Abstract:
A heat exchanger system for a gas turbine engine includes: (a) a fan having at least two stages of rotating fan blades surrounded by a fan casing, the fan operable to produce a flow of pressurized air at a fan exit; (b) at least one heat exchanger having a first flowpath in fluid communication with the fan at a location upstream of the fan exit; and (c) a fluid system coupled to a second flowpath of the at least one heat exchanger. The first and second flowpaths are thermally coupled to each other.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Provisional Patent Application Ser. No. 61/091,553 filed Aug. 25, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to gas turbine engines and methods for oil cooling in such engines. 
         [0003]    Gas turbine engines are commonly provided with a circulating oil system for lubricating and cooling various engine components such as bearings, gearboxes, and the like. In operation the oil absorbs a substantial amount of heat that must be rejected to the environment in order to maintain the oil at acceptable temperatures. Commonly, the oil is circulated through an oil-to-fuel heat exchanger where heat from the oil is rejected to the fuel, which acts as a heat sink. The fuel is subsequently injected into the engine&#39;s combustor and burned. 
         [0004]    In many operating conditions, aircraft gas turbine engines have more oil heat load than heat sink from the fuel which will be burnt in the engine. The typical solution to this is to either cool engine fuel or engine oil with engine fan air, or to pump fuel through the oil-to-fuel heat exchanger at a higher rate than required for combustion, with the excess fuel flow being recirculated from the engine back to the aircraft fuel tanks. Low-bypass military turbofan engines have too many fan stages (typically three) to make fan air cooling a viable solution, because the fan duct discharge air is too hot. Therefore, tank recirculation is used. 
         [0005]      FIG. 1  depicts an example of a prior art aircraft gas turbine engine  10  with a fuel tank recirculation system. The engine  10  has a fan  12 , a high pressure compressor  14 , a combustor  16 , a high pressure turbine  18 , and a low pressure turbine  20 , all arranged in a serial, axial flow relationship. The engine  10  is operable to generate a core flow of exhaust gases as well as a bypass flow in a conventional manner. In the illustrated example, the engine  10  is a low-bypass turbofan in which a portion of the flow from the fan  12  is directed around the core in a bypass duct  22 . The bypass flow and the core flow both exit into an afterburner duct  24  which has an afterburner flameholder  26  disposed at its upstream end. 
         [0006]    A fuel-to-oil heat exchanger  28  is coupled to the lubrication system  30  of the engine  10 . A feed pump  32  pumps fuel from the tanks  34  of the aircraft (not shown) through the fuel-to-oil heat exchanger  28  where it absorbs heat from the oil. The fuel then passes downstream where it is metered into the combustor  16  and burned. In many cases the heat load required to be rejected from the oil is greater than the heat sink capacity of the fuel at the required fuel flow for the engine operating condition. For example, this can occur when the oil is at a high temperature and the fuel flow is low (e.g. flight idle). Accordingly, to get sufficient cooling, fuel is supplied to the fuel-to-oil heat exchanger  28  at the required rate for cooling, then the excess above that needed for engine operation is routed back to the tanks  34 . 
         [0007]    During ground idle the fuel in the tanks  34  may become very hot and it may become necessary to use ground support equipment to cool the fuel. During flight, tank fuel temperature increases at an ever increasing rate as the tanks  34  become near empty. Furthermore, this raises the temperature of the fuel as subsequently supplied to the engine  10 . The engine  10  may have limits on the acceptable input fuel temperature. For example, the fuel may be needed at a relatively low temperature for cooling a full authority digital engine control (FADEC) or other electronics. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    These and other shortcomings of the prior art are addressed by the present invention, which provides a method and apparatus for using fan bleed air to cool oil in a gas turbine engine. 
         [0009]    According to one aspect of the invention, a heat exchanger system for a gas turbine engine includes: (a) a fan having at least two stages of rotating fan blades surrounded by a fan casing, the fan operable to produce a flow of pressurized air at a fan exit; (b) at least one heat exchanger having a first flowpath in fluid communication with the fan at a location upstream of the fan exit; and (c) a fluid system coupled to a second flowpath of the at least one heat exchanger. The first and second flowpaths are thermally coupled to each other. 
         [0010]    According to another aspect of the invention, a gas turbine engine includes: (a) a fan having at least two stages of rotating fan blades surrounded by a fan casing, the fan operable to produce a flow of pressurized air at a fan exit; (b) a heat exchanger having a first flowpath in fluid communication with the fan upstream of the fan exit; (c) at least one heat source disposed in the engine remote from the heat exchanger; and; (d) a fluid circuit coupled between the at least one heat source and a second flowpath of the heat exchanger, and operable to circulate a working fluid therebetween. The first and second flowpaths are thermally coupled within the heat exchanger. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0012]      FIG. 1  is a schematic view of a gas turbine engine incorporating a prior art heat exchanger system; 
           [0013]      FIG. 2  is a schematic view of a gas turbine engine incorporating a heat exchanger system constructed according to an aspect of the present invention; 
           [0014]      FIG. 3  is cross-sectional view of a portion of the fan section of the engine shown in  FIG. 2 , having a heat exchanger mounted thereto; 
           [0015]      FIG. 4  is a plan view of the heat exchanger of  FIG. 3 ; 
           [0016]      FIG. 5  is a view taken along lines  5 - 5  of  FIG. 4 ; 
           [0017]      FIG. 6  is a view taken along lines  6 - 6  of  FIG. 4 ; 
           [0018]      FIG. 7  is an enlarged view of a portion of  FIG. 3 ; and 
           [0019]      FIG. 8  is a view taken along lines  8 - 8  of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 2  depicts an exemplary gas turbine engine  110  incorporating a fan bleed heat transfer system constructed in accordance with an aspect of the present invention. The basic engine  110  is a low-bypass turbofan configuration substantially similar in construction to the engine  10  described above. For illustrative clarity only the three-stage fan  112  and the fan casing  114  are shown in detail. The fan  112  discharges pressurized air to the downstream compressor at a fan exit  115 . 
         [0021]    One or more bleed air heat exchangers  116  are mounted to the fan casing  114  in close proximity to the fan  112 . The heat exchangers are of the air-to-liquid type and are as described in more detail below. Scavenge pumps  118  are provided which remove heated oil from sumps  120  and a gearbox  122  of the engine  110  and pump it to an oil tank  124 , after removal of air in an air/oil separator  126 . While the sumps  120  and gearbox  122  are commonly found in gas turbine engines, oil or another liquid could also be used to remove heat from any other heat source within the engine  110 . 
         [0022]    The hot engine scavenge oil flows from the oil tank  124  to the bleed air heat exchangers  116  where heat is removed from the oil. A bypass valve  128  is provided to assure continuous oil flow in the oil system in the event oil congeals in the bleed air heat exchanger  116  (for example, due to exceptionally cold fan bleed air passing through the bleed air heat exchanger  116 ). 
         [0023]    The fan bleed air is used to cool the engine oil. As shown in  FIG. 2 , the engine oil may be used directly as the liquid-side working fluid for the fan bleed heat exchangers  116 . Optionally another fluid, such as fuel or a water-glycol mixture, may be used as an intermediate medium to transfer heat from the engine oil to the bleed air heat exchanger  116 . 
         [0024]    After exiting the bleed air heat exchangers  116 , the oil may pass through a conventional oil-to-fuel heat exchanger  130  where, depending on operating conditions, heat is transferred from the oil to the fuel, or from the fuel to the oil. The oil is then returned to the sumps  120  and gearbox  122  by a supply pump  132 . 
         [0025]      FIG. 3  is a side view of the engine  110  showing the location of the bleed air heat exchanger  116 . The bleed air heat exchanger  116  is mounted to the exterior of the fan casing  114  and is positioned to receive airflow bled from the fan  112  upstream of the fan exit  115 , as shown generally by the large arrow. For illustrative purposes only a single bleed air heat exchanger  116  is shown, but it will be understood that a plurality of them could be positioned around the periphery of the fan casing  114 . 
         [0026]      FIGS. 4-6  illustrate the bleed air heat exchanger  116  in more detail. It is an air-to liquid configuration and has fore and aft plenums  134  and  136  including an inlet  138  and an outlet  140 , respectively. The plenums  134  and  136  communicate with a series of parallel liquid channels  142 , which may include fins  144  (see  FIG. 5 ) or other heat transfer enhancements. The liquid channels  142  are separated by air channels  146  which may also include fins  148  (see  FIG. 6 ) or other heat transfer enhancements. Within the bleed air heat exchanger  116 , the liquid channels  142  constitute a first flowpath and the air channels  146  constitute a second flowpath. As with all heat exchangers, the two flowpaths are mutually thermally coupled, that is, they are arranged such that heat energy can flow from one flowpath to the other. 
         [0027]      FIG. 7  illustrates the bleed air flow path. As shown by the solid arrows, air discharged from the fan first stage blades  149  passes aft and radially outboard past the outer platforms  150  of the second stage vanes  152 , through existing gaps between the periphery of the outer platforms  150  and the fan casing  114 . A radial gap  154  between the outer platforms  150  and the fan casing  114  allows air flow in a circumferential direction. The air then bleeds through the fan casing  114  through one or more bleed apertures  156 . The number, shape, size, and position of the bleed apertures  156  may be selected in a known manner to permit adequate mass air flow to the bleed air heat exchanger  116  with an acceptable pressure loss, and to throttle the bleed flow to prevent excessive loss from the fan air flow. A plenum  158  may be provided between the fan casing  114  and the bleed air heat exchanger  116  to permit fore-and-aft air flow. Depending on the specific engine and fan configuration it may be possible to bleed air from another stage of the fan  112 . 
         [0028]    While the air temperature at the tip of the fan first stage blades  149  is relatively low and thus suitable for cooling, the air discharge pressure is quite small, and only the static pressure is available for bleed air cooling. The available heat exchanger air pressure drop is the blade tip discharge pressure less the pressure drop through the second stage vane outer platforms  150 , the bleed apertures  156  and the bleed air heat exchanger  116 , minus the fan cowl static pressure outside the bleed air heat exchanger  116  (which is essentially ambient pressure). In order to obtain adequate air side heat transfer with this very low pressure drop, the bleed air heat exchanger  116  uses a large ratio of air frontal face area to air flow depth (i.e. radial thickness). A secondary advantage of this configuration is that hot air from the bleed air heat exchanger  116  is directed radially outboard, away from temperature-sensitive components such as electrical cables. 
         [0029]    With proper selection of the various components described above the total heat sink available in the bleed air and the fuel scheduled for combustion will be equal to or greater than the heat load required to keep the oil at an acceptable temperature. Therefore, no heat will be transferred to the aircraft tanks in the form of heated, recirculated fuel. This includes the most critical operating conditions where combustion fuel flow is low, for example, ground idle, cruise, and flight idle conditions. Furthermore, in some flight conditions, the bleed air heat exchanger  116  not only dissipates heat which would otherwise return to the tanks, it also cools the engine fuel at several flight conditions (negative fuel-oil heat exchanger heat transfer) thus providing lower fuel temperature to the engine fuel nozzles with less likelihood for nozzle fuel coking. 
         [0030]    The foregoing has described a heat exchanger for a gas turbine engine and a method for its operation. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only.