumped overboard or sent to another system for use. Third heat exchanger  22  actively cools (or removes heat from) the fuel in order to reduce fuel temperature before it flows back to tank  16 . Since expanded air from turbine  30  (“active” air) is significantly cooler than ram air (“passive” air), third heat exchanger  22  of fuel system  28  provides a much more effective heat sink for fuel than third heat exchanger  22  of fuel system  10 . 
         [0018]      FIG. 3  is a schematic flow chart depicting a second embodiment of fuel system  40  in accordance with the present disclosure. Fuel system  40  contains many of the same components as fuel systems  10  &amp;  28  described above, and like numerals designate like components. Depicted in  FIG. 3  are gas turbine engine  12 , fan  14 , and components of fuel system  40 : tank  16 , first heat exchanger  18 , second heat exchanger  20 , third heat exchanger  22 , and tank-to-engine conduit  24 . Also shown are turbine  30 , energy absorber  32 , shaft  34 , pressurized air conduit  36 , and expanded air conduit  38 . Third heat exchanger  22 , which is located on tank-to-engine conduit  24 , actively cools fuel with expanded air from turbine  30 . 
         [0019]    Components of fuel system  40  are similar to components of fuel systems  10  &amp;  28  described above, but are arranged in a different order to negate the need for return-to-tank conduit (item  26  in  FIGS. 2 &amp; 3 ). Tank  16 , third heat exchanger  22 , first heat exchanger  18 , and second heat exchanger  20  are positioned along tank-to-engine conduit  24  in flow series. Accordingly, third heat exchanger  22  is located downstream of tank  16  and upstream of first heat exchanger  18 . Fuel flows from tank  16 , through tank-to-engine conduit  24 , and into third exchanger  22 . As described above for fuel system  28 , pressurized air conduit  36  connects a source of pressured air to an inlet of turbine  30 , and expanded air conduit  38  connects an outlet of turbine  30  to third heat exchanger  22 . Within third heat exchanger  22 , fuel is placed in a heat exchange relationship with expanded air from turbine  30 . Heat from the fuel is absorbed by the expanded air, such that fuel exits third heat exchanger  22  at a lower temperature than it entered third exchanger  22 . Fuel continues along tank-to-engine conduit  24  to absorb a heat load from an aircraft (within first heat exchanger  18 ), and absorb a heat load from the engine  12  (within second heat exchanger  20 ), as described above for fuel systems  10  &amp;  28 . Fuel system  40  actively cools fuel with expanded air in third heat exchanger  22  to pre-cool fuel, thereby allowing for a greater temperature difference between fuel and heat loads in first and second heat exchangers  18  &amp;  20 . The architecture of fuel system  40  actively modulates the cooling in third heat exchanger  22  to ensure no temperature limits are exceeded and eliminates the need for a return-to-tank conduit (item  26  in  FIGS. 2 &amp; 3 ). 
         [0020]      FIG. 4  is a schematic flow chart depicting a third embodiment of fuel system  42  in accordance with the present disclosure. Fuel system  42  contains many of the same components as fuel systems  10 ,  28 , &amp;  40  described above, and like numerals designate like components. Depicted in  FIG. 4  are gas turbine engine  12 , fan  14 , and components of fuel system  42 : tank  16 , first heat exchanger  18 , second heat exchanger  20 , third heat exchanger  22 , tank-to-engine conduit  24 , return-to-engine conduit  26 , and fourth heat exchanger  44 . Also shown are first and second turbines  30 A &amp;  30 B, shaft  34 , pressurized air conduit  36 , compressor  46 , first branch air conduit  48 , second branch air conduit  50 , valves  52 , compressed air conduit  54 , return air conduit  56 , and expanded air conduit  58 . Fuel exiting second heat exchanger  20  either is heated by fourth heat exchanger  44  prior to use by engine  12  or cooled by third heat exchanger  22  prior to returning to tank  16 . 
         [0021]    Components of fuel system  42  are similar to components of fuel systems  10 ,  28 , &amp;  40  described above. Tank  16 , first heat exchanger  18 , and second heat exchanger  20  are positioned along tank-to-engine conduit  24  in flow series. Third heat exchanger  22  and tank  16  are positioned along return-to-tank conduit  26  in flow series. Fourth heat exchanger  44  is located downstream of second heat exchanger  20  and upstream of both engine  12 . As fuel exits second heat exchanger  22 , it is either heated by fourth heat exchanger  44  and sent to engine  12  for use, or cooled by third heat exchanger  22  and sent back to tank  16  via return-to-tank conduit  26 . 
         [0022]    Pressurized air conduit  36  provides a source of pressured air for use by both third heat exchanger  22  and fourth heat exchanger  44 . Pressurized air conduit  36  connects a source of pressurized air (e.g. ram, fan, bleed, etc.) with both first branch air conduit  48  and second branch air conduit  50 , each having valve  52 . Second branch air conduit  50  directs pressurized air to second turbine  30 B, where it is expanded and sent to cool a heat load or exhausted overboard. First branch air conduit  48  directs pressurized air to compressor  46  for use by fourth heat exchanger  44  and subsequently, to first turbine  30 A for use by third heat exchanger  22 . Accordingly, fuel system  42  includes a three-cycle air machine (including first turbine  30 A, second turbine  30 B, and compressor  46 ) and third and fourth heat exchangers  22  &amp;  44  to better modulate fuel temperature within fuel system  42 . 
         [0023]    Pressurized air from first branch air conduit  48  enters compressor  46 , and is compressed. This compressed air exits compressor  46  and is sent through compressed air conduit  54  to fourth heat exchanger  44 . Within fourth heat exchanger  44 , compressed air is placed in a heat exchange relationship with fuel, thereby increasing fuel temperature. After dumping heat into the fuel, compressed air exits fourth heat exchanger  44  and travels along return air conduit  56  to first turbine  30 A. Compressed air is subsequently expanded within first turbine  30 A, and the extracted energy is sent along shaft  34  to compressor  46  for use, where compressor  46  is attached to first turbine  30 A and optionally second turbine  30 B by shaft  34 . (Compressor  46  can also be powered, in part, by second turbine  30 B.) Expanded air exits first turbine  30 A and is directed along expanded air conduit  58  to third heat exchanger  22 . Within third heat exchanger  22 , fuel is placed in a heat exchange relationship with expanded air. Heat from the fuel is absorbed by the expanded air, thereby decreasing the fuel temperature. After use by third heat exchanger  22 , expanded air is exhausted overboard or sent to cool another heat load. Fuel exits third heat exchanger  22  and is sent back to tank  16  via return-to-tank conduit  26 . 
         [0024]    Fuel system  42  uses a pressurized air source, a three-wheel air cycle machine, and third and fourth heat exchangers  22  &amp;  44  to better manage fuel temperature across a mission. Pressurized air is sent to compressor  46  for compression, and then fourth heat exchanger  44  to increase fuel temperature prior to use by engine  12 . Air exiting fourth heat exchanger  44  is sent to first turbine  30 A for expansion, and then third heat exchanger  22  to decrease fuel temperature prior to storage in tank  16 . Fuel system  42  provides the flexibility to sub-cool fuel and actively control fuel temperature for gas turbine engine  12 , which is absent in prior art systems. 
         [0025]    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.