Abstract:
A gas turbine engine has a fan, a compressor section, a combustor, and a turbine section. The fan delivers a portion of air into the compressor, and into a duct, as bypass air. A bleed air system bleeds a quantity of air from the compressor into a chamber at least at low power conditions of the engine. The bleed air system has an opening which may be selectively closed to block bleed air, or opened to allow bleed flow from the compressor into the chamber. A heat exchanger is positioned such that a first surface of the heat exchanger is contacted by bypass air in the duct, and a second surface of the heat exchanger is contacted by bleed air when the bleed air system directs air from the compressor into the chamber.

Description:
BACKGROUND 
     This application relates to a cooler for use in a gas turbine engine. 
     Gas turbine engines are known, and typically include a fan delivering air into a compressor. The air is compressed and passed into a combustion section. From the combustion section, the air is mixed with fuel and ignited, and then passes over turbine rotors. 
     A number of accessories are associated with gas turbine engines. As an example, a generator generates electricity. Various fluid supply systems such as oil supply, fuel supply, etc. deliver fluids around the gas turbine engine. Many of these accessories require some degree of cooling and may receive lubricant, which also requires cooling. Thus, there are a number of heat exchangers in a gas turbine engine. 
     Typically, so-called air-to-fluid heat exchangers have been placed in a location where a single source of air will pass over the heat exchanger. 
     As one example, a heat exchanger may be placed in a bypass air duct, such that cooling air being driven by the fan will pass across the heat exchanger. 
     Alternatively, heat exchangers have been placed in other locations where air may be driven through the gas turbine engine. 
     The current known location for such heat exchangers have resulted in unduly large heat exchangers. 
     SUMMARY 
     In a featured embodiment, a gas turbine engine has a fan, a compressor section, a combustor, and a turbine section. The fan delivers a portion of air into the compressor, and a portion of air into a duct, as bypass air. A bleed air system bleeds a quantity of air from the compressor into a chamber at least at low power conditions of the engine. The bleed air system has an opening that may be selectively closed to block bleed air, or opened to allow bleed flow from the compressor to the chamber. A heat exchanger has fluid containing passages to be cooled. The heat exchanger is positioned such that a first surface is contacted by bypass air in the duct, and a second surface is contacted by bleed air when the system is directing air from the compressor into the chamber. 
     In a further embodiment according to the previous embodiment, fins are formed on the first surface of the heat exchanger extending into the bypass air flow. 
     In a further embodiment according to the previous embodiment, fins are also formed on the second surface of the heat exchanger and extend toward the bleed air flow. 
     In a further embodiment according to the previous embodiment, the fins on the second surface extend for a greater height away from the fluid containing passages than do the fins on the first surface. 
     In a further embodiment according to the previous embodiment, the fins on the first surface extend for a greater length along a flow path of the bypass air than do the fins on the second surface. 
     In a further embodiment according to the previous embodiment, the heat exchanger selectively cools a fluid associated with a generator for the gas turbine engine. 
     In another featured embodiment, a gas turbine engine has a heat exchanger with a first convective cooling feature and a second convective cooling feature. The first convective cooling feature is disposed at least partially in fluid communication with a bypass air flow of the engine, wherein the second convective cooling feature is disposed at least partially in a bleed air chamber of the engine. 
     In a further embodiment according to the previous embodiment, a selectively controllable valve controls bleed air flow from a compressor of the engine to the bleed air chamber. 
     These and other features may be best understood from the following drawings and specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a gas turbine engine. 
         FIG. 2  schematically shows an oil cooling system for an accessory in a gas turbine engine. 
         FIG. 3  shows the location of a heat exchanger. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  22 , a compressor section  24 , a combustor section  26  and a turbine section  28 . Alternative engines might include an augmentor section (not shown) among other systems or features. The fan section  22  drives air along a bypass flowpath while the compressor section  24  drives air along a core flowpath for compression and communication into the combustor section  26  then expansion through the turbine section  28 . Although depicted as a turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. 
     The engine  20  generally includes a low speed spool  30  and a high speed spool  32  mounted for rotation about an engine central longitudinal axis A relative to an engine static structure  36  via several bearing systems  38 . It should be understood that various bearing systems  38  at various locations may alternatively or additionally be provided. 
     The low speed spool  30  generally includes an inner shaft  40  that interconnects a fan  42 , a low pressure compressor  44  and a low pressure turbine  46 . The inner shaft  40  is connected to the fan  42  through a geared architecture  48  to drive the fan  42  at a lower speed than the low speed spool  30 . The high speed spool  32  includes an outer shaft  50  that interconnects a high pressure compressor  52  and high pressure turbine  54 . A combustor  56  is arranged between the high pressure compressor  52  and the high pressure turbine  54 . A mid-turbine frame  57  of the engine static structure  36  is arranged generally between the high pressure turbine  54  and the low pressure turbine  46 . The mid-turbine frame  57  further supports bearing systems  38  in the turbine section  28 . The inner shaft  40  and the outer shaft  50  are concentric and rotate via bearing systems  38  about the engine central longitudinal axis A which is collinear with their longitudinal axes. 
     The core airflow is compressed by the low pressure compressor  44  then the high pressure compressor  52 , mixed and burned with fuel in the combustor  56 , then expanded over the high pressure turbine  54  and low pressure turbine  46 . The mid-turbine frame  57  includes airfoils  59  which are in the core airflow path. The turbines  46 ,  54  rotationally drive the respective low speed spool  30  and high speed spool  32  in response to the expansion. 
     The engine  20  in one example is a high-bypass geared aircraft engine. In a further example, the engine  20  bypass ratio is greater than about six (6), with an example embodiment being greater than a ratio of approximately 10:1, the geared architecture  48  is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3:1 and the low pressure turbine  46  has a pressure ratio that is greater than about 5:1. In one disclosed embodiment, the engine  20  bypass ratio is greater than about 10:1, the fan diameter is significantly larger than that of the low pressure compressor  44 , and the low pressure turbine  46  has a pressure ratio that is greater than about 5:1. Low pressure turbine  46  pressure ratio is pressure measured prior to inlet of low pressure turbine  46  as related to the pressure at the outlet of the low pressure turbine  46  prior to an exhaust nozzle. The geared architecture  48  may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.5:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine is applicable to other gas turbine engines including direct drive turbofans. 
     A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section  22  of the engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft, with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFC’)”—is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point. 
     To make an accurate comparison of fuel consumption between engines, fuel consumption is reduced to a common denominator, which is applicable to all types and sizes of turbojets and turbofans. The term is thrust specific fuel consumption, or TSFC. This is an engine&#39;s fuel consumption in pounds per hour divided by the net thrust. The result is the amount of fuel required to produce one pound of thrust. The TSFC unit is pounds per hour per pounds of thrust (lb/hr/lb Fn). When it is obvious that the reference is to a turbojet or turbofan engine, TSFC is often simply called specific fuel consumption, or SFC. 
     “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tambient deg R)/518.7)^0.5]. The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second. 
     This application relates to a heat exchanger in such a gas turbine engine which utilizes cooling air from two distinct sources such that the heat exchanger may be made smaller than the prior art. 
       FIG. 2  shows a circuit for cooling oil such as may be associated with an accessory  152 . In one example, accessory  152  may be a generator. Hot oil leaves the generator  152  after cooling the generator, and enters an inlet  156  associated with a heat exchanger  154 . The oil passes through a series of passages  155  from the inlet manifold, to an outlet manifold, and then the outlet  158  of the heat exchanger. This oil is then returned to the generator  152 , having been cooled by air. 
       FIG. 3  shows a location for the heat exchanger  154 . The location may be approximately at area X as shown in  FIG. 1 . As shown, the heat exchanger  154  is located such that one side has convective cooling features  160  that are disposed at least partially within the duct  100  that is in fluid communication with bypass airflow B (see  FIG. 1 ). The convective cooling features  160  may be fins, pins, projections, ribs, etc. The features  160  provide surface area for convective cooling. These features  160  may be relatively small, as they will extend into the bypass air flow, and it may be desirable to limit obstruction to this flow. The size and geometry of the features  160  may be optimized to consider both the weight of the heat exchanger, drag within the bypass air flow, and convective cooling magnitude. 
     The opposed side of the heat exchanger  154  has features  162  which tend are disposed at least partially within a bleed air supply chamber  200 . 
     As known, under bleed conditions, bleed air is air downstream of a compressor rotor  164  (which may be part of the low pressure compressor  44 , see  FIG. 1 ) that is diverted out of the core engine flow and into the chamber  200 . This typically occurs at low power conditions, and serve to reduce the load of downstream compressor stages, and the rest of the engine by not driving unnecessary air through the engine. This may occur while the aircraft is idling on the ground, as an example. 
     The heat exchanger  154  is located such that the features  162  are in fluid communication with the bleed air flow. The size and geometry of the features  162  may be optimized to consider both the weight of the heat exchanger, drag within the bleed air flow, and convective cooling magnitude. These low pressure conditions are also coincidentally a most challenging time for the heat exchanger  154  to be adequately cooled by the bypass air alone in duct  100 . As an example, there may be a limited amount of bypass air under those conditions. In the past, the heat exchangers cooled by bypass air have been necessarily unduly large, as they must still adequately cool the fluid even under the low power conditions. 
     The features  160  extend away from a surface of the heat exchanger  154  for a lesser distance than the features  162  extend away from the opposed surface. This is because the features  160  extend into the bypass air flow, and as mentioned above, it would be desirable to limit their height. 
     On the other hand, the features  160  may extend for a greater distance along a flow path of the bypass air B than do the features  162 . This is generally as illustrated in  FIG. 3 . 
     In one embodiment, the features  160  extend away from a surface of the heat exchanger for 0.40″ (1.02 cm), and extend along the path of the bypass air flow B for 6″ (15.2 cm). In this same embodiment, the features  162  extend away from the surface for 0.5″ (1.27 cm), and extend along the flow path for 4″ (10.2 cm). 
     As is known, a mechanism  68  can selectively close off the passage  166  and block further bleed air flow. Examples of mechanisms include valves and gates that may be mechanically and/or electromechanically controlled. This occurs as the engine moves more toward full power operation. However, under those conditions, the bypass airflow will be greatly increased in volume, and should adequately cool the fluid in the heat exchanger  154  on its own. 
     Although an embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the true scope and content of this disclosure.